In the power system, the terms "overcurrent" and "overload" are not unfamiliar to everyone. Today I will talk to you about the difference between "overcurrent protection" and "overload protection".

First of all, we have to know what overcurrent protection and overload protection are. Over-current protection is actually over-current protection, that is, when the current exceeds a predetermined maximum value, a protection method that causes the microcomputer protection device to operate. Overload protection refers to protection measures when the protected area exceeds the specified load, and is also called overload protection.

The working current of the over-current protection is set according to the maximum load current, which cannot meet the sensitivity requirement in some cases. Therefore, in order to increase the sensitivity of the overcurrent protection and improve the conditions for avoiding the maximum load current when a short-circuit fault occurs, this is why a low-voltage lockout device is installed in the overcurrent protection.

Overload usually refers to power, and overload current usually indicates failure. Load is just a generic name, it is a quantified value, that is, the power written in the book. Overload means that the power consumed by the load exceeds its rated value. It can be known from power equals voltage multiplied by current. When the power exceeds the rated value, if the power supply voltage does not change, the current will also exceed the rated value, that is, overcurrent occurs. The high-voltage overload protection is equivalent to the delay protection of the low-voltage system. Generally, the setting time of transformer overload protection is 9-15 seconds, and the working current is slightly larger than the rated current of the transformer. In addition, high-voltage capacitors and motors can also be used for overload protection. The circuit is usually not used. Overcurrent protection is usually aimed at short-circuit protection.

Take generator protection as an example. Generator overcurrent has certain timeliness, which will cause tripping; generator overload protection also has certain timeliness, which will affect signals and alarms. The setting value is less than the overcurrent setting value. Therefore, overcurrent is usually caused by an external fault, such as an external short circuit, which belongs to backup protection and usually has a certain delay; overload is caused by overload or asymmetry. From an outlet point of view, overcurrent protection usually trips, while overload usually sends a trip signal, and negative sequence overload usually delays the trip.

There are two main differences between overcurrent protection and overload protection: one is a different set action value, and the other is the action time. The setting value of the overload protection is far lower than the setting value of the overcurrent protection, but greater than the normal load to prevent the equipment from overload operation. A certain delay should be added to avoid the starting current of the equipment. When the overcurrent protection refuses to operate for some reason, the overload protection can also be used as a backup protection for the overcurrent protection. However, overload protection is usually divided into two areas: definite time limit and anti-time limit. When the rated time is exceeded, the main equipment can withstand less overload capacity for a period of time. At this time, there is no need to trip and stop if the rated value is exceeded.

In many cases, the main equipment cannot withstand a large load for a long time, so the action must be tripped and stopped, otherwise the main equipment will be greatly damaged. The inverse time limit action time can be calculated based on the overcurrent heat dissipation capacity of the primary equipment (the overload is mainly due to the increase of current and the generation of heat. To a large extent the equipment that causes combustion). Overcurrent is an external fault that requires the protection to take immediate action and trip.

The fixed value of overload protection is low, usually 1.2 times the rated value, and the delay time is longer. The main protective measure is to prevent malfunctions such as torque increase. The characteristic of this type of fault is that it will not cause a large current increase, it can be delayed for a long time without causing overheating damage to the coil, and the action will trip. Overcurrent protection has a relatively high fixed value, usually 3-4 times. This value is mainly used for locked-rotor, but also used for three-phase short-circuit backup protection. When the current protection is activated, it means that the system has a short-circuit fault and the voltage will drop a lot, but the voltage will not drop when the overload signal is sent. Therefore, overcurrent protection uses low-voltage shutdown, while overload protection does not use overcurrent protection. Collect three-phase or two-phase current value, while overload protection only needs to collect any one-phase current value. Overcurrent protection will act on the outlet, while overload protection only sends a signal!