WIND ENERGY SYSTEMS - Cd3wd

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Chapter 5—Electrical Network 5–20 The angle θ is |I| = P 25 × 103 = |V | cos θ 277(0.85) = 106.2 A The phasor operating current is then θ =cos −1 (0.85) = +31.79 o The reactive power supplied per phase is I = |I|/ − θ = 106.2/ − 31.79 o =90.3 − j55.9 A Q = (277)(106.2) sin 31.79 o =15, 500 var/phase The generator is then supplying a total reactive power of 46.5 kvar to the grid in addition to the total real power of 75 kW. The voltage E is given by Kirchhoff’s voltage law. E = V + IZ s = 277/0 o + 106.2/ − 31.79 o (0.033 + j4.073) = 508 + j366 = 626/35.77 o V/phase Since the terminal voltage V has been taken as the reference (V = |V |/0 o ), the power angle is just the angle of E, or35.77 o . The total stator ohmic loss is P loss =3I 2 R s = 3(106.2) 2 (0.033) = 1.117 kW This is a small fraction of the total power being delivered to the utility, but still represents a significant amount of heat which must be transferred to the atmosphere by the generator cooling system. The pullout power, given by Eq. 33 with sin δ =1is P = |E||V | = 626(277) =42.6 kW/phase X s 4.073 or a total of 128 kW for the total machine. As mentioned earlier, if the input shaft power would rise above the pullout power from a wind gust, the generator would lose synchronism with the power grid. In most systems, the pullout power will be at least twice the rated power of the generator to prevent this possibility. This larger pullout power represents a somewhat better safety margin than is available in the MOD-0 system. One advantage of the synchronous generator is its ability to supply either inductive or capacitive reactive power to a load. The generated voltage |E| is produced by a current Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 5—Electrical Network 5–21 flowing in the field winding, which is controlled by a control system. If the field current is increased, then |E| must increase. If the real power is fixed by the prime mover, then from Eq. 33 we see that sin δ must decrease by a proportional amount as |E| increases. This causes the reactive power flow to increase. A decrease in |E| will cause Q to decrease, eventually becoming negative. A synchronous generator rated at 125 kVA and 0.8 power factor can supply its rated real power of 100 kW and at the same time can supply any value of reactive power between +75 kvar and −75 kvar to the grid. Most loads require some reactive power for operation, so the synchronous generator can meet all the requirements of a load while requiring nothing from the load. It can operate in an independent mode as well as intertied with a utility grid. The major disadvantage of a synchronous generator is its complexity and cost, as well as the cost of the required control systems. Some of the complexity is shown by the synchronization process, as illustrated in Fig. 13. From a complete stop, the first step is to start the rotor. The sensors will measure wind direction and actuate the direction controls so the turbine is properly directed into the wind. If the wind speed is above the cut-in value, the pitch controls will change the propeller pitch so rotation can occur. The generator field control is activated so a predetermined current is sent through the field of the generator. A fixed field current fixes the flux Φ, so that E is proportional to the rotational speed n. The turbine accelerates until it almost reaches rated angular velocity. At this point the frequency of E will be about the same as that of the power grid. The amplitude of E will be about the same as V if the generator field current is correct. Slightly different frequencies will cause the phase difference between E and V to change slowly over the range of 0 to 360 o . The voltage difference V d is sensed so the relay can be closed when V d is a minimum. This limits the transient current through the relay contacts, thus prolonging their lives, and also minimizes the shock to both the generator and the power grid. If the relay is closed when V d is not close to its minimum, very high currents will flow until the generator is accelerated or decelerated to the rotational position where E and V are in phase. Figure 13: AC generator being synchronized with the power grid. Once the relay is closed, there will still be no power flow as long as E and V have the same Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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