In the self-excitation excitation system, the generator excitation mode includes DC power supply excitation, AC power supply excitation, and residual voltage excitation. The first two are the process of injecting a certain direct current into the rotor coil after the generator has a boosting condition, magnetizing the rotor, reacting the stator armature of the generator rotor, and generating a voltage at the generator end to build the generator. . The residual pressure excitation is a process in which the generator automatically builds pressure when the residual voltage of the generator reaches a certain value, using the residual pressure excitation device, with or without external magnetic assistance.
When there is no residual voltage excitation function, the AC or DC excitation needs to supply a certain current to the rotor. The current is allowed to reach 10% of the no-load rated current of the generator. When the excitation is started, it will impact the DC or AC power supply system of the power plant. . Because there is residual magnetism in the rotor of the starter generator, there is residual voltage at the end of the generator after idling, which creates conditions for the residual pressure to start. Therefore, it is better to have the residual pressure excitation function when the generator set starts. The power supply excitation is only input when the generator is first magnetized and the residual voltage at the machine is insufficient for residual voltage excitation. Residual pressure excitation has a good application prospect.
2 residual voltage excitation principle 2.1 switch control diode rectification type residual voltage excitation switch control diode rectification type residual voltage excitation work principle is simple, only need to put K1 (see) when the generator starts excitation, K1 is automatically started The control mechanism inputs and cuts. The cutting conditions can be combined with the terminal voltage (such as 30% terminal voltage) or the excitation time limit (such as 5s).
Diode rectification type residual voltage excitation is simple, but it has the following disadvantages: It needs to increase the main loop control device, such as diode rectifier bridge V1 and contactor Q1, Q2. As the unit capacity increases, the excitation current becomes larger. As the specifications of the main circuit diode and the contactor increase, the installation position of the device needs to be additionally considered, which increases the cost of the device.
Since the residual voltage excitation circuit (V1, Q1, Q2) and the control loop are added, the reliability of the entire excitation device is reduced, so the diode rectification residual voltage excitation is gradually replaced by the thyristor control type residual voltage excitation.
2.2 Schematic diagram of the main circuit of the thyristor control type residual voltage starting thyristor control type residual voltage excitation as shown in the dotted line. It is actually the main circuit of the generator's self-excitation excitation, without the need to add another device. Compared with the diode rectified residual voltage excitation ratio, it has the characteristics of simple wiring and cost saving, and is more and more widely used in the self-excitation excitation system.
The principle can be described as follows: After the generator has the excitation condition, it idling to near the rated speed, and the residual voltage is induced at the machine end due to the existence of the residual magnetism of the rotor. When the residual voltage is greater than the voltage drop of the rotor circuit, the residual voltage is excited, that is, the thyristor is continuously triggered to make the thyristor conduct, and the residual voltage is used to charge the generator rotor to achieve the purpose of residual voltage excitation. When the generator voltage rises to the residual voltage and the excitation exceeds the setting value, the residual pressure excitation function is automatically cut off to restore the normal excitation adjustment.
3 The primary circuit that achieves residual voltage excitation with residual voltage excitation is relatively simple, both diode rectification type and thyristor trigger type. The realization of residual voltage excitation is mainly secondary control loop. The diode type control is a simple relay, and the operation has been less and less used in the excitation system due to the above analysis. This paper mainly introduces the control loop of the thyristor residual pressure excitation type and the problems in the implementation.
The hardware diagram of the secondary control loop of the typical thyristor-controlled residual voltage excitation is shown in the figure. Only one pulse circuit schematic is shown in the figure. The other five pulse control principles are the same as the current one, except that the phase of the pulse is different during normal control. And already.
Thyristor control type residual voltage excitation secondary circuit schematic diagram Thyristor residual voltage excitation hardware is shown by the primary circuit and the control circuit group-secondary circuit as shown in the dotted line box. The residual voltage of the secondary return thyristor is composed of a primary circuit and a control loop, and the primary circuit is as shown in the dotted line box. As shown in the secondary circuit, during normal operation, the residual voltage excitation unit exits. The double pulse signal is amplified by D3 and output to the primary side of the pulse transformer T1, and then amplified by the pulse transformer and output to the control pole of the thyristor V11. When the residual voltage is excited, the control unit starts the high-frequency pulse generator to generate the high-frequency pulse immediately after receiving the residual pressure excitation command, and simultaneously closes K1 to input the pulse in parallel to the base of Q1, thus obtained at the gate of the thyristor. It will be a high-frequency pulse train, which will cause the thyristor to conduct at low voltage, and achieve the purpose of residual voltage excitation. After the residual voltage starts, the excitation regulator automatically cuts off the residual pressure excitation circuit and resumes normal regulation.
4 residual pressure excitation and residual pressure excitation mechanism 4.1 normal double pulse trigger circuit can not be used for residual voltage excitation to see the conduction mechanism of the thyristor, the thyristor can be seen as two transistor interconnection as shown The gate is injected with a trigger current %. When the external load is small enough, Ig will form a strong positive feedback, so that the two tubes are saturated. The process of positive feedback is: the current amplification factor of one Ia-Icl-Ig thyristor varies with the change of the emitter current, that is, the current amplification factor is small when the emitter current is small, and vice versa. The emitter current is also subjected to the thyristor operating principle of the applied voltage and the external load. The thyristor is turned off. The schematic diagram of the thyristor current is lower than that of the thyristor rectifier bridge for power supply to the generator rotor can be simplified as (a) Show.
The rotor excitation schematic (b) is a simplified diagram of a thyristor conduction loop, U2 is the commutating secondary side voltage, R is the rotor loop resistance, and L is the rotor inductance.
We only care about the dynamic process of the rotor current at the instant of thyristor conduction. We can use the following differential equation to describe Vt as the tube voltage drop, brush contact and other voltage drops. U2 is a sinusoidal function. For the sake of simplicity, the instantaneous value of the conduction start voltage can be replaced in the range of 60 degrees. U2 is the effective value of the excitation side line voltage. To solve the problem, the unit frequency is 50Hz. In a sinusoidal period, since the pulse width of the double pulse is very narrow, if the pulse of one pulse of the thyristor is insufficient to make the current flowing through the thyristor forward, the residual voltage of the double pulse fails.
4.2 Analysis of the excitation mechanism of high-frequency pulse residual voltage From the conduction mechanism of the thyristor, the conduction of the thyristor is formed by the trigger current. When the conduction current of the thyristor reaches the holding current, the thyristor is turned on. A high-frequency pulse trigger with a wide pulse or a sufficiently high frequency can be used to flow through the crystal when the thyristor forward voltage is present and high enough, and the thyristor current is maintained to conduct current. ,lishingHouse.rightsreserved,http:// SRM Common Power Converter Main Circuits and Principles Yang Yuefeng, Zhang Yihuang (School of Electrical Engineering, Northern Jiaotong University, Beijing 100044, China) 1 Introduction SRM Switched Reluctance The winding of the motor only needs unidirectional current, but it should be able to quickly receive power from the power supply and quickly return energy to the power supply. Since the SRM power converter only needs to provide a unidirectional current to the motor, it is simpler and more reliable than the asynchronous motor PWM inverter. However, the operating current and voltage waveforms of the SRM are not sinusoidal, and the waveform is difficult to accurately predict due to the operating conditions of the system and the motor design parameters. This makes the calculation of the rating of its main switching device more complicated.
Starting from the basic requirements of power converter matching with motor structure, high efficiency, convenient control, simple structure and low cost, an ideal power converter main circuit structure should have the following conditions: minimum number of main switching components; All the power supply voltage is applied to the motor phase winding; the voltage rating of the main switching device is close to the motor; the ability to quickly add phase winding current; the main switching device can be modulated to effectively control the phase current; while the winding flux linkage is reduced It can return energy to the power supply.
The following is a brief introduction to several common circuits of SRM power converters, focusing on the comparison of the rating of the main switching devices in these different lines, the number of components, the method of energy feedback and the applicable occasions. 2 Motor double winding type is the double winding structure used in the early stage. Usually, the main and auxiliary windings are in the form of double wire winding to obtain the maximum mutual inductance. After the main winding switching element 31 is disconnected, the energy of the main winding passes through mutual inductance. To the secondary winding, and then through the diode to continue D,. The rated operating voltage of the main switching element of the circuit is 2 (1+D)V, where V is the rectifier bridge that can be connected in parallel in the rotor circuit. It is suitable for Pientice-Hall2W1 in August.
Assume that the residual voltage of the unit is as follows: the voltage drop of the series two thyristors is 2.5V, the voltage drop of other circuits is 0.5V, the rotor loop resistance is 1 ohm, the rotor inductance is 2 hen, the thyristor current is 80mA, and the rectification becomes the secondary line. The voltage rms value U2 is 8V, and the minimum time t at which the thyristor reaches the sustain current can be calculated according to equation (2) to be t=1.9ms. Under the above conditions, the actual time from trigger to conduction is greater than t. At the same time, the pulse train is used, and its duty ratio is 1. If the conduction cannot be completed in one pulse, then in the 0 segment of the pulse, the thyristor has not been turned on, and then turns into the cutoff until the next pulse "1 comes. At this time, the current in the rotor has not decayed to 0. In this way, after several pulse cycles, the thyristor will reach the holding current and conduct. 4.3 The residual voltage can not be taken to improve the trigger voltage and current, that is, appropriate reduction Small trigger loop resistance R1 or change T1 åŒ ratio (see), plus T1 secondary side coil turns.
5 Practical application and field test A power station uses the residual voltage to start the excitation. The unit parameters are: rated power 12500kW, rated excitation voltage 261V, rated excitation current 460A, Td0=3.29s; high frequency pulse train frequency is 2K, triggering during normal adjustment The pulse adopts 60 double pulses, and the T1 turns ratio is 1:2. When the unit starts to idling to the rated speed, the actual measured excitation residual secondary residual voltage is greater than 8V. Under the above conditions, the two sets of residual pressure excitation test are as follows: R1=1(1) When the residual pressure is applied, the excitation fails. When R1 is changed to 50 ohms, the residual pressure is applied and the excitation is successful.
R1=100 ohms are unchanged, and a resistor is connected in parallel with the generator rotor side. When the parallel resistance value is 134 ohms, the residual voltage is excited and the excitation is not successful. When the parallel resistance is changed to 67 ohms, the excitation is successful.
The excitation of the 3 machine adopts the method that the R1 resistance is 50 ohms, and the residual excitation is applied. In the excitation of the 24th machine, the residual force is applied to the rotor side and the resistance is applied. The DC excitation auxiliary power supply can be omitted, and satisfactory results are obtained.
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