Secondary circuit circuit schematic diagram and explanation

Secondary circuit definition: all low-voltage circuits such as measurement circuits, relay protection circuits, switch control and signal circuits, operating power circuits, electrical blocking circuits of circuit breakers and isolating switches. The electrical circuit consisting of secondary equipment connected to each other to monitor, control, regulate and protect primary equipment is called a secondary circuit. It is a circuit composed of the secondary winding of the transformer, measuring and monitoring instruments, relays, automatic devices, etc., connected through control cables in the electrical system. It is used to control, protect, regulate, measure and monitor the working conditions of various parameters and components in the primary circuit. The electrical connection circuits used to monitor measuring meters, control operation signals, relay protections and automatic devices are all called secondary circuits or secondary wiring.

Detailed explanation of secondary loop circuit

21. According to Figure 23, explain the composition and working principle of the overcurrent protection of a single power supply three-winding transformer.

Answer: When there is an external fault of the three-winding transformer, the overcurrent protection of the transformer should selectively disconnect the circuit breaker on the fault side. The remaining two sides can continue to operate normally. To this end, over-current protection should be implemented according to the following principles.

(1) For single-sided power three-winding transformer (as shown in Figure 23), two sets of over-current protection should be installed. One set is on the load side, such as winding I II III, and its action time limit TIII is the smallest, and the protection action only trips QF3. The other set is on the power side, such as winding I, which has two levels of time limit TI and TII, TII=tIII+Δt , used to cut off QF2; and tI=tII+Δt, used to cut off the high, middle and low side circuit breakers.

(2) For transformers with two-terminal or three-terminal power supplies, overcurrent protection should be installed on three sides, and directional components should be installed on the power supply side with the smallest action time limit according to the calculated value to ensure the selectivity of the action.

Figure 23 Single power supply three winding overcurrent protection principle wiring diagram

22. According to Figure 24, explain the composition and working principle of the transformer zero-sequence current protection.

Answer: The grounding zero-sequence current protection installed on the transformer in the high-current grounding system serves as the backup protection for the main protection of the transformer and the backup protection for grounding short circuit of adjacent components. As shown in the figure, under normal circumstances, 3I=0, no current flows through TA, and the zero-sequence current protection does not operate. When a ground short circuit occurs, a zero-sequence current occurs. When it is greater than the operating current of the protection, it should be greater than the zero-sequence current of the outlet on that side. Protect the operating current of the backup section. The action time limit of protection is also one Δt larger than the latter.

Figure 24 Transformer zero sequence current protection principle wiring diagram

23. According to Figure 25, explain the composition and working principle of the transformer neutral point direct grounding zero sequence current protection and neutral point gap grounding protection.

Answer: Currently, graded insulation transformers are commonly used in large current grounding systems. When two or more graded insulation transformers are running in parallel in a substation, it is usually only considered that the neutral points of some transformers are operated through gap grounding to prevent faults during the fault process. The resulting overvoltage destroys the insulation of the transformer. In order to ensure the stability of the number of grounding points, when the grounding transformer exits operation, the transformer grounded through the gap should be converted to grounding operation. It can be seen that the graded insulation transformers operating in parallel have two operating modes: grounding and gap grounding. For this purpose, neutral point direct grounding zero sequence current protection and neutral point gap grounding protection should be configured. The principle wiring diagram of these two types of protection is shown in Figure 25.

Neutral point directly grounded zero-sequence current protection: Neutral point directly grounded zero-sequence current protection is generally divided into two sections. The first section consists of current relay 1, time relay 2, signal relay 3 and pressure plate 4. Its setting and outlet The first section of the ground protection is matched with the busbar circuit breaker for 0.5s. The second section is composed of current relay 5, time relay 6, signal relays 7 and 8, pressure plates 9 and 10 and other components. The setting value is matched with the last section of the outlet grounding protection, and the bus tie circuit breaker and the high-voltage side circuit breaker of the main transformer are cut off with a short delay, and the circuit breakers on the three sides of the main transformer are cut off with a long delay.

Neutral point gap grounding protection: When a grounding short circuit occurs in the busbar or line of the substation, if the protection of the faulty component refuses to operate, the zero-sequence current protection action of the neutral point grounding transformer will disconnect the bus tie breaker. If the fault point is in the middle In a system with a transformer whose neutral point is grounded through a gap, this local system becomes an ungrounded neutral point system. At this time, the neutral point will rise to the phase voltage, and the insulation of the graded insulation transformer will be destroyed, and the neutral point will be destroyed. The task of point gap grounding protection is to reliably cut off the transformer before the neutral point voltage rises to endanger the neutral point insulation to ensure that the insulation of the transformer is not destroyed. Gap ground protection includes zero-sequence current protection and zero-sequence overvoltage protection, and the two protections are mutually backup.

The zero sequence current protection is composed of current relay 12, time relay 13, signal relay 14 and pressure plate 15. A starting current is usually about 100A and the time is 0.5s. The neutral point discharge gap length of the 110kV transformer can be 115-158mm according to its insulation, and the breakdown voltage can be 63kV (effective value). When the neutral point voltage exceeds the breakdown voltage (it has not reached the voltage that endangers the neutral point insulation of the transformer), the gap breaks down, and a zero-sequence current flows through the neutral point. After the protection starts, the three sides of the transformer are cut off after a delay of 0.5s. device.

The zero-sequence voltage protection consists of an overvoltage relay 16, a time relay 17, a signal relay 18 and a pressure plate 19. The voltage setting is set according to the highest zero-sequence voltage that appears on the bus to avoid ground faults. The 110kV system generally takes 150V; when the grounding point If the selection is difficult or the ground fault bus 3U voltage is relatively high, it can also be set to 180V and the action time is 0.5s.

Principle wiring diagram of transformer neutral point direct grounding zero sequence current protection and neutral point gap grounding protection

Editor's comment: The biggest feature of the secondary circuit diagram is that it is very logical. The actions of its equipment and components are strictly in accordance with the order of design. Therefore, when looking at the diagram, you only need to grasp certain rules: first, then twice; communicate first. , then DC; first power supply, then wiring; first coil, then contact; first up, then down; first left, then right.