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TEMPUS ENERGY: SYNCHRONOUS GENERATORS 1: TURBO GENERATORS AND SALIENT-POLE GENERATORS In the majority of the synchronous generators, a sinusoidal voltage is induced in the stator by driving a rotor. This rotor produces, with respect to this rotor, a fixed magnetic field. This fixed magnetic field is due to permanent magnets mounted in the rotor or this magnetic field is due to electromagnets mounted in the rotor and fed by a DC current. By controlling this DC current, the induction of this magnetic field can be controlled (which is not possible when using permanent magnets). Synchronous generators are commonly used in large electrical power stations (in nuclear power stations and in power stations based on fossil fuels) where they inject their active power into the high voltage grid. Synchronous generators can be turbo generators or salient-pole generators. Turbo generators are fast rotating generators (for instance 1500 rpm or 3000 rpm) and have a rotor with a limited diameter with a small number of pole pairs π. Turbo generators are mainly used in thermal power stations driven by steam turbines. The high synchronous speed of rotation ππ = 60 π π is inversely proportional with the number of pole pairs π in the generator (and proportional with the generated frequency π). Figure 1: Synchronous turbo generator in an electrical power station Salient-pole generators have a large rotor diameter which enables to realize a larger number of pole pairs. This limits the synchronous speed. These slowly rotating salient-pole generators are commonly used in hydraulic power stations (Figure 2 which visualizes a scale model of the pump station of Coo in Belgium having salient-pole generators) and in wind turbines (Enercon wind turbine of Figure 3). Figure 2: Scale model of the pump station of Coo with salient-pole generator Figure 3: Synchronous generator in a wind turbine 2: EQUIVALENT CIRCUIT OF A TURBO GENERATOR AND THE POWER FLOW A synchronous generator behaves as a non-ideal voltage source having the equivalent circuit visualized in Figure 4. There is a generated voltage π(π‘) and an internal ohmic inductive impedance containing π π and πΏπ . Figure 4 also visualizes a switch which allows to connect and disconnect the generator with the grid voltage π’π π ππ(Ξ©π π‘) (having an amplitude π’π and pulsation Ξ©π ). Assume the generator is connected with a strong grid meaning the generator is not able to change the grid voltage π’(π‘) = π’π π ππ(Ξ©π π‘). The current π(π‘) satisfies the differential equation π’(π‘) + π π π(π‘) + πΏπ π π(π‘) ππ‘ = π(π‘). The solution of this differential equation mainly depends on the amplitude of π(π‘) in comparison with the amplitude of π’(π‘) and on the phase of π(π‘) in comparison with the phase of π’(π‘). As visualized in Figure 5, the amplitude πΈ of π(π‘) and the phase difference π are very important to determine the power flow between the generator and the grid. Figure 4: Equivalent circuit of a synchronous generator The instantaneous power π(π‘) = π’(π‘) π(π‘) contains two terms. The first term has an average value π= 1 π’π πΈ π ππ(π) 2 Ξ©π πΏπ and the second term has an amplitude π=β π’π (π’π β πΈπππ (π)) Ξ©π πΏπ . Here, π is the active power injected into the grid by the generator for each phase (the three phase power is three times larger)1. In case a synchronous machine is generating, the angle π is positive. The voltage π(π‘) leads the grid voltage π’(π‘) = π’π π ππ(Ξ©π π‘) (in case the synchronous machine is motoring the angle π is negative and π(π‘) lags the grid voltage). 3: REACTIVE POWER The DC excitation of the rotor windings is very important. By increasing the excitation current in the rotor windings, the amplitude of the induced voltage π(π‘) increases. This implies the torque and therefore also the active power can be larger. The excitation also determines the reactive power (for each phase, the three phase power is three times larger) as specified by2 π=β 1 π’π (π’π β πΈπππ (π)) Ξ©π πΏπ . In case of a salient-pole synchronous generator, another formula is valid but the main conclusions and the philosophy remains the same. 2 In case of a salient-pole synchronous generator, another formula is valid but the main conclusions and the philosophy remains the same. By feeding the rotor windings by an excitation current which is sufficiently large (over excited generator having a large πΈ), this generator will generate reactive power (contrary to an asynchronous generator which consumes reactive power). By feeding the rotor windings by an excitation current which is smaller (under excited generator having a small πΈ), this generator will consume reactive power (similar with an asynchronous generator which also consumes reactive power). By adjusting the excitation current, the generated or the consumed reactive power can be adjusted. By having a sufficiently large excitation current, the synchronous generator is able to supply reactive power to the loads, contrary to an asynchronous generator which has problems to supply reactive power. 3.1: REMARKS Especially when considering smaller wind turbines, the synchronous generator can be grid connected without the use of a power electronic converter (which reduces the capital investment). In this situation, a strong grid with a fixed frequency gives the generator and the blades of the wind turbine a fixed speed of rotation. Since the generator is grid connected, the excitation of the generator is important since this excitation determines the reactive power generated or consumed by this generator. When there is a frequency converter between the synchronous generator and the grid as visualized in Figure 5, the speed of the generator and the rotor blades is not fixed. Due to the frequency converter, the frequency of the voltage generated by the generator is allowed to depend on the wind speed. Moreover, the frequency converter is able to inject reactive power into the grid or to consume reactive power irrespective of the excitation of the synchronous generator. 4: BRUSHLESS SYNCHRONOUS MACHINE The rotor of a classical synchronous machine (functioning as a motor or as a generator) is fed by an excitation current. This requires slip rings and brushes which implies a lot of maintenance. Figure 5 visualizes a wind turbine equipped with a classical synchronous generator having slip rings and brushes. Figure 5: Wind turbine equipped with a synchronous generator In order to reduce the maintenance costs related with these slip rings and brushes, a brushless synchronous generator can be used as visualized in Figure 6. Actually two generators, an auxiliary generator and a main generator, are built in one single machine. The auxiliary generator contains an excitation winding in the stator which is easily fed by a DC excitation current giving a fixed magnetic field. The rotor contains a three phase winding which rotates in this fixed magnetic field and an AC voltage is induced in this three phase winding. Diodes, mounted inside the rotor, rectify this generated voltage. Using this rectified voltage, the excitation winding (in the rotor) of the main generator is fed giving a synchronously rotating magnetic field. Due to this rotating magnetic field, a sinusoidal voltage is induced in the three phase stator winding of the main generator. Figure 6: Brushless synchronous generator Figure 7: Rotors of brushless synchronous generators Figure 8: Stator and rotor of a brushless synchronous generator (4 poles)