AC auxiliary supply

A substation needs its own supply for the equipment that operates and protects it: breaker and isolator motors, transformer coolers, lighting, air conditioning, the battery chargers and the fire pumps. This supply is the auxiliary supply, called the station service in the ANSI / IEEE set. Its alternating current (AC) part is a low-voltage (LV) system of 415 V in the IEC + CEA / CBIP set and 480Y/277 V in the ANSI / IEEE set. The design builds its load list from the substation it has laid out, then sizes the transformers, the standby generator and the switchboard from that list.

Read first: Power transformers · Single-line diagram (SLD)

1Sources and switchboardCopy link to Sources and switchboard

The AC auxiliary supply has auxiliary transformers and a standby generator. The design provides 2 auxiliary transformers, one on duty and one on standby. Where the medium-voltage (MV) bus has a single section, from one transformer with one LV winding, it provides one. In the IEC + CEA / CBIP set the standby generator is a diesel generator (DG) set, started by an automatic mains failure (AMF) panel when the transformer supply is lost.

The transformers feed an AC distribution board (ACDB) of two sections joined by a bus coupler, which is normally open. Transformer 1 feeds section 1 and transformer 2 feeds section 2. The DG set's incomer is on section 2. The coupler and an interlock keep one incomer per section in service at a time. The auxiliary transformers are Dyn11, so the LV side has an earthed neutral for single-phase loads.

Where the transformers take their supply depends on the substation, as the table lists. A pooling substation takes it from its MV bus sections 1 and 2. A switching substation has no MV bus. It takes the supply from the tertiary winding of each interconnecting transformer (ICT), with an 11 kV feed from the local distribution network and a third transformer as back-up. Where the ICTs have no tertiary, both transformers take the 11 kV feed.

The LV side of the IEC + CEA / CBIP ratio is the no-load voltage of the 415 V board. In the ANSI / IEEE set the transformer's LV side is 480Y/277 V and the distribution feed is 13.8 kV.

2The load listCopy link to The load list

The load list is the basis of the sizing. The design builds it from the substation it has laid out, so the list follows the project. The quantities come from the design's counts: the ICTs, the HV breakers and motorised isolators, the HV bays, the MV switchgear panels and the MV sections. Area-based loads come from the yard and the building. The flood lighting follows the fenced area, the road lighting the fence's perimeter, the air conditioning the control, relay and battery rooms, and the ventilation the switchgear hall.

Each item has a quantity, a rating in kW per unit, a power factor and a duty. The duty sets a duty factor $d$: continuous loads count in full (1.0), intermittent loads at half (0.5) and standby loads not at all (0.0). A standby load runs only when another is out, such as the standby charger, or only in an emergency or for maintenance, such as the main fire pump and the oil filtration outlet. The ratings are the design's values. The vendor's data replaces them at detail design, through the load list on the Auxiliary supply step.

the table is the load list of the default 220 kV project. It has 25 items, with a connected load of 248.4 kW.

The list changes with the project. An indoor gas-insulated switchgear (GIS) hall adds its ventilation, its crane and the local control cubicles of its bays. Outdoor MV switchgear replaces the panel loads with bay kiosks and motors. A switching substation has no MV switchgear and no reactive power equipment, so those loads are absent. The designer can change the quantity, rating, power factor or duty of an item, remove an item or add one. The design keeps these edits per item for the next run.

3Maximum demandCopy link to Maximum demand

The maximum demand is the apparent power the auxiliary supply carries with the loads at their duty. The active demand sums the items' kW with their quantity $q_i$, rating $P_i$ and duty factor $d_i$: $P=\sum q_i{P}_i{d}_i$. The reactive demand takes each item's demand kW at its power factor $\cos {\phi }_i$: $Q=\sum q_i{P}_i{d}_i\tan {\phi }_i$. The maximum demand is their vector sum, $S=\sqrt{P^2+Q^2}$.

The duty factor is the design's allowance for loads that do not run at the same time. Breaker and isolator motors run for seconds after an operation, so they count at half. The standby charger, the main fire pump and the maintenance outlets count at zero, since they replace or follow other loads. A designer who expects a standby load to run with the others changes its duty to continuous in the load list.

For the default 220 kV project the active demand is 133.0 kW, the reactive demand 74.0 kvar and the maximum demand 152.2 kVA.

4Auxiliary transformersCopy link to Auxiliary transformers

Each auxiliary transformer carries the full auxiliary load alone, since the other is on standby. The design takes the maximum demand with a margin of 1.2, $S_{\mathrm{TR}}=1.2S$, and rounds it up to the next size of its list: 250, 315, 400, 500, 630, 800, 1000, 1250, 1600 and 2000 kVA. The margin leaves room for loads added after the design and for the vendor's ratings.

On Auto, the Aux transformer setting takes this size. A value entered there replaces it, for example to match a client's standard rating. The switchboard's incomer ratings then follow the entered rating. The ratio is set by the supply, as the table lists.

5DG set and motor-start dipCopy link to DG set and motor-start dip

The DG set supplies the full auxiliary load when both transformers are out, not only an essential part of it. The design sizes it on two conditions. It takes the maximum demand with a margin, $1.2S$. It also takes the active demand at the set's rated power factor of 0.8, with the same margin, $1.2P/0.8$. The larger of the two, rounded up to the next size of its list, is the size the load needs. The design's DG sizes are 62.5, 82.5, 125, 160, 200, 250, 320, 380, 500, 625, 750, 1010, 1250 and 1500 kVA.

A motor started direct-on-line (DOL) draws a starting current several times its full-load current, at a low power factor. On a DG set this current causes a transient voltage dip, larger than on a transformer supply, since the set's sub-transient reactance $x_d^{\prime \prime }$ is high. The design checks the start of the largest motor in the load list, standby motors included. It takes the starting kVA as 6.0 times the motor's full-load kVA, with an efficiency $\eta$ of 0.9:

• starting kVA: $S_{\mathrm{start}}=6.0P_m/(\eta \cos {\phi }_m)$, for a motor of rating $P_m$ and power factor $\cos {\phi }_m$
• short-circuit kVA of the set: $S_{\mathrm{DG}}/x_d^{\prime \prime }$, with $x_d^{\prime \prime }=0.15$ per unit
• voltage dip: $\Delta U=S_{\mathrm{start}}/(S_{\mathrm{start}}+S_{\mathrm{DG}}/x_d^{\prime \prime })$

The dip must be 15 % or less, the design's limit, which its checklist credits to performance class G3 of ISO 8528-5. Where the dip of the size the load needs is above the limit, the design takes the next size until the dip is within it, up to the largest size of its list. A soft starter on the motor is the alternative where the vendor's data allows it.

6Switchboard ratingsCopy link to Switchboard ratings

The design gives a rated current to the ACDB's incomers, its bus coupler, its bus and the feeders of its load groups. A device's rating is the first size at or above its full-load current, with no further factor. The full-load current of a three-phase rating $S$ at the board's voltage $U$ is $I=S/(\sqrt{3}U)$.

Each incomer of the transformers and of the DG set is an air circuit breaker (ACB), in the IEC 60947-2 rated currents of the design's list: 630, 800, 1000, 1250, 1600, 2000, 2500, 3200 and 4000 A. The bus coupler and the ACDB's bus take the rating of the largest incomer, since one incomer can feed both sections through the coupler. Each load group has one feeder, sized on the group's demand from its kW and kvar. A feeder is a moulded-case circuit breaker (MCCB), of 100, 125, 160, 200, 250, 320, 400 and 630 A, or an ACB where the group's current is above 630 A.

The ratings are rated currents only. The design does not calculate the LV fault level, the breaking capacity of the LV devices or the cable sizes. Those are set at detail design, with the route lengths and the installation conditions.

the table lists the feeders of the default 220 kV project. The largest group current is 41.3 A, so each feeder takes the smallest MCCB, 100 A.

The connected load of a group can be far above its demand, as the fire fighting and utilities groups show. Their large items are standby loads.

7The ANSI / IEEE station serviceCopy link to The ANSI / IEEE station service

The ANSI / IEEE set follows the same method with its own voltages, names and size lists. The station service is 480Y/277 V three-phase, four-wire, a secondary voltage RUS Bulletin 1724E-300 §15.1 lists among the manufacturers' standard ones. The station-service transformers are pad-mounted three-phase units, which §15.1 gives for ratings up to 2500 kVA. The design's sizes are 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000 and 2500 kVA.

The standby generator replaces the DG set and starts through an automatic transfer switch (ATS). Its sizing and the motor-start check are those of the DG set. Each incomer, the coupler and each feeder above the molded-case breaker range is a low-voltage power circuit breaker (LVPCB), in the frame sizes of IEEE C37.13-2015: 800, 1600, 2000, 3200, 4000, 5000 and 6000 A. The molded-case breakers are 100, 150, 225, 250, 400, 600, 800 and 1200 A, as the design's list of ratings.

the table compares the default 220 kV project with a new 230 kV pooling project of the ANSI / IEEE set. The two load lists hold the same items, at their own counts.