Application of Residual Current Protection Relay in Low Voltage Distribution System

Residual Current Protection Relay Is a Type of Leakage Protection Device

Residual current protection relay serves as the main or backup protection in low-voltage distribution systems, playing a critical role in eliminating hazards to human health and fire risks.

In the TT system, residual current protection relay is used for ground fault protection; while in the TN system, quick action and leakage protection can meet the sensitivity requirements for ground fault protection. As the scale of China's low-voltage electrical system and economic construction continues to expand, increasing power supply capacity and safety has become especially important. This is particularly true in systems powered by dedicated independent transformers. Due to weak supervision, maintenance personnel safety cannot be guaranteed, leading to frequent accidents involving line equipment electric shocks, leakage, ground short-circuits causing fires, and economic losses.

Therefore, it is imperative to develop innovative intelligent leakage protection devices. Residual current protection relay, as an effective protective device, its main duty is to prevent human injury or death due to electric shocks and to prevent fires caused by electrical equipment or line leakage.

The Basic Principle of the Residual Current Protection Relay

The basic principle of the residual current protection relay lies in: during the normal operation of the circuit, based on the circuit current theorem, the current flowing in and out of one end of the network is equal in magnitude, opposite in direction, and thus sums to zero. At this time, the shock protector is in a stationary state, with the zero-sequence current transformer acting as the detection element.

The vector sum of phase currents equals zero (assuming the zero-sequence current protection does not consider unbalanced current), thus, there is no signal output from the secondary side winding of the zero-sequence current transformer, and the operating executive mechanism takes no action.

When a ground fault occurs, the vector sum of phase currents is not zero, the fault current induces magnetic flux in the ring-shaped iron core of the zero-sequence current transformer, and the induced voltage on the secondary side triggers the corresponding action of the operating executive mechanism, such as tripping a molded case circuit breaker or an air circuit breaker to achieve ground fault protection.

When a person touches a live wire, their body, acting as a conductor, causes the total current on the right side of the shock protector to be non-zero. When the leakage current reaches the operating current, the operating executive mechanism acts to keep the current flowing through the body small and controlled within 30mA, ensuring personal safety and achieving the effect without power interruption.

Similarly, in the event of a same-phase electric shock, the reactance control of the operating executive mechanism keeps the shock current within 30mA; in the event of a different-phase second electric shock, the operating executive mechanism trips the molded case circuit breaker or the air circuit breaker, ensuring personal safety.

Characteristics of the Residual Current Protection Relay

Residual current protection relay for low-voltage distribution system lines and equipment powered by independent transformers, it realizes real-time monitoring, early warning of leakage accidents, and leakage protection. Under certain conditions, it can also provide backup protection for electric shocks, reducing property loss and power outage impacts caused by frequent tripping of leakage switches.

Residual current protection relay features continuous real-time monitoring of the insulation and leakage status of system lines and equipment, utilizing high-tech methods to enhance safety management levels. Through remote monitoring, it enables leakage protection without tripping, predicting, warning, and troubleshooting in advance, effectively reducing the incidence of safety accidents.

Currently, violative use of non-standard electrical lines and equipment still exists in the buildings under construction and already built in China. Coupled with uncivilized and violative electrical installation construction, the risk of electric shock accidents and electrical fire remains high.

Additionally, many old large and medium-sized public buildings, including ancient buildings, show declining insulation performance of lines and devices due to aging cables and non-standard construction. The original design load capacity is limited, while power consumption increases year by year. Residual current protection relay becomes an effective protective device.