Today, we will introduce a key component of relay protection: overcurrent protection with low-voltage blocking.

Low-voltage blocking overcurrent protection is a type of power-system relay protection scheme that combines a low-voltage blocking element with overcurrent protection. It is designed to enhance the sensitivity and reliability of overcurrent protection and to ensure the safe and stable operation of power equipment and systems.

Components of overcurrent protection with low-voltage blocking:

1. Overcurrent protection section: This is primarily composed of a current relay (KA). The current relay is used to monitor the current level in the line in real time; when the current exceeds its preset setting value, the current relay operates, causing its normally open contact to close.

2. Low-voltage blocking section: This includes low-voltage relays (KV), intermediate relays (KM), and other components. The low-voltage relay monitors the busbar voltage; when the busbar voltage falls below its set value, the low-voltage relay operates, causing the normally open contact to close. The intermediate relay serves to transmit signals and perform logical decision-making.

Operating logic of overcurrent protection with low-voltage blocking:

1. Normal operating condition: During normal operation of the power system, the line current remains within the range of normal load current and does not exceed the setting value of the current relay; simultaneously, the busbar voltage is also normal and exceeds the setting value of the low-voltage relay. Under these conditions, neither the current relay nor the low-voltage relay operates, the protection device remains in a locked state, and no trip signal is issued.

2. Startup status of high-capacity equipment: When high-capacity equipment starts up, a significant inrush current is generated. This current may exceed the setting value of the current relay, causing the current relay to trip and its normally open contacts to close. However, because the busbar voltage has not yet fallen significantly at this point, the low-voltage relay remains inactive; consequently, its normally open contacts remain open, the intermediate relay cannot receive power to operate, and the protection device will not trip erroneously.

3. Fault condition: When a short circuit or other fault occurs in the protected line, the line current rises sharply, exceeding the setting value of the current relay, causing the current relay to operate. Simultaneously, the short-circuit fault causes the busbar voltage to drop significantly below the setting value of the low-voltage relay, causing the low-voltage relay to operate; its normally open contact then closes, energizing the intermediate relay. Once the intermediate relay operates, it closes the circuit to the time relay (KT); after a specified time interval, the time relay's delayed closing contact closes, energizing the trip coil (YR), causing the circuit breaker to trip and thereby isolating the faulty line.

Design purpose of overcurrent protection with low-voltage blocking:

1. Improve sensitivity: In traditional overcurrent protection, the operating current must exceed the maximum load current, resulting in a relatively high setting value that may render the protection insensitive to certain small fault currents. In overcurrent protection with low-voltage blocking, the setting value can be configured based on the normal continuous load current, thereby reducing the setting value and making the protection device more sensitive to fault currents, enabling it to detect faults and operate more promptly.

2. Enhanced reliability: During actual operation, factors such as the inrush current generated when high-capacity equipment is started may cause overcurrent protection to trip erroneously. A low-voltage blocking device can block the overcurrent protection when voltage is normal, preventing erroneous protection trips caused by non-fault conditions such as inrush current. Protection will only operate when the line current exceeds the set trip current of the current relay and is accompanied by a significant voltage drop; this thereby enhances the reliability of the protection device's operation.

3. Adaptability to complex operating conditions: The operating conditions of power systems are complex and variable, and both load current and voltage may change at any time. Overcurrent protection with low-voltage blocking can adapt to these complex operating conditions, enabling accurate fault identification and the implementation of appropriate protective measures under different operating conditions.

Application scenarios for overcurrent protection with low-voltage blocking:

In substations, it typically protects critical equipment such as transformers and busbars. Transformer faults can affect substation operation; this protection can promptly isolate the faulty transformer. In the event of a busbar short circuit, it prevents damage to the busbar and ensures continued power supply.

In the power distribution systems of industrial enterprises, a large number of power-consuming devices have stringent requirements for power supply stability. Overcurrent protection with low-voltage blocking protects distribution lines and equipment, thereby preventing equipment damage and production interruptions caused by overcurrent and low voltage conditions.

Overcurrent protection with low-voltage blocking combines low-voltage blocking and overcurrent protection, thereby addressing the shortcomings of conventional protection, improving fault response speed and accuracy, and enhancing system adaptability. As the power system continues to evolve, this protection will also undergo ongoing optimization, providing robust safeguards for power system safety.