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(1)AGH University of Science and Technology The Faculty of Electrical Engineering, Automatics, Computer Science and Biomedical Engineering. mgr inż. Tomasz Kuczek. Vacuum circuit breaker switching in medium voltage networks with photovoltaic panels Operacje łączeniowe wyłącznikiem próżniowym w sieciach średniego napięcia z uwzględnieniem paneli fotowoltaicznych Ph. D. Thesis Under the supervision of: Ph. D. Eng. Marek Florkowski Kraków, 2015.

(2) I would like to express my thanks to Mr Ph. D. Eng. Marek Florkowski for his supervision of this thesis. I am grateful for all the guidance and suggestions.. I am also thankful for valuable remarks given by my colleagues from ABB Corporate Research Center in Kraków.. 2.

(3) TABLE OF CONTENTS List of symbols and abbreviations ....................................................................................... 5 1.. 2.. PREFACE ..................................................................................................................... 8 1.1. Introduction ............................................................................................................. 8. 1.2. Literature research – state of the art.......................................................................... 9. 1.3. Motivation for thesis’ research ............................................................................... 12. 1.4. Thesis contents and structure ................................................................................. 13. TRANSIENTS IN POWER SYSTEMS ..................................................................... 15 2.1. Basics of high frequency transient effects .............................................................. 15. 2.2. Transients classification according to international standards ................................. 17. 2.3. Possible transient scenarios in PV power plants ..................................................... 19. 3. VACUUM CIRCUIT BREAKERS AND ASSOCIATED TRANSIENT PHENOMENA ............................................................................................................... 24 3.1. Overview of current breaking technologies ............................................................ 24. 3.2. Dielectric properties of arc in vacuum .................................................................... 25. 3.3. Construction and application properties ................................................................. 27. 3.4. Chopping current effect during breaking operation................................................. 29. 3.5. Multiple arc re-ignitions phenomenon .................................................................... 31. 3.6. Arc initiation during closing operation of VCB ...................................................... 35. 4. PHOTOVOLTAIC POWER PLANTS – BACKGROUND INFORMATION AND MODELING TECHNIQUES ........................................................................................ 36. 5.. 4.1. Solar energy – status, trends, market share ............................................................. 36. 4.2. PV cells ................................................................................................................. 39. 4.3. DC/DC converters.................................................................................................. 47. 4.4. DC/AC converters.................................................................................................. 51. 4.5. Transformers.......................................................................................................... 58. 4.6. Medium voltage cables and transmission lines ....................................................... 61. 4.7. Overvoltage mitigation .......................................................................................... 64. 4.8. Modular central inverters ....................................................................................... 72. LABORATORY TEST STAND SETUP AND MEASUREMENT RESULTS ........ 73 5.1. Introduction ........................................................................................................... 73 3.

(4) 6.. 7.. 8.. 5.2. Test stand description ............................................................................................ 75. 5.3. Measurement of steady state voltage, no-load and inrush current ........................... 79. 5.4. LC filters at the 20 kVA transformer at the network load side ................................ 81. 5.5. LC filters at the 250 kVA transformer at the network supply side........................... 87. 5.6. Operation of low voltage AC circuit breaker .......................................................... 96. SIMULATION RESULTS........................................................................................ 103 6.1. Introduction ......................................................................................................... 103. 6.2. Validation process description ............................................................................. 104. 6.3. Model description ................................................................................................ 107. 6.4. LC filters at the 20 kVA transformer at the network load side – simulation results 110. 6.5. Simulation results – additional verifications ......................................................... 123. 6.6. Simulations of transient conditions in PV power plants ........................................ 128. 6.7. Discussion on possible overvoltage mitigation methods ....................................... 140. DISCUSSION AND CONCLUSIONS ..................................................................... 147 7.1. Summary ............................................................................................................. 147. 7.2. Analyses of transient states in PV Power Plants ................................................... 148. 7.3. Overvoltage mitigation methods – conclusions and recommendations .................. 153. 7.4. Reference to main thesis ...................................................................................... 154. 7.5. Further activities related to transients on PV power plants.................................... 156. BIBLIOGRAPHY ..................................................................................................... 158. 4.

(5) LIST OF SYMBOLS AND ABBREVIATIONS A BIL C0 Cd Cpg Cpp Cps Csg CSI CZ du/dt EMI EMTP-ATP ESS ETAP f FF fn GIS GSTC GTO HF HV I0 ich IEC Ig IGBT Imp IN1 IN2 ISC ISC,%/K k l. diode ideality factor (quality factor, emission coefficient) Basic Insulation Level equivalent capacitance of cable-transformer compartment DC link capacitor phase-to-ground capacitance of transformer primary winding phase-to-phase capacitance of transformer primary winding capacitance between primary and secondary winding of the transformer phase-to-ground capacitance of transformer secondary winding Current Source Inverter network capacitance overvoltage steepness Electromagnetic Interference Electromagnetic Transients Program Alternative Transients Program Energy Storage Systems Electrical Power System Analysis frequency of system voltage Fill Factor natural frequency of voltage oscillations Gas Insulated Substation irradiance at Standard Test Conditions Gate Turn-off Thyristor High Frequency High Voltage transformer no-load current chopping current in vacuum circuit breaker International Electrotechnical Commission total current generated by entire series-parallel PV string Insulated Gate Bipolar Transistor current at Maximum Power Point nominal current at primary winding of transformer nominal current at secondary winding of transformer short circuit current of PV cell temperature coefficient for short circuit current of PV cell Boltzmann’s constant length of cable 5.

(6) L(f) L(ψ) L0 LCL Ld Lp LP1, LP2 LV LZ ma MCB mf MOSFET MPP MPPT MV n Np Ns PCC PF Pmp PSCAD PV PWM R(f) R0 RDDS RFe RRDS S SCR SPWM T tdel topen TRV TSTC. frequency dependent winding inductance of the transformer nonlinear magnetization characteristic equivalent inductance of cable-transformer compartment inductive-capacitive-inductive filter DC link inductor transformer inductance at primary side inductances of connections at both sides of vacuum circuit breaker Low Voltage network side equivalent inductance amplitude modulation index in Sinusoidal Pulse Width Modulation Miniature Circuit Breaker frequency modulation index in Sinusoidal Pulse Width Modulation Metal-Oxide Semiconductor Field-Effect Transistor Maximum Power Point of PV cell Maximum Power Point Tracker Medium Voltage transformer ratio number of parallel connections of PV cells number of series connections of PV cells Point of Common Coupling power factor maximum power output of PV cell Power System Computer Aided Design Photovoltaic Phase Width Modulation frequency dependent winding resistance of the transformer per unit resistance of cable Rate of Decrease of Dielectric Strength of vacuum circuit breaker contact gap iron core losses Rate of Rise of Dielectric Strength of vacuum circuit breaker contact gap apparent power Semiconductor Controlled Rectifier Sinusoidal Phase Width Modulation temperature delay time opening time instant of a circuit breaker Transient Recovery Voltage temperature at Standard Test Conditions 6.

(7) UB U1, U2, U3 U8/20;10 uk% UN1 UN2 Up UR v V0 VCB Vd VFT Vg VLL Vmp VOC VOC,%/K VRT VSC Xp ΔPCu ΔPFe ψ(t). breakdown voltage voltage of propagating transient waves residual voltage at 8/20 µs current impulse and maximum peak of 10 kA transformer impedance voltage nominal voltage at primary winding of transformer nominal voltage at secondary winding of transformer maximum overvoltage peak value dielectric withstand of the vacuum circuit breaker contact gap overvoltage wave propagation speed output voltage of the converter Vacuum Circuit Breaker input voltage of the converter Very Fast Transients total voltage generated by entire series-parallel PV string line-to-line RMS voltage voltage at Maximum Power Point of PV cell open circuit voltage of PV Cell temperature coefficient for open circuit voltage of PV cell Voltage Ride Through Voltage Source Converter transformer impedance transformer load losses transformer no-load losses flux linkage in transformer core. 7.

(8) 1.. PREFACE. 1.1 Introduction Instant growth of electrical power systems, distribution, transmission and generation capacity is one of the most important research issues for many companies and universities. Growing demand for safe and reliable power supply networks forces ones to investigate various ways of energy harvesting. Nowadays, it can be seen that solar energy might be one of the most important energy sources in the world, with instant growth of generation capacity of power plants as well as development of intelligent systems. It can be stated that considering all kinds of solar power plants the photovoltaic ones can be seen as the future. It is approximated that by year 2020 energy generated in solar power plants will have been equal to 20% of the output all alternative energy sources [32]. This will force development of systems and methods that will prevent any damage or failure of such power plant. Ensuring safety is essential at the distribution as well as at the transmission level [32, 62, 84, 110, 120]. There are several significant advantages that favour enormous potential of solar energy. Most of all, it is totally pollution free, unlike any fossil fuels. However, greenhouse effect is not a concern here. Also maintenance costs are very low, since the fuel is “free of charge”. Moreover, researchers estimate that the Sun should be stable for approximately next 5 billion years, until entire hydrogen in the core will be exhausted [97]. Thus, technically it can be stated that this is an infinite source of energy. In the photovoltaic (PV) power plant energy is generated by series-parallel connected PV cells that work under various irradiations and ambient temperatures, thus generate different levels of voltage and current. The output is controlled by low voltage DC/DC and DC/AC converters (boost/buck converters and inverters). The interconnection with the transmission system is done by medium voltage switchgear equipped with appropriate apparatus for measuring, switching, protection and overvoltage mitigation [7, 9]. At certain conditions there is a need for shutting down part of the PV power plant, either from the network operational point of view or for the protection purposes – in case of emergency switching or faults. For this purpose, vacuum or SF6 circuit breakers (both commonly present in modern medium voltage systems) are utilized. This work is mainly focused on switching operations performed by vacuum circuit breaker. It uses vacuum as a quenching medium for electrical arc suppression, which appears across breaker contacts during any switching operation. Thanks to this method, the dielectric withstand between circuit breaker contacts is approximately 10 times larger than in air at atmospheric pressure. As a result, it is possible to decrease the gap between contacts inside the vacuum chamber of the circuit breaker. Furthermore, dimensions of entire medium voltage switchgear can be significantly decreased. Moreover, vacuum circuit breaker is characterized by large mechanical withstand and lack of poisonous effect of the arc, so negative influence on the environment is eliminated. Finally, maintenance of this apparatus is 8.

(9) much easier than traditional oil or SF6 breakers. However, there is one significant disadvantage of this device – during the opening operation there is a possibility of current chopping effect. In an ideal circuit, soon after contacts’ separation, an electric arc ignites and burns until first zero crossing on sinusoidal current waveform, when it is finally extinguished. In vacuum circuit breakers the arc becomes unstable at low currents (3 to 5 A for modern constructions) due to low quantity of electric carriers, which leads to sudden chopping to zero within several microseconds. For specific network conditions it may lead to overvoltages, which are characterized by high peak value, significant steepness and high frequency oscillations. In the worst cases such effect multiplies, leading to so-called multiple arc reignitions. This may speed up ageing processes of insulation systems of cables, transformers, machines and other electrical apparatus [122]. Since some PV installations (and other, where vacuum circuit breaker is utilized) require frequent switching on and off operations (even several times per day) they should be investigated with main focus on generation of overvoltages in different topologies and working conditions. This thesis presents a comprehensive approach to the phenomena described above. It outlines research work conducted in the laboratory as well as in the simulation software. Overvoltages resulting from vacuum circuit operations were measured in the real set-up for several scenarios. Results served as an input for preparation of computer models that were used for analyses of situations that were impossible to obtain in the laboratory due to network limitations. Computer models were prepared both in PSCAD and EMTP-ATP software packages, which allowed to indicate differences in modelling techniques with special focus on high frequency models of cables, transformers and vacuum circuit breakers. Results from laboratory set-up were compared to the ones achieved during simulations. Finally, several methods for overvoltages mitigation are presented and discussed.. 1.2 Literature research – state of the art Power systems transients were discussed in the past by means of traditional approach during electromagnetic circuit theory course. Since the very beginnings of transmission and distribution power systems development, all tests have to be conducted on site or in the laboratory. Nevertheless, several important theories for power systems’ analyses were developed over the end of 19th and beginning of 20th century [13]. First work that was related to power systems transients was conducted in 1854. Kelvin described wave propagation on a distributed parameter line in order to derive signal distortion for Trans-Atlantic telephone cable [64]. His research was extended and confirmed by Heaviside’s transform publication [49]. Significant contribution was given by symmetrical components theory that served as the basis for the switching and fault studies in 3-phase circuits [39]. Formulas for theories related to travelling waves [16], conductor internal impedance [116], earth-return impedance [21] and admittance [139] calculations for overhead lines were developed. Most of those methods are still utilized. Analyses of lightning current parameters’ calculation for insulation coordination studies were also conducted [23, 58, 88]. Last but not least, there are methods related to 9.

(10) determination of transformer parameters for numerical studies. One of the most important topics is calculation of losses in windings and other parts of the transformer [142 – 144]. There was no extensive method developed that was necessary for studies of complicated issues such as nonlinearities of machines, transmission line’s frequency dependent characteristics or travelling waves behaviour. Two milestone books can be distinguished in this areas, namely H.A. Peterson’s Transient Performance in Power Systems and A. Greenwood’s Electric Transients in Power Systems, published in 1951 and 1971 respectively [137]. Those books addressed a lot of basics in area of system studies as well as provided fundamental concepts based on the Laplace transform. It served for future work, since the mid-1960s researchers started to use computers for power systems simulations. First electromagnetic transients programs (EMTP) were based on Dommel’s computation algorithms [27] published in 1969. After that many interesting milestone papers were published which treated about various aspects of transients, both related to laboratory measurements as well as to simulations. Authors addressed such aspects as capacitor bank switching, transformers energization, insulation coordination, Very Fast Transients and vacuum circuit breaker switching in medium voltage networks. Since that time various software packages have been developed over the years. Nowadays, the market is shared among several vendors. One of the most popular software are PSCAD, EMTP-ATP, EMTPRv, DigSilent PowerFactory and ETAP. Currently these software packages are very sophisticated tools which are used not only for studies related to transients, but also for load flow, voltage stability, power electronics, harmonics and other. All of these areas are related to applications of photovoltaic power plants. Photovoltaic cells are based on photo-electric effect that was discovered in 1839 by a French physicist Edmond Becquerel. Then, in 1888, first US patent was granted to Edward Weston for his solar cell [138]. However, for over 100 years it was not used in any commercial application. First practical PV module was patented and constructed by BELL company in US in order to provide power source for spacecrafts and artificial satellites [22]. Since then, development of PV modules has significantly increased on a large market scale. First PV cells achieved efficiency in the range of 10%. Over the years this level has raised up to 25% for Si mono-crystalline cells and up to 27.6% for GaAs thin film-cells [46]. However, it has to be added that such efficiency is achieved in laboratory set-ups for strictly selected operating conditions. In real installations it can be stated that nowadays the average efficiency is in the range of 5-17%. PV cells were also extensively studied over the past years by means of appropriate simulations. It was necessary in order to determine working characteristics of PV cells in the domain of Sun’s irradiation, temperature and load connected to its terminals. For that purpose, researchers use Matlab [45], EMTP-ATP [114, 115] or PSCAD [91] in most frequent cases. Another part of research that is extensively conducted these days is modelling and simulation of grid-connected or standalone inverters that supply the load from irradiated PV modules. First papers related to PV systems analyses were published in 1980s. They 10.

(11) contained both on-site measurement results as well as some simulations. Harmonics and power factor in residential installations with single-phase inverters were analysed in [20] and [89]. In further years more and more papers were published in various areas. Main interests were focused on controlling algorithms of Maximum Power Point Trackers (MPPT) with real and reactive power flow control [33, 141] and application of various topologies of solar inverters [43, 109]. As mentioned earlier, power generated in large photovoltaic installations has to be transmitted to the medium voltage grid. Most often the vacuum circuit breaker is used there for breaking and making operations – it is installed in the medium voltage switchgear as a fully integrated device. First publications related to vacuum contactors are dated at 1890, when the first patent for vacuum switch was awarded [31]. It is visible that it is another example of technology described in this paper (along with PV cells), which was invented in 19th century, but over 100 years had to pass until it was well established on the market. In the mid-1960s GE developed its first working power vacuum interrupter, rated at 15 kV of nominal voltage and 12 kA of breaking current [100]. After that other companies began their development of the vacuum circuit breakers on a larger scale. Moreover, there were several attempts to deliver vacuum interrupter able to break high currents at voltages at transmission level to the market. Such circuit breaker was demonstrated in 1967 and was rated at 132 kV and 15.3 kA. What is interesting, it was equipped with 8 vacuum breaking chambers in series [134]. However, experience has shown that while interruption of high currents is still possible, withstanding of high voltages at transmission level is extremely difficult. Thus, further work in this area was ceased and more effort was put to develop low cost vacuum circuit breakers that were suitable for medium voltage levels [122]. Transient overvoltage effects related to switching operations with use of vacuum contactors have been recognized since 1970s, when those devices became dominant at distribution transformers and motors. Important guide was published in 1971 [48], where applications of vacuum circuit breakers were discussed along with some corrective measures and remarks. In 1974 mitigation overvoltages resulting from vacuum switch operations in motor applications were discussed in [93]. Later, in 1977 researchers discussed issues related to chopping current levels in vacuum circuit breakers as well as other types of interrupters, such as minimum oil, air blast and SF6 [94]. Important paper was published in 1988 by Greenwood and Glinkowski [47], where voltage escalation due to multiple arc re-strikes was analysed during unsuccessful motor starting. Particular attention was given to the di/dt interrupting capability of the vacuum circuit breaker as well as to the rate of rise of the dielectric recovery of the contact gap. Several papers were also published in the area of failure analysis that was caused by voltage escalation during opening operations. It was conducted in the area of distribution transformers [85, 99, 118] shunt reactors [95] and arc-furnace transformers [119]. Also studies of rate of rise of dielectric recovery during de-energization of transformer with secondary side faulted were conducted, for example in [124]. Resonance conditions as well as overvoltage distribution along transformer’s windings were extensively studied in [36] and [37]. Papers related to computer analyses of behaviour of vacuum circuit breaker transients were also 11.

(12) published, starting from late 1990s when simulation software had reached a more established position in the market. An EMTP model was presented in 1995 with implementation of chopping current effect and dielectric withstand rise [68]. Such approach was also used in many other papers, for example in [44, 80, 101, 140]. Lately, extensive reports have been published by CIGRE study group. Part 1 [29] contains general expertise about vacuum circuit breaker related transients, whereas part 2 [30] is devoted to exemplary case studies for various topologies and applications. However, none of the present available papers or reports was strictly related to transient overvoltage effects caused by VCB switching operations with PV installation involved at the low voltage side of the operated transformer. Several papers in this area were published by the author of this thesis [71 – 81].. 1.3 Motivation for thesis’ research The purpose of conducting the research presented in herein thesis is based on several reasons. As mentioned in the chapter above, nowadays large photovoltaic plants’ market is significantly growing, thus the need for confident and reliable control, measuring and switching devices will also increase. This thesis will mainly focus on effects related to transient overvoltages resulting from switching operations performed by vacuum circuit breaker (VCB) in medium voltage switchgears. It has to be mentioned that this switching device and phenomena related to it are not a novelty and are well described in literature, technical papers standards etc. However, as will be reported later, the switching conditions are significantly dependent on many factors, especially related to the network topology, not only at the medium voltage side, but also at low voltage. This is due to the fact that in large photovoltaic installations transformer is very often connected directly to the inverter that is equipped with various inductive-capacitive LC filters. These may change the impedance seen from operated terminals of VCB, thus influencing the natural frequency of switched circuit. It impacts resulting overvoltages that are generated during breaking and making operations, both in terms of maximum overvoltage peak values as well as their steepness. Due to the fact described above, there exists a special need for laboratory research as well as computer modelling of such networks in order to determine if significant overvoltages are possible in designed topology. There is a need to extend the knowledge in this area in order to strengthen the capabilities of the recognition of possible risky topologies. Thanks to that unfavourable switching conditions may be avoided. If this was not possible due to technical or economical limitations, appropriate overvoltage mitigation devices may be proposed. It has to be added that vacuum circuit breaker transients do not occur only on large PV power plants. Work conducted in this thesis may be also important for other applications, such as distributions transformers, motors, wind farms and arc furnaces. Based on the above motivation, laboratory test stand was designed and prepared in high voltage laboratory at AGH University of Science and Technology in Kraków, Poland. Several measurements of transformer energization and de-energization by means of vacuum circuit breaker were conducted. The purpose of these tests was to measure transient overvoltages in 12.

(13) several network configurations. Thanks to experiments, well fitted computer simulation models were prepared in PSCAD and EMTP-ATP. This will allow to use these tools for more extensive studies as well as for recognition of other potential risk scenarios. This is very useful since not all scenarios can be studied experimentally due to limitations related to maximum network loading or short circuit capability. Moreover, credible simulations can save time and money during base design preparation of new or modernized medium voltage networks, where appropriate precautions against transient overvoltages have to be proposed.. 1.4 Thesis contents and structure Based on the motivation described above as well as on conducted research studies, thesis for this work was prepared. It is necessary to determine topologies and scenarios for grid connected large photovoltaic power plants that can be risky from the point of view of switching operations. For the working transformers and apparatus, overvoltages that are characterized by high overvoltage peak values and significant steepness (du/dt) are the most hazardous. Thus, several aspects have to be taken into account in order to study possible transients on PV power plant. For all possible scenarios the topology of the studied network is the most important. By topology one can understand the arrangement and lengths of interconnecting MV cables as well as transformer ratings. Moreover, overvoltage mitigation devices have to be also incorporated. Finally, crucial aspect is to include the low voltage side in the study, thus the power electronic converters along with their LCL filters, cables and LV circuit breakers and disconnectors. Another aspect is the power generation during steady state operation – hence the environmental conditions, namely sun’s irradiation and ambient temperature. For the emergency states, along the above mentioned factors, switching sequences that follow the established grid codes have to be respected. If possible, more reasonable switching scenarios can be suggested, which could prevent or decrease hazardous transient overvoltages generation. Justification presented above results in the following thesis that is proposed for this work: It is possible to determine operating conditions and topologies of large photovoltaic grid connected power plants, where switching of vacuum circuit breaker in medium voltage switchgear may result in generation of multiple arc re-ignitions and high overvoltage peak values. Inclusion of electrical apparatus installed at operated transformer low voltage side is critical from the point of view of proper overvoltage analysis. Teza: Istnieje możliwość określenia warunków eksploatacyjnych oraz topologii elektrowni fotowoltaicznych przyłączonych do zewnętrznej sieci dystrybucyjnej, w których operacje łączeniowe dokonywane wyłącznikiem próżniowym w rozdzielni średniego napięcia mogą doprowadzić do generacji ponownych zapłonów łuku oraz wysokich wartości przepięć. Uwzględnienie aparatury zainstalowanej po stronie niskiego napięcia transformatora sprzęgającego z siecią dystrybucyjną jest istotne z punktu widzenia poprawnej analizy przepięciowej. 13.

(14) Chapter 2 starts with an overview of transient effects that are present in the high frequency domain. Phenomena of wave reflections and resonances are briefly described. Furthermore, classification of transients according to international standard IEC 60071-1 was presented. Both low frequency and high frequency transients that can occur in modern power systems were described. Finally, possible hazardous transient states that can occur in photovoltaic (PV) power plants are presented. Subjects related to vacuum circuit breaker are discussed in chapter 3. Firstly, an overview of current breaking technologies was provided. Other technologies than vacuum are discussed, namely SF6, air blast and oil. Regarding the vacuum itself, subjects related to voltage breakdown mechanisms and construction details were addressed. Finally, phenomena associated with current breaking and making were discussed. Effects of current chopping as well as multiple arc re-ignitions occurrence are explained. Chapter 4 comprehensively describes PV power plants. At first, solar market energy was discussed. Also, other types of solar plants were mentioned, namely thermal concentrated ones. PV cells were in details described in terms of operating principles, types of PV panels and numerical representation. Separate subsections are dedicated to description of DC/DC and DC/AC converters. Basics of operations as well as typical topologies are mentioned. Finally, description of all measuring, switching and protective devices that are installed in typical PV farm was provided. Technical details regarding modelling techniques were also provided. Section 5 was dedicated to description of laboratory setup that was designed and prepared in the high voltage laboratory at AGH University of Science and Technology. Conducted experiments regarding overvoltages resulting from vacuum circuit breaker switching were described. Distribution transformers rated at 20 kVA and 250 kVA were energized and de-energized under various configurations. For each scenario, primary side voltage of transformer was measured. All conducted tests are described and results are given in the form of current and voltage oscillograms. Tabularized summary was also provided. Simulation results which were based on laboratory measurements were described in Chapter 6. PSCAD and EMTP-ATP software packages were utilized for the modelling purposes. Process of models fitting was explained. The goal was to achieve satisfying convergence of measurement and simulation results in terms of overvoltage peak value, steepness and natural frequency of oscillations. Chapter 6 contains verification of achieved laboratory measurement results as well as some additional simulations of scenarios that could not be performed in the laboratory due to technical limitations also described there. Finally, discussion about possible overvoltage mitigation methods was attached. Chapter 7 contains discussion as well as conclusions with special focus on most hazardous scenarios in PV plants from the point of view of vacuum circuit breaker operations. Details regarding possible overvoltage mitigation methods were provided. Issues related to transients’ suppression in terms of overvoltage peak values as well as steepness were discussed for all studied methods. Entire work content was summarized with respect to thesis. Finally, further activities related to transients in PV power plants were proposed. 14.

(15) 2.. TRANSIENTS IN POWER SYSTEMS. 2.1 Basics of high frequency transient effects Transient overvoltages are associated with several effects, which may occur even during one singular switching operation. Overvoltages may be generated during any lightning or switching event. Energization and de-energization of transmission lines, capacitor banks, transformers, motors and shunt reactors is a concern. Various aspects and phenomena are involved during mentioned events. This chapter is mainly focused on interaction of transients with transformers. Overvoltages can be divided into two groups, namely internal and external. The following sections will discuss both with main focus on external incoming overvoltages causing wave reflections as well as internal resonances and voltage breakdowns. Finally, since this work is related mainly to switching operations, the capabilities of current breaking with respect to Transient Recovery Voltage between the operated contacts are discussed. 2.1.1 Wave reflections Due to the fact that any switching operation generates an overvoltage wave, it is important to take wave reflections into consideration during the modelling and analyses. It is well known that propagating overvoltage wave behaves differently in parts of power systems with various surge impedances [24, 34, 42, 70]. It can be reflected, reduced or amplified at any point of discontinuity (Figure 2.1). Z1 represents surge impedance of transmission line or cable, whereas Z2 stands for surge impedance of the transformer. Figure illustrates also overvoltage waves U1, U2 and U3 – incoming wave, reflected wave and transmitted wave. U2 U1 U3 Z1 Z2 Figure 2.1. Wave reflections phenomenon, Z1 –transmission line surge impedance, Z2 – transformer’s surge impedance, U1 – incoming wave, U2 – reflected wave, U3 – secondary side transmitted wave [29] The reflected and transmitted wave’s magnitudes can be calculated with equations [51]: = =. − +. 2 +. (2.1) (2.2). Based on above equations one can note that every time an overvoltage wave encounters an open end (or very large impedance) the voltage at this point is doubled. As a practical 15.

(16) example, one can imagine a circuit where transformer is supplied from MV switchgear through a cable. Transformer’s surge impedance is in the range of 3÷5 kΩ, whereas for MV cables it accounts for approximately 40÷50 Ω. This will results in doubled voltage at the transformers medium voltage terminals. It has to be added that such calculations should be treated as an approximation, especially in terms of overvoltage transferred to the secondary side of the transformer. Such winding response shall be measured in the real tests or simulated. However, in order to conduct credible simulations, a wideband model of the transformer has to be prepared based on frequency sweep measurement [29]. 2.1.2 Resonance Severe overvoltages may occur when transformer interacts with cable or transmission line. This is due to the resonating energy between the inductive and capacitive parts of the transformer-network compartment. At certain frequency that is specific for each system, capacitive and inductive reactances are cancelling out each other. In other words, the corresponding RLC circuit consumes only real power, since the imaginary part of the network impedance “vanishes”. This may lead to significant rise of voltage or current. One has to also distinguish external and internal resonances. The first one is composed out of the network and transformer parameters, whereas the latter is mainly related to analyses of overvoltages within the windings. Those do not always coincide, since for high frequency transients winding appears to the system as a frequency dependent impedance [38]. Inner transformer resonances may arise for various frequencies, not one only. Voltage distribution across the windings is non-uniform and is also dependent on the secondary side loading of the transformer. Therefore, frequency sweep measurements shall be conducted, as it was done in example in [29] and [82]. Exemplary frequency characteristic of single phase separating transformer rated at 230V/230V and 0.2 kW is presented in Figure 2.2. Measurement was conducted with three techniques: with use of Solatron SI1260 Impedance/Gain phase analyzer as well as with designed circuit based on measuring resistors (27 Ω and 136 Ω). Several resonant peaks can be seen. Satisfactory level of measurement convergence is visible for all three methods.. Y, [S]. f, [Hz] Figure 2.2. Exemplary transformer frequency characteristic, three measuring techniques: 1 – Solatron SI1260 analyzer, 2 – measuring resistor 27 Ω, 3 – measuring resistor 136 Ω [82] 16.

(17) Computer modelling of the transformers subjected to steep front impulses is very complicated. An extensive research was conducted in this area and most frequent model that is proposed is based on RLC components connected in cascades (Figure 2.3). Accuracy for higher frequencies can be achieved by increased number of subdivided circuits. The model is composed out of “w” number of windings that are split into “n” sections. The following components are incorporated here: · C – capacitances: to earth, longitudinal and insulation’s, · G –conductance, · R – resistance, · (L11)1 – self inductance, · L11-12, L11-1w, (L1n)1 – mutual inductances.. Figure 2.3. Transformer winding model for high frequency studies [38]. 2.2 Transients classification according to international standards In section 2.1 basic information about transients’ interaction with transformers was indicated. Similar reasoning may be applied to all kinds of transformers (power, distribution, arc furnace) as well as shunt reactors and motors. However, the origin of transient overvoltages can be different. The insulation of working machines and apparatus can be stressed by different phenomena. Overvoltages can be generated during various types of phenomena and are mainly characterized by their magnitude, rise time and frequency. The IEC 60071-1 standard provides comprehensive classification of overvoltages that can occur in power systems [53]. It is divided into two subgroups, namely for low frequency and high frequency transients. Table 2.1 is an extract from standard mentioned above – it provides frequency range, standard voltage shape and standard withstand voltage test (if applicable) for all types of transients. 17.

(18) Table 2.1. Classification of overvoltages in power systems, according to IEC 60071-1 [53] Class Category. Low frequency Continuous. High frequency. Temporary. Slow Front. Fast Front. Very Fast Front. Shape. Tf ≤ 100 ns Frequency range. f = 50 Hz or 60 Hz Tt > 3 600s. 10 Hz < f < 500 Hz 0.02 s ≤ Tt ≤ 3 600 s. 20 μs < Tp ≤ 5 000 μs T2 ≤ 20 ms. 0.1 μs < T1 ≤ 20 μs T2 ≤ 300 μs. 0.3 MHz < f1 < 100 MHz 30 kHz < f2 < 300 kHz. Standard voltage shape. (*). Standard withstand voltage test. n/a. short duration power frequency test. switching impulse test. lightning impulse test. (*). Origin. long terms load variations. faults, load rejection, ferroresonance. inductive or capacitive currents switching. lightning surges. SF6 disconnector operating inside GIS switchgear. (*)To be specified by the relevant apparatus committees. Low frequency transients are divided into continuous and temporary ones. Continuous overvoltages are caused by long term load variations and are normally controlled by appropriate load flow management by transmission system operator. Shunt reactors can be also utilized in order to compensate capacitive reactive power that causes voltage rises. Temporary transients are the power frequency overvoltages caused by oscillations during phase to ground and phase to phase faults, as well as load rejection and ferroresonance effects. Their frequency is in the range of 10 to 500 Hz and magnitudes around 2.0 p.u. The class of high frequency transients is related to any switching or lightning events. Slow front transients are caused by switching operations, such as line or transformer energization and have a frequency in the range of 200 up to 20000 Hz. Their magnitudes and rise times are very much dependent on various factors, like system earthing, type of load that is switched as well as presence of overvoltage mitigation devices. Lightning overvoltages are in the group of fast-front transients caused by atmospherical electric discharge into an overhead transmission line. Frequency of this phenomenon is in the range of 20 kHz up to 1 18.

(19) MHz. Those overvoltages are a significant concern in high voltage power systems, since they generate overvoltage waves, which propagate along the system and can cause failures of working machines and apparatus. Due to that fact, surge arresters with appropriate ratings are installed, which can significantly reduce magnitudes of these overvoltages. Oscillations with the highest frequencies are called Very Fast Transients and are a result of disconnector operating or internal faults inside Gas Insulated Substations (GIS). The Very Fast Transients (VFT) in power systems cover a frequency range from 100 kHz up to hundreds of MHz. Their magnitude is in the range of 1.5 to 2.0 p.u. of the line-to-neutral voltage crest, but they can also reach values as high as 2.5 p.u. to 3 p.u. in case of ultra high voltage systems. These values are usually below the Basic Insulation Level (BIL), but VFT can accelerate aging of insulations and degradation processes due to their frequent occurrences.. 2.3 Possible transient scenarios in PV power plants Various switching operations are possible in photovoltaic power plants, some of them are planned and some may be emergency ones. This chapter presents possible switching sequences that may lead to transient overvoltages [74]. Not all of them are hazardous for working transformers and apparatus. Such PV plant, apart from solar modules, has to be equipped with control, monitoring and protection systems and apparatus. General layout that this thesis is concerned with is presented in Figure 2.4 [7, 9, 76, 77, 112]. There are multiple topologies available for PV strings, inverters and transformer. The purpose of the presented diagram is to indicate general idea of photovoltaic plant operating principles. It can be fully adapted to the grid connected PV plants. However, partially the diagram also refers to the stand-alone installations like houses, traffic lights etc. – medium voltage transformer and interconnection to the external grid is neglected then. Instead of that one may encounter energy storage systems (ESS) at LV side [15], which guarantee electric energy supply also when lighting is poor or when it is dark. However, ESS are out of scope of work of this thesis. PV strings. DC disconnectors. DC/DC. DC/AC. Sine wave LCL filter. LV/MV transformer AC CB. DC CB. MV cables. VCB. external grid or fixed load control and grid monitoring. Figure 2.4. General grid connected photovoltaic power plant layout All events have been more extensively analysed during numerical simulations presented in section 6. More conclusions about potential overvoltage and damage threats were discussed. Every photovoltaic power plant consists of several essential apparatus that provide high performance for the power generation. All switching sequences were described in relevant figures, numbers indicate the order of closing/opening operations. 19.

(20) A. PV power plant start sequence The start sequence of the photovoltaic plant can be considered as always planned, not accidental. Typical switching sequence is presented in Figure 2.5 and it is as follows: 1. DC switch closing, DC link charging due to PV panels being irradiated by the Sun. 2. Vacuum circuit breaker closing, transformer at no-load state. 3. AC switch closing – reference voltage from network side provided at inverter control. 4. Phase angle and frequency of current or voltage of the fundamental component is recognized by the inverter control – inverter is synchronized “online” with network, power flow can be controlled by appropriate adjustment of inverter’s output voltage phase angle and magnitude [86]. PV. power electronic converters. (1st) DC CB DC. (4th). (3rd) AC CB. DC. DC. (2nd). LV/MV transformer. external grid. VCB MV cables. MV cables. AC. Figure 2.5. PV power plant start sequence From the point of view of vacuum circuit breaker operation, the one and only issue is the transformer energization at no load state, since voltage breakdown between closing contacts can be expected during terminals closing, leading to arc pre-strikes. However, this is commonly considered as a low hazard for the transformers insulation. B. PV power plant unsuccessful start sequence – VCB opening on inrush current The unsuccessful transformer energization is inevitably related to flow of inrush current. It may happen that sometimes due to wrong coordination of protection devices, transformer will be switched off several tens of miliseconds after energization, as described below according to Figure 2.6. 1. Vacuum circuit breaker closing, transformer not loaded. 2. Vacuum circuit breaker opening, breaking of inrush current. Such sequence will results in very unfavourable conditions that are hard to predict, since high value of inrush current is switched off, which leads to possibility of chopping of several Amperes of current. Possibility of multiple arc re-ignitions and voltage escalation is very high in such scenario. PV. power electronic converters. DC CB DC. AC CB. DC. DC. (1st) and (2nd). LV/MV transformer. VCB MV cables. external grid. MV cables. AC. Figure 2.6. PV power plant unsuccessful start sequence – VCB opening on inrush current of the transformer 20.

(21) C. PV power plant scheduled shut down sequence The sequence of PV plant shut down is not often discussed in the literature. From the point of view of avoiding of large current breaking, at first inverter should be blocked. This results in no load state of the LV/MV transformer, thus vacuum circuit breaker can be operated with decreased hazard of overvoltage generation. It is important to switch off the DC breaker at the last sequence, under no-load conditions. Finally, following switching sequence can be distinguished according to Figure 2.7: 1. 2. 3. 4. PV. Inverter shut down, blocking signals for power electronic switches AC circuit breaker opening, transformer at no-load state Vacuum circuit breaker opening, transformer de-energized DC switch opening under no-load condition power electronic converters. (4th) DC CB DC. (1st). (2nd) AC CB. DC. DC. (3rd). LV/MV transformer. external grid. VCB MV cables. MV cables. AC. Figure 2.7. PV power plant shut down sequence – planned D. PV power plant shut down – VCB opening at variable loads Sometimes transformer is switched off by means of vacuum circuit breaker under load conditions that are different due to variable Sun’s irradiation. Such accidental tripping should be avoided since even several dozens of Amps can be interrupted, which results in hazardous conditions in terms of earlier mentioned overvoltage escalation and multiple arc re-ignitions. power electronic converters. PV DC CB. AC CB DC. DC. DC. (1st). LV/MV transformer. external grid. VCB MV cables. MV cables. AC. Figure 2.8. PV power plant shut down, VCB opening at variable loads E. VCB emergency switching off during short circuit conditions The protection relay’s coordination is always well-defined in the international standards as well as in the grid codes, which are often different for each country. In this chapter vacuum circuit breaker switching transients have been investigated under short circuit conditions, both when earth fault occurred at low voltage or medium voltage side [56]. At this point, the so-called grid code has to be discussed. Each country has its own rules regarding the strategies for management of decentralized power sources (such as PV or wind farms). PV power plants must not be disconnected from the MV network immediately after fault occurrence – they are required to stay connected with the system for a specified time. This is often referred to as Voltage Ride Through (VRT). It is due to the fact that nowadays 21.

(22) distributed energy sources play more and more important role in medium and high voltage power systems, thus their contribution to the power flow and voltage stability becomes more significant. PV plants are required to be connected during voltage sags (i.e. resulting from short circuit) and inject capacitive current that should depend on the minimum voltage measured at the PCC (Point of Common Coupling) in order to increase the voltage [25]. Exemplary grid code’s fault conditions for Germany are presented in Figure 2.9 [131].. Figure 2.9. Dynamic grid support during fault conditions – Germany [131] Figure above presents percentage voltage sag (vertical axis) versus time on horizontal axis. As can be seen, three possible areas of operation of PV power plant were distinguished: 1) cannot be disconnected – it has to be connected with the system for 1-ph, 2-ph and 3-ph earth faults with near-zero voltage for at least 150 ms; 2) may be disconnected but does not have to – it is up to an agreement between the network and plant operator; 3) has to be disconnected by protection relays. Since this thesis is focused on vacuum circuit breaker operation, author has concentrated on area 3, where PV plant has to be disconnected. Two different scenarios were distinguished, according to diagrams in provided Figures: -. VCB tripping during 3-phase fault at MV side – Figure 2.10, VCB tripping during 3-phase fault at LV side at AC – Figure 2.11. power electronic converters. PV DC CB. AC CB DC. DC. DC. (2nd). LV/MV transformer MV cables. AC. PCC. VCB. external grid. MV cables. (1st). Figure 2.10. VCB opening during short circuit at MV side If the short circuit occurs at the MV side, mainly vacuum circuit breaker at MV side is subjected to opening operation. During such event (Figure 2.10), especially if earth fault is metallic (low impedance), transformer’s impedance has small contribution to the earth fault loop. However, it is strongly dependent on the number of transformers connected to the same 22.

(23) bus since this influences the short circuit power of the network. If the transformer’s side impedance is neglected, than almost pure resistive current is being interrupted. Short circuit occurring at low voltage side can be also problematic. Assuming that earth fault occurred at the low voltage DC side, first action would be DC circuit breaker triggering (or fuse triggering, if used instead of DC circuit breaker). They are utilized for overcurrent protection of cables as well as power electronic switched in DC/DC and DC/AC converters. Moreover, earth fault currents that are fed only from PV plant are not higher than the maximum short circuit current resulting from PV panel’s characteristics (section 4.2), so their peak values are close to the rated ones. In case of fault at DC side with multiple PV strings, the resulting fault current may be also fed from other strings. Overcurrent protection of cables is necessary then since the fault current may exceed the service current. Depending on the requirements of the customer as well as considered standards, appropriate fuses or DC switchdisconnectors (circuit breakers) may be utilized. It can be also mentioned that the short-circuit at DC side may be also supplied from MV side through the inverter’s capacitors and parallel diodes. This is due to the fact that when the inverter’s transistors are blocked, it is seen from the AC side as diode bridge rectifier. Thus the inverter acts as a rectifier. However, this thesis considers a scenario when PV plant is also connected to the external grid, hence the short circuit would be still supplied from the MV side. This can result in VCB emergency tripping (according to rules described in Figure 2.9), which technically is de-energization of the LV/MV transformer with short-circuited LV terminals. Such situation is illustrated in Figure 2.11. At first, AC CB disconnected the PV side, but the transformer is still connected to the medium voltage network. Several dozens of Amps are interrupted (depending on the transformer’s rated power and impedance), which can also be a threat from the point of view of transient overvoltage escalation. power electronic converters. PV DC CB. AC CB DC. DC. DC. (2nd). LV/MV transformer MV cables. AC. PCC. VCB. external grid. MV cables. (1st). Figure 2.11. VCB opening during short circuit at LV side at AC It has to be finally mentioned that VCB opening under short circuit conditions is associated not only with photovoltaic power plants. All the possible scenarios described in chapters above can be adopted with some modifications to other applications, such as distribution transformer, motors, arc furnaces, wind farms etc.. 23.

(24) 3.. VACUUM CIRCUIT BREAKERS AND ASSOCIATED TRANSIENT PHENOMENA. 3.1 Overview of current breaking technologies There are several types of current interrupters, which are distinguished by means of arc quenching medium that is used therein. The following types of circuit breakers can be pointed out: oil, air blast, magnetic blast, SF6 and vacuum. Typically, each interrupter consists of main current bus, arc quenching chamber and drive with appropriate control systems. Oil-type current interrupters are using insulating oil as a quenching medium [63, 127]. The arc is burning inside the quenching chamber filled with oil, which is rapidly heated up. The oil vaporizes and is decomposed mainly to the hydrogen, which has very high thermal conductivity. Thus, the arc is suppressed and current is interrupted at next zero crossing. Oiltype current interrupters are divided into two subgroups: bulk and minimum oil circuit breakers. The bulk type uses oil for both arc suppression as well as insulation of active parts of the breaker, whereas the minimum type uses oil only for arc quenching. All other contacts and poles are insulated with porcelain. Oil-type circuit breakers were used in the past thanks to their simple construction, low costs and easy maintenance. However, breaking capabilities were at certain point insufficient for continuously developing power systems. Moreover, accidents of oil explosions were recorded. Due to the above facts, oil circuit breakers were successfully replaced by SF6 and vacuum types. The air circuit breakers were used in the past mainly for high voltage networks as well as for generators at medium voltage. They use compressed air at 1.5 to 3 MPa as an arc quenching medium – increased pressure provides higher dielectric withstand. During the opening operation, once the contacts are moving, a compressed air is blasting at the burning arc. The arc is elongated and extinguished. At some constructions additional damping parallel resistors are connected in order to decrease the rate of rise of transient recovery voltage, which can be very rapid. It is an actual disadvantage of air blast construction and comes out of the fact that during first cycle of arc quenching the dielectric withstand is small and rises later. This may result in arc re-ignition, which is suppressed by additional damping resistors. Nowadays, in modern medium voltage networks two technologies are dominant, namely SF6 and vacuum. They use different techniques and are characterized by advantages and disadvantages. The SF6 current interrupter utilizes pressurized sulphur hexafluoride as an insulator and arc quenching medium. It is characterized by very high dielectric strength and lower costs [98]. Moreover, following a breakdown, SF6 is able to regenerate itself thus restoring the original withstand. It is used in modern high voltage substations (up to 1100 kV of nominal voltage), which thanks to SF6 properties, require less amount of space for installation when compared to traditional air insulated units (reduction even by 90%) [5]. 24.

(25) Their main disadvantage is related to the fact that during breaking of high currents, temperature is rising and SF6 is being decomposed to SF4, SF2 fluorides as well as toxic S2F10. The other competitive type of circuit breaker is the vacuum interrupter, which is the focal point of this paper. This device utilizes advantages of air at very low pressure (in the range of 10-6 mbar), at which the amount of free electrons is negligible. Moreover, the mean free path is long, which results in very good arc suppression capabilities. Thanks to those facts, the breaking chamber is very compact and can be easily mount with entire drive in the medium voltage substation. Moreover, it is pollution and maintenance free. However, at certain conditions vacuum circuit breaker may produce multiple arc re-ignitions leading to overvoltage escalation. There is no simple answer to which circuit breaker, SF6 or vacuum type, is better. They are both suitable for switching of transformers, capacitor banks, shunt reactors or transmission lines. They are also comparable in terms of short-circuit current breaking capabilities as well as achieved dielectric withstand once the contacts are fully open. Engineering, economical, service and environmental issues have to be taken into account during selection between those two types of current interrupters in the design process of the medium voltage switchgear. Basic comparison is provided in Table 3.1. Table 3.1. Comparison of SF6 and vacuum circuit breakers Criteria SF6 interrupter Vacuum interrupter cumulative breaking current. 50 x rated short circuit current 10000 x rated continuous current. number of operations between servicing. 5000 to 20000 C–O operations. overhaul of interrupter. complete dismantling of the interrupter, high labour costs, low material cost. suitability for single and multishot auto-reclose cycles environmental and pollution issues switching applications. 100 x rated short circuit current 20000 x rated continuous current 10000 to 20000 C–O operations vacuum level check, if necessary interrupter replace, low labour cost, high material cost. well-suited. well-suited. yes. no. overhead and cable feeders, motors, transformers, shunt reactors. 3.2 Dielectric properties of arc in vacuum Vacuum circuit breakers are commonly used in modern medium voltage networks due to the fact that dielectric properties that characterize vacuum have many advantages when it comes to insulation and electric arc quenching capabilities. At first, some facts about dielectric behaviour of gases should be pointed out. It was established by Friedrich Paschen that the breakdown voltage between two electrodes is a function of the product of gas pressure p and distance d between those electrodes – Figure 3.1 illustrates theoretical and experimental curves. At normal atmospheric pressure (or above), the breakdown voltage UB grows along with the product of p and d. It can be explained by the 25.

(26) fact that electrons must acquire sufficient energy between two collisions in order to ionize gas molecules, thus creating other electrons [19]. UB [kV]. theoretical characteristic. experimental characteristic. UB min. (p·d)mi p·d [hPa cm]. Figure 3.1. Paschen curve – initial breakdown voltage UB of air in function of product of gas pressure p and gap d [35] In vacuum this rule no longer exists – it is possible for electrons to acquire high energy because of their significant mean free path, but the probability that they can encounter another molecule is very low. A typically utilized vacuum chamber in circuit breakers operates under pressure in range of 10 -3 to 10-7 hPa. In example, under pressure equal to 10-6 hPa, a 1 mm3 volume still contains around 27·106 gas molecules. However, as mentioned, path between two collisions is in the range of hundreds of meters and their interactions are negligible. It was stated above that in vacuum possibility of electrons’ avalanche causing breakdown occurrence decreases significantly according to Paschen’s curve, thus dielectric withstand between electrodes rapidly improves. Breakdown voltage UB of vacuum in function of distance between electrodes d is presented in Figure 3.2 [19].. UB [kV]. d [mm] Figure 3.2. Breakdown voltage in vacuum, function of gap between contacts, 10 -6 hPa [19] 26.

(27) Although vacuum has been proven to be a robust insulation medium, an electric breakdown between electrodes can occur causing electric discharges. In vacuum interrupters this is related mainly to phenomenon called field emission. Electrons are extracted from metal surface of electrodes as a result of very high temperature rise [19, 122]. Plasma is made up of electrons and high speed ions. This phenomenon occurs during breaking of current. Despite fast contacts movement (range of 1 m/s), electric arc starts to burn instantaneously when circuit breaker opens. As an effect of that, temperature rises significantly, causing electrons’ field emission from metal surfaces, which provide “conducting path” along vacuum gap.. 3.3 Construction and application properties Like all circuit breakers, vacuum interrupter consists of two contacts: fixed and moving one. Exemplary concept of vacuum interrupter is presented in Figure 3.3. Contacts are inside a vacuum chamber, which works as an insulation medium. Entire chamber is embedded in an external insulation, typically porcelain or epoxy. Upper and lower contact terminals are used to connect the interrupter inside the medium voltage switchgear. Finally, the operating mechanism works as an energy-storing device, based on latches and springs. 1 – upper contact terminal 2 – vacuum chamber 3 – epoxy resin enclosure 4 – lower contact terminal 5 – flexible connector 6 – contact force spring 7 – insulated coupling rod 8 – opening spring 9 – shift lever 10 – drive shaft 11 – release mechanism 12 – mechanism enclosure with spring operating mechanism Figure 3.3. Vacuum circuit breaker in embedded pole technique, construction schema [11] When moving contact starts to open, an electric arc is formed and flows through the evaporated surface particles from the cathode. The electric arc in vacuum can occur in two different forms: diffuse mode and constricted mode. The diffused mode adopts for a current range from a few Amperes to a few kA. Mainly it is characterized by several arc spots at the cathode that emit neutral plasma made up of electrons (high speed ions). Typically cathode spot is very small with radius in the range of 5 to 10 μm. Moreover, at those spots very high temperature and electric field exist in the 27.

(28) range of 5000 K and 5 GV/m, respectively. As shown in Figure 3.4a, anode reacts as a passive charge collector since its entire surface is immersed in the plasma. In the situation when current rises significantly, a constricted mode starts to form (Figure 3.4b). Arc is being contracted first at the anode, then at the cathode. Furthermore, more and more electrons are attracted by the anode resulting in appearance of a luminous spot larger when compared to that at the diffuse mode (range of ~1 cm2). This phenomenon causes also arc contraction on cathode side due to the fact that preferential current path is created. Finally, large cathode spot that corresponds to anode spot is created and constricted arc is established (Figure 3.4c). a). b). c). A – anode, C – cathode. Figure 3.4. Electric arc, transformation from diffuse to constricted mode; a – diffused arc, b – contraction at anode, c – contraction at anode and cathode (constricted arc) [11] As previously mentioned, electric arc is characterized with very high temperature that can cause overheating and melting of the surface of contacts. In order to mitigate erosion effects, arc is rotated inside the vacuum chamber thanks to special spiral geometry of contact surface, as presented in Figure 3.5.. Figure 3.5. Electrical arc in vacuum chamber with radial magnetic field [11] Thanks to utilized geometry a radial magnetic field is created in entire arcing area. Electromagnetic forces caused by current flow have azimuthal direction, which causes rotation of the arc around the contacts’ axis. As a result of that arc length increases, providing 28.

(29) high quenching capability. Moreover, thanks to arc movement and rotation, erosion effect is significantly decreased. For the correct functioning of the described arc length increase technology, a satisfactory compromise has to be achieved – for too large width of slots arc propagation from one slot to another may be difficult, which results in stationery mode at one side causing unnecessary overheating of the spot. On the other hand, too small slots can be filled by the fusion of contact material leading to immobilization of the current path [19].. 3.4 Chopping current effect during breaking operation Despite advantages of vacuum in terms of dielectric withstand properties a lot of various conditions have to be fulfilled in order to successfully interrupt the current. It was reported in many references that vacuum circuit breaker switching may pose significant overvoltage hazards for switched devices, like machines, transformers or shunt reactors. Several factors have influence on overvoltages’ generation. Figure 3.6 presents simplified circuit of transformer being switched off with use of vacuum circuit breaker [103]. Such circuit can be easily adapted to switching of shunt reactors or electrical machines, too.. +. Figure 3.6. Transformer switching off – simplified single line diagram; U –network source voltage, LZ, CZ – network inductance and capacitance, LP1, LP2 – inductance of connections at both sides of vacuum circuit breaker W, C0, R0, L0 – equivalent capacitance, resistance and inductance of transformer During current breaking an electric arc starts to conduct between electrodes as a result of electrons field emission. This is common behaviour for all kinds of circuit breakers, which take advantage of the natural zero crossing of the current (twice per period – so each 10 ms for 50 Hz frequency). In an ideal situation, once an arc is ignited it should burn until first zero crossing when it immediately quenches. However, in certain circuits arc can become unstable, which results in so-called chopping current effect, as illustrated by waveforms in Figure 3.7. At this stage, a single arc ignition is discussed. Chopping of the current before its natural zero crossing results in oscillations of energy trapped in circuit formed of transformer’s capacitance and inductance (formula (3.1). This results in overvoltage peak value Up and a frequency of oscillations fn that are specified by equations (3.2) and (3.3) respectively: 1 2. =. 1 2. (3.1). 29.

(30) ∙. = =. 2. 1. (3.2). (3.3). where: ich – chopping current level, [A], L0 – equivalent inductance of de-energized circuit, [H], C0 – equivalent capacitance of de-energized circuit, [F]. u, i u(t). i(t) Up. UN. IN t. ich. current chopping. oscillations fn. Figure 3.7. Inductive current breaking, chopping current effect, IN, UN – nominal current and voltage during steady state, ich – chopping current, Up – maximum overvoltage peak value, fn – frequency of voltage oscillations after current breaking Value of chopped current ich is mainly dependent on metals selected for contacts surface (Table 3.2) as well as on the level and form of interrupted current. Analytical prediction of this value is very complex. Smeets approached this problem in [123]. It was found that the chopping current is dependent on the load current amplitude. The explicit expression that was proposed for chopping current calculation is provided below: = (2 ∙. where: f – frequency of system voltage, [Hz], i – amplitude of load current, [A], α = 6.2·10-16, [s], β = 14.3, q = (1 – β)-1.. ∙. ∙ ∙. ∙ ). (3.4). 30.

(31) Utilization of this equation with proposed constants results in values of the chopping current that corresponds to those presented in Table 3.2 for Ag or Cr contacts (approximately for 3.5 to 7 A of ich). Moreover, this approach is commonly considered in studies and often provided by literature, i.e [50, 103, 122]. Nowadays, research is focused on copper based contact materials. Most of modern vacuum circuit breakers are equipped with surfaces made of Cu-Cr alloy. Table 3.2. Chopping current levels for different surface materials of electrodes [122] Electrodes Average chopping current Maximum chopping current material [A] [A] Ag 3.5 6.5 Cu 15 25 Cr 7 16 Ni 7.5 14 W 16 350. 3.5 Multiple arc re-ignitions phenomenon Waveforms presented in section above illustrate successful current breaking near first zero crossing. However, very often this process is not so effective, especially when the Transient Recovery Voltage (TRV) has higher rate of rise than the dielectric withstand of vacuum circuit breaker’s contact gap. In the studies it is commonly assumed that this dielectric withstand curve may be approximated by linear rate of rise of dielectric strength (RRDS). Typically, it is reported that RRDS value accounts for between 2 to 50 V/µs [67]. The dielectric recovery UR(t) of vacuum gap as a function of time can be calculated according to formula (3.5): ( )=. −. +. (3.5). where: UR – dielectric withstand, [kV], A – rate of rise of dielectric strength (RRDS), [V/µs] or [kV/ms], B – constant, often referred to as initial dielectric withstands, [kV], topen – circuit breaker’s opening time instant, [s]. Based on equation and explanation it can be deduced that the rate of rise of recovery voltage of vacuum gap is essential from the point of view of breaking capabilities in terms of multiple arc re-ignitions’ occurrence. Unfortunately, manufacturers do not provide such data, so it has to be determined by means of laboratory measurements. However, some theoretical clarification can also be provided. Two withstand curves are known, namely cold and hot. The cold withstand curve characteristic’s slope is mainly determined by contact’s movement velocity during opening operation. At the moment when contacts start to separate the gap is cold in the sense of room temperature, especially at low current flows (below several hundreds of Amperes). Although in reality dielectric withstand does not rise perfectly 31.

(32) linearly, it is commonly assumed that it may be approximated by linear rise. Going further, once the contacts start to separate electric arc is ignited. Perfect vacuum is no longer present due to metallic particles that are emitted from electrodes as a result of thermionic and field emissions. The cold gap withstand characteristic is the most important one from the point of view of simulations [18, 90]. Results of conducted research are provided in Figure 3.8 [111]. UB [kV]. t [µs]. Figure 3.8. Cold gap breakdown characteristic of the vacuum gap, U B – breakdown voltage, t – time after contacts started to separate [111] The hot gap characteristic plays more important role once the vacuum arc is extinguished. After last spark, the gap is still hot due to temperature rise. Moreover, metal vapours and ions are still present in the gap, which causes certain time delay in the recovery of the dielectric strength. Exemplary hot gap characteristic is provided in Figure 3.9. Time zero referred to moment when arc is extinguished and 40 kV is the full withstand when the contacts are fully separated. It has to be mentioned that typically this effect is not considered in the studies and plays less important role in the analyses of multiple arc re-ignitions’ occurrence. UB [kV]. t [µs]. Figure 3.9. Hot gap breakdown characteristic of the vacuum gap, UB – breakdown voltage, t – time after contacts started to separate [111] Figure 3.10 illustrates the process of voltage breakdown during opening operation of VCB. At the moment when contacts start to open just before current zero crossing, a previously mentioned current chopping effect may occur, which results in transient 32.

(33) overvoltage. At the time instant when the recovery voltage TRV becomes too high and exceeds the dielectric withstand UR, a voltage breakdown occurs. This results in repetitive, multiple arc re-ignitions and continues until the dielectric withstand of the gap becomes higher than the TRV, as indicated in Figure 3.10. Resulting high frequency (HF) currents are superimposed on the system frequency current. Although they pass through high frequency current zeros, quenching capability of the circuit breaker may be insufficient. Therefore it is possible that several current zeros pass before the arc will be fully extinguished. Once the dielectric withstand is fully built-up, voltage oscillates according to natural frequency resulting from inductance and capacitance of the switched off circuit (formula (3.3)). Another overvoltage effect may be recorded in three phase systems, where certain stray capacitances are present between phases. At the moment when current is chopped at one phase, high frequency transient currents may be produced in other two phases, which is referred to as virtual current chopping. In certain conditions these capacitive coupled currents may be chopped near their zero crossing, causing voltage to oscillate in other phases. a). I. t. ich. UR. b) U. TRV. A. B. C. D. t. Figure 3.10. Multiple arc re-ignitions during VCB opening, A – first current chopping, B, C – arc re-ignitions, D – successful interruption; a – current under interruption, b – voltage between the operated VCB contacts (TRV); U R – dielectric withstand, TRV – Transient Recovery Voltage, ich – chopping current [122] At this point another issue has to be pointed out. As discussed above, during the opening operation Transient Recovery Voltage rises between the opening contacts of the operated circuit breaker. This is strictly related to the so-called circuit breaker’s breaking capability envelope (or TRV envelope). Standards for AC circuit breakers provide parameters of the TRV that have to be fulfilled by the circuit breaker, especially for breaking of faults. Explanation of that is provided i.e. in IEEE Std. C37.011 standard [57], as presented in 33.