Converting a generating unit into a synchronous condenser removes most of its active-power export, but the GSU can still carry substantial current. Reactive-power support may become the main continuous duty, and the transformer can spend more hours energized at light load. Set the general power-transformer specification under IEC 60076-1 [1].
For balanced steady operation, current depends on √(P² + Q²), not P alone. A condenser operating near zero P with large Q can approach a transformer winding-current limit. Small active import also supplies machine and transformer losses; zero exported MW is not zero energy flow.
Compare the duty rather than the nameplate labels
| Design input | Generating service | Condenser conversion review |
|---|---|---|
| Power envelope | Active export plus reactive support | Continuous Q extremes and loss import |
| Voltage envelope | Generation dispatch cases | Grid-support setpoints, low/high grid voltage |
| Cooling operation | Linked to unit load or plant logic | Availability during low-MW, high-current service |
| Light-load duration | Original operating pattern | Long energized intervals and core heating |
| Starting sequence | Turbine-driven synchronization | Actual replacement drive and supply paths |
Use measured or calculated winding currents at each new reactive endpoint. Include tap position and terminal voltage. At lower voltage, the same reactive power requires higher current. A comparison at only nominal voltage can miss the binding case.
Cooling logic deserves a separate test
A cooling control that uses MW as a convenient plant-load signal may no longer track the transformer’s copper losses. Inspect the actual input signals and interlocks. Confirm that pumps or fans remain available when the turbine and its auxiliaries are retired or reconfigured.
Loss distribution also changes. Core losses persist while the transformer is energized; winding losses follow the current and harmonic content. A reduced annual energy throughput does not describe the hottest winding condition or the time spent near it.
Check the low-voltage reactive endpoint
Assume condenser operation at P = −2 MW and Q = +100 Mvar, balanced at 50 Hz. Apparent power is √(2² + 100²) = 100.02 MVA. At 20 kV line-to-line, current is 2887 A; at 18 kV it is 3208 A. An assumed 3000 A LV continuous limit covers the first point but misses the second by 208 A. Restrict the low-voltage reactive envelope or establish a revised qualified duty; low MW and the same Mvar request do not justify unchanged cooling.
Use the conversion model to define the remaining restrictions
Evaluate continuous reactive supply, continuous reactive absorption, startup and shutdown, and the specified grid-disturbance response. Confirm the new voltage range against both machine excitation capability and transformer limits. Starting-converter harmonics require their own current paths and duration; they cannot be inferred from the condenser’s steady Q rating.
Reuse depends on the new voltage, current and cooling envelope. A lower MW transfer can still create a duty outside an original GSU limit. Check each changed case against the retained design evidence and identify the restriction that prevents unrestricted conversion.
For mineral-oil equipment, relate the loading assessment to operating temperature and thermal ageing [2].
References
- IEC, IEC 60076-1:2011, Power transformers – Part 1: General.
- IEC, IEC 60076-7:2018, Power transformers – Part 7: Loading guide for mineral-oil-immersed power transformers.