I. Overview of Secondary Circuits in Switchgear
The main components of the switchgear include: vacuum circuit breakers, current transformers, locally installed microprocessor-based protection devices, operating circuit accessories (handles, indicator lights, pressure plates, etc.), and various position auxiliary switches.
The circuit breaker and current transformer are installed inside the switch cabinet. The microprocessor protection, accessories, and electricity meter are installed on the panel of the relay room (using the old name, but there are actually no relays anymore). The terminal block and various power circuit breakers are installed inside the relay room. The terminal block is connected to the circuit breaker mechanism through control cables or special sockets.
To understand the secondary wiring of the switchgear, we need to find two sets of drawings:
Protection schematics and wiring diagrams provided by the microcomputer integrated protection device manufacturer;
The switchgear manufacturer provides secondary schematic diagrams, wiring diagrams, terminal block diagrams, and circuit breaker mechanism schematic diagrams. These integrated protection system drawings serve as the original design basis for the switchgear manufacturer and are also the basis for our review of the manufacturer's drawings.
Switchgear manufacturers typically modify the schematic diagrams of microprocessor-based protection systems, then schematically depict the inputs of current, voltage, and signal quantities, as well as the outputs of control quantities.
II. Secondary wiring of the central switchgear
The KYN28A medium-voltage switchgear is the most commonly used 10kV switchgear type in urban substations. It consists of two parts: the cabinet and the withdrawable components (medium-voltage trolley). The cabinet is divided into the trolley compartment, main busbar compartment, cable compartment, and relay instrument compartment.
1. Relay Room
The relay room panel is equipped with a microcomputer protection device, operating handle, protection output pressure plate, and indicator lights (red light for closed position, green light for open position, and yellow light for energy storage completion).
2. Handcart (circuit breaker) room
The most commonly used circuit breaker in a medium-voltage switchgear is the VS1 vacuum circuit breaker. The wiring within the circuit breaker mechanism connects to the terminal blocks in the relay compartment via a dedicated socket. One end of the plug is fixedly connected to the circuit breaker mechanism, while the other end is a dedicated plug. The matching socket is installed in the upper right corner of the circuit breaker compartment, and wires are led out from the socket to the terminal blocks in the relay compartment. To understand the secondary circuit, we need to have some understanding of the operating process.
The circuit breaker trolley in the central switchgear has three positions: disconnect, test, and operation (it should be noted that the circuit breaker trolley and the circuit breaker are two different concepts; the circuit breaker trolley is actually the circuit breaker and its mounting base).
During normal operation, the circuit breaker trolley is in the running position, the circuit breaker is in the closed position, and the secondary line plug and socket are connected.
After manual tripping, the circuit breaker is in the open position and the trolley is in the running position. Use the special crank to move the circuit breaker trolley out to the test position, and the secondary plug can be unplugged (it cannot be unplugged when the trolley is in the running position).
Continue cranking until the trolley exits the circuit breaker compartment and is in the disconnected position.
Disconnected position: The circuit breaker is not connected to the primary equipment (busbar), has lost operating power (the secondary plug has been unplugged), and is in the open position;
Test location: The secondary plug can be inserted into the socket to obtain power. The circuit breaker can be closed and opened, and the corresponding indicator light will illuminate.
The circuit breaker is not connected to the primary equipment and can be operated in various ways, but it will not have any impact on the load side, so it is called the test position.
Operating position: The circuit breaker is connected to the primary equipment. After closing, the power is transmitted from the bus to the transmission line through the circuit breaker.
The central switchgear does not have the traditional concept of a disconnecting switch. When the handcart is in the test position, it is equivalent to the traditional disconnecting switch being open, that is, the circuit breaker and the main circuit (primary bus) have a clear disconnection point.
3. Relay Room
The 95X series microprocessor-based protection device is a microprocessor-based protection device designed by Shanghai Guozhi Electric Power Technology Co., Ltd. for medium-voltage switchgear. It features small size and comprehensive functions. It can perform conventional relay protection functions such as three-stage overcurrent, overvoltage, undervoltage, three-phase reclosing, and inverse time protection.
(1) Current loop
As shown in the diagram below, the analog quantities to be connected to the CT are the currents of phases A, B, and C. (Of course, in some projects, only phases A and C are connected, which is not a complete star connection.) The microcomputer protection device can also display the primary electrical quantities, which are taken from the protection level. If there are special requirements for accuracy, an ammeter can be connected in series with the measuring level.
(2) Voltage circuit
As shown in the diagram above, the analog quantities that need to be connected to the PT are phase voltages A, B, and C. Of course, if necessary, zero-sequence voltage can be connected for zero-sequence voltage protection. The diagram shows that this connection is a star/star configuration. For a V/V connection, simply short-circuit D9 and D12.
(3) Operating circuit
The operating circuit of the 95X is relatively simple and intuitive. It is more convenient to operate and use than traditional integrated protection systems. It is easy to understand and operate. The equipment within the red box is not part of the GZP-95X; it is provided by the switchgear manufacturer and installed on the relay room panel, connected to the closed position of the circuit breaker.
(4) Input circuit
The above describes the remote signaling (switching) circuit of the device. Unlike the protection devices for 110kV voltage level equipment, the protection and control of 35/10kV equipment are integrated into a single device. Except for the circuit breaker position, all other inputs can be defined by the user. An alarm input signal is also provided, enabling non-electrical trip signal inputs.
III. Regarding Phase B Current Transformer
The function of B-phase current transformer: It is generally mentioned that only A-phase and C-phase current transformers are installed, so why is B-phase CT installed now?
This is mainly to take into account the function of selecting the line when a 10kV overhead line is grounded in a single phase. However, it is necessary to determine which line is grounded so that the line can be checked and repaired as soon as possible, i.e., "line selection".
The basis for selecting a fault line is whether the zero-sequence current of the line is zero. The zero-sequence current is the vector sum of the three-phase currents. Under normal conditions, its theoretical value is zero. When a single-phase ground fault occurs, the measured zero-sequence current is not zero, which indicates a fault in the line. Therefore, a three-phase current transformer (CT) is required to calculate the zero-sequence current.
For cable outgoing lines, a dedicated zero-sequence current transformer is configured on the high-voltage (10kV) cable, so it is no longer necessary to use the sum of the three-phase CTs for calculation. Therefore, it is sufficient to configure two-phase CTs, A and C, to achieve the protection function.
Disclaimer: This article is a reprint. If there are any copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
