Standby generator on a UPS load: rating, ATS neutral and earth fault protection
A telecommunications hub has a 400/230 V, 50 Hz standby diesel generator rated 100 kVA / 80 kW at 0.8 pf; the alternator has X″d = 12% and its rating assumes a 40 °C ambient. The load is a 30 kVA double-conversion UPS with a six-pulse rectifier (input displacement factor 0.82, THDi 32% at full load), 15 kW of AHU motors including one 11 kW direct-on-line unit, and 5 kW of lighting, giving 45 kW steady demand. The generator room runs at 45 °C. During a recent outage the ATS transferred and the set picked up the block load, but busbar voltage fell to 310 V for about 600 ms and frequency to 47.5 Hz; the 11 kW AHU contactor dropped out and did not restart. After 40 minutes at only 48 kW the alternator winding reached 155 °C. Records show the original four-pole ATS was replaced by a three-pole unit two years ago 'to stop nuisance tripping', and the generator star point is bonded to a local electrode at the generator while the main board has its own neutral-earth bond.
1. Determine whether the 100 kVA set is adequate for this load, and explain the 155 °C winding temperature at only 48 kW.
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What the load really is. The UPS draws 30 kVA at 0.82 displacement, i.e. 24.6 kW fundamental; with 32% THDi the distortion factor is 1/√(1 + 0.32²) = 0.95, so its true input is about 31.5 kVA. Add 20 kW of motors and lighting for a total of 45 kW. During recharge after an outage a double-conversion UPS will pull 110–125% of rated input, so the block step is nearer 56 kVA.
Three capacity reductions the designer ignored.
- Non-linear load. A six-pulse rectifier draws 5th and 7th harmonic currents that produce a reverse-rotating field, inducing 6th-harmonic currents in the damper winding and rotor surface. Manufacturers require an alternator derating of roughly 0.8 (or about 1.4× oversizing) at 30–35% THDi. The UPS alone therefore looks like about 39 kVA to the alternator rather than 31.5 kVA.
- Ambient. The rating is at 40 °C but the room runs at 45 °C, worth roughly 0.95, so the 100 kVA standby figure is really about 95 kVA before harmonics and nearer 76 kVA after.
- Duty. The machine is a standby-rated set; a 40-minute continuous run must be assessed against the prime rating, typically one step below.
Taking the derated capability as roughly 54–60 kW, a 48 kW load is at 80–90% of it, which with the rotor harmonic heating and field forcing explains 155 °C (110 K rise at 45 °C ambient, above the Class F limit). The set is not adequate. Specify a 150 kVA / 120 kW prime-rated machine, X″d ≤ 12%, and either require the UPS to meet THDi ≤ 10% with a DC-link choke or oversize the alternator by about 1.5× for the rectifier load.
2. Specify the correct ATS pole configuration and neutral-earthing arrangement, and explain why the three-pole unit is causing trouble.
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What the three-pole ATS does. With the neutral solid through the ATS, the generator star point bond at the generator and the neutral-earth bond at the main board are both in service simultaneously. The load neutral current then divides between the neutral conductor and the earthing system, producing stray current in bonding conductors and structural steel, corrosion at joints and circulating currents in parallel paths. Any residual-current device upstream of the ATS sees this diverted current as an imbalance and trips - precisely the 'nuisance tripping' the change was supposed to cure.
Correct arrangement. Use a four-pole ATS that switches the neutral, making the generator a separately derived system. Then:
- The generator neutral is bonded to earth at one point only, in or adjacent to the generator terminal box, and that is the sole neutral-earth link for the generator source.
- The main board's neutral-earth link serves the transformer source only; the fourth pole separates the two neutrals so the two links are never in parallel.
- Both earth electrodes and all bonding conductors must still be connected into one common equipotential zone. It is the neutral-earth link that is switched, never the protective conductor.
Checks. Confirm the neutral pole is fully rated for the load current and is genuinely simultaneous or make-before-break as the ATS is designed to be; confirm no second neutral-earth link exists in any downstream panel; and record the two source-side links on the schematic so the next electrician does not 'restore' the three-pole unit.
3. The generator earth fault protection has never been verified. Explain what happens for a downstream line-to-earth fault on generator supply and recommend a protection arrangement.
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The fault current. On a 100 kVA, 400 V alternator the line-to-earth fault current is limited by the machine's zero-sequence and subtransient reactances and by AVR forcing, not by the transformer. In practice a bolted fault gives only about 2.5–3.5 times rated current - rated current is 144 A, so roughly 400–500 A, decaying as the subtransient flux collapses. A 160 A moulded-case breaker with a 10× magnetic setting (1,600 A) will never see it.
Consequence. For a TN system the disconnection time depends on Ia ≤ U0/Zs. At 400 V with a 230 V line-neutral voltage and a 500 A fault, Zs ≈ 0.46 Ω; that is comfortably met for a 32 A final circuit only if the circuit's own breaker has a low enough magnetic threshold, and it is not met for submains relying on the generator's contribution. The result is a fault that persists until something upstream trips on overload or a person is exposed.
Recommendation.
- Fit a residual current device of 300 mA, time-delayed, on the generator outgoing way, or a residual earth-fault relay on a core-balance CT set at 10–20% of generator rating (say 20 A) with a 0.5 s delay for discrimination against the 30 mA final-circuit devices.
- Locate the sensing on the generator side of the ATS so protection is not lost when the neutral pole changes over.
- Verify by test: inject at the core-balance CT, and measure or calculate Zs on the generator source with the set running, not on the transformer source.
- Confirm that the generator neutral-earth link is upstream of the sensing so the relay does not see its own bonding current as a fault.