What Is a Current Loop?

Loop is a basic concept of electricity. It generally refers to a current path composed of a power source, a key, and an electrical appliance.

Single circuit refers to the circuit where one load has one power supply; dual circuit refers to the circuit where one load has two power supplies. Generally, enterprises with high requirements for power supply reliability, or important substations in the region, use double-circuit power supply, which can protect one power supply from power failure for another reason and the other power supply can continue to supply power. High and small users often use a single power supply.

Analysis of Typical Misoperation and Misoperation Principle of Current Loop

Typical misoperations of the current loop include misopening, misshorting, misgrounding, and accidentally adding test quantities. If the current circuit in operation is open, it will endanger the safety of personnel and equipment, the maintenance personnel attaches great importance to it, and the probability of misopening is extremely small. However, most of the faults such as short circuit, ground fault, and incorrect test load occur during power failure maintenance. The maintenance personnel s risk awareness is relatively low, which leads to a higher probability of protection malfunction. For example, the above three typical malfunction events and related historical events . Therefore, the principle of protection malfunction caused by faulty short circuit, faulty grounding, and incorrectly added test quantity should be mainly analyzed.

The nature of protection malfunction caused by faulty short circuit, faulty grounding, and improperly added test quantity is that the current operation during the power failure interval causes the current circuit current in the operation interval to change. Specifically, it can be subdivided into AC current incorrectly added to the bus differential protection or stability control device, DC current incorrectly added to the bus differential protection, flowing into the line or main transformer protection, and the current is shunted and the current circuit is grounded at two points. A detailed analysis of the protection malfunction principle is as follows.

AC current is incorrectly added to the bus differential protection or stability control device

In the 220kV and above line protection or main transformer protection replacement acceptance, measurement and control device regular inspection, line protection or main transformer protection expansion project, etc., during the secondary up-current test, there is an erroneous addition of the AC test current to the bus differential protection or stability. The risk of the control device is shown in Figure 1.

figure 1

If the test current is added to the bus differential protection by mistake, when the test current is larger and the differential current is larger than the bus differential protection setting value, it may cause the bus differential protection to malfunction.

If the test current is added to the stability control device by mistake, when the test current is greater than the overload setting of the stability control device, it may cause the stability control device to cut the load by mistake.

Incorrectly adding DC current to bus differential protection

In 220kV and above line protection or main transformer protection replacement acceptance, line protection or main transformer protection expansion project, the newly laid current loop cables will be checked for cable cores. If the positive pole of the dry battery is contacted with the current loop terminal of the mother differential group during the check of the cable core, the mother differential protection may malfunction, as shown in Figure 2. For example, the voltage of two dry batteries is 3V, and the resistance of the current loop is generally about 3, and the DC current generated by it is as high as 1A. Especially for 500kV bus differential protection, its protection outlet is not blocked by bus voltage, and the probability of bus differential protection misoperation is extremely high.

figure 2

Operational protection and current is shunted

In the two-thirds wiring method, it often involves the work of power failure maintenance of the middle switch. Working on the medium switch current circuit, if the current circuit of the line differential protection group is short-circuited by mistake, it will cause the line differential protection to malfunction. Taking Figure 3 as an example, if the current circuit terminals of the medium-switch line differential protection group are mistakenly shorted, the differential current of the line protection is approximately equal to half of the load current. When the line load is heavy, the probability of misoperation of the line differential protection is high.

image 3

Two points of current loop are grounded

Working on the middle switch current circuit of three-half wiring mode, if the current circuit of the line protection differential group is incorrectly grounded, it may cause the line differential protection to malfunction. Taking Figure 4 as an example, if the current circuit of the middle switch line differential protection group is incorrectly grounded, it may cause the line differential protection to malfunction. The line protection differential current comes from two aspects: first, the two ground points form a loop, and the C-phase current IA6 is shunted; second, the potentials of the two ground points are not equal, and the differential current generated by the unbalanced voltage of the ground network. Similarly, when the side switch is overhauled, it may cause the mother differential protection to malfunction.

Figure 4

Current loop current loop safety measures

The safety measures for the secondary maintenance work are mainly concentrated in the outlet circuit and the voltage circuit, and there are few secondary measures in the current circuit. The main reason for these four types of current loop misoperation is protection current misoperation.

In the technical renovation and acceptance project, the construction period is tight and the work intensity is high. The mental state of the maintenance personnel cannot always maintain a good level, and it is difficult to fully control the quality and skill level of the construction personnel. The safety measures of the current loop should be done as far as possible. In the case of a human error, the protection can still be prevented from malfunctioning. Therefore, perfect safety measures must be implemented in the current loop, and double breakpoints must be set in the current loop during the power failure interval. The first breakpoint is set in the switch local control cabinet, and the second breakpoint is set at the protection screen. Taking the 50kV side circuit breaker 5021 switch protection replacement work as an example, the current circuit safety measures to prevent the 500kV 1M bus differential protection from malfunctioning are explained in detail.

(1) Permit work ticket and complete security account;

(2) At the local control cabinet of the side circuit breaker 5021, cut the middle connecting piece of the current loop of the bus differential group according to the drawing, and then seal the current terminals of the bus differential group to the main control room side with insulating tape (the first Breakpoint);

(3) At the 500kV 1M bus differential protection screen in the main control room, use a clamp meter to confirm that there is current in the running switch interval current circuit, and then seal the running interval current circuit with insulating tape;

(4) After confirming that there is no current in the 5021 current circuit with a clamp meter at the 500kV 1M bus differential protection screen in the main control room, short the 5021 current loop to the terminal of the bus differential protection device with a short wire, and then cut off the corresponding current connection piece. (Second breakpoint);

(5) During the acceptance process, it is strictly forbidden to change the safety measures.

(6) After the acceptance work is completed, the safety measures are restored.

Through the above safety measures, it is possible to effectively prevent the 500kV 1M bus differential protection from malfunctioning due to misoperation of the current loop.

Conclusion of the current loop

In the secondary maintenance work of relay protection or technical renovation and expansion projects, the safety measures for the secondary circuit of the power failure interval are mainly for the outlet circuit and the voltage circuit, and rarely involve the current circuit. The protection misoperation event caused by multiple current loop misoperations shows that the safety measures of the current loop are very important. This article analyzes in detail the four typical situations and principles of protection maloperation caused by the current loop, and proposes to implement a "double breakpoint" safety measure in the current loop, which can avoid the protection maloperation caused by the misoperation of the current loop, which effectively guarantees the safety of the power grid. Stable operation. ^[1]