Electricity distribution

5.4 .1 Conceptual layout

The concept for the design o f the HE-LHC electrical network is driven by four factors:

- The existing electrical infrastructure that is used to supply the LHC and its extension for HL-LHC;

- The estimated electrical power requirements o f HE-LHC (Tab. 9.1 in Sect. 9.2) ; - The location and type o f equipment to be supplied;

- The expected level o f electrical network availability and operability.

The electrical network is com posed o f a transmission and a distribution level. The transmission level transmits the power from one source to all 8 o f the HE-LHC points at voltages o f 400 kV and 66 kV. The distribution level distributes the power from the transmission level to the end users at medium and low voltage levels comprised between 18 kV and 400 V.

5 .4 .2 Source of electrical energy

The estimated 213 M W electrical power requirement will be supplied from the Euro­

pean grid through the existing 400 kV connection to the Bois Tollot electrical sub­

station. The source is self-redundant and, according to French network provider RTE (Reseau Transport Electricite), is capable o f providing 200 M W on top o f its current load by the year 2035 (the present maximum daily load by CERN on source I is 191 M W ).

5 .4 .3 Transm issio n network topology The transmission network includes:

- The 400 kV transmission line connecting the 400 kV source on the European grid to the incoming substation;

- The existing 400/66 kV transformer substation on the CERN Prevessin campus;

- Six existing 66 kV transmission lines connecting the transformer substation radi­

ally to six points;

- A new 66 kV transmission ring com posed o f 8 segments connecting each o f the 8 points to its two neighbouring points.

- A 66/18 kV transformer substation at each point;

F i g . 5 . 5 . S im p lifie d s c h e m e o f t h e 4 0 0 k V in c o m i n g s u b s t a t io n a n d t h e c o n n e c t i o n t o t h e s t e p - d o w n t r a n s fo r m e r s .

Figure 5.4 shows a schematic view o f the transmission network.

Analysing the power requirements o f the machine for each point and nomi­

nal operation with beam, the highest power demands occur in 6 points (1, 2, 4, 5, 6 and 8) where the cryogenic and RF systems are located, each requiring between 12 and 31 M W . These points are radially connected to the main substa­

tion. The remaining points 3 and 7 only host cryogenic systems requiring 7 M W each. They are powered through one o f the adjacent points via the transmission line segments.

This transmission network layout provides full redundancy, enhanced availabil­

ity and operability in case o f a fault on one o f the transmission line segments. A redundant scheme o f 400/66 kV voltage step-down transformers supplies the trans­

mission line segments connecting two adjacent points. In points 1, 2, 4, 6 and 8 a substation will receive the incoming 66 kV transmission line segments. In all points 66/18 kV step-down transformers supply the distribution networks level. Redundant step-down transformers and switchgear provide the required level o f availability and maintainability. Figure 5.5 shows a simplified scheme o f the 400 kV incom­

ing substation and the connection to a point with the corresponding step-down transformers.

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F i g . 5 . 6 . D i a g r a m o f t h e b a s e lin e d i s t r i b u t i o n n e t w o r k o f o n e H E - L H C p o i n t i n c lu d in g t h e a lt e r n a t iv e p o w e r s o u r c e s .

5 .4 .4 D istribution network topology

The distribution networks connect the transmission network to the surface and under­

ground equipment and systems. During normal operation, the transmission network supplies the distribution network. Alternative sources o f supply are needed to meet the required level o f network availability and to cope with a degraded scenario such as a disruption o f the general or local power supply. Therefore, the distribution network includes a second supply that already exists, rated between 2 and 4 MVA, fed from a regional grid node, a third source o f supply rated 1-3 M VA from local, diesel-powered generator power stations and a fourth source which provides uninterruptable power.

Figure 5.6 shows the single line diagram o f the baseline distribution network o f one point including the alternative power sources.

The distribution network is com posed o f a primary indoor substation comprising five bus bars located on the surface. The incoming feeders are the two redundant 66/18 kV transformers supplied from the transmission network, the second supply from a regional source and the third supply from the local diesel power station. The outgoing feeders supply secondary substations. These are located either on the surface or underground, near the load. The operating voltage o f the distribution network is 18kV corresponding to the current operating voltage o f the LHC. This voltage is used for the power distribution over distances greater than 750 m. Voltage step-down transformers feed end users from the secondary substations over a maximum cable length o f 750 m. End users are supplied from the secondary substations at voltage levels between 400 V for wall plug equipment and 3.3 kV for high power motors for cooling, ventilation and cryogenic systems.

Fig. 5 .7 . Typical distribution of transient voltage dips recorded within the existing CERN network (collected between 2011 and 2017); the design zone covers most of the transient voltage dips, which are within 0-150 ms and 0-50% magnitude.

5 .4 .5 Power quality and transient voltage dip m itigation

The main issues concerning power quality are voltage stabilisation, harmonic filtering and reactive power compensation as well as the mitigation o f transient voltage dips.

Transient voltage dips as shown in Figure 5.7, which are typically caused by lightning strikes on the 400 kV network overhead lines, often cause undesired stops o f C E R N ’s accelerators. The powering system design must include mitigation measures against these.

The following mitigation measures are being studied:

- Dynamic Voltage Restorer (D V R ) technology: the voltage will be restored by dynamic series injection o f the phase voltage between the distribution network and the loads. An integrated energy storage system provides the required energy to restore the load voltage during transient voltage dips (Fig. 5.8a).

- High-Voltage DC (H V D C ) back-to-back link: H VD C is a well-established tech­

nology for long distance transmission o f large powers and for decoupling different high voltage networks. Combined with energy storage, an H VD C system provides performance similar to a very large uninterruptable power supply (UPS). Such a system would prevent transient voltage dips in the 400 kV transmission network from entering the collider network. In addition it would allow the reactive power (Fig. 5.8b ) to be controlled.

- Static Synchronous Compensator (STA TC O M ): this technology is already used for reactive and active power compensation. STATCO M would fully restore the load voltage during transient voltage dips by dynamic shunt (parallel) injection, com bined with an integrated energy storage system (Fig. 5.8c ).

- Motor-Generator Set: such a system would decouple the network from the load.

During transient voltage dips, the load voltage is restored by using the energy stored in a rotating mass (Fig. 5.8d ).

- Medium-Voltage DC (M V D C ) distribution network: the principle o f this approach is the distribution o f power using DC. In com bination with energy storage, this technology mitigates transient voltage dips, eliminates the reactive power, reduces the distribution losses and, com pared to AC distribution, permits a larger spacing between electrical substations in the tunnel. This promising technology is still in

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F i g . 5 . 8 . S im p lifie d la y o u t o f v a r io u s m e t h o d s f o r t r a n s ie n t v o l t a g e d ip m it ig a t io n .

its early stage o f development and would require considerable R&D efforts before its use (see Sect. 12.10) .