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What is The Regulation of Synchronous Generator 2

Aug. 15, 2022

We will continue to introduce the regulation of synchronous generators. The regulation of the output voltage of the synchronous generator: the purpose of regulation is to stabilize the output voltage within the rated load range of the synchronous generator. The idea of the control technology is to obtain the voltage and current from the main generator armature in real time, and then supply it to the stator coil of the exciter after rectification and negative feedback adjustment, so that it generates a DC electromagnetic field whose variation law is opposite to that of the output voltage of the main generator. The magnetic field must also cause the output voltage of the exciter rotor armature and the DC current supplied by the rotating rectifier to the main generator rotor coil to change according to the same law. Therefore, it can adjust the size of the rotor magnetic field of the main generator in real time, so that the main generator can maintain a good output characteristic within the rated load range.


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The adjustment process of the generator output voltage can be represented by the following process; due to the increase of the load, the main generator armature voltage ↓ (drop) → after the negative feedback adjustment, the exciter stator current and magnetic field ↑ → the exciter rotor armature output voltage ↑→Rotary rectifier output current ↑→Main generator rotor magnetic field ↑→Make main generator armature voltage ↑. If the voltage of the main generator increases, its feedback regulation reduces the effect of the above links, causing the voltage to return to the rated value.


It can be seen that the output voltage can be stabilized by adjusting the size of the rotor magnetic field of the main generator in real time by the exciter. Among them, the negative feedback regulation unit plays an important role, which is usually referred to as a constant voltage excitation device and an automatic voltage regulator.


Automatic voltage regulator: Modern AC synchronous generators often use automatic voltage regulator AVR, an electronic component to adjust the strength of the exciter stator magnetic field. Although there are many types of AVRs, their performance is similar; they all sample the output voltage value of the main generator in real time and compare it with a preset value, and use the comparison result to adjust the pulse width modulator PWM; if the output voltage value is high, the modulator output The pulse width is narrow, and vice versa. Then, these pulses are used to control the time when the high-power switching device, that is, the triode or the field effect tube, controls the current sent into the stator coil of the exciter. Therefore, its magnetic field strength changes inversely with the change of the output voltage of the main generator; that is, when the output voltage increases, the magnetic field of the exciter stator decreases, and when the output voltage decreases, the magnetic field of the exciter stator increases. So as to achieve the purpose of negative feedback regulation.


The circuit principle of automatic voltage regulator is a commonly used type of AVR. The signal of sampling the output voltage of the main generator is input to the voltage measurement and comparison unit from both ends of 8 and 9, and is compared with the internally preset voltage value (for example, 380V). The comparison result is sent to the pulse width modulation unit PWM with the output voltage UA, and the output voltage UC is sent to the low frequency protection unit. L, S, H of the voltage measurement and comparison unit are the three terminals connected to the output voltage amplitude adjustment potentiometer of the main generator.


The pulse width modulator is powered by the DC voltage UCC output by the regulator to ensure its stable performance. Its output voltage UB controls the modulation tube VT3. If the UA sent by the voltage measurement and comparison unit is large, it indicates that the output voltage of the main generator increases, and the large UA will narrow the width of the pulse UB output by the pulse width modulator. A narrow pulse will make VT3 turn on for a short time and pass less current. On the contrary, when the voltage of the main generator decreases, UA becomes smaller, and the width of the pulse UB output by the pulse width modulator becomes wider, so that the conduction time of VT3 becomes longer and the current passing through it increases.


One end of the exciter stator coil is connected to the terminal X1, and the other end is connected to the XX1 terminal. The three-phase voltages of EA, EB and Ec sent by the armature of the main generator are rectified by three diodes VD10, VD11 and VD12, and the current flows from the X1 end into the stator coil of the exciter, flows out from XX1, and then passes through the modulation tube VT3 and The XN terminal flows back to the main generator armature, forming the excitation current path for the exciter stator coil. VT3 is a switch on this path. It has a long conduction time, and the stator coil flows for a long time, and the stator magnetic field strength is large; VT3 has a short conduction time, and the stator coil current is small, and the stator magnetic field strength is small.


This is how AVR regulates the voltage of the main generator; the output voltage of the main generator increases due to the load, and the UA output by the voltage measurement and comparison unit increases with the increase, and the output pulse UB width of the pulse width modulator controlled by UA narrows, and the switch The conduction time of the tube VT3 is short, the stator magnetic field of the exciter is weakened, the rotor armature voltage and the output current of the rotating rectifier are reduced accordingly, resulting in the reduction of the excitation current supplied to the rotor of the main generator, and the main generator is due to the reduction of the rotor magnetic field. and reduce the output voltage. On the contrary, the negative feedback regulation function of AVR will increase the output voltage of the main generator.


When the main generator outputs a very large current due to the load exceeding the rated value, the diesel engine also needs to output a huge power so that its speed is lower than the rated value. The function of the low frequency protection unit is to limit the excess current increase in the stator coil of the exciter in this case. It pre-sets a low-frequency protection point with the charging and discharging branch composed of resistance and capacitance. When the load of the main generator is normal, the UC from the voltage measurement unit is less than the low-frequency protection point, the voltage Ud output by the low-frequency protection unit is high, and the diode VD8 If it is cut off, Ud can't reach the PWM and can't work. If the main generator is overloaded, Ud becomes low, VD8 is turned on, Ud and UA can act on the pulse width modulator at the same time, so that the output pulse UB becomes narrower with the decrease of Ud, and the conduction time of the modulation tube VT3 changes accordingly. Short, the excitation current decreases and the exciter stator magnetic field becomes weaker, resulting in a decrease in the main generator rotor magnetic field. The generator output voltage drops and the current decreases. The low frequency protection unit plays the role of protecting the exciter and the main generator.


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