1. Concept of main electrical wiring

In the substation, the main circuit composed of the generator, transformer, circuit breaker, disconnector, transformer and other high-voltage electrical equipment, as well as the high-voltage cables and buses that connect them together, according to their functional requirements, is called the electrical primary system, also known as the electrical main wiring.

When selecting the main electrical connection, it is necessary to meet the requirements of power supply reliability, dispatching flexibility, economy, etc. according to the status of the substation in the power grid, the number of incoming and outgoing lines, voltage grade, load nature and other conditions.

2. Type of main electrical wiring

The main body of the main electrical wiring is the power (incoming) circuit and the line (outgoing) circuit. It can be divided into two categories: busbar with busbar and busbar without busbar. In this issue, we mainly focus on the connection mode of busbar. The basic classification of main electrical wiring is as follows:

  Summary of main electrical wiring mode

3. Basic form of main electrical wiring

(1) Single bus connection

As shown in the figure, it is a single bus connection, and all power supplies and generators are connected to a common bus W. Circuit breakers and disconnectors are installed in each circuit. The one close to the bus side (such as QS2) is the bus disconnector, and the one close to the line side (such as qs3) is the line disconnector.

When the circuit breaker QF2 is overhauled, the power-off operation sequence is as follows: first disconnect QF2, and then successively open the disconnectors qs3 and QS2 on both sides. Then hang grounding wires on both sides of QF2 to ensure the safety of maintenance personnel. The operation sequence for QF2 to restore power transmission is: close QS2, qs3, and then close QF2.

  Summary of main electrical wiring mode

advantage:

Simple and clear wiring, less equipment, low investment and convenient operation.

Disadvantages:

Low reliability and inflexibility. Specifically:

a. When any circuit breaker is overhauled, the circuit must be powered off;

b. In case of bus or bus disconnector failure or maintenance, all circuits connected to the bus will be powered off;

Scope of application:

6~10kv outgoing line number ≤ 5 circuits;

Number of 35kV outgoing lines ≤ 3 circuits;

Number of 110kV outgoing lines ≤ 2 circuits.

(2) Single bus section

Compared with the single bus connection, a bus section breaker (or disconnector) is added to the single bus section to divide the single bus into two sections.

QF is closed and busbar operates in parallel:

It is equivalent to single bus connection without sections. If power supply 1 stops power supply, power supply 2 supplies power to section I bus through QF closure, which will not affect the power supply to the load;

If section I bus fails, the protection device will automatically trip QF, section I bus will be cut off, and section II bus will continue to supply power.

QF is disconnected and busbar operates separately:

It is equivalent to two non segmented single busbar connections. If power supply 1 stops and section I bus loses voltage, QF can be automatically switched on by automatic reclosing device, and section I bus restores power supply;

If section I bus fails, it will not affect section II, and section II bus will continue to supply power.

  alternator

advantage:

From the above description, it can be seen that single bus sectioning improves power supply reliability and dispatching flexibility. In case of bus failure or maintenance, the scope of power failure is reduced by half.

Disadvantages:

a. When any circuit breaker is overhauled, the circuit must be powered off;

b. In case of fault or maintenance of any section of bus or bus disconnector, all circuits connected to this section of bus will be powered off;

Scope of application:

6~10kv outgoing line number ≥ 6 circuits;

The number of 35kv~60kv outgoing lines is 4~8;

The number of 110kv~220kv outgoing lines is 3~4 circuits.

(3) Single bus with bypass

Single bus with bypass bus connection can be divided into single bus with bypass connection and single bus with bypass connection.

Single busbar non sectionalized with bypass connection

As shown in the figure, a group of bypass buses WP are added outside the working bus W and connected to the outside of each line through the bypass disconnector. Another group of bypass circuit breakers QF is set between the working bus and bypass bus.

When the circuit breaker (such as QF1) of any circuit needs to be powered off for maintenance, first close the bypass circuit breaker QF to charge the bypass bus, then close the bypass disconnector QS, and finally disconnect the line circuit breaker QF1 and the disconnectors on both sides. At this time, the line can obtain power from the working bus through the bypass disconnector QS and then the bypass circuit breaker QF, as indicated by the red arrow. We can understand that the bypass circuit breaker is the common standby circuit breaker of each circuit breaker. However, it should be noted that the bypass circuit breaker can only replace one line circuit breaker at the same time.

  Summary of main electrical wiring mode

It can be seen that this connection mode can maintain the circuit breaker without power failure, so the power supply reliability is improved. However, in case of bus failure or maintenance, all outgoing lines will still be cut off. In order to solve this problem, single bus sectioning with bypass connection can be adopted.

Single bus section with bypass connection

This wiring method takes into account the advantages of bypass bus and bus section. During normal operation, the bypass circuit breaker QF is disconnected. At this time, the system operates in a single bus section mode. When a certain outgoing circuit breaker needs to be repaired, it will supply power to the line through the red arrow path through the same switching operation above.

Summary of main electrical wiring mode

  advantage:

The biggest advantage of the single bus with bypass connection is that the line can be powered through the bypass circuit breaker without power failure during the maintenance of the circuit breaker, which improves the reliability of power supply.

Disadvantages:

a. Many equipment have been added, with high cost and complex operation;

b. In case of fault or maintenance of any section of working bus or bus disconnector, all circuits connected to this section of bus will be powered off.

Scope of application:

As the power grid becomes stronger and stronger, important users often supply power for double circuits, so less wiring with bypass is used. Generally, the bypass bus is set only when the outgoing circuit breaker is not allowed to be powered off for maintenance.

We can find that when the primary system of single bus, working bus or bus disconnector is in fault or maintenance, the circuit connected to the bus will be powered off. This is the common defect of various wiring modes of single bus. In order to solve this problem, we can adopt the main wiring mode of double bus.

(4) Double bus connection

The figure shows double bus connection. Each outgoing line is connected to two groups of buses through a circuit breaker and two groups of bus disconnectors. Two groups of busbars are connected through bus tie breaker QF.

During normal operation, bus I is the working bus and bus II is the standby bus. All outgoing lines are connected to the working bus (the working bus disconnector is closed and the standby bus disconnector is disconnected), and QF is disconnected. This is equivalent to single bus operation. When the working bus fails, all power and loads can be transferred to the standby bus in a short time, and the power supply can be quickly restored after a short-time power failure.

Summary of main electrical wiring mode

In order to improve the reliability of power supply, another operation mode is often adopted, that is, the working bus and the standby bus take part of the power supply and load, and the bus coupler QF is closed. This is equivalent to single bus section operation. When any group of bus fails, QF jumps off, the circuit connected to another group of bus is not affected, and the circuit connected to the faulty bus can also be quickly transferred to the intact bus to restore power supply.

  During the maintenance of the bus disconnector, it is only necessary to disconnect the circuit and the bus connected to the disconnector, so the rest of the circuits can be transferred to another group of buses to continue operation without power failure.

The normal power supply of the line can not be affected through “switching operation” during bus maintenance:

(a) Close the disconnectors QS1 and QS2 on both sides of the bus coupler circuit breaker;

(b) Close the bus tie breaker QF to charge the standby bus;

(c) At this time, the two groups of buses are at equipotential. According to the principle of “on before off”, the “switching operation” shall be carried out one by one: the standby bus disconnector shall be closed first, and the working bus disconnector shall be disconnected;

(d) Finally, disconnect the bus coupler circuit breaker QF, and open the disconnectors on both sides;

(f) The working bus has been cut off and isolated. After there is no power, it can be grounded with grounding switch for maintenance.

advantage:

a. The bus can be overhauled in turn without affecting the normal power supply;

b. When the bus disconnector of any circuit is overhauled, only the power supply of the circuit is affected;

c. After the working bus fails, all circuits can quickly restore power supply after short-time power failure;

d. During maintenance of any circuit breaker, bus tie circuit breaker can be used to replace outgoing line QF;

Disadvantages:

a. The switching operation process is complex and easy to cause misoperation;

b. When the working bus fails, it will still cause short-time power failure;

c. A large number of disconnectors are used and the investment is increased.

Scope of application:

The number of 35 ~ 60kV outgoing circuit exceeds 8; The number of 110 ~ 220kV outgoing circuit is 5 or more.

(5) Double bus section

The double bus section connection is mainly to reduce the scope of short-time power failure in case of bus failure. Its principle is similar to that of single bus section wiring. You can refer to the above to understand it. There is no need to repeat it here.

Summary of main electrical wiring mode

  (6) Double busbar with bypass

The purpose of double bus with bypass connection is to realize uninterrupted maintenance of any circuit breaker on the basis of double bus connection. This kind of wiring is flexible, convenient and reliable, but the investment is large. The principle can refer to single bus with bypass connection.

  Summary of main electrical wiring mode

(7) 3/2 circuit breaker wiring

Wiring of 3/2 circuit breaker is shown in the figure. There are two groups of busbars. Each two incoming and outgoing stations form a string with three circuit breakers, which are connected between the two groups of busbars. Since the ratio of the number of circuit breakers to the number of circuits is 3/2, it is called 3/2 circuit breaker wiring.

It should be noted that this connection mode can be configured into at least 3 strings to form a multi loop power supply. The double circuit lines of the same user shall be arranged in different strings during string distribution.

Summary of main electrical wiring mode

  advantage:

a. High reliability and flexible dispatching: during normal operation, all circuit breakers are closed, and any bus failure will not affect the power supply of each circuit;

b. Simple operation: avoid knife switch operation with disconnector, which is not easy to cause misoperation;

c. When any bus or line circuit breaker is overhauled, it is only necessary to disconnect the corresponding circuit breaker and disconnector, which will not affect the normal power supply of each circuit.

Disadvantages:

a. More circuit breakers are occupied and the investment is large;

b. Secondary wiring and relay protection are complicated;

Scope of application:

3/2 circuit breaker wiring is a common wiring used in modern large-scale power plants and substations as EHV systems. It is generally used in 500kV substations.

  Summary of main electrical wiring mode

To sum up, the main electrical wiring of various modes is gradually improved according to the needs of the system for reliability, flexibility and economy. Finally, a simple summary from the perspective of reliability is given with a picture

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