UFC760BE143 3BHE004573R0143 uses a transition diode to detect positive and negative overvoltages in the excitation circuit
Brand ABB height 50mm rated current 250mA
Protection level adapted to motor power Applicable site Material code
IP45 28KW Power industry HIEE401782R0001
Part number UFC760BE143 3BHE004573R0143
Applicable pipe whether imported is weighing 3.7 kg can be sold nationwide
UFC760BE143 3BHE004573R0143 uses a transition diode to detect positive and negative overvoltages in the excitation circuit
UFC760BE143 3BHE004573R0143 Description of transient overvoltage protection devices in AC and DC systems
The reason for the overvoltage, in addition to the atmospheric overvoltage, is mainly due to the circuit breaker operation process in the system, and the controlled silicon component itself to change the related breaking process, in the circuit to stimulate the electromagnetic energy conversion and transmission caused by the overvoltage. The overvoltage protection measures adopted in the excitation system of Dongfang Motor are resistance-capacitance absorption protection on the AC side and non-linear resistance on the DC side.
AC overvoltage protection
An AC overvoltage protection circuit is placed on the AC input side of each rectifier bridge to absorb voltage spikes caused by thyristor commutation. Ac overvoltage protection basically consists of a three-phase diode rectifier bridge and a capacitor connected to its DC output side. For high frequency spikes the capacitor behaves as a low impedance and acts as a filter. The voltage at both ends of the capacitor cannot change, and the basic characteristic of storing electrical energy and the capacitor in parallel has a charge and discharge resistance that absorbs energy when the capacitor is discharged. The capacitors used in the application should be able to withstand high di/dt. The AC side of the diode bridge is protected by a fuse with a detecting microswitch. Its connection mode adopts reverse blocking type connection as shown in the figure.
DC overvoltage protection
UFC760BE143 3BHE004573R0143 Negative excitation current induced by generator side faults such as short circuit, error synchronism and/or asynchronous operation, which may cause high voltage in the excitation circuit. Such overvoltages must be limited to a range that is safe enough for the insulation voltage (test voltage) of the excitation coil and the PIV(peak reverse voltage) value of the rectified thyristor. The so-called CROWBAR circuit is often used for this purpose. The CROWBAR circuit uses a transition diode to detect positive and negative overvoltages in the excitation circuit. Whenever it is activated, the associated thyristor is immediately connected to the nonlinear magnetic field protection resistor in parallel with the rotor.
Magnetic field protection
UFC760BE143 3BHE004573R0143 The standard scheme of magnetic field protection is the magnetic field protection scheme using SCR rectifier bridge with DC side magnetic field circuit breaker +CROWBAR circuit, based on the logic control of magnetic field circuit breaker, CROWBAR thyristor trigger and pulse blocking. The CROWBAR circuit uses a crossover diode to detect positive and negative overvoltages in the excitation circuit. Whenever it is activated, the relevant thyristor is immediately connected to the nonlinear magnetic field protection resistor and the rotor parallel logic control and sends the command to: The magnetic field breaker is switched on, and the magnetic field protection is activated immediately after receiving a tripping command from the generator protection system or the internal excitation system protection. The CROWBAR thyristor trigger immediately connects the field protection resistor in parallel with the rotor, allowing the field current to be commutated to the field protection resistor.
UFC760BE143 3BHE004573R0143 Estimates and adjusts the value of the field resistance by the arc voltage of the field breaker and the maximum allowable voltage between the field coil. The energy to be absorbed in the field coil is also estimated from the excitation current induced by the three-phase short circuit at the generator end and/or the top value of the excitation current during no-load operation.
2. Description of parallel thyristor current sharing method
There are two reasons for the uneven current distribution between thyristor parallel branches:
a in the transient state, due to the difference in the opening time of the parallel elements, the transient current is uneven;
b After the conduction enters the steady state, due to the difference in the volt-ampere characteristics (forward voltage drop) of the parallel element in the on-state, the steady state current is uneven. Over the years, UFC760BE143 3BHE004573R0143 improving the distribution of current in parallel thyristor Bridges has always been a problem. There is no practical method, such as the position of the AC inlet, the current equalization reactor at each thyristor bridge inlet is not fully effective. In fact, in the best case, the acceptance of 10% of the rated current misweighing is beyond the theoretical requirements. Dongfang Electric adopts the dynamic current distribution as a software function, so that the current flowing through each thyristor bridge is almost equal. In most large current applications, a thyristor bridge can be saved by passing the dynamic current distribution, since the current imbalance does not have to be calculated. In addition, the AC inlet lines of a group of thyristor rectifier Bridges can be entered on the right side or the left side. This is because there is no need to consider the appropriate electromagnetic path from the secondary side of the transformer to the thyristor bridge. Thus, the internal interconnection of the AC busbar is unified and standardized. As a result, the work and costs of internally interconnected AC busbars are greatly reduced.
UFC760BE143 3BHE004573R0143 Dongfang Electric adopts the following methods to solve the above uneven current distribution:
a. A dynamic current distribution is adopted in the regulator as a software function so that the difference in current flowing through each thyristor bridge is equal.
b. In the selection of parallel branch components, ensure that the opening characteristics of the selected components are basically consistent with the forward voltage drop, that is, the opening time deviation is not more than 20 microseconds, the forward voltage drop deviation is not more than 0.05 volts, and the rated current of the components is reduced to less than 0.8 times.
c. In the trigger circuit design, increase the rise rate and amplitude of the trigger current as much as possible, so that the opening time deviation is reduced;
d. In the arrangement of components and the position of the leading bus, strive to make the resistance of each branch equal, the self-inductance equal, and the mutual inductance roughly equal; In the selection of busbars and processing technology, we strive to achieve the consistency of parallel branches.
UFC760BE143 3BHE004573R0143 uses a transition diode to detect positive and negative overvoltages in the excitation circuit
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