I. Introduction

With the rapid development of power systems and the advancement of computer technology and communication technology, relay protection is further developing rapidly in the direction of computerization, networking, integration of protection, measurement, control, data communication and artificial intelligence. At the same time, more and more new technologies and new theories will be applied in the field of relay protection, which requires us relay protection workers to continue to learn, explore and make progress to achieve the purpose of improving power supply reliability and ensuring the safe and stable operation of the power grid.

2. The role of relay protection in power supply system obstacles

(1) Ensuring the reliability of the relay system is the prerequisite for the relay protection device to play its role

The reliability of the relay system is the prerequisite for the relay protection device to play its role. Generally speaking, the reliability of relay protection is mainly guaranteed by relay protection devices with reasonable configuration, excellent quality and technical performance, and normal operation, maintenance and management.

(2) The role of relay protection in the safe operation of power systems

The main functions of relay protection in the safe operation of power systems include the following three points:

1. Ensure the safety of the power system. When a protected power system component fails, the relay protection device of the component should quickly and accurately issue a trip command to the circuit breaker closest to the faulty component, so that the faulty component can be disconnected from the power system in a timely manner to maximize the Reduce damage to the power system components themselves, reduce the impact on the safe power supply of the power system, and meet certain specific requirements of the power system (such as maintaining the transient stability of the power system, etc.).

2. Prompt the abnormal operation of the power system. It reflects the abnormal working conditions of electrical equipment, and sends a signal according to the abnormal working conditions and equipment operation and maintenance conditions (such as whether there are regular personnel on duty), so that the personnel on duty can handle it, or the device automatically adjusts, or changes those Electrical equipment that may cause an accident if continued operation shall be removed. Relay protection devices that respond to abnormal working conditions are allowed to operate with a certain delay.

3. Monitor the operation of the power system. Relay protection is not only an accident handling and response device, but also a device for monitoring the normal operation of the power system.

3. Common faults of relay protection

The failure of the secondary voltage circuit of the voltage transformer during operation is a weak link in the relay protection work. As the starting point of relay protection measurement equipment, voltage transformers are very important for the normal operation of the secondary system. There are not many PT secondary circuit devices and the wiring is not complicated, but faults on the PT secondary circuit are not uncommon. The serious consequence caused by a fault on the PT secondary voltage circuit is that the protection malfunctions or refuses to operate. According to operating experience, abnormalities in the PT secondary voltage circuit are mainly concentrated in the following aspects: abnormal grounding method of the PT secondary neutral point; manifested as secondary ungrounding (virtual connection) or multi-point grounding. Secondary ungrounding (virtual connection) is not only caused by the substation grounding network, but also caused by the wiring process. In this way, a voltage is generated between the PT secondary grounding phase and the ground grid, which is determined by the voltage imbalance of each phase and the contact resistance. This voltage is superimposed on the voltage of each phase of the protection device, causing the amplitude and phase of each phase voltage to change, causing the impedance element and directional element to refuse to operate or malfunction. The PT open delta voltage circuit is abnormal; the PT open delta voltage circuit is disconnected due to mechanical reasons, and the short circuit is related to some common practices. In electromagnetic bus and transformer protection, in order to achieve a fixed zero-sequence voltage, the current-limiting resistor in the voltage relay is often short-circuited. Some use small-scale current relays, which greatly reduces the open delta loop impedance. When there is a ground fault in the substation or at the outlet, the zero sequence voltage is large, the loop load impedance is small, the loop current is large, and the insulation of the voltage (current) relay coil is overheated and a short circuit occurs. If the short circuit lasts too long, the coil will burn out, causing the PT open delta voltage circuit to be disconnected there. This has happened in many areas. PT secondary voltage loss; PT secondary voltage loss is a classic problem that troubles the use of voltage protection. The root cause is the imperfect performance of various breaking equipment and secondary circuits.

Current transformer is an important component that provides relay protection and monitoring systems to determine the operating status of the system. As a basic requirement for current transformers in relay protection, the current transformers are required to truly reflect the waveform of the primary current. Especially in the event of a fault, they are required to reflect not only the size of the fault current, but also the phase and waveform of the current, and even the rate of change of current. The traditional electromagnetic current transformer uses the principle of electromagnetic induction to achieve primary and secondary current conversion through core coupling. Since the iron core has magnetic saturation characteristics and is a nonlinear component, when the primary current is large, especially the presence of non-periodic components in the primary current will cause severe saturation, the excitation current will increase by dozens or hundreds of times, and it will contain a large number of non-periodic components. and high-order harmonic components, causing serious distortion of the secondary current, seriously affecting the correct operation of relay protection. It can be known from basic electrical engineering theory that when a current transformer is severely saturated, the DC component in its primary current is very large, causing its waveform to deviate to one side of the time axis. There is residual magnetism in the iron core, and the direction of the residual magnetism is the same as the direction of the magnetic flux generated by the DC component of the excitation current. Under the combined action of the DC component of the short-circuit current and the residual magnetism, the iron core becomes saturated in less than half a cycle after the short circuit. As a result, all the primary current becomes the excitation current, and the secondary current is almost 0. Because the current transformer is severely saturated, its transmission characteristics deteriorate and even the output is 0, which leads to the refusal of the circuit breaker protection and the backup of the main transformer. Protection override trip.

In view of the characteristics of the current microcomputer relay protection device itself, the faults of the microcomputer protection device are generally caused by the following reasons: power supply problems, such as insufficient output power of the power supply, which will cause the output voltage to drop. If the voltage drop is too large, it will cause the comparison circuit reference value changes, shortening of charging circuit time and a series of other problems, thus affecting the logic cooperation of microcomputer protection and even misjudgment of logic functions. Especially when an accident occurs, the outlet relay, signal relay, reset relay, etc. will act one after another, requiring the power supply output to have sufficient power. If an accident occurs on site and the microcomputer protection fails to provide a background signal or reclosing fails, it should be considered whether the output power of the power supply has declined due to component aging. On-site management of the inverter power supply should be strengthened, and the inverter power supply must be inspected according to the regulations during regular inspections. Interference and insulation problems, the anti-interference performance of microcomputer protection is poor, and the use of walkie-talkies and other wireless communication equipment near the protection screen will cause some logic components to malfunction. The microcomputer protection device has high integration and tight wiring. After long-term operation, due to static electricity, a large amount of electrostatic dust accumulates around the wiring solder joints of the plug-in, which can form a conductive channel between the two solder joints, thereby causing a relay protection failure.

4. Relay protection fault handling methods

(1) The replacement method uses the same component that is good or considered normal to replace the suspected or faulty component to judge its quality and quickly narrow the scope of finding the fault. This is the most common method to deal with internal faults of comprehensive automatic protection devices. When some microcomputer protection fails, or some unit relays with complex internal circuits can be replaced with nearby spare plug-ins or relays that are temporarily under maintenance. If the fault disappears, it means that the fault is in the replaced component. Otherwise, you have to continue to check the fault in other places.

(2) The reference method compares the technical parameters of normal and abnormal equipment to find out the fault points of abnormal equipment from different places. This method is mainly used to check for faults such as wiring errors. During the fixed value verification process, it is found that the test value is significantly different from the expected value and the cause cannot be determined. When the secondary wiring cannot be restored correctly after circuit modification and equipment replacement, refer to the wiring of similar equipment. When checking the relay setting value, if it is found that the test value of a certain relay is far from its setting value, do not easily judge that the characteristics of the relay are not good at this time, or adjust the scale value on the relay immediately. You can use the same meter to check the relay value. Measure other similar relays in the same circuit for comparison.

(3) The short-circuit method uses a short wire to connect a certain section or part of the circuit to a short circuit to determine whether the fault exists within the short wire range or elsewhere, thereby reducing the scope of the fault. This method is mainly used for electromagnetic lock failure, open current loop, switching relay inaction, and judgment of whether the contacts of transfer switches such as control are good.

(4) The intuitive method handles some situations where it is impossible to test point by point with instruments, or when a certain plug-in fails and there is no spare parts to replace, but you want to eliminate the fault. Troubleshooting when the 10KV switch refuses to open or close. After the operation command is issued, it is observed that the closing contactor or tripping coil can operate, indicating that the electrical circuit is normal and the fault exists within the mechanism. If you go to the scene and directly observe that the inside of the relay is obviously yellow, or which component emits a strong burnt smell, you can quickly identify the fault and replace the damaged component.

(5) The item-by-item dismantling method sequentially disconnects the secondary circuits connected in parallel, and then puts them back in order. Once a fault occurs, it will indicate which circuit the fault exists. Then use the same method to find smaller branch roads in this road until the fault point is found. This method is mainly used to check DC grounding, AC power fuse failure and other faults. Such as DC ground fault. First, through the pull method, according to the importance of the load, each DC load loop supplied by the DC panel is opened for a short time. The cutting time should not exceed 3 seconds. When a certain loop is removed and the fault disappears, it means that the fault is in that loop. , and then further use the pull method to determine the branch where the fault is located. Then disassemble the power terminals of the ground branch separately until the fault point is found. If the secondary fuse of the voltage transformer is blown, there is a short-circuit fault in the loop, or the secondary AC voltages cross each other, etc., the terminals can be separated from the general lead of the secondary short-circuit phase of the voltage transformer, and the fault is eliminated. Then restore them one by one until the fault occurs, and then check the branch roads one by one. If the protection fuse of the entire device is blown or the power air switch cannot be closed, the fault scope can be narrowed down by unplugging and plugging each plug-in and observing changes in the fuse blowing situation.

XZTJB-I 3 Phase Relay Protection Tester