Detailed analysis of problems in relay protection and solutions

1. Quality in the line

1.1 Influence of excitation induction in the line on the relay protection device

The excitation surge current is due to the transformer airborne delivery, the magnetic flux in the core cannot be mutated, and the non-cyclical component magnetic flux is present, so that the dielectric core is saturated, and the excitation current is drastically increased. The transformer is the maximum value of the transformer, which can achieve 6 to 8 times the rated current of the transformer, and related to the capacity size of the transformer, the smaller the transformer capacity, the larger the excitation surgery. The exciting surge current has a large non-cyclical component, and attenuated attenuated attenuates, the time constant of the attenuation is also related to the size of the transformer, the larger the capacity, the greater the time constant, the longer the flow of the inrush flow. The 10kV line is equipped with a large amount of distribution transformer. When the line is put into the line, the power distribution transformer is hung on the line. In the case of the closing, the excitation surge flow generated by each transformer is equal to each other on the line, reflected back and forth, producing a complex The electromagnetic transient process, when the system impedance is small, there will be a larger inrush flow, and the time constant is also large. Current speed protection in two-stage current protection, due to the intensive sensitivity, the operating current value is often smaller, especially when the long line or system impedance is large. Excitation inductive flow values may be greater than the device's set value, making protection misunderstanding. This situation is less than the number of line transformers, small capacity, and system impedance are not protruding, so it is easily ignored, but it may appear when the number and capacity of the line transformer increases. The North Power Supply Bureau of Guiyang City has a problem that 10KV lines have not been properly input due to the incurred stream after the increase in the power generation.

1.2 Preventing the flow of inflows

The excitation surge flow has a significant feature that contains a large number of second harmonics. This feature is used in the main protection of the main transformer to prevent the exciting surge flow from causing the protection malfunction, but if used in 10kV line protection, the protection device must be performed Reconstruction, will greatly increase the complexity of the device, so practicality is very poor. Another feature of the excitation bond is that its size is attenuated, and the surge in the spring is very large. After a period of time, the current flow attenuation is zero, and the current flowing through the protection device is the line load current, using the incurred flow, in the current speed-off protection A short time delay, it can prevent false motion caused by the excitation flow, which is the biggest advantage that it does not have to transform the protection device (or simply transformed), although it will increase the fault time, but for system stable operation like 10kV. The impact is also applicable. In order to ensure reliable escaping of the excitation boom, the acceleration circuit in the protective device is also to join the delay. Through a few years of exploration, 0.15 ~ 0.2 s time limit is added in the 10kv line current speed and acceleration circuit, in recent years, run safety, and can avoid the protection of the protective device due to the induction boom due to the line in the line. action.

2. TA saturation problem

2.1TA saturation effects on protection

The short circuit current at the 10KV line exit is generally small, especially the power generation in the farnet, often away from the power supply, and the system impedance is large. For the same line, the short-circuit current size at the exit will vary from the system scale and operating mode. As the system scale continues to expand, the short circuit current of 10kV system will change, and can reach several hundred times more of the rated current of TA. There are some TAs that can be used normally in the system. On the other hand, Short-circuit failure is a transient process, and the short circuit current contains a large number of non-cycle components, and further accelerates TA saturation. When the 10kV line is short, due to the saturation of TA, the current induction the secondary side is small or close to zero, so that the protection device is refused, the fault is removed from the mother-in circuit breaker or the main transformer reserve protection, not only the fault time, not only The fault range will be expanded, affecting the reliability of power supply, and seriously threatening the safety of running equipment.

2.2 Method for avoiding TA saturation

TA saturation, in fact, the magnetic flux saturation in the TA core, and the magnetic flux density is proportional to the induction potential. Therefore, if the TA secondary load impedance is large, in the same current, the secondary circuit inductive potential is large, or in the same Under load impedance, the larger the secondary current, the greater the induction potential, and both cases make the magnetic flux density in the core, when the magnetic flux density is large, TA is saturated. When TA is severely saturated, the primary current becomes the excitation current, the secondary side induction current is zero, and the current flowing through the current relay is zero, and the protection device will refuse. Avoiding Ta saturation mainly starting from two aspects, one is when selecting TA, the ratio cannot be selected too small, and it is necessary to consider the TA saturation problem when the line is short-circuiting. Generally, the 10kV line protection TA ratio is preferably greater than 300/5. On the other hand, try to minimize TA secondary load impedance, try to avoid protection and measure sharing TA, shorten the length of the TA secondary cable and increase the secondary cable section; for the integrated automation substation, 10KV line as possible to use protection, test One product, and installed on the control screen so that the secondary loop impedance can be effectively reduced to prevent TA saturation.

3. Transformer protection used

3.1 Problems in the prevention of transformers used

The transformer used is a relatively special device, the capacity is small but the reliability requires very high, and the installation position is also very special. Generally, it is connected to the 10kV bus, and its high-voltage side short circuit is equal to the system short circuit current, which can reach more than ten thousand. The low-voltage side outlet short circuit current is also large. It has always been ignorant of the reliability of the transformers used, which will cause a big threat to the transformer used until the safety operation of the entire 10kV system. The traditional transformer protection uses fuse protection, and its safety reliability is also relatively high, but with the increase in short circuit capacity, and this method has gradually satisfied it. Nowadays, new or transformed substations, especially integrated automation, mostly configured the transformer switchgear used, and the protection configuration is similar to 10kV line, but often ignored the protection TA saturation problem. Since the transformer capacity used is small, the rated current is small, and the measurement is shared TA, to ensure the accuracy of the measurement, the design is small, and some places even select 10/5. In this way, when the transformer is faulty, Ta will be severely saturated, inductive secondary circuit current is almost zero, so that the transformer protection device is refused. If it is a high voltage side fault, the short circuit current is sufficient to disconnect the mother-in-one protection or main transformer backup protection, if it is a low voltage side fault, the short circuit current may not reach the start value of the mother-in-one protection or the main transformer reserve protection, so that the fault Unable to remove, finally burned the transformer used, and severely affecting the safe operation of the transformer.

3.2 Solution

Separate TA to solve the transformer protection tripping problem, should reasonably configuring protection start that selects TA to consider saturation transformer failure used, while metering a TA must be used with protection, the protection of TA mounted on the high side to ensure the protection of the transformer, the metering means TA low voltage side of the transformer used to improve the measurement accuracy. In terms of the setting value, the current protection may be short-circuited low-voltage transformer tuning outlet with overload protection by setting the capacity of the transformer used.

4. A distribution transformer protection

4.110kV distribution transformer protection problems

10kV distribution transformer protection configuration of the main circuit breaker or load switch plus fuse load switches and the like. Load switch investment, but not short-circuit breaking current, rarely used; good technical performance of the circuit breaker, but the higher investment in equipment, the use of complex, widely unrealistic; plus load switch protection fuse arrangement in combination, can avoid using complex operations, expensive circuit breakers, load switches can not compensate for the disadvantages of short circuit current breaking, but also to meet the needs of actual operation, this configuration can be used as protection of distribution transformers. But for a relatively large capacity distribution transformers, equipped with gas relays, circuit breakers need to be compatible with the Buchholz relay can effectively protect the transformer, if necessary, there should be zero-sequence protection, these problems are worthy of attention problems .

4.2 Solution

In the ring network, whether 10kV power supply unit, high voltage distribution or an end user unit, using load switch heightening interrupting capacity back-up protection arranged current limiting fuse combination, can provide the rated load current, but also short-circuit current is disconnected, and with the opening and closing load transformer performance, can effectively protect distribution transformers. To this end, recommended load switch disposed heightened interdiction capacity fuse combination back-flow restriction, as a distribution transformer protection protection. When standard GB14285 \"automatic protection and safety device Technical Specification\" provides select distribution transformer protection devices, when the capacity is equal to or greater than 800kVA, it should be used with circuit breaker protection device. For this requirement, it can be understood as based on the following two aspects need.

When the distribution transformer capacity of 800kVA and above, in the past mostly use oil-immersed transformers, and is equipped with gas relays, circuit breakers can cooperate with gas relay, thus effectively protect the transformer.

Means for capacity greater than 800kVA users, due to various causes single-phase ground fault protection action resulting in zero sequence, so that the circuit breaker is tripped, the fault partition, not cause substation feeder circuit breakers, normal supply other users. Standard also stipulates that even a single transformer does not reach this capacity, but if the total capacity of distribution transformers user reaches 800kVA, also to comply with this requirement.

5. Line Protection

5.110kV Distribution Line Protection Problems

Whether it is within the urban distribution network lines, or lines of rural distribution network, are the main 10kV voltage level, but the structural characteristics of 10kV distribution lines are poor consistency, if any, for the subscriber line, with only one or two users, similar to the transmission line; some radially, dozens of transformer T connected to the respective branches of the same line; short circuit to some hundred meters to some tens of kilometers long lines; some lines of 35kV substation outlet, some lines 110kV line from the substation; distribution transformer capacity on some lines is very small, but the largest 100kVA, there are thousands of kVA transformer on some lines.

5.2 Solution

Protection 10kV distribution line, the general trip current, over current and three-phase reclosing configuration. Special wiring structure or special load protection circuit, when not meet the requirements, can be considered to increase further protection, such as protection Ⅱ period, voltage blocking like. Setting calculation process, it should be considered a special case and the conventional case, and the sensitivity checking. For 10kV distribution line, although the protection device configuration is relatively simple, but because of the complexity and variability of the load line, the normal and exceptional cases, protection and protection selection value computing means is worthy of attention. The grid protection configuration where the cities and operating experience, the use of standard protection setting calculation method, various situations can be calculated, can generally meet the requirements.