Automatic frequency unloading

The frequency of the electrical network is 50 Hz, for the correct operation of consumers of electrical energy and the energy system as a whole, the frequency must be within this value. If the amount of energy generated in power plants is less than the amount of energy consumed by consumers, then a sharp drop occurs power grid frequency.

Automatic frequency unloading (AFR) — an element of automation of emergency control of distribution substations, which is designed to prevent a drop in the frequency of the power system in the event of a sharp reduction in the amount of active power in the electric network.

Thanks to AFC, in the event of a deficit in the generated capacity in the power plants, the energy system remains in operation and provides power to the most critical users, whose disposal is inadmissible, as it can lead to various negative consequences.

First of all, it is users of the first categorydisconnection that poses a danger to human life or may result in major material damage.The second most important are the users of the second category of reliability of the power supply, whose interruption leads to disruption of the normal work cycle of enterprises, various systems and communications of settlements.

In addition, a sharp drop in frequency in the power system can disrupt the normal operation of power plants. That is, if you do not take any measures, the reduction in frequency will continue, leading to a complete breakdown of the power system.

Automatic frequency unloading, in the event of a decrease in frequency below the set value, automatically disconnects some consumers from the power grid, thus reducing the deficit of generated active power in the power grid. Reducing the shortage of electricity, in turn, contributes to increasing the frequency of the electricity grid to the required value of 50 Hz.

Automatic frequency unloading devices operate in stages. The first stage, which has the shortest delay of 0.3-0.5 s and is triggered when the frequency drops to 49.2 Hz (or lower, depending on the characteristics of the power system), turns off the least important users of the substation. As a rule, for this stage of the ACR, user lines are created that feed users of the third power category.

The next stage of AFC is designed to prevent an avalanche process of falling frequency, which can occur in case of insufficient discharge from the first stage of AFC, when the frequency of the mains starts to fall below 49 Hz. The delay of a given AFC stage can vary from a few seconds to a few tens of seconds.This stage of unloading is carried out by excluding users of the second category.

Along with automatic frequency unloading devices, devices can be installed to automatically reclose consumers who have been disconnected from the frequency unloading action — CHAPV. ChAPV devices restore power to exhausted consumers as soon as the grid frequency is normalized.

An increase in the frequency of the power grid occurs with an increase in the amount of generated power in the power system. The restoration of power supply to consumers must be phased, taking into account the current state of the power system. If the reason for the drop in frequency is an outage in the power system of a large power plant, it means that the power supply to all consumers disconnected from the action of ACR can be restored only after the resulting power shortage is overcome.

Often, after the operation of the FAR, the frequency drops again, therefore, in case of serious emergency situations in the power system, the FAR is taken out of operation and the restoration of compensated consumers is done in manual mode.

AChR and CHAPV devices can be implemented on an electromechanical type relay, as well as using more advanced microprocessor devices.

AChR devices are powered by voltage transformers.As a rule, the power supply is provided by two different sources (voltage transformers) to ensure the possibility of operation of this device in case of need to withdraw one of the voltage transformers for repair.