Necessity of power supply backup automatic switching device tester-EEWORLD

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Abstract: The automatic switching device of the backup power supply can improve the reliability and continuity of power supply. However, many automatic switching devices used in actual production cannot be officially put into operation because the operation mode and logical relationship do not meet the requirements. One of the means is to conduct a comprehensive inspection of the automatic switching device of the power supply from aspects such as the operation mode and logical relationship to ensure the correctness of the automatic switching device of the power supply. In the current production practice, there is no special tester for the inspection of the automatic switching device of the power supply. Generally, the test is carried out by borrowing other auxiliary devices, which will lead to many inaccurate tests, missing items, and coordination difficulties, thus burying safety hazards, which is not allowed in power production. 1. Introduction Automatic switching of the backup power supply and automatic switching of the backup equipment are generally referred to as "automatic switching" on site. Automatic switching can improve the power supply capacity of the power grid during normal operation, reduce the load of the overloaded line, limit the short-circuit current, and improve the reliability and continuity of power supply. With the development of the power system, the role and status of the automatic switching device of the backup power supply are becoming more and more important. The test of the device must be reliable and safe to eliminate the safety hazards. This article analyzes the difficulties and defects in testing the automatic switching device of the backup power supply, and proposes an instrument that can test the device, hoping to make a difference to the safety hazards. 2. Complex wiring and complex logic methods ① Complex wiring: The automatic switching device of the backup power supply has to face various changes in the system operation mode, resulting in complex logical relationships and complex wiring methods. The complexity of the logical relationship is as high as more than 70 combinations of power grids in Shenzhen. For example, the typical wiring of a step-down substation is 2 incoming lines, 2 main transformers split operation or one operation and one standby, but there are many types of substation wiring. For example: there are 3 or more main transformers, and there may be 3 or more high-voltage incoming lines; double mother wiring on the high-voltage side; expanded internal bridge wiring on the high-voltage side; low-voltage side section switch also serves as a bypass switch, etc. Complex wiring brings a variety of complex automatic switching methods, which requires each switching device to automatically identify the system operation mode and automatically select the corresponding automatic switching scheme. ② Logic of automatic switching: a. Automatic switching charging. When the working power supply is in the normal power supply state, the standby power supply is in the hot standby state (bright standby), or both are in the normal power supply state (dark standby), the standby automatic switch device determines whether the primary equipment is in this state based on the collected voltage, current and switch position signals, and completes the charging process after a delay of 10s to 15s. b. Standby automatic switch discharges electricity. When the standby automatic switch exits operation, the working circuit breaker is tripped by manual operation, the standby circuit breaker is not in the standby state, the circuit breaker refuses to trip or close, the standby object fails, etc., which do not allow the standby automatic switch to act, the standby automatic switch will be energized to terminate its behavior. c. After the standby automatic switch is charged, its start-up conditions are met, and its tripping logic is executed with or without delay (the circuit breaker may have tripped), and there may be multiple tripping objects. d. After the standby automatic switch executes the tripping logic, its closing conditions are met, and its closing logic is executed with or without delay, and there may be multiple closing objects. 3. Other issues of power backup automatic resetting ① Circuit breaker refusal to move inspection and automatic (remote manual) reset problem: Circuit breaker refusal to move means that the backup automatic resetting action logic is correct, but the circuit breaker does not move due to the circuit breaker, the main power supply circuit breaker is not disconnected or the backup power supply circuit breaker is not closed, so that the backup power supply device is in a waiting state. There are two ways to deal with it: one is to keep the tripping and closing pulses, and the device will not reset until the circuit breaker is activated or personnel intervene to reset the entire group for restart; the other is that the device will time a certain period of time after sending the tripping and closing pulses. If the circuit breaker does not move, the tripping and closing pulses will be retracted and a signal will be sent at the same time. The author once encountered a situation: the backup power supply circuit breaker of a backup automatic resetting device produced by a certain factory refused to move after the main power supply circuit breaker was tripped, so that the backup automatic resetting device program entered an infinite loop of waiting for closing, and the tripping command of the main power supply circuit breaker could not be retracted, resulting in the main power supply unable to be restored and the tripping coil burned. Generally speaking, there is a tripping self-holding relay in the circuit breaker operation circuit. After the standby automatic throw action trips, the relay can be started as long as the tripping pulse is output for 200 ms. Later, the manufacturer was required to improve the standby automatic throw logic: the main power supply tripping command was issued and it was promptly withdrawn 200 ms later. After the standby power circuit breaker failed to close, it should have the functions of circuit breaker refusal alarm, automatic reset of the standby automatic throw device and remote manual reset. ② The problem of handling circuit breaker tripping: For substations without PT on the bus, there are two ways to start the standby automatic throw: using the three-phase voltage of the incoming PT to replace the bus PT voltage after connecting in series through the position contacts of the incoming circuit breaker and starting the standby automatic throw using the circuit breaker position. The author does not recommend the circuit breaker position start method, which is affected by the DC power supply and the single circuit breaker auxiliary contact has relatively poor reliability, and it is easy to cause the standby automatic throw to malfunction due to vibration. ③ The problem of putting into the bus tie protection during the closing process: For the incoming line protection of the single busbar segmented wiring, the short delay generally trips the segment switch and the long delay trips the incoming line switch. When the backup power is applied to the faulty bus, the faulty bus must be quickly cut off to reduce the short-circuit impact and prevent the protection from overstepping and causing power outages on the two bus sections. For this reason, some manufacturers produce backup automatic switching devices that briefly switch on the bus tie switch quick-break protection before the closing process, so that when the backup power is applied to the faulty bus quick-break protection, it will be quickly cut off. This design is equivalent to the duplication of bus tie protection, which has a positive effect on preventing the expansion of accidents, but the backup automatic switching must introduce bus tie current, and its wiring and logic are complex. The actual operation shows that the relative reliability of this method is poor. We believe that if the bus protection or bus backup protection (such as main transformer composite voltage lockout overcurrent, etc.) can be used in the station to lock the backup automatic switching, there is no need to add bus tie quick-break; if it is inconvenient to use the bus protection or bus backup protection (such as main transformer composite voltage lockout overcurrent, etc.) The output contact locks the backup automatic switching, it must be considered to switch on the bus tie protection during the closing process to avoid the expansion of accidents. ④ PT disconnection monitoring function of the backup power supply: If the PT of the backup power supply line is disconnected or the fuse is blown, the voltage detection condition will be false, causing the backup automatic start to fail. Therefore, the PT disconnection monitoring function of the backup power supply line is very important. In principle, when the three-phase PT is disconnected or only a single-phase PT is installed, the disconnection and the power outage on the opposite side cannot be distinguished by only checking the voltage, so the auxiliary contacts of the air switch of the PT secondary circuit should be monitored at the same time, but fundamentally speaking, the reliability of the PT secondary circuit should be strengthened. ⑤ The coordination problem of multi-level backup automatic start: When there are multiple levels of backup automatic start in the system, the relationship between each level of backup automatic start should be considered. In principle, the backup automatic start with high voltage level, high reliability and large impact surface will act first, and the backup automatic start with low voltage level, low reliability and small impact surface will be set to avoid the upper level backup automatic start. However, the accident rate of low voltage level lines is high. In the case of high reliability requirements, in order to shorten the delay, the author advocates that each backup automatic start can act at the same time, and consider avoiding the multi-level backup automatic start in sequence from the operation mode. ⑥ The currently used microcomputer-based automatic backup power supply device is widely used in 110kV voltage level system substations. Most of the busbars in 110kV voltage level system substations are not equipped with bus differential protection. There is no bus differential protection to open the locked backup automatic switch. At this time, it is impossible to determine whether the bus power outage is caused by a line fault or a bus fault. In this way, when the busbar and its connected equipment (such as voltage transformer) fail and cause the busbar to lose power, the automatic backup power supply device will operate and automatically switch on the backup power supply, which will impact the faulty bus again and expand the fault. For example: A 110kV substation caused the main power supply to trip because the 110kV bus voltage transformer was damaged due to overvoltage, causing the voltage transformer to heat up and expand the accident. 4. Problems existing in the acceptance and regular inspection of the automatic backup power supply device

According to the requirements of relay protection technical supervision, the newly commissioned automatic switching device and the regular inspection of the automatic switching device must pass the switch transmission test to ensure the reliability of the device and the correctness of the external circuit. This ensures that the device can operate correctly and ensure power supply after the working power supply or equipment is disconnected due to a fault. At the same time, according to the practice of relay protection testing, before doing the switch transmission test, a simulation test must be carried out to ensure that the device itself is reliable to improve the test efficiency, and at the same time, the number of switch actions can be reduced to facilitate the safe operation of the primary equipment. In the phased construction of the substation, the primary wiring is incomplete and the regular inspection under the normal operation of the switch after commissioning, the contradiction is more prominent, and the transmission test cannot be carried out at all. In the actual site, there are no suitable simulation test means and equipment. The reliability and correctness of the logic of the automatic switching device and its circuit cannot be guaranteed.

5. Comprehensive analysis
In the process of power production and supply, in order to ensure the reliability of power supply and minimize power outages to users, substations and important users generally use dual power supplies or multiple power supplies as backup power supply. The automatic switching device of the backup power supply is an intelligent control device to realize this function. Its safe and reliable operation is the key to ensure the success of power backup.
Due to the existence of the above-mentioned problems and defects, power outages caused by the refusal and malfunction of the automatic switching device still account for a considerable proportion of power outages in the power system. Because the equipment powered by dual or multi-power systems is complex and the operation mode is changeable, the actual on-site simulation test of the automatic switching device cannot be carried out due to the need to change the power supply mode or power outage. The whole set of transmission experiments has many links, the wiring is complicated, time-consuming and labor-intensive, and prone to errors. Therefore, it is a very tedious and difficult task to determine and verify whether the automatic switching device can operate normally. The GMT3101 automatic switching tester solves the problem of on-site inspection of the automatic switching device, fills the domestic gap in the calibration instrument of the automatic switching device, and greatly facilitates the on-site work of the equipment commissioning personnel.
The GMT3101 automatic switching tester can provide and display the electrical and position signals of the lines and switches in various modes according to the primary wiring of various types of automatic switching circuits in the substation and power supply system, simulate abnormal conditions under various operating modes, and provide a basis for the long-term operation, maintenance and management of the device.

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