رله بازگشت توان بر اساس جهت توان موثر عبوری از ترمینال ژنراتور تنظیم می شود. این تنظیمات به بخش گرداننده مکانیکی ژنراتور بستگی دارد، همچنین دقت اندازه گیری ترانسهای اندازه گیری به حساسیت مورد نیاز در این خصوص بستگی دارد.
When the prime mover energy input to a generator is removed, the losses must be supplied from the power system. A power relay at the generator terminals looking into the generator can detect this condition. The type of prime mover determines the sensitivity requirement for the relay.
Gas turbines represent a large compressor load when motoring. The power consumed by the unit in kW during motoring can be up to 100% of the name plate kVA rating for a single-shaft machine or as low as 10 to 15% for a double shaft installation. On the other hand, hydro turbines with their blades above the tail-race water level require only 0.2 to 2% of their nameplate rating expressed in kW. When the impeller is fully submerged, motoring power can reach 100% nameplate.
Hydro turbines with Kaplan adjustable blades can require less than 0.2% kW when the blades are in the flat position.
Diesel generators, with no cylinders firing, require about 15% or 25% kW for four- and two cycle engines, respectively. Steam turbines under full vacuum require 0.5 to 3.0% kW. Steam turbines without condensers or condensing turbines that lose vacuum will require more that 3% motoring power. Manufacturers should be consulted for exact motoring power requirement and the maximum permissible motoring time for their units.
The above discussion shows that the sensitivity of the reversed power relay can become the key application issue on large steam turbines and hydro turbines. Electromechanical and electronic reverse power relays are available with a large diversity of designs. These relays may be calibrated in amps or in secondary watts. They may measure single-phase or three-phase power.
The application of single-phase relays is predicated on the assumption that motoring losses are supplied equally by all phases. This is not always the case. On very large units, authors have reported variations of 0.2 to 0.3% of rated output between phases.3 This is very significant, because the total three-phase motoring power for such units can be as low as 0.5%. When microprocessor relays are applied, unbalance is not a problem, because these relays usually measure three-phase power.