Analysis on the misoperation of overvoltage protection after the protection change of generator set

With the rapid development of microcomputer relay protection, the microcomputer protection alternative type rectifier protection and electromagnetic protection, and more and more shows the reliability and superiority of microcomputer relay protection, greatly improve the safe operation of the level of the equipment and systems. However, in the process of technical transformation, often the field personnel only pay attention to the transformation of the protection device itself and ignore the attention to the secondary circuit, which brings the hidden danger to the normal operation of microcomputer protection. In this paper, the actual together with our factory, because of the voltage transformer secondary circuit generator overvoltage protection misoperation caused by jumping machine accident as an example, the analysis reason, formulate measures, for your reference and reference.

1. Pre-accident situation

One power plant no. 5 (type QFQS - 210) generator - transformer set (200 mw) protection for LCD original rectifier type relay protection device, in this kind of type rectifier protection device, in order to prevent the generator TV secondary fuse three-phase fuse at the same time, especially in normal operation to remove three-phase secondary fuse or withdraw from the TV, low voltage protection misoperation caused cannot protect the atresia related. So for rectifier type of protection relay protection and automatic device anti-accident measures in [1] has done the following requirements: requirements at the second fuse generator TV on a capacitance in parallel to that one in three phase, to ensure that the generator TV three-phase wire break or exit the TV at the same time, the generator TV break protection can reliable action closure relevant protection, thus to prevent maloperation of the protection. Take phase C as an example and its schematic diagram is shown in figure 1.

FIG. 1 secondary circuit of rectifier type protective voltage
In figure 1: J is the voltage relay; C is the capacitance; TVA, TVB and TVC are secondary winding of generator TV.

With the development of microcomputer protection, the factory replaced the above rectifier protection in 2008 with the dual protection device of xu ji wfb-100 microcomputer. However, in a generator TV operation in early 2004, there was a generator overvoltage protection misoperation jump accident.

2. Accident history

, on March 10, 2009 10 points, just parallel operation of generator stator grounding "omega 3" 5 signal, spot checks found power pattern around the TV open delta voltage is 2 v, voltage generator neutral TV nearly 1 v, a system voltage is normal, before starting the generator insulation is qualified. According to the empirical analysis: it could be generating pattern in the TV high side fuse three-phase resistance, causing the machine end of the generator TV open delta voltage imbalance, the stator ground fault protection action. Therefore, the operator is required to quit the TV at the generator end and check the value of the high voltage side fuse resistance and the voltage secondary circuit. In front of the TV exit power pattern in our end, the voltage of the generator field loss protection, low voltage over-current protection, reverse power protection run out, but because the generator overvoltage protection increment protection, so there is no exit. At 14:30, when the operator took down the second fuse of A and B phases of TV and then the second fuse of C phases, the over-voltage protection action of the generator caused the no. 5 transformer group and the system disconnection. The protection device shows the AC phase and BC phase overvoltage of the generator.

3. Analysis of accident causes

3.1 analysis of protective actions

After the accident, a detailed inspection of the protection was carried out immediately:

(1) check the action record of the protection, and the protection device shows the AC phase and BC phase overvoltage of the generator, and the action voltage exceeds the setting value;
(2) the fixed value of the protection shall be checked by the power on, and the protective action value and the setting value 136V shall be exactly consistent.
(3) the monitoring voltage before the operator's panel is: the voltmeter swings instantaneously and exceeds the meter measuring range of 25kV (corresponding to the secondary voltage exceeding 167V), and there is no other operation before the accident;

According to the above analysis, the second measurement of the generator does appear overvoltage, the action of the protection device is correct.

3.2 investigation and analysis of accident causes

Although protection action correctly, but due to the generator operating conditions at the time of the accident completely normal, and normal system and the main transformer voltage, generator and showed no sign of any overvoltage at the same time, so the analysis of the secondary circuit voltage could be the cause, cause the protection misoperation. Then, according to the voltage circuit diagram, check one by one from the protection device to the generator TV. Except the parallel capacitance is found at the secondary c-phase fuse of the generator TV, all other circuits are correct. After further analysis, it is found that the overvoltage is caused by the capacitor.

Specific analysis is as follows: after the replacement for the microcomputer protection, due to the microcomputer protection using ac transformation plug-in isolation transformer (middle), after turn TV secondary voltage generator for weak current use, the protection and isolation transformer the middle there is a winding inductance L, so the microcomputer relay protection secondary circuit voltage diagram is shown in figure 2.

FIG. 2 voltage secondary circuit of microcomputer protection
By the figure 2 shows, the above operations, in take off TV secondary side fuse fu 1 and 2 after fu, then remove the 3 fu fuse, capacitance C, the voltage transformer microcomputer protection a winding inductance L and generator TV making secondary voltage measurement E series resonance circuit, as shown in diagram as shown in figure 3.

FIG. 3 series resonant circuit
According to the value of the capacitance C 20 mu (F) and xuchang relay manufacturers provide the voltage transformer primary winding inductance L value (0.5 H), just can satisfy the f0 = 1 / (2 PI series resonance conditions (f0 = 1 / (2 PI = 50 hz) and because a winding inductance impedance voltage converter XL > > R (loop resistance), so the resonance occurs, the voltage converter voltage on the UL can achieve potential E L (100 v) several times (UL =), or even higher. It can be seen that in this case, the overvoltage protection action of the generator is also taken for granted.

4. Problems of accident exposure and discussion

From the above analysis results, it can be seen that the real reason for the generator over-voltage protection misoperation is the capacitance C in parallel at the TV station. Because the capacitance is the relay protection and automatic device anti-accident measures required to install, so now the rectifier type of TV secondary circuit protection fuse on the shunt capacitance, if there is no need to exist in the microcomputer protection were discussed.

(1) in the former rectifier protection device, the voltage-related generator demagnetization protection and forced excitation device are protected by TV broken line protection and locked, so as to prevent the protection from misoperation due to pressure loss. Therefore, it is necessary for the rectifier type protection device to parallel a capacitance with a suitable capacity on a certain phase fuse of TV secondary three phases.
(2) however, principle of microcomputer protection TV bolt has changed a breakthrough, even though all three fuse fusing or withdraw from the TV, also can detect the break line loss of pressure, which can effectively locking relevant protection. (3) due to the microcomputer protection ac voltage loop are feeling type load, access the TV voltage, capacitance, when XL = Xc, namely will produce voltage harmonic, voltage will be added to the protection of the power supply voltage for several times. It will not only damage the secondary equipment directly, but also cause overvoltage protection misoperation.
(3) when a short circuit fault occurs between the phase connected with capacitance and the phase connected with zero, even if the phase fuse is fused, there is still current flow through the fault point of capacitor C, and the fault cannot be eliminated.

By the above analysis: the original "relay protection and automatic device anti-accident measures" of the shunt capacitance of TV secondary test measures, already can not adapt to modern microcomputer protection, so the capacitance in microcomputer protection has should no longer exists.

5. Corrective measures

(1) remove the capacitance C of the secondary measurement and parallel connection of the original generator TV.
(2) replace the three-phase fuse on the second side of the generator TV with a quick air switch to realize simultaneous and rapid disconnection of the three-phase, so as to achieve the purpose of quick removal of faults and avoiding parasitic circuits.
(3) the TV break criterion of the voltage loop USES the principle of the voltage balance relay in the microcomputer protection, so as to reliably avoid the misoperation and refusal of the TV break protection of the voltage loop.
(4) re-check all secondary circuit wiring of the protection, especially the problem of historical residuals, to prevent similar things from happening again.

Since the implementation of these measures in the later several generator TV operation, the generator of over-voltage of secondary loop never happen, fundamentally solves the reformed generator overvoltage protection misoperation.

Responsible editor: Randy