96
WM 064-6 PWRD, T-pwRD January 1997An
Algorithm for Compensating Secondary
Currents
ofCurrent Transformers
XC. Kang (Student Member, IEEE; Department of Electrical Engineering, Seoul National University; Seoul, Korea), J.K. Park (Member, IEEE; Department of Electrical Engineering, Seoul National University), S.H. Kang (Member, IEEE; Department of Electrical Engineering, Myong-Ji University, Yong-In, Korea), A.T. Johns (Senior Member, IEEE; School of Electronic & Electricd Engineering, University of Bath, Bath, UK), R.K. Aggarwal (Senior Member, IEEE; School of Electronic & Electrical Engineering, University of Bath)
Current transformer (CT) saturation may cause a variety of protec- tive relays to malfunction. The conventional method to deal with the problem is overdimensioning of the core so that CTs can carry up to
20 times the rated current without exceeding 10 percent ratio correction. However, this not only reduces the sensitivity of relays, but also in- creases the CT core size.
Hitherto there is no satisfactory method available that would fully cope with the problem of CT saturation; protective relay engineers have thus to take account of this drawback when laying down the specifica- tions in terms of relay performance.
This paper presents a technique of estimating the secondary current corresponding to the CT ratio and it can be incorporated within the digital protective relay algorithm.
The equivalent circuit of a CT is shown in Figure 1. In Figure 1, at any instant the relationship between ip(t) and is(t) is:
1 .
n
-zp(t) = i,(t)
+
is@).As is(t) is the measured secondary current, if ie(t) can be deduced, then the secondary current corresponding to the CT ratio -zp(t) can be estimated.
If the burden of a CT is a resistive inductive burden of the type(&
= Rb
+
jwLb), at any time the relationship between the core flux q(t) and the secondary current is(t) is given by:1 . n
W - c p ( t 0 )
As all the secondary circuit parameters are known, the core flux can be calculated. The initial core flux cp(to) can also be calculated using a full cycle window of the secondary current in the steady state. The magnetization curve and the hysteresis curve are shown to- gether in Figure 2. Beyond the positive and negative saturation points, the magnetizing current is the same as the exciting current. Between the points, however, the exciting current has two different values for every one value of the flux. The difference between them is equal to or less than half the width of the major loop. As the latter is small, the difference between them is even smaller so that exciting current can be assumed to,be the same as the magnetizing current. Thus, -zp(t) can be calculated by simply adding the magnetizing current to the measured secondary current.
This paper proposes a novel compensating algorithm for secondary currents using the magnetization curve, and which accurately estimates the secondary current corresponding to the CT ratio even when the CT is saturated. It is clearly shown that the error is maintained at a low level even when there is severe CT saturation such as under large offset primary currents and in the presence of remanent flux. It has a number of significant attributes such as improvements in the sensitivity of relays to low level internal faults, maximizing the stability of relays for 1 .
n
I
Figure 1. Equivalent circuit of a current transformer
Figure 2. Hysteresis curve and magnetization curve
external faults, and making a reduction in the required core cross section possible.
Discussers: W.C. Kotheimer; P.G. McLaren
96 WM 065-3 P W m , T P W m January 1997
The Influence of Protection System
Failures
and
Preventive Maintenance
on Protection Systems in Distribution Systems
J.J. Meeuwsen (Delft University of Technology, Delft, The Netherlands), WL. Kling (Delft University of Technology),
WA.G.A. Ploem (NV PNEM Facilitair Bedrijf Hertogenbosch, The Netherlands)
An electric power system comprises generation, transmission and distribution. Although distribution systems have received less attention than generating systems and composite generating and transmission systems, analysis of the customer failure statistics shows that distribu- tion systems are responsible for as much as 90 percent of the unavail- ability of supply to a customer. Such statistics reinforce the need to be concerned with the reliability evaluation of distribution systems.
An important aspect of the reliability of load points in a distribution system is the reliability of protection systems. A protection system is designed to fulfill two major functions:
Quick isolation of faults after their occurrence
Protection of major power system components from possible damage by abnormal voltage or current
Protection systems in power systems can fail either by not respond- ing when they should (failure to operate) or by operating when they should not (false tripping). The former type of failure is particularly serious since it may result in the isolation of large sections of the network. However, the probability of a failure to operate can be reduced
by carrying out preventive maintenance on protection systems. This paper describes an approach to determine the impact of the various failure modes of protection systems on the reliability indices of supply to a customer. The impact of preventive maintenance on protection systems is considered in this article. The proposed approach is based on Markov models.
The paper also reports on some results obtained from studies of a distribution system of the PNEM, a utility in the southern part of the Netherlands. Figure 1 shows some results.
Figure 1 shows that the expected energy not supplied per year (EENS) in the chosen distribution system can considerably be reduced by performing preventive maintenance on protection systems, How- ever, it can be seen that preventive maintenance must be performed carefully (low values of the probability of unsuccessful maintenance, Pum). Carelessly performed preventive maintenance on protection sys-
tems can even lead to an increase in the expected energy not supplied
(Pum=0.20). It should be noted that all lines in the graph converge to the line which corresponds with never carrying out preventive mainte- nance (dotted line).
An optimal maintenance interval or frequency can be determined for component protections, when preventive maintenance is carried out carefully. From the viewpoint of the expected energy not supplied (EENS), this optimal maintenance frequency is about 2 inspections per year for the chosen system. When maintenance costs and unsupplied energy costs are taken into consideration, it is illustrated in the paper that the relatively high value of 2 inspections per year is reduced to 1 inspection per 2 years.
Discusser: J.J. Kumm
EENS
~1
0.1 [Mwh/yearli I&-0.20\
\
\
\
no preventive maintenance\ \
//
-
,-0.05/
0.01 0.1 1 10Maintenance interval [years]
-
Figure 1. Results f o r a certain case study96
WM 016-6Wm,
T-PWm Janua y 1997Proposed Statistical Performance Measures for
Microprocessor-Based Transmission-Line Protective
Relays,
Part1: Explanation of the Statistics
Working Group D5 of the Line Protection Subcommittee, Power System Relaying Committee: E.A. U d r e n (chair), J.A. Zipp (vice-chair), G.L. Michel, K.K Mustaphi, S.L. Nilsson, A.G. Phadke, R. Ramaswami, G.D. Rockefeller, M.S. Sachdev, WM. Strang, J.S. Thorp, D.A. Tziouvaras, V Varneckas, C.L. Wagner
The paper presents practical calculations and novel techniques for collecting performance data from protective relays. The methods are focused on but not limited to, microprocessor-based transmission line relaying systems.
Standard definitions are presented for Availability, Dependability, Security, Hardware MTBF, Relaying MTBF, Repair dime, and other measures of interest for specific relay types. The paper explains the novel concept of Exposures, the key to a standardized security measure. A companion paper, Part 2: Collection and Uses of Data, gives
practical guidelines for gathering and calculation of results. With these standard measures, relay users will be able to compile the first consis- tent industry-wide database for relay performance assessment.
Discusser: C.R. Heising
96 WM 127-2 PWRD, T-PWm J a n u a q 1997
Proposed Statistical Performance Measures for
Microprocessor-Based Transmission-Line Protective
Relays, Part
2:Collection and Uses of Data
Working Group D5 of the Line Protection Subcommittee, Power System Relaying Committee: E.A. Udren (chair), J.A. Zipp (vice-chair), G.L. Michel, K.K Mustaphi, S.L. Nilsson, A.G. Phadke, R. Ramaswami, G.D. Rockefeller, M.S.
Sachdev, W.M. Strang, J.S. Thorp, D.A. Tziouvaras, I? Varneckas, C.L. Wagner
A companion paper,
Part
1: Explanation of the Statistics, presented practical calculations and novel techniques for collecting performance data from protective relays. Standard definitions were presented for Availability, Dependability, Security, Hardware and Relaying MTBF, and other measures.The present paper gives practical guidelines and complete examples of actual data collection procedures. It focuses on methods for users to compile exposure counts needed for the novel Security measurement. Also reasons for collection and uses for the statistical results are described.
Discusser: C.R. Heising
96 SM 379-8 PWRD, T-WRD January 1997
Frequency Estimation by Demodulation
of
'IRyoComplex Signals
Magnus Akke (Sydkraft AB, Malmo, Sweden)
This paper presents a method for frequency estimation in power system by demodulation of two complex signals. In power system analysis, the ab-transform is used to convert three phase quantities to a complex quantity where the real part is the in-phase component and the imaginary part is the quadrature component. This complex signal is demodulated with a known complex phasor rotating in opposite direction to the input. The advantage of this method is that the demodu- lation does not introduce a double frequency component. For signals with high signal to noise ratio, the filtering demand for the double frequency component can often limit the speed of the frequency esti- mator. Hence, the method can improve fast frequency estimation of signals with good noise properties. The method looses its benefits for noisy signals, where the filter design is governed by the demand to filter harmonics and white noise. The method has been previously published, but not explored to its potential. The paper presents four examples to illustrate the strengths and weaknesses of the method.
96 SM 389-7 PWRD, T-WXLl January 1997
Current Transformers and Coupling-Capacitor
Voltage Transformers in Real-Time Simulations
J.R. Mati (The University of British Columbia, Vancouver, B.C., Canada), L.R. Linares (The University of British Columbia), R.W Dommel (The University of British Columbia)
Under normal transient conditions, instrument transformers do not affect the behavior of the electric power system. It is then possible to
