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Loading of a hydro generator consist of two components:
  1. Loading with active power: could be modified by changing the admission of the fluid to the hydraulic turbine (direct proportions).
  2. Loading with reactive power: modifying generator's field current (direct proportions).

P [MW] - active power

Loading a hydro generator with ACTIVE POWER by changing the admission of the fluid (water) there could be some situations, according to the operating state of the generator: working independently or working connected to a power network (grid). If the generator is connected directly (alone) to the consumers (a small network or grid), then with the active power, the speed will also be modified (direct proportions). If the generator is connected with a power grid, then the admission of the water to the hydraulic turbine will determine only the load with active power. In this (general) case the speed will remain the same being held by the frequency of the power system (the frequency is unique in a power system):
f = p * n or: f = p * n/60
f = frequency
p = pairs of poles
n = speed [rev./s] n = speed [rev./min.]

There are two ways to modify the active power produced by the generator:

  • manually by direct command operated by the authorized personnel, and
  • automatically by the speed regulator. If the frequency is modifying in the power system, considering the formula above, the speed will change in the same proportion, so the regulator will intervene and will adjust the amount of water flow to the turbine resulting the nominal (rated) speed to the unit (turbine - generator).

The maximum active power of the generator is limited by the maximum mechanical power of the hydraulic turbine, and this limit is drawn on top of the P-Q diagram (spare power).

The lower limit of the active power depends on the hydraulic turbine, considering for instance the cavitation phenomena.


Q [MVAr] - reactive power

Loading a hydro generator with REACTIVE POWER by changing the field current, we could obtain either capacitive reactive power, or inductive reactive power, depending on the level of the field current.

The minimum field current corresponds to the stability limit of the hydro generator. Under this limit, the generator becomes unstable and it will exit from synchronism. This is an unacceptable operating state of a synchronous generator (hydro and turbo also). The cage (damper) winding could burn, because the inductive currents in these windings could reach levels higher than the maximum designed limit. There are two such limits:

  • a theoretical one and
  • a practical one.
This current limit corresponds to the minimum reactive power of the generators: it is the maximum capacitive reactive power. This situation tells us that the generator absorbs reactive power from the system.

The maximum field current corresponds to the temperature limit of the rotor winding caused by Joule effect. This current limit corresponds to the maximum reactive power of the generators: it is the maximum inductive reactive power. This situation tells us that the generator supplies reactive power into the power system.

These limits (theoretical and practical minimum, and maximum) are drawn on the left and right side of the P-Q diagram (spare power).

During the operation of the unit, the personnel will take care to the unit's limits, although the unit has a lot of warning signals and a lot of protections which prevent all the undesired operating states or damaging phenomena of the unit. Some times the unit could operate with larger values than the rated ones of its parameters. As an example, if we want to increase the reactive power over the rated one (with 10%), then we have to decrease the active power in order to keep the apparent power of the generator:


S2 = P2 + Q2




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