Parallel Transformers, a 3-Pole ATS and a Hospital Imaging Load
A hospital redevelopment adds a new imaging suite to an existing 11 kV/400 V substation. The substation has two 1,500 kVA transformers (Z = 6.25%) fed by a ring from the utility, each with a 2,000 A LV ACB rated Icu 50 kA, and a 1,250 kVA standby generator (X″d = 0.15 pu) connected through an automatic transfer switch. The contractor's proposal uses a 3-pole ATS with the generator and transformer neutrals both bonded to the main earth terminal at the LV panel. The new suite draws 620 A continuously and includes a 300 kW chiller with direct-on-line starting and a 400 kVA UPS. The electrical inspector has asked you, as the contractor's responsible engineer, to confirm the design before first energisation. The project director wants energisation on Friday; the utility's 11 kV relay has a 0.3 s grading margin requirement.
1. Calculate the prospective fault current at the LV terminals of one transformer and at the generator terminals. State, with reasons, whether the existing 2,000 A ACB and the proposed ATS are adequately rated, including the case where both transformers are paralleled.
Read the worked answer
Transformer LV fault: Isc = S / (√3 × V × Z) = 1,500,000 / (1.732 × 400 × 0.0625) ≈ 34.6 kA symmetrical. Generator: 1,250,000 / (1.732 × 400 × 0.15) ≈ 12.0 kA. The ACB's 50 kA Icu appears adequate for one transformer, but that is not the governing case. If the two transformers are paralleled at the LV busbar, the busbar fault level is approximately the sum of both sources, about 69 kA plus motor contribution, which exceeds the 50 kA ACB and almost certainly the ATS withstand. The design must therefore interlock the LV busbar coupler to prevent paralleling, or the switchgear and ATS must be upgraded. For the ATS, a 2,000 A frame rating says nothing about short-circuit rating: require certified Icw for the 0.3 s grading time, Icm peak making, and Ics. Also check the ACB short-time withstand for 34.6 kA for at least 0.3 s plus relay overshoot. Motor contribution from the chiller and UPS should be added for the first few cycles. The utility source impedance reduces the 34.6 kA only slightly, so do not rely on it to justify a lower rating.
2. The contractor argues that a 3-pole ATS is cheaper and 'the neutral is earthed anyway'. Explain the earthing and neutral problems with the proposal and state what you would require before energisation.
Read the worked answer
With a 3-pole ATS, both source neutrals remain connected to the LV panel neutral bar and both source neutral points remain bonded to earth. When the generator runs, neutral current can divide between the transformer neutral bond and the generator neutral bond, creating parallel earth paths, circulating currents and elevated earth potential. Earth fault current can return by an unintended path, and residual current devices or earth fault relays may operate unpredictably or fail to operate. The installation also loses the clear separation between a TN-S arrangement on utility supply and a separately derived generator source. A 4-pole ATS, or an interlocked switched-neutral arrangement, ensures only one neutral-earth bond is in service at any time. I would require: a single neutral-earth bond associated with the source in service; the generator neutral earthed at the generator or through a link that is only closed when the generator is the source; neutral treated as a live conductor with full insulation and a rated ATS neutral pole; a schematic and switching programme showing the bond positions; and a test that confirms no parallel neutral path in either mode. The medical locations may also need dedicated medical IT systems and insulation monitoring, which should be checked separately.
3. Assess the impact of the 300 kW direct-on-line chiller on transformer voltage dip and on the 11 kV feeder protection grading, and state your recommendation.
Read the worked answer
Assume the chiller has pf 0.85 and efficiency 0.94: FLC ≈ 300,000 / (1.732 × 400 × 0.85 × 0.94) ≈ 542 A. At six times DOL starting current, the starting current is about 3,250 A, or 2.25 MVA. On a 1,500 kVA transformer with 6.25% impedance, the approximate busbar dip is 6.25% × 2.25 / 1.5 ≈ 9.4%, before utility impedance and before other loads. That is likely to disturb medical equipment, drop contactors and cause unacceptable flicker, and the dip on the generator would be worse because its subtransient reactance is higher. I would require a soft starter or VFD, a motor-starting study with the actual allowable dip agreed with the hospital, and verification that the UPS and imaging equipment ride through. For the 11 kV relay, transformer primary full-load current is 78.7 A. With a 100/1 CT, the relay pickup must be above the new continuous load and transformer inrush, but low enough to detect minimum faults, and the LV ACB must clear LV faults within the 0.3 s margin. A coordination study is required: check the new load does not push the relay pickup above the available fault current, and check cold-load pickup and motor contribution do not cause nuisance tripping.