630kVA Transformer Protection Coordination: Fuse Selection, ACB Settings, and REF Protection Design

Introduction

If 500 kVA is the most frequently purchased transformer rating, then 630 kVA is the most frequently protected one in a non-trivial way. At 630 kVA, the transformer crosses the threshold where a simple fuse on the HV side is no longer the only protection considered — you need a coordinated protection scheme that spans the HV fuse or circuit breaker, the transformer's mechanical protection devices (Buchholz, temperature), the LV air circuit breaker (ACB), and possibly a Restricted Earth Fault (REF) relay.

I have witnessed the aftermath of poor protection coordination more times than I care to count: an ACB that refuses to trip on a downstream fault, allowing the HV fuse to operate and blacking out an entire substation for a fault in a single LV feeder. A Buchholz relay connected to the wrong trip circuit, so that when gas accumulated, it signalled but never tripped — and the transformer burned. An REF relay set so sensitively that every motor start caused a nuisance trip.

This article is a protection coordination design walkthrough for a 630 kVA distribution transformer, covering the protection devices from HV to LV in the order fault current flows.

The 630 kVA Protection Architecture

At 630 kVA, the protection scheme typically includes:

                    ┌─────────────────────┐
    11 kV (or 10 kV)│   HV Fuse / HV CB   │ ← First line: phase overcurrent
         incoming   └─────────┬───────────┘
                              │
                    ┌─────────▼───────────┐
                    │     Buchholz Relay   │ ← Gas / oil surge detection
                    │  Winding Temperature │ ← Thermal overload
                    │     Oil Temperature  │ ← Top-oil monitoring
                    └─────────┬───────────┘
                              │
              ┌───────────────▼───────────────┐
              │    630 kVA, 11/0.415 kV        │
              │    Dyn11, Uk = 4.5%            │
              └───────────────┬───────────────┘
                              │
                    ┌─────────▼───────────┐
                    │      REF Relay       │ ← Internal earth fault
                    └─────────┬───────────┘
                              │
                    ┌─────────▼───────────┐
                    │   LV ACB (400 V)     │ ← L/S/I/G protection
                    └─────────────────────┘

HV Fuse Selection

For a 630 kVA transformer on an 11 kV system, the HV rated current is:

> I_HV = 630,000 / (√3 × 11,000) = 33.1 A

Fuse Rating Selection per IEC 60282-1

The HV fuse must:

• Carry the transformer's rated current continuously without deterioration.
• Withstand the transformer's magnetizing inrush current without melting.
• Clear a secondary-side fault reflected to the primary side.
• Co-ordinate with the LV ACB so that LV faults clear at the ACB, not at the HV fuse.

Criterion	Calculation	Result
Rated full-load current	33.1 A	—
Magnetizing inrush (12×, 0.1 s)	12 × 33.1 = 397 A peak	Fuse must not operate at this level for ≥ 0.1 s
Maximum continuous overload (150%, limited by thermal)	1.5 × 33.1 = 49.7 A	Fuse must carry this without deterioration
Recommended fuse rating	1.3–1.5 × I_FL = 43–50 A	Select 50 A or 63 A fuse

The typical fuse selection for a 630 kVA, 11 kV transformer is a 50 A or 63 A IEC 60282-1 current-limiting fuse. A 50 A fuse provides tighter protection but risks nuisance operation on inrush if the transformer has a low inrush design. A 63 A fuse provides more margin but sacrifices sensitivity to low-level overloads inside the transformer.

Fuse Co-ordination with LV ACB

The critical co-ordination requirement: the LV ACB must clear any fault on the LV bus or LV feeders before the HV fuse melts. If the HV fuse operates for an LV fault, the entire transformer is disconnected, and the fault is not isolated — bad for selective coordination.

The reflected fault current to the HV side for a three-phase fault on the LV terminals:

> I_fault_LV = 630,000 / (√3 × 415 × 0.045) = 19.5 kA > > I_fault_HV (reflected) = 19,500 × (415 / 11,000) = 735 A

A 63 A HV fuse takes approximately 0.3–0.5 seconds to clear at 735 A (refer to the fuse manufacturer's time-current curve). The LV ACB's instantaneous setting must operate at 19.5 kA in less than 0.2 seconds (ideally < 0.1 seconds) to ensure coordination. If the ACB's maximum clearing time is 0.12 seconds and the HV fuse's minimum melting time at 735 A is 0.3 seconds, coordination is achieved with a safety margin of 0.18 seconds.

> ☑ Specification blank: HV fuse type: IEC 60282-1 current-limiting, rated voltage 12 kV, rated current _____ A (recommend 50 A or 63 A), breaking capacity ≥ _____ kA (minimum 20 kA for most 11 kV networks).

LV ACB Settings: L / S / I / G

For a 630 kVA, 415 V transformer, the LV rated current is:

> I_LV = 630,000 / (√3 × 415) = 877 A

Select an ACB rated at 1000 A or 1250 A frame, 1000 A rating plug.

L – Long-Time (Overload) Protection

Parameter	Setting	Basis
Pickup (Ir)	0.9 × In = 0.9 × 1000 = 900 A	1.03 × I_FL — just above full load to avoid nuisance tripping
Time delay (tr)	10–20 seconds at 6× Ir	Standard inverse-time characteristic; co-ordinates with transformer thermal damage curve per IEC 60076-7
Characteristic	I²t = constant (IEC 60255-3, Normal Inverse or similar)	Mimics transformer thermal time constant

S – Short-Time (Delayed Overcurrent) Protection

Short-time protection covers LV busbar faults and provides time-graded coordination with downstream MCCB/feeder breakers.

Parameter	Setting	Basis
Pickup (Isd)	4–6 × Ir = 3,600–5,400 A	Above motor starting inrush, below transformer damage current
Time delay (tsd)	0.2–0.4 seconds	Co-ordination step with downstream devices (0.1–0.2 s for feeder MCCBs + 0.2 s grading margin)
I²t	ON or OFF	ON provides thermal imaging for the busbar; OFF gives definite-time flat response

I – Instantaneous Protection

Parameter	Setting	Basis
Pickup (Ii)	10–12 × In = 10,000–12,000 A	Above the maximum through-fault current from the transformer but below the HV fuse operating point
Time	< 40 ms (inherent breaker operate time)	Instantaneous — clears LV terminal faults immediately

Setting logic: The transformer's maximum through-fault current at the LV terminals is approximately 19.5 kA. The ACB's instantaneous pickup must be set above the maximum expected load inrush (motor starting, transformer energization of downstream units) but below the maximum fault current. A setting of 10 × In (10,000 A) provides a 50% margin above 6× Ir (5,400 A) for motor inrush while still being well below the 19.5 kA maximum fault level.

G – Earth Fault Protection

Parameter	Setting	Basis
Pickup (Ig)	0.2–0.3 × In = 200–300 A	Earth fault current limited by system earthing — with a solidly earthed neutral, the earth fault current can reach phase-fault levels. A sensitive setting (0.2 × In) detects high-resistance earth faults.
Time delay (tg)	0.2–0.4 seconds	Graded below the short-time delay for earth faults; co-ordinates with downstream earth fault protection
I²t	OFF (definite time)	Earth faults do not follow a thermal damage curve — definite time is appropriate

REF (Restricted Earth Fault) Protection

REF protection is the most misunderstood protection element for distribution transformers. It is a differential protection scheme applied specifically to earth faults within the transformer's LV winding and LV bushing zone. It is "restricted" because the protection zone is bounded by current transformers (CTs) on the LV phase leads and in the LV neutral connection — faults outside this zone (on the LV feeders) are not detected by REF.

Do You Need REF for a 630 kVA Transformer?

REF protection is not mandated by any IEC standard for transformers below 1 MVA. However, I recommend it for a 630 kVA unit when:

• The transformer is solidly earthed on the LV neutral: A solidly earthed neutral ensures that any winding-to-earth fault produces a detectable residual current. Without solid earthing (e.g., IT system), a single earth fault does not produce fault current and REF is ineffective.

• The transformer supplies critical loads: If an unplanned transformer outage costs more than the incremental cost of REF protection ($800–1,500 for relay + CTs + wiring), it is justified.

• The transformer is remotely located: If a winding fault develops slowly (turn-to-turn short progressing to earth fault), the HV fuse may not detect it until significant damage has occurred. REF detects earth faults at a much lower current level.

REF CT Configuration

The REF scheme requires:

• Three phase CTs on the LV bushing conductors (one per phase, U, V, W).
• One neutral CT on the neutral-to-earth link.
• The phase CT secondaries are connected in parallel (Holmgreen connection) and compared to the neutral CT secondary.

Under normal balanced-load conditions, the vector sum of the three phase currents = 0, so the residual current in the Holmgreen connection is zero (ignoring CT errors). The REF relay compares this residual current to the neutral CT current:

• External earth fault (downstream of the LV CTs, e.g., on a feeder): The phase CTs see the fault current in the faulted phase and zero in the unfaulted phases; the neutral CT sees the same current returning via the neutral. The REF relay sees equal currents and does NOT operate.
• Internal earth fault (within the transformer winding or LV bushings): The neutral CT sees the earth fault current flowing from the tank to earth, but the phase CTs do NOT see it because the fault is upstream of the phase CTs. The REF relay sees a differential current and trips.

REF Settings

Parameter	Setting
Pickup	0.1–0.2 × I_n (CT secondary)
Operating time	100–200 ms
High-set element (if fitted)	1.0 × I_n, 30 ms

A setting of 0.1 × I_n provides sensitivity down to 10% of rated secondary current for high-resistance earth faults, while the 200 ms operating time prevents nuisance tripping on CT saturation during external phase faults. The high-set element provides fast clearance for solid internal earth faults.

Buchholz and Temperature Protection Wiring

The Buchholz relay and temperature indicators are mechanical protection devices that detect faults inside the transformer tank — they are complementary to the electrical protection (fuses, ACB, REF).

Buchholz Relay

A double-float Buchholz relay provides two contacts:

Contact	Detection	Action
Alarm (upper float)	Slow gas accumulation from incipient fault (partial discharge, hot spot, arcing in oil)	Alarm only — do NOT trip. The operator investigates via DGA sampling.
Trip (lower float)	Rapid oil surge from major internal fault (winding short-circuit, phase-to-phase arc)	Instant trip — trip HV circuit breaker (or initiate shunt trip on ring-main unit / circuit breaker). Trip LV ACB via shunt trip coil.

Critical wiring rule: The Buchholz trip contact must be wired to trip the HV interrupting device (fuse-switch, circuit breaker, or RMU), not just the LV ACB. A fault inside the transformer tank is fed from the HV side — tripping the LV ACB does not interrupt the fault current. The Buchholz trip circuit must be a direct trip (no time delay, no interposing logic) hardwired to the HV breaker trip coil.

Winding Temperature Indicator (WTI)

The WTI measures the hottest-spot winding temperature using a CT-heated sensing bulb (thermal image technique) and provides:

Contact	Setpoint	Action
Alarm	105–110°C (depending on insulation class)	SCADA alarm, operator investigation
Trip	120–125°C	Trip LV ACB (load shed), trip HV CB after time delay (30–60 s)
Fan start (if ONAF)	85–90°C	Start forced cooling fans
Fan stop	70–75°C	Stop fans (hysteresis to prevent hunting)

FAQ

Q: My 630 kVA transformer has a 50 A HV fuse. The 50 A fuse keeps blowing on transformer energization, but the transformer test report shows normal magnetizing current. What's wrong?

A: Transformer magnetizing inrush current is not a single value — it depends on the point-on-wave of the voltage at the instant of switch-on, and the residual flux in the core from the previous de-energization. Worst-case inrush (energizing at voltage zero-crossing with maximum residual flux of opposite polarity) can reach 12–15× rated current for 0.1 second. Your 50 A fuse sees a peak of 50 A × 12 = 600 A for 0.1 s. Check the fuse manufacturer's time-current curve: if the pre-arcing I²t at 0.1 s is exceeded by the inrush I²t, the fuse will melt. Solutions: (a) uprate to 63 A fuse (check coordination first), (b) install a point-on-wave closing controller on the HV switch (closes at voltage peak to minimize inrush), (c) specify a transformer with controlled inrush design (e.g., gapped core, or a design that limits residual flux). Option (a) is the fastest and cheapest but must not compromise protection sensitivity.

Q: My ACB has L/S/I/G settings but the manual doesn't explain what "I²t ON/OFF" does for the S-curve. Which should I use?

A: "I²t ON" means the short-time delay follows an inverse-time curve: the higher the fault current, the faster the trip (like a fuse curve). "I²t OFF" means a definite (flat) time delay: regardless of fault current magnitude (above pickup), the ACB trips after exactly tsd seconds. For a transformer LV ACB, the choice depends on what it's coordinating with: if the downstream feeder breakers (MCCBs) have inverse-time (thermal-magnetic) trips, set I²t = ON so the S-curve mimics the downstream device's thermal curve and provides selective coordination. If the downstream devices are definite-time electronic releases, set I²t = OFF for a clean selectivity step. For most mixed commercial installations with both thermal-magnetic MCCBs and electronic releases, I²t = ON is the safer default.

Q: Is REF protection worth installing on a transformer that already has a differential relay (87T)?

A: No. A transformer differential relay (87T) already provides protection for phase-to-phase and phase-to-earth faults within its zone, which typically covers the entire transformer — windings, bushings, and lead connections. The 87T is sensitive to inter-turn faults because it detects the resulting current imbalance, which REF may miss if the inter-turn fault does not involve earth. Adding REF to an 87T-protected transformer is redundant. REF is most valuable on transformers that are too small (< 5 MVA) to economically justify a full 87T scheme — which is exactly the 630 kVA class. Install REF when the transformer is below the 87T economic threshold and requires earth-fault sensitivity beyond what the HV fuse and LV ACB earth fault element can provide.

Q: The temperature trip contact — should it trip only the LV ACB or the HV breaker as well?

A: Trip strategy for transformer thermal overload should be staged. Stage 1 (alarm, _{105°C): SCADA alarm, operator investigates but no tripping. Stage 2 (LV shed,} 115°C): trip the LV ACB only — this removes the load that is causing the thermal overload. The transformer remains energized, cooling naturally — often this alone brings the temperature back within limits within 15–30 minutes. If the temperature continues to rise despite LV load shedding, Stage 3 (HV trip, ~125°C): the fault is likely internal (a developing shorted turn, blocked oil circulation, cooler failure) and the transformer must be de-energized from the HV side. The key question to ask your protection engineer: does the WTI really need to trip the HV breaker? In a substation where the transformer is remotely monitored (SCADA), Stage 1 alarm + operator intervention is usually sufficient, and Stage 3 can be implemented as a SCADA-initiated trip rather than a hardwired trip. In an unmanned substation, hardwire all stages.

Q: My ACB's earth fault protection picks up and trips randomly. The insulation resistance of the transformer and cables tests fine. What's causing this?

A: Intermittent earth fault trips with clean insulation test results point to one of three causes. (1) Third-harmonic residual current: In a 4-wire LV system with single-phase non-linear loads (LED lighting, switch-mode power supplies), the third-harmonic currents in each phase sum in the neutral rather than cancelling. If the neutral CT is part of your earth fault measurement (via a residual connection of phase + neutral CTs), this third-harmonic residual appears as spurious earth fault current. The fix is to filter the earth fault measurement or increase the pickup setting. (2) CT saturation during motor starting: A large motor start (e.g., a 100 kW DOL motor) draws 6–8× rated current on one phase momentarily, potentially causing the phase CT on that phase to saturate. The saturated CT's secondary current is lower than the actual primary current, creating an apparent residual current (the vector sum of the three phase CTs ≠ 0) that the earth fault element interprets as a fault. The fix is to increase the earth fault time delay to 0.3–0.5 s to ride through the motor starting transient. (3) Neutral-to-earth interconnection downstream: If a downstream sub-distribution board has its neutral connected to a separate earth electrode (creating a parallel neutral-to-earth path), part of the load current returns via the earth path instead of the neutral conductor. This shows up as earth fault current at the transformer's earth fault CT. The fix is to verify that the neutral is earthed at one point only (the transformer LV neutral bushing), per IEC 60364-5-54.

References and Standards

Standard	Title
IEC 60076-5:2006	Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-7:2018	Power transformers – Part 7: Loading guide for mineral-oil-immersed transformers
IEC 60076-22-7:2020	Power transformer and reactor fittings – Accessories and fittings
IEC 60255-3	Electrical relays – Single input energizing quantity measuring relays
IEC 60255-151	Measuring relays and protection equipment – Functional requirements for over/under current protection
IEC 60282-1	High-voltage fuses – Part 1: Current-limiting fuses
IEC 60947-2	Low-voltage switchgear and controlgear – Part 2: Circuit-breakers
IEC 60364-5-54	Low-voltage electrical installations – Earthing arrangements and protective conductors
IEEE C37.91	Guide for Protecting Power Transformers

About the Author

Du Fu is a Production Engineer at ZY POWER. He has designed protection coordination schemes for distribution transformers from 100 kVA to 5 MVA, and regularly liaises with protection relay manufacturers and utility protection engineers to validate coordination studies for transformer procurement projects.