Generator vs. Utility Paralleling for a Hospital Load
A hospital has a 1.2 MVA standby diesel generator, currently configured for open-transition (break-before-make) operation with the utility supply. The facility manager wants to run the generator in parallel with the utility during peak tariff periods to reduce energy cost, using the existing generator and a new synchronising panel. The utility's interconnection rules require reverse-power protection, anti-islanding, and a visible isolation point. The hospital has a 400 V main switchboard with a fault level of 25 kA, and the generator's subtransient reactance gives a fault contribution of about 8 kA at the point of common coupling. The generator has no reverse-power relay and its governor is a mechanical droop type.
1. Assess the technical feasibility of paralleling the existing generator with the utility.
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Paralleling is technically possible but the existing generator is not ready as-is. Key issues:\n\n1. Synchronising: a mechanical droop governor can be synchronised, but load sharing in parallel with the utility requires the generator to export a controlled amount of power. With droop control and no isochronous or import/export controller, the generator will tend to take a share of load determined by its droop, not a set export. A load-sharing/export controller is needed to hold a defined kW export.\n\n2. Protection: the utility requires reverse-power protection and anti-islanding. The generator has no reverse-power relay. Anti-islanding must be provided (e.g. a directional power relay plus under/over voltage and frequency protection, or a dedicated interface protection relay).\n\n3. Fault level: the combined fault level at the PCC rises. 25 kA utility plus 8 kA generator contribution is not simply additive in phase, but the switchboard and cabling must be rated for the combined fault current; 25 kA equipment with an 8 kA generator contribution may exceed the 25 kA rating depending on phase angle. This must be calculated, not assumed.\n\n4. Visible isolation: the utility requires a visible isolation point; the existing open-transition changeover may not provide a compliant visible break for parallel operation.
2. Explain the safety consequence of losing the anti-islanding function.
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If the utility supply is lost and the generator continues to energise the network (islanding), the following occur:\n\n- Utility lines that are expected to be dead become live, endangering line workers and the public.\n- The generator may attempt to carry the whole hospital load plus the connected utility network, causing overload, frequency and voltage excursions, and possible generator damage.\n- On utility restoration, the generator and utility can be out of synchronism, causing a severe out-of-phase closure with high currents and mechanical stress on the generator and switchgear.\n- Protection settings designed for the utility source may not operate correctly on the generator source, so faults in the island may not be cleared.\n\nAnti-islanding must therefore be independent, fail-safe, and tested. A reverse-power relay alone is not sufficient; loss-of-mains detection with under/over voltage and frequency, and a transfer-trip scheme to the generator breaker, is the standard approach. The hospital's life-safety loads must be transferred to the generator on loss of utility, so the anti-islanding scheme must not conflict with the essential-supply changeover logic.
3. Recommend the minimum scope of work before parallel operation is permitted.
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Minimum scope: (1) install a synchronising and export-control panel with an import/export controller capable of holding a set kW export against a droop governor; (2) install a utility-grade interface protection relay providing reverse-power, loss-of-mains, under/over voltage and frequency protection, with a transfer trip to the generator breaker; (3) provide a visible isolation point compliant with the utility's rules; (4) recalculate the combined fault level at the PCC and verify switchboard and cable ratings, upgrading if the 25 kA rating is exceeded; (5) agree the protection settings and interconnection agreement with the utility in writing; (6) commission with a witnessed test of the anti-islanding and reverse-power functions, including a simulated loss of mains. Until these are complete, the generator must remain in open-transition mode. Cost and tariff savings should be compared against this scope before proceeding.