Common bus wiring 
1. Single busbar wiring: Single busbar wiring has the advantages of simple and clear, less equipment, small investment, convenient operation and expansion, and favorable expansion, but the reliability and flexibility are poor. When the busbar or busbar disconnector fails or is serviced, all power to the busbar must be disconnected.
2. Double busbar wiring: Double busbar wiring has the advantages of reliable power supply, convenient maintenance, flexible scheduling or easy expansion. However, there are many devices used in this kind of wiring (especially the isolating switch), the power distribution device is complicated, and the economy is poor. In operation, the isolating switch is used as an operating device, which is prone to misoperation and is inconvenient for automation; especially when the busbar system fails. It is necessary to cut off more power and wiring in a short time, which is not allowed for large power plants and substations that are particularly important.
3, bus plus bypass: its power supply reliability is high, flexible and convenient to operate, but the investment has increased, the economy is slightly worse. Especially when the bypass breaker is used to lead the way, the operation is complicated, and the chance of misoperation is increased. At the same time, due to the addition of bypass breakers, the corresponding protection and automation systems are complicated.
4, 3/2 and 4/3 wiring: high power supply reliability and operational flexibility. No bus power failure or overhaul will result in no power failure; except for the two-circuit line connected to the circuit breaker in case of short-term power failure, the failure or maintenance of any other circuit breaker will not interrupt the power supply; even the two busbars will be faulty at the same time (or one In the extreme case of another set of faults during group maintenance, power can continue to be delivered. However, there are many devices used for this connection, especially circuit breakers and current transformers. The investment is large, and the secondary control wiring and relay protection are more complicated.
5. Busbar-transformer-generator unit wiring: It has simple wiring, few switchgears, easy operation, suitable for expansion, and because there is no generator outlet voltage busbar, the short-circuit current of the low-voltage side of the generator and main transformer is reduced. Features.
The specific meaning of stability
1. The static stability of the power system means that the power system does not undergo non-periodic loss of synchronization after being subjected to small interference, and automatically returns to the initial running state.
2. The transient stability of the power system means that after a sudden large disturbance of the system in a certain operation mode, the system reaches a new stable operation state or returns to the original stable state after an electromechanical transient process.
3. The dynamic stability of the power system means that the power system is out of step without the oscillation of increasing amplitude after being disturbed. Mainly include: low-frequency oscillation of power system, sub-synchronous oscillation of electromechanical coupling, and self-excitation of synchronous motor.
4. The voltage stability of the power system refers to the ability of the power system to maintain the load voltage within a specified operating limit. It is related to power supply configuration, network structure and operation mode, load characteristics and other factors in the power system. When the voltage is unstable, it will cause the voltage to collapse, resulting in a large area power outage.
5. Frequency stability refers to the ability of the power system to maintain the system frequency within a specified operating limit. When the frequency is lower than a certain critical frequency, the balance between the power supply and the load will be completely destroyed, and some units will successively exit the operation, causing a large-scale power outage, that is, the frequency collapse.
Arrangement of transformer neutral grounding method
The arrangement of the neutral point of the transformer should be kept as far as possible to keep the zero sequence impedance of the substation basically unchanged. In the case of special operation modes that cause large changes in the zero-sequence impedance of the substation due to transformer maintenance and other reasons, it shall be temporarily disposed according to the regulations or actual conditions.
1. If there is only one transformer in the substation, the neutral point should be directly grounded. When calculating the normal protection setting, only the normal operation mode of the neutral point of the transformer can be considered. When the transformer is overhauled, it can be handled in a special operation mode, such as changing the value or deactivating according to regulations, and starting the relevant protection section.
2. When changing all two or more transformers, only one neutral point of the transformer should be directly grounded. When the transformer is out of service, change another neutral point ungrounded transformer to direct grounding. If, for some reason, the substation normally has two transformer neutral points directly grounded, when one of the transformers whose neutral point is directly grounded is out of service, if there is a third transformer, the third transformer is changed to The neutral point is directly grounded. Otherwise, it is handled in a special mode.
3. When all three or more transformers are operated by double busbars, they should be operated according to the neutral grounding of the two transformers, and they should be connected to different busbars respectively. When one of the neutral points is directly grounded, the transformer will be stopped. When operating, connect another neutral point ungrounded transformer directly to ground. If it is not possible to maintain a grounding point on each busbar, it is treated as a special operation mode.
4. In order to improve the protection coordination relationship, when a short line is overhauled, the number of neutral point grounding transformers can be increased to offset the impact of line outage on the zero-sequence current distribution relationship.
5. The neutral point of the autotransformer and the transformer with the required insulation must be directly grounded.
Why should zero sequence protection be installed in a large current grounding system?
The overcurrent protection of the three-phase star connection can also protect the ground short circuit, but its sensitivity is lower and the protection time is longer. This problem can be overcome by using zero sequence protection because:
1. When normal operation and phase-to-phase short circuit occur, zero sequence current and zero sequence voltage will not appear, so the action current of zero sequence protection can be set smaller, which is beneficial to improve its sensitivity;
2, Y / â–³ wiring step-down transformer, the fault after the â–³ side will not reflect the zero-sequence current on the Y side, so the action time limit of the zero-sequence protection can be matched with the line protection of the transformer to take a shorter Action time limit.
Zero sequence current protection in operation
1. When the current loop is broken, it may cause protection malfunction. This is a common weakness of generally sensitive protection and needs to be prevented during operation. In terms of the probability of disconnection, it is much less likely to break the line than the protection voltage loop. If it is necessary, it can also be prevented by the method of zero-sequence current blocking of adjacent current transformers.
2. When the power system is in asymmetrical operation, zero sequence current should also occur, such as asymmetric operation caused by different three-phase parameters of the transformer, two-phase operation in single-phase reclosing, three-phase reclosing and manual closing When the three-phase circuit breaker is in different phases, when the circuit breaker is connected in parallel with the isolating switch or the normal circuit of the circuit breaker is running, the zero-sequence circulation occurs due to the inconsistent three-phase inconsistency of the isolating switch or the circuit breaker, and the airdrop The unbalanced magnetizing inrush current generated by the transformer, especially in the case where the neutral point grounding transformer of the busbar where the air-dropping transformer is located is in operation, the unbalanced magnetizing inrush current and the DC component may occur for a long time, etc., and the zero sequence may be caused. Current protection is activated.
3. Parallel lines with geographical proximity. When one of the lines fails, the other line may cause induced zero-sequence current, causing the relay in the reverse direction side zero-sequence direction to malfunction. If this is possible, the negative sequence direction relay can be used instead to prevent the above direction relay from misjudging.
4. Since the zero-sequence direction relay AC circuit usually has no zero-sequence current and zero-sequence voltage, the loop disconnection is not easy to be found; when the relay zero-sequence voltage is taken from the open triangle side of the voltage transformer, it is not easy to check with a more intuitive simulation method. The correctness of the direction is therefore more likely to cause protection rejection and malfunction in the event of a grid failure due to a problem with the AC loop.
Line protection mid-check and check-free settings
If one side is used for inspection without pressure, the other side is inspected for the same connection method. Then, on the side where the non-pressure is used, when the circuit breaker trips due to some reason (such as accidental collision, protection misoperation, etc.) under normal operation, since the opposite side does not operate, there is The voltage, and thus the inability to achieve coincidence, is a big drawback.
In order to solve this problem, it is common to check the same period on the side where the pressure is not detected, and the contacts of the two are connected in parallel, so that the circuit breaker with the false trip can be re-introduced.
In order to ensure the same working conditions of the circuit breakers on both sides, the test is also installed without pressure on the synchronous side. After switching, it is used according to the specific conditions. However, it should be noted that when one side is tested for no pressure and the same period is detected, the other side can only be used for the same period. Otherwise, simultaneous voltage-free reclosing on both sides will result in non-synchronous closing. In addition, during the inspection period, it is necessary to check the conditions under which the line is pressurized.
Therefore, the line protection mid-check and the check-free no-pressure settings are: one side checks no pressure and the same period, while the other side checks the same period.
Where does the unbalanced current of the transformer differential protection come from?
Transformer differential protection always has some differential current during operation (including out-of-zone faults), which is caused by unbalanced current.
Unbalanced current in steady state conditions (need to escape by differential threshold):
1. Due to the different current transformer types on each side of the transformer, that is, the unbalanced current caused by the saturation characteristics and excitation current of each side of the current transformer. It must meet the requirements of the 10% error curve of the current transformer.
2. The unbalanced current caused by the actual current transformer ratio and the calculated ratio.
3. Unbalanced current caused by changing the transformer voltage regulating tap.
Unbalanced current in transient conditions (requires ratio braking or second harmonic to escape):
1. Since the non-periodic component of the short-circuit current is mainly the excitation current of the current transformer, the core is saturated, and the error increases to cause an unbalanced current.
2. The magnetizing inrush current of the transformer no-load closing, only current on the side of the transformer.
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