Читать книгу Handbook of Large Hydro Generators - Geoff Klempner - Страница 33

For all intents and purposes, during reactive power support operation, there is no water flowing through the turbine, thus no MW are being supplied to the system. However, by adjusting the excitation higher (over excitation), the machine can deliver reactive power and by decreasing the excitation (under excited), the machine can absorb reactive power. This mode of operation for a conventional generator (that normally supplies MW as well) is only during times where there may be an abnormal system condition and the generator will need to provide this reactive support or there is system emergency and extreme reactive support is required.

On one end of extreme operation, there is almost no current supplied to the DC field winding of the rotor and large amounts of reactive power (MVARS) are being absorbed by the generator. The capability curve, which is discussed later in the book, will have limits set as to how many MVARS the generator may absorb on a continuous basis without damaging effects. This mode of operation is not far from operating with a loss of excitation. Protections are put in place to prevent migration into this potentially damaging area of operation. Depending on the design of the generator and excitation system, rotating or static, the minimum excitation value can approach 10 A or less before the excitation system bottoms out and/or protections operate.

On the other end of extreme operation, the generator will need to supply large amounts of reactive power (MVARS) to the system, again, for abnormal system events or emergencies. Again, the capability curve from the manufacturer will outline what the maximum capability is and over excitation limits can be put in place to protect the generator from damaging effects from rotor overheating.

Figure 1.7-5 Steady‐state power angle characteristic of a salient pole synchronous machine (with negligible armature resistance).

It is now appropriate to expand on the effects of “saliency” on generator operation. Power developed in a salient pole machine can be written as Equation (1.1) [1]:

(1.1)

The term on the left‐hand side of the “+” sign is the power developed due to the field excitation. The term on the right‐hand side of the “+” sign is the power developed due to saliency. A graphic representation of the resultant power is shown in Figure 1.7-5 [1].

Even if no current is supplied to the DC field winding, there is a torque generated known as reluctance torque, due to the saliency effect. It is this torque which keeps the rotor in synchronism with the system, and prevents the machine from slipping poles.