Transformer Auxiliary Power — Station Service Transformer, AC/DC Dual Supply & UPS Battery Sizing

Introduction

A 200 MVA power transformer itself cannot operate without auxiliary power. Cooling fans, oil pumps, on-load tap changer motor drives, control panels, protection relays, SCADA RTUs, and circuit breaker spring-charging motors all need reliable low-voltage power. When the transformer you are protecting is the source of auxiliary power, a clever design paradox emerges: the auxiliary system must remain operational through a transformer fault to enable its protection and restoration. This article covers the complete auxiliary power design for substation transformers based on IEC 61936-1 and IEEE 1818.

1. Auxiliary Load Identification

1.1 Load Categories

Category	Examples	Supply Priority
Protection & Control	Relays, RTU, SCADA, communication	DC (battery-backed) — Uninterruptible
Transformer Cooling	Oil pump, fan motors, OLTC MDU	AC 380–400 V — Essential
Switchgear Operation	CB spring charging, disconnector motors	DC or AC — Essential
Lighting & HVAC	Substation lighting, air conditioning	AC — Normal
Fire Protection	Fire detection, suppression control	DC (battery-backed) + AC (pumps)

1.2 Typical Auxiliary Load of a Power Transformer

Load	Power Rating	Quantity	Total (kVA)
Oil pump (forced cooling)	2.2 kW	4	8.8
Cooling fan (OFAF)	0.75 kW	12	9.0
OLTC motor drive	0.75 kW	1	1.3 (intermittent)
Breather heater	0.05 kW	1	0.05
Marshalling kiosk heater	0.15 kW	1	0.15
LVAC distribution panel	—	—	5.0 (misc. loads)
Total AC load			~25 kVA

For a typical 110/220 kV substation with 2–3 transformers, the total station auxiliary load is 50–150 kVA.

2. Station Service Transformer (SST)

2.1 Configuration Options

Configuration	Reliability	Cost	Use Case
Tertiary winding of main TX	Low (lost when TX out)	Lowest	Small distribution substations
Dedicated SST from MV bus	Medium (lost with bus)	Low	Standard industrial
Dedicated SST with MV dual-feed	High (bus transfer)	Medium	Important substations
Dual SSTs + ATS	Very high (N-1)	High	Transmission substations
SST + diesel generator	Highest (N-2)	Very high	Black-start capable stations

2.2 SST Sizing

S_SST ≥ S_total × (1 + margin)

Where margin = 25–30% for future expansion and motor starting.

Example: 25 kVA × 1.3 = 32.5 kVA → select 50 kVA standard size for a single transformer's auxiliary needs. For a multi-transformer substation, size for simultaneous worst-case operation of all transformers.

2.3 Voltage Selection

System	Typical Voltage	Purpose
LV AC	380/400 V, 3-phase, 4-wire	Cooler motors, pumps, general services
DC control	110 V or 220 V DC	Protection, control, CB trip/close coils
UPS AC	220/230 V, 1-phase	SCADA, RTU, communications, critical PLCs

3. DC Supply System

3.1 Why DC?

• Reliability: Battery-backed DC remains available even during complete AC loss
• Simplicity: No phase synchronization, no frequency concerns
• Speed: DC trip coils on circuit breakers provide fast, reliable tripping
• Legacy: The substation DC bus is the lynchpin of all protection — it must work when everything else fails

3.2 DC Voltage Selection

DC Voltage	Advantages	Disadvantages
48 V	Low shock hazard, telecom-compatible	Higher current (I²R losses), limited relay compatibility
110 V	Compromise — manageable voltage, good relay compatibility	Requires ~55 cells
220 V	Lower current, smaller cable CSA	Higher shock hazard, more cells

110 V DC is the most common choice for transmission substations globally. 220 V DC is common in regions with legacy European influence (IEC standard).

3.3 Battery Sizing — IEEE 485 Method

The battery must supply:

• Continuous load: Relay burden, SCADA RTU, panel indication — for the entire duty cycle (typically 8–24 hours)
• Momentary load: Breaker trip/close coils, motor-operated disconnectors — high current, short duration

The one-minute and one-second discharge rates determine the battery capacity:

C = (L_cont × T × k_t × k_a × k_d) / DOD

Where:

• L_cont = continuous DC load (A)
• T = autonomy time (h) — typically 8 h for attended stations, 24 h for unattended
• k_t = temperature correction factor (1.19 at −10°C for VRLA)
• k_a = aging factor (1.25 for 80% end-of-life capacity)
• k_d = design margin (1.10)
• DOD = depth of discharge (80% for VRLA, 80% for NiCd)

Example: Continuous load = 20 A, T = 8 h, VRLA battery at 25°C:

C = (20 × 8 × 1.0 × 1.25 × 1.10) / 0.80 = 220 / 0.80 = 275 Ah

Select: 110 V, 300 Ah VRLA battery with 55 cells.

3.4 Battery Charger Sizing

I_charger = L_cont + 0.1 × C_10

For C₁₀ = 300 Ah:

I_charger = 20 + 0.1 × 300 = 50 A

Select a 50 A battery charger with float/boost modes. The charger must also supply the continuous DC load while recharging a discharged battery.

4. UPS System

4.1 When Is a UPS Required?

A UPS is required for loads that cannot tolerate even 10 ms of power interruption:

• Numerical protection relays (reboot time 30–120 seconds)
• SCADA servers and HMIs
• Digital fault recorders (DFR)
• Communication equipment (modem, switch, firewall)
• Substation automation controllers (IEC 61850 merging units)

4.2 UPS Topology

Power path:

AC Input → Rectifier → DC Bus → Inverter → Critical AC Load
                              ↓
                         Battery Bank

The DC bus is shared between the rectifier, battery, and inverter. On AC loss, the battery seamlessly supplies the inverter with zero transfer time.

4.3 UPS Sizing

Load	Power (VA)	Power Factor	Real Power (W)
SCADA server	500	0.95	475
HMI workstation	300	0.95	285
Ethernet switch × 4	60	0.90	54
Modem/router	50	0.80	40
Protection relay × 8	200	0.70	140
DFR	150	0.70	105
Total	1260 VA		1099 W

Select a 2 kVA UPS (allowing 1.6× margin for growth and inrush).

Battery autonomy for a 2 kVA UPS at full load:

• Internal battery (standard): 10–15 minutes
• External battery bank: 2–8 hours (as required)

5. Dual Supply and Automatic Transfer

5.1 AC Supply Redundancy

[SST-1] ─── [LVAC Bus-1] ─── [ATS] ─── Critical AC Loads
[SST-2] ─── [LVAC Bus-2] ─── [ATS] ─── Critical AC Loads

[DG] ──────────────────────── [ATS] ─── Critical AC Loads (optional)

The ATS monitors voltage on both sources and transfers within 0.5–5 seconds. Critical loads downstream of the UPS are unaffected by this transfer time.

5.2 DC Supply Redundancy

DC redundancy is typically achieved through dual battery banks or a battery + charger redundancy scheme:

[Battery Charger 1] ─┬─ [DC Bus-1] ── [Battery 1]
                     ├─ [Diode Decoupler] ── Critical DC Loads
[Battery Charger 2] ─┴─ [DC Bus-2] ── [Battery 2]

The diode decoupler prevents either battery from back-feeding the failed charger, ensuring true independence.

6. Testing and Maintenance

Test	Frequency	Action Limit
Battery discharge test	Annual	Capacity <80% → replace
Cell voltage check	Quarterly	Any cell deviating >0.05 V from average
Specific gravity (flooded)	Quarterly	Deviation >0.025 from baseline
Charger output ripple	Annual	>2% RMS → rectify
UPS transfer test	Semi-annual	Any glitch on output
ATS transfer test	Annual	Total transfer time >5 s

FAQ

Q: Can I use the transformer's tertiary winding to power the auxiliary system?

Tertiary-connected auxiliary supply is the lowest-cost option but has a critical flaw: when the transformer trips or is taken out of service, the auxiliary supply is lost. This means the protection relays, SCADA communication, and remote control for that transformer are dead — exactly when they are needed most. Tertiary supply is acceptable only for small distribution transformers where a dedicated SST cannot be justified and the consequence of auxiliary loss is minimal.

Q: Why 110 V DC instead of 24 V DC like industrial control systems?

Substation DC loads require higher voltage because (1) circuit breaker trip coils need substantial current (5–20 A) delivered over cable runs that can be 100+ meters, and (2) the voltage drop in the DC distribution must be kept below 5% of nominal. At 24 V, a 5% drop is only 1.2 V — an 0.6 Ω cable resistance (25 m of 2.5 mm² Cu round-trip) would violate this limit at just 2 A.

Q: What is the difference between VRLA and NiCd batteries for substation applications?

VRLA (valve-regulated lead-acid) batteries are lower cost, maintenance-free (no watering), and require minimal ventilation. They are the default choice for indoor substations with controlled temperature. NiCd (nickel-cadmium) batteries tolerate extreme temperatures (−20 to +50°C), have longer calendar life (20+ years vs. 8–12 for VRLA), and are immune to thermal runaway. They cost 2–3× VRLA and produce explosive hydrogen during charging, requiring forced ventilation. Use NiCd for outdoor, unconditioned, or extreme-climate substations.

Q: How do I size the station service transformer for motor starting?

Motor starting can draw 6–8× the motor's rated current. The SST must maintain ≥85% terminal voltage during motor starting. A simplified check: I_start × Z_SST (in p.u.) ≤ 0.15. A 50 kVA, 4% impedance SST: Z = 0.04 p.u. on 50 kVA base. A 9 kW (15 kVA starting) motor: 15/50 = 0.3 p.u. → 0.3 × 0.04 = 0.012 p.u. voltage drop. This is well within limits. For multiple simultaneous motor starts, sum the starting kVA.

Q: Can I share the DC battery between multiple substation bays?

Yes, and most substations use a single station battery for all bays. However, the DC distribution must be radial from the main DC panel with individual MCB protection for each bay. A fault in one bay's DC wiring should not trip the entire station's DC supply. For EHV substations (>220 kV), consider dual DC buses with automatic transfer — the incremental cost is small compared to the consequence of a total DC failure.

Q: What maintenance can be performed on VRLA batteries while the DC bus is live?

VRLA batteries are maintenance-sealed and require no watering. While the DC bus is live, you can: (1) measure individual cell float voltages, (2) measure inter-cell connection resistance (tightness check), and (3) perform thermal imaging to identify cells with elevated temperature (sign of internal short). Do NOT perform a discharge test on a battery connected to a live DC bus unless a second independent battery source is available.

References & Standards

Document	Title	Relevance
IEC 61936-1	Power installations exceeding 1 kV AC	Substation auxiliary supply requirements
IEEE 485	Recommended practice for sizing lead-acid batteries	Station battery sizing methodology
IEEE 1818	Guide for the design of LV AC and DC auxiliary systems	LV auxiliary system design
IEEE 446	Emergency and standby power systems	UPS and generator integration
IEC 62040	Uninterruptible power systems (UPS)	UPS performance and testing
IEC 60896	Stationary lead-acid batteries	VRLA battery specifications

*Du Fu, ZY POWER Production Engineer — The auxiliary system is the nervous system of the substation. When the main circuits fail, the auxiliaries must still operate.*