DC system

The direct current (DC) system supplies the equipment that must work when the AC auxiliary supply is lost: the protection relays, the trip and closing coils of the breakers, the control and indication, the substation automation and the emergency lighting. A battery carries these loads, and chargers keep it charged and supply the continuous load in normal service. The design sizes the battery by the duty-cycle method of IEEE Std 485-2020 and the chargers by IEEE Std 946-2020.

Read first: AC auxiliary supply

1The DC systemCopy link to The DC system

The IEC + CEA / CBIP set takes a DC system of 220 V, with two batteries and two float-cum-boost chargers. Each battery and each charger is rated for the full load. The batteries are of the valve-regulated lead-acid (VRLA) type. The CEA Construction Regulations 2022 require two battery sets with separate float-cum-boost chargers at 132 kV and above (Reg. 47(1)(c)). The design provides two sets for the substation.

The DC distribution board (DCDB) has two sections joined by a normally open coupler. One charger and one battery feed each section. The board's outgoing feeders supply the HV relay and control panels, the HV breakers' control supply, the MV switchgear, the substation automation and the emergency lighting. A charger holds its battery at the float voltage in normal service. After a discharge, it recharges the battery at the higher boost voltage.

The telecom equipment has its own 48 V DC battery and charger, which the telecom supplier provides. Its charger is a load of the AC auxiliary supply (AC auxiliary supply, The load list), as are the two float-cum-boost chargers.

2The duty cycleCopy link to The duty cycle

The duty cycle is the current the battery must deliver, period by period, over the time it supports the substation without a charger. The design takes a backup time of 10 h in the IEC + CEA / CBIP set, in these periods:

• the first minute: the continuous load, the emergency lighting, and the tripping of the breakers
• to the end of the first hour: the continuous load and the emergency lighting
• to the last minute: the continuous load
• the last minute: the continuous load and the closing of the largest breaker

The loads come from the design's counts, at the design's values per item. The continuous load is 150 W per HV bay, 40 W per MV panel and 500 W common to the substation, divided by the nominal voltage. The emergency lighting is 1500 W for the first hour. The tripping takes 5 A per HV breaker and 2 A per MV breaker, with both trip coils. The closing of the largest breaker takes 10 A, for its closing coil and its spring-charging motor.

Figure 1 shows the duty cycle of the default 220 kV project and of a new 230 kV project of the ANSI / IEEE set. The tripping current of the first minute is the largest, and the continuous load sets the long middle period.

3Cell countCopy link to Cell count

The cell count sets the battery's voltage at each stage of its charge and discharge. The equipment on the DC system accepts a range of voltage about its nominal value. The highest voltage of the battery is its voltage on charge, so the design takes the largest number of cells whose charge voltage is within the top of the range:

In the IEC + CEA / CBIP set the range is 0.85 to 1.10 of the nominal voltage, 187 V to 242 V. The VRLA cell floats at 2.25 V, takes a boost charge at 2.30 V and is discharged to 1.75 V per cell. The discharged battery must stay at or above the bottom of the range. The cell count of Eq. (1) is 107, which gives an end-of-discharge voltage of 187.25 V, at or above the bottom of the range.

4Cell sizing sectionsCopy link to Cell sizing sections

A battery's capacity depends on how long it delivers a current. A cell delivers a high current for one minute that it could not hold for an hour. IEEE Std 485-2020 therefore sizes the cell by sections of the duty cycle. Section $S$ covers the periods from the first to period $S$. Its required capacity sums the changes of current at the start of each period, each multiplied by the capacity rating factor for the time from that start to the end of the section:

A fall in current gives a negative term, so a later section can need less capacity than an earlier one. The design calculates each section, and the largest governs. The capacity rating factor $K_t$ comes from the cell's discharge curve to the end-of-discharge voltage. In the IEC + CEA / CBIP set the design holds a VRLA curve to 1.75 V per cell on the C10 basis, the table, and interpolates it on logarithmic scales. The battery vendor's own curve applies at detail design.

5Battery capacityCopy link to Battery capacity

The governing section's capacity is the capacity of a new cell at the reference temperature, with no allowance for the future. The design multiplies it by the correction factors for aging, margin and temperature:

The aging factor allows for the fall of the battery's capacity over its life (IEEE Std 485-2020 §6.3.4). The design margin is an allowance on the calculated capacity (§6.3.3). The design takes a temperature factor of 1.00, for an air-conditioned battery room. The design selects the next standard capacity at or above $C$, from 100, 150, 200, 300, 400, 500, 600, 800, 1000, 1200, 1500 and 2000 Ah.

6ChargersCopy link to Chargers

A charger supplies the continuous load in normal service and recharges the battery after a discharge. IEEE Std 946-2020 rates its output current from the continuous load and the charge to be returned in the recharge time:

The design selects the next standard rating at or above $I$, from 25, 40, 50, 63, 80, 100, 125, 160, 200 and 250 A. The charger's AC input is $N{V}_{\mathrm{boost}}I/\eta$, with an efficiency $\eta$ of 0.85. That input enters the AC load list, once for the duty charger and once, as a standby load, for the second.

7DC fuses and busCopy link to DC fuses and bus

The charger and battery circuits of the DCDB are protected by fuses of the IEC 60269-1 gG sizes the design holds: 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200 and 250 A. A fuse takes the first size at or above its full-load current. A charger's fuse is sized on the charger's rated output. A battery's fuse is sized on the larger of the charger's output and the peak current of the duty cycle, the tripping current of the first minute.

The DCDB's bus carries one charger's output and one battery fuse's rating, and takes the first of 315 and 400 A at or above that sum, as IEC 61439-2 rated currents. The outgoing DC feeders carry no rating in the design, since it holds no load per feeder. Their fuses and cables are sized at detail design.

8The ANSI / IEEE setCopy link to The ANSI / IEEE set

The ANSI / IEEE set takes a 125 V DC system with a vented lead-calcium battery. RUS Bulletin 1724E-300 §15.2 rates such a battery at the 8 h rate to 1.75 V per cell, and asks for at least 8 h of duty. The design takes a duty cycle of 8 h, with the same loads in amperes at 125 V.

The cell count follows from the 125 V DC control range of IEEE C37.04-2018 Table 23. For closing and energy storage the range is 100 V to 140 V, the stricter of the lower bounds the table gives for the breaker classes. On an equalize charge of 2.33 V per cell, Eq. (1) gives 60 cells, at 139.8 V. The discharged battery ends at 105.0 V, above the bottom of the range. The float voltage of 2.25 V and the equalize voltage are the design's values per cell.

The capacity rating factor comes from the cell of RUS Bulletin 1724E-300 Example 15-1, from its capacities at the 8, 5, 3 and 1 h rates and for 1 minute. The sections, the correction factors and the charger rule are those of the IEC + CEA / CBIP set. The fuses are class J, of 30, 60, 100, 200, 400 and 600 A, and the DC panelboard's bus is 225 and 400 A, as the design's lists of ratings.

the table compares the two sets' DC systems.