Content:
- What is AChR for?
- Classification
- Operating principle
- Conclusion
What is AChR for?
Let's start with the fact that an alternating current with a frequency of 50 Hz flows in domestic power grids. This is its nominal value, deviations of no more than 1%, that is, 0.5 Hz, are permissible. With a shortage of active power consumption and an excess of reactive power, a frequency drop may begin. Where it leads? To an avalanche-like further decline in frequency and accidents, let us consider them in more detail:
- The speed of rotation of asynchronous and synchronous motors will decrease with the frequency. At a power plant, this will lead to a decrease in the performance of water pumps for their own needs - for filling boilers and cooling powerful electrical machines.
- When steam turbines operate at a reduced frequency, the following can happen - a group of blades will enter into resonance and begin to vibrate and collapse.
Due to a decrease in the speed of the electric motors of the pumps and mechanisms of the power plant, its power also decreases, because of this, the frequency decreases even faster, which is why it is called an avalanche of frequency.
These problems lead to accidents up to a global scale, if this happens at a nuclear power plant, for example. Therefore, to resume work in the nominal mode, the consumer groups are switched off one by one. For this, an automatic frequency unloading system - ACHR is used.
Output: AFC is installed where there may be a shortage of active power.
Classification
Before we look at how automatic frequency unloading works, let's take a look at its types:
- AChR-I - fast-acting protection. The frequency setting range for the AChR-1 is within 46.5-48.8 Hz, and for the AChR of a special queue is 49.0-49.2 Hz. By response time - 0.15-0.3 s. Wherein selectivity of protection should exclude the operation of automatic frequency unloading in case of short circuit.
- AChR-II. It can be combined and uncombined. A part of consumers, which is disconnected by the combined one, is included in the volume of AChR-1. The frequency range of AChR-2 is within 48.7-49.1 Hz. It coincides with the range of the first frequency response, but differs in time setting, namely from 5 to 90 seconds. This response time is acceptable if the equipment can operate at a frequency of 49.2 Hz or more without causing damage to systems and components. In this case, turning them off to speed up the process of returning to the rated frequency does not make sense.
- ZACHR - installed at nuclear power plants, operates at a frequency of 49.2 Hz for 0.5 s.
The figure below shows the response graph of the AChR-1 and AChR-2:
And the response graph depending on the rate of frequency reduction:
Operating principle
Automatic frequency unloading has the following operating principle. When the frequency drops below the nominal, consumers of the third or second category of power supply begin to turn off. This is necessary to return the first category to a normal diet. The shutdowns take place in steps at different frequency and time settings.
The steps are needed to minimize interruptions in the power supply to consumers, since not in all situations the frequency drop process develops so critically that it is necessary to make global shutdowns. And also in order to exclude its operation in the event of a short circuit on the line. Consumers are evenly distributed over the steps according to their power.
Automatic frequency unloading was based on a frequency relay. To organize the disconnection of consumers, it is necessary to distribute them by categories and, accordingly, by TP and RI according to their intended purpose. Otherwise, selective shutdown is more difficult to implement.
In addition, it is necessary to disconnect consumers from the mains, it is necessary to ensure their re-inclusion for this there is a system of frequency automatic reclosing - CHAPV. It is also worth noting that at present, purely relay circuits are rarely used; they can be found on old objects that have not been reconstructed. Microprocessor-based automatic frequency unloading systems are mainly used. They allow you to more accurately adjust the response parameters in case of disturbing influences.
Conclusion
Let's summarize and answer in simple language, where and in what cases the automatic frequency unloading system is used. AChR is used in the power industry to prevent accidents at power plants due to an avalanche-like decrease in the frequency of electric current in the network. This can happen due to the fact that the balance of active and reactive power is disturbed.
The main task before automatic frequency unloading is the alternate disconnection of consumer groups in accordance with categories of power supply, so as not to entail significant downtime and accidents on those powered by this line enterprises.
To consolidate the studied material, we recommend watching a useful video on the topic:
Related materials:
- What is the automatic reclosing for?
- How to measure AC frequency
- Zero sequence overcurrent protection
- Malfunctions in electrical installations and their causes
