Among the four 220kV lines of the 220kV Xintian boosting station, one of the line main protections uses the LFCB-102 type microwave phase separation differential protection 'Al differential protection produced by Alstom. The protection device has a phase selection function, the relay is all digital, the design uses a microprocessor, and is compatible with modern communication systems. Because digital information can easily modulate and carry data, all three-phase current signals can be transmitted over the same channel. The current is compared according to the phase separation, and the phase selection capability is corresponding to different failure modes, thereby avoiding the asymmetry problem of the current transformer (hereinafter referred to as TA) comprehensive quantity comparison scheme. At the same time, regardless of the fault current at one end of the line, or the fault current at all ends, the relays at each end of the line can act simultaneously to quickly remove the fault. The protection is a single-phase full ratio differential, and the relay has two ratio braking characteristics, as shown in Figure 1. The initial slope ensures that the sensitivity of the low level fault increases with the fault level; if the TA saturation results in an additional error, the slope is increased to compensate. |Idiff|=|IA-L1+IB-L1|, |Ibias|=(|IA-L1|+|IB-L1|)/2. Idiff in the formula - differential current; Ibias - bias current; IA-L1 - L1 phase current at line A; IB-L1 - L1 phase current at line B. According to the ratio differential curve, the trip criterion is: When |Ibias| When |Ibias|》IS2, |Idiff|》K2|Ibias|-(K2-K1)IS2+IS1. Where IS1 - differential gate current; IS2 - biased gate current. Manufacturers recommend IS2=2.0In (where In is the rated current), K1=30, K2=150, only IS1 is set by the user, generally depends on the line capacitance current IC, recommended IS1"2.5IC, can guarantee to avoid the no-load line The increase in capacitive current caused by system overvoltage and external faults when charging current and avoiding normal load. When the new South A line of the 220kV line was put into operation, it was found that the TA ratio of the Xintian station side was 1200/1, and the TA ratio of the Nanhai station side was 1500/1. Thus, under normal load conditions, differential relays flow on both sides of the differential relay, and the setting of the set value is difficult. The basic method to solve this problem is to install a secondary converter on one side of the line so that the current flowing into the protection device on both sides of the line is exactly the same. However, the secondary converter cannot be equipped in the short-term, and in order to ensure the correct operation of the protection, the line is put into normal operation, and we have made adjustments. According to the manufacturer's relevant data, when the TA mismatch is greater than 15, K1 is selected as twice the mismatch, that is, K1 is 50, and IS1 is 0.3In from the original value. Change to 0.25In. For such adjustments, we investigate the situation of the differential relay in several modes of operation by the following calculations. 2.1 Influence of unbalanced current on protection device Available by IA/IB=1500/1200=1.25, ie IB=0.8IA Idiff=IA-0.8IA=0.2IA Ibias=(IA 0.8IA/2=0.9IA In the middle IA - Xintian station side current (secondary); IB - Nanhai station side current (secondary). Thanks to |Ibias| |Idiff|》K1|Ibias|+IS1, The protected braking current is K1|Ibias|+IS1=0.45IA+0.25In, The differential current is less than the protective braking current, and the differential current at this time is in the protected braking zone. Therefore, the unbalanced current generated due to the difference in the TA ratio does not cause the protection device to malfunction. 2.2 Impact of through faults on protection devices |Idiff|=|1.1IA+0.9IB|=0.38IA, |Ibias|=(|1.1IA|+|0.9IB|)/2=0.91IA. When |Ibias| K1|Ibias|+IS1=0.455IA+0.25In, The differential current is less than the protective braking current and the protection will not operate in the braking zone. When |Ibias|》IS2, ie IA》2.198In, the guaranteed braking current is K2|Ibias|-(K2-K1)IS2+IS1= 1.365IA-1.75In. According to the protection action criterion, IA "1.777In is contradicted with IA" 2.198In, so the protection will not operate. From the above calculation results, when the TA ratios on both sides of the protection device are different, under certain circumstances, the secondary converter can be temporarily considered. (Adding a secondary converter also changes the secondary characteristics of TA. , causing protection misoperation). However, the setting of the setting should be accurate, especially the choice of IS2=2.0In is very important to prevent the protection device from malfunctioning when the system passes through the fault. After the commissioning test, it can basically meet the needs of operation. 3.1 Protection against refusal LFCB-102 Microwave Phase Separation Protection '》Differential protection has a latching contact in the exit trip circuit. The contact is taken from the 94VX1 relay, and the 94VX1 relay is started by the zero sequence relay 50N. The 50N zero sequence relay current is taken from the other winding of the line TA, and its function is to determine whether there is a current loop disconnection in the differential circuit, the value is 0.1In; 94-1 relay, when L1, L2, L3 three When any phase of the phase difference relay operates, the relay operates; 94A, 94B, and 94C are differential relay three-phase outlet relays, respectively. It can be seen from the logic circuit diagram that when a fault occurs in the protection zone, the differential relay of the corresponding phase acts, but only the 94VX1 action (ie 50N action) can open the exit trip circuit. Therefore, if the protection device can operate reliably, the 50N must operate. That is to say, the protection device can operate reliably only when there is zero-sequence current flowing on the line (the designer must have instantaneous zero-sequence current when considering the line fault). However, when a three-phase short circuit or a two-phase short circuit occurs in the line, since the zero-sequence current does not flow through the line (such as a lightning bypass between the line towers), the zero-sequence relay 50N does not operate, even if it is poor. When the relay is activated, the protection device will not trip the outlet, causing the protection to refuse. 3.2 Solution The protection device has a function of starting the three-phase outlet in two phases of the software, that is, when any two-phase differential relay is simultaneously operated, the three-phase differential relays are all exported. To this end, we started the loop of the 94VX1 relay at the 50N open contact, and incorporated the series circuit of the open contact of the three-phase differential relay, as shown by the dotted line in Figure 2. It ensures that the protection device can start the 94VX1 relay in the case of three-phase short circuit and two-phase short circuit, thus opening the exit trip circuit. The protective device can be reliably operated by testing and simulating various short-circuit faults. After adding this series circuit, when the two-phase current loop is disconnected at the same time, the protection will be mis-operated, but this phenomenon will not occur under actual operation conditions (single-phase disconnection, the device software is designed to be blocked). As for the open circuit of the current loop caused by the power outage (such as the leakage wiring, etc.), the protection device that may appear during the power transmission of the line may malfunction, which has little effect on the line or the system. LFCB-102 Microwave Phase Separation Differential Protection '》Differential protection was put into operation at 220kV New Purple (South) Line A after the above adjustment. In 1997, a single phase ground fault occurred in the new Zijia B line. The protection device is correct. action.
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