Defect analysis and solutions for coordination between power supply CT and passive relay protection

　　introduction:

　　The function of passive relay protection is: when a fault or abnormal operating condition occurs in the power system, the faulty equipment is automatically removed from the system in the shortest possible time and smallest area, or a signal is sent to the on-duty personnel to eliminate the root cause of the abnormal condition, so as to reduce or avoid damage to the equipment and the impact on the power supply of adjacent areas. Among them: "Passive" means that the relay protection device can drive the circuit through the internal battery and the power taken from the cable, and does not require other forms of power drive. The commonly used cable power supply method mainly uses current transformer power supply, which has the advantages of easy installation and reliable performance.

　　1. Problems caused by conventional power supply mode:

　　1. Example:

　　Recently, a company used the power input of the CT power supply intelligent self-powered protection device. After running for a period of time, the internal circuit of the passive relay protection burned out and the equipment could not work. For this reason, the technicians went to the accident site for technical analysis and confirmation, see Figure 1:

　　figure 1

　　2. Preliminary fault diagnosis

　　Obviously, the circuit board in the picture was burned due to excessive voltage. The technicians conducted the following investigations:

　　1) The open-circuit voltage of the power supply CT was measured with a multimeter, and the effective value was about 25V, which is far from causing damage to the relay protector.

　　2) The technicians carefully read the instruction manual of the passive relay protector. Table 1 shows the requirements of the passive relay protector manufacturer for the power supply CT:

　　It can be seen that the parameters of the current transformer are designed strictly in accordance with the requirements of the passive relay manufacturer, but why does the power supply equipment burn out?

　　3. In-depth troubleshooting:

　　Our technicians noticed that the parameter values ​​provided in the manual are all multimeter measurements, that is, effective values. However, in actual use, the voltage waveform cannot be a standard sine wave. The fluctuation of grid current and environmental interference will superimpose multiple harmonics on the existing sine wave. Therefore, the peak value of the actual voltage is much higher than the peak value of the standard sine wave. In response to this problem, our technicians conducted multiple sets of comparative tests after returning to the factory and finally found the problem:

　　We found the core corresponding to this model of CT, wound several coils with different ratios, and measured the effective value and peak-to-peak value of the open-circuit voltage (using an oscilloscope). We obtained Table 2:

　　Analyze the parameters in the chart:

　　1) When the current transformer is open-circuited, the voltage waveform is not a sine wave, but a peak with obvious harmonic interference. The peak-to-peak value at this time is generally 10 times greater than the effective value.

　　2) When the number of coil turns increases, the ratio of peak-to-peak value to effective value increases.

　　Therefore, it can be basically inferred that due to the fluctuation of the power grid, the power CT produces a high peak-to-peak value, and the circuit board is burned out due to long-term operation in an overvoltage environment. According to the effective value requirements of the passive relay manufacturer and the above test data, in order to meet the technical requirements of the passive relay manufacturer for the effective voltage value, a manufacturing solution with a transformation ratio of 400:1 must be selected. At this time, the peak-to-peak value is bound to be too high, and the voltage signal that is not processed will inevitably cause great harm to the equipment.

　　4. Common solutions:

　　There are two ways to fundamentally reduce the peak-to-peak value: increase the cross-sectional area of ​​the core and reduce the number of turns of the coil. Due to space limitations and voltage RMS requirements, the above two methods cannot make significant improvements. Therefore, the secondary voltage induced by the power CT is mixed with large harmonics and can only be detuned through subsequent circuits.

　　2. Conventional solutions and their defects:

　　There are many kinds of passive relays on the market now, so our technical staff has studied and analyzed the circuit structures of several commonly used passive relays. Most passive relay manufacturers use TVS and NMOS to form a DC voltage limiting circuit on the DC side, and use varistors at the AC interface to prevent surges. This method requires that the capacity of the rectifier bridge is sufficient, and there are certain requirements for the switching speed of the rectifier bridge. However, due to cost issues, a slow rectifier bridge is selected, and the switching speed is slow, which will burn the varistor, but generally will not have much impact on the operation of the equipment; if the capacity of the selected rectifier bridge is insufficient due to space constraints, the rectifier bridge will burn out, causing the equipment to stop powering on and unable to operate normally;

　　3. Solutions provided by our technical staff:

　　In order to make the protection range of the protection module more comprehensive and the protection performance more stable, the protection module can be separated and placed before the rectifier bridge. After this design, the switching performance of the rectifier bridge will not affect the protection performance of the protection module. Even if a fault occurs and the protection module burns out, the subsequent circuit will not be affected, and the replacement of the protection module will be more convenient.

　　IV. Conclusion:

　　1. Since the power-taking CT works in the semi-saturated area, coupled with environmental factors such as power grid fluctuations, the problem of high peak-to-peak voltage at the secondary end of the CT cannot be fundamentally eliminated. It is recommended that, if the installation space permits, a power-taking CT with a larger core cross-sectional area be selected, and the number of turns of the coil be reduced accordingly, which can effectively reduce the peak-to-peak value of the CT output.

　　2. In addition to confirming and requiring the effective value of the voltage of the power CT, passive relay manufacturers should also make specific parameter requirements for the peak-to-peak value of the voltage and design more stable and reliable protection circuits.

　　Article author: Wei Neng Hui Tong Li Ming, Sun Xuegang, Cheng Xin

　　references:

　　[1] GB1208-2006 Current Transformer [S]. China Standards Press, 2007.

　　[2] Ling Zishu. High Voltage Transformer Technical Manual [M]. China Electric Power Press, 2005.

　　[3] Research on switching power supply for current transformer[D].

　　[4] Ni Haidong, Jiang Yuping. Design and application of special circuits for switching power supplies [M]. China Electric Power Press, 2008.

　　[5] Zhou Shuangxi. Power System Voltage Stability and Control[M]. China Electric Power Press, 2004.