The 7 Most Common Knowledge Points in Power Systems

Common busbar wiring methods

1. Single busbar wiring: Single busbar wiring has the advantages of simple and clear, less equipment, small investment, convenient operation and conducive to expansion, but its reliability and flexibility are poor. When the busbar or busbar isolating switch fails or is repaired, 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, this kind of wiring uses a lot of equipment (especially isolating switches), the power distribution device is complex, and the economy is poor; during operation, the isolating switch, as an operating electrical appliance, is prone to misoperation and is inconvenient for automation; especially when the busbar system fails , it is necessary to cut off more power supplies and lines in a short time, which is not allowed for particularly important large power plants and substations.

3. Busbar plus bypass: It has high power supply reliability, flexible and convenient operation, but the investment has increased and the economy is slightly worse. Especially when using a bypass circuit breaker to lead the circuit, the operation is complicated and increases the chance of misoperation. At the same time, the installation of bypass circuit breakers complicates the corresponding protection and automation systems.

4, 3/2 and 4/3 wiring: high power supply reliability and operational flexibility. Failure or maintenance of any busbar will not cause a power outage; except for the short-term power outage of the two circuits connected to it when the contact circuit breaker fails, any other circuit breaker failure or maintenance will not interrupt the power supply; even two sets of busbars fail at the same time (or one In the extreme case of failure of another group during maintenance of one group), power can still be transmitted. However, this wiring uses a lot of equipment, especially circuit breakers and current transformers, which requires a large investment, and the secondary control wiring and relay protection are relatively complex.

5. Busbar-transformer-generator unit unit wiring: It has simple wiring, less switching equipment, easy operation, and is suitable for expansion. Since there is no generator outlet voltage busbar, the short-circuit current on the low-voltage side of the generator and main transformer is reduced. Features.

The specific meaning of stable

1. The static stability of the power system means that the power system does not experience non-periodic desynchronization after being subjected to small disturbances and automatically returns to the initial operating state.

2. The transient stability of the power system means that after the system is suddenly subjected to a large disturbance in a certain operating mode, it reaches a new stable operating state or returns to the original stable state through an electromechanical transient process.

3. The dynamic stability of the power system means that the power system will not lose synchronization due to oscillations with increasing amplitude after being disturbed. Mainly include: low-frequency oscillation of power system, subsynchronous oscillation of electromechanical coupling, self-excitation of synchronous motor, etc.

4. The voltage stability of the power system refers to the ability of the power system to maintain the load voltage within a certain specified operating limit. It is related to factors such as power supply configuration, network structure and operation mode, and load characteristics in the power system. When voltage instability occurs, it will cause voltage collapse and cause widespread power outages.

5. Frequency stability refers to the ability of the power system to maintain the system frequency within a certain specified operating limit. When the frequency is lower than a certain critical frequency, the balance between power supply and load will be completely destroyed, and some units will cease operation one after another, causing large-scale power outages, that is, frequency collapse.

Arrangement of transformer neutral point grounding method

The arrangement of the transformer neutral point grounding method should try to keep the zero sequence impedance of the substation basically unchanged. When encountering a special operating mode in which the zero-sequence impedance of the substation changes significantly due to transformer maintenance or other reasons, temporary handling should be carried out according to 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 transformer neutral point being grounded can be considered. When the transformer is inspected, special operation methods can be implemented, such as changing the setting value or deactivating and activating the relevant protection sections according to regulations.

2. When substituting two or more transformers, only one transformer should be operated with its neutral point directly grounded. When the transformer is out of operation, the other transformer with an ungrounded neutral point should be changed to directly grounded. If for some reason, the substation must have two transformers with directly grounded neutral points for normal operation. When one of the transformers with directly grounded neutral points is out of service, if there is a third transformer, change the third transformer to The neutral point is directly connected to the ground. Otherwise, handle it in special operation mode.

3. When a substation operating with double busbars has three or more transformers, the neutral points of the two transformers should be directly grounded, and they should be connected to different buses. When one of the neutral points is directly grounded, the transformer stops. During operation, connect another transformer with ungrounded neutral point to ground directly. If it is not possible to maintain a grounding point on different buses, it will be treated as a special operation mode.

4. In order to improve the protection cooperation relationship, when a short line is out of service for maintenance, the number of neutral point grounding transformers can be increased to offset the impact of the line outage on the zero-sequence current distribution relationship.

5. The neutral point of autotransformers and transformers with insulation requirements must be directly grounded.

Why should zero sequence protection be installed in large current grounding systems?

Although the overcurrent protection of three-phase star connection can also protect against ground short circuit, its sensitivity is lower and the protection time limit is longer. This deficiency can be overcome by using zero sequence protection because:

1. During normal operation and phase-to-phase short circuit, zero-sequence current and zero-sequence voltage will not appear, so the operating current of the zero-sequence protection can be set smaller, which is beneficial to improving its sensitivity;

2. For Y/△ connection step-down transformers, faults on the △ side will not reflect zero-sequence current on the Y side, so the action time limit of the zero-sequence protection does not need to be coordinated with the line protection after the transformer and should be shorter. Action time limit.

Problems with zero-sequence current protection during operation

1. When the current loop is disconnected, it may cause protection malfunction. This is a common weakness of generally more sensitive protection and needs to be taken care of during operation. In terms of the probability of disconnection, it is much smaller than the probability of disconnection of the distance protection voltage circuit. If necessary, the zero-sequence current blocking method of adjacent current transformers can be used to prevent this malfunction.

2. When asymmetric operation occurs in the power system, zero sequence current will also occur, such as asymmetric operation caused by different three-phase parameters of the transformer, two-phase operation during single-phase reclosing, three-phase reclosing and manual closing. When the three-phase circuit breaker is in different phases, during the busbar switching operation, the circuit breaker and the isolating switch are connected in parallel, or when the circuit breaker is running in parallel, zero sequence circulation occurs due to the inconsistent contact resistance of the isolating switch or circuit breaker in the three phases, as well as air drop. The unbalanced excitation inrush current generated by the transformer, especially when the busbar where the air-dropped transformer is located has a neutral point and the grounded transformer is in operation, unbalanced excitation inrush current and DC components may occur for a long time, which may cause zero sequence. Current protection starts.

3. For parallel lines that are geographically close together, when one of the lines fails, it may cause an induced zero-sequence current in the other line, causing the zero-sequence direction relay on the opposite side to malfunction. If this is indeed possible, a negative sequence direction relay can be used instead to prevent misjudgment by the above direction relay.

4. Since the AC circuit of the zero-sequence direction relay usually does not have zero-sequence current and zero-sequence voltage, circuit breakage is not easy to be found; when the zero-sequence voltage of the relay is taken from the opening triangle side of the voltage transformer, it is not easy to use a more intuitive simulation method to check The correctness of its direction makes it easier for the protection to refuse to operate or malfunction when the power grid fails due to problems with the AC circuit.

Settings for line protection mid-check synchronization and no-voltage checks

If one side is put into inspection without voltage, the other side is put into inspection at the same time. Then, on the side that is used to check for no voltage, when its circuit breaker trips due to some reason (such as accidental touch, malfunction of protection, etc.) under normal operating conditions, since the opposite side does not operate, there is a line on the line. voltage, so coincidence cannot be achieved, which is a big flaw.

In order to solve this problem, the side that checks for no voltage is usually put into checking the same period at the same time, and the contacts of the two work in parallel, so that the circuit breaker that accidentally tripped can be put back in.

In order to ensure that the working conditions of the circuit breakers on both sides are the same, the non-voltage detection side is also installed on the synchronization side of the detection. After switching, it can be used according to the specific situation. However, it should be noted that when one side is put into pressure-free inspection and synchronization inspection, the other side can only be put into inspection for synchronization. Otherwise, non-synchronous closing will occur if both sides realize non-voltage verification reclosing at the same time. In addition, during the inspection period, the voltage condition of the line must be checked.

Therefore, the setting of synchronization and no-voltage detection during line protection is: one side checks no-voltage and synchronization, while the other side checks the same period.

Where does the unbalanced current of transformer differential protection come from?

There is always some differential current during operation of transformer differential protection (including external faults), which is caused by unbalanced current.

Unbalanced current under steady state conditions (requires differential threshold to escape):

1. The unbalanced current is caused by the different models of current transformers on each side of the transformer, that is, the saturation characteristics and excitation current of the current transformers on each side are different. It must meet the requirements of the 10% error curve of the current transformer.

2. Unbalanced current caused by the difference between the actual current transformer ratio and the calculated ratio.

3. Unbalanced current caused by changing the voltage regulating tap of the transformer.

Unbalanced current under transient conditions (requires ratio braking or second harmonic to avoid):

1. Since the non-periodic component of the short-circuit current is mainly the excitation current of the current transformer, its core is saturated and the error increases, causing unbalanced current.

2. The excitation inrush current when the transformer is closed without load has current only on one side of the transformer.

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