How does undervoltage protection work?
Device and principle of operation
The responsive organ of the system is a relay that controls the minimum voltage. The relay is connected to a sectional voltage transformer. The protection also includes a time relay, indicator relaysignaling protection operation, intermediate relays.
The purpose of the protection that responds to the minimum voltage is to turn off the engines of less critical mechanisms to ensure the successful self-start of the more important ones.
To understand what this means and why protection is needed, consider its principle of operation in thermal power plants. The electric motors of the mechanisms of each boiler unit are connected to their own station needs section. Each section has a working input of power from its own transformer. In addition, the sections are interconnected by a sectional switch. A circuit is considered normal when the sections are powered by the inputs of the auxiliary transformers, while the sectional switch is turned off. Imagine a situation where the voltage at the power input section disappears (for example, as a result of damage to the auxiliary transformer). The working input is turned off, the automatic transfer switch (automatic switch on reserve) is activated, including a section switch. After that, the section is powered from another auxiliary transformer through a sectional switch. The minimum operating time of the ATS consists of the delay in the system that controls the voltage of the working input, the time of operation of the intermediate relays, the time of switching off and on of the switches of the working and backup inputs. During this time, braking of electric motors powered by the section occurs.
After applying power, a group self-start of electric motors connected to the section begins. At the same time, depending on the depth of the braking that has occurred, a voltage landing (reduction) takes place to a greater or lesser extent.
Note. When the boiler is started in normal mode, the mechanisms are switched on sequentially at large intervals. Therefore, with the simultaneous start (even if not completely braked) of the mechanisms, the total value of the starting current significantly exceeds the rated current of the supplying TSN. This can cause a deep voltage drop across the sections.
Undervoltage protection has two steps. The operation of the first stage occurs if the decrease reaches 0.7 * Un with a time delay of 0.5 s. The second stage has a setting of 0.5 * Un and a response time of up to 9 s.If during the non-current pause there was minimal braking of the mechanisms and the voltage did not reach 70% of the nominal, self-starting of all section electric motors is successful, the boiler continues to work.
If the voltage drops to 70% or lower for a period of 0.5 seconds, the protective equipment starts the first stage. The least important mechanisms for the operation of the boiler are switched off. This is done to prevent further voltage reduction in order to enable responsible mechanisms to start.
Conclusion. The principle of operation of the first stage of protection of the minimum voltage serves to keep the boiler in operation by turning off mechanisms of secondary importance.
A further decrease in voltage (after operation of the 1st stage of protection) and reaching its level of 50% of the nominal for a time of up to 9 seconds means that the self-starting of the critical mechanisms of the boiler failed. At this stage, the question of the operation of the boiler is no longer standing. The circuit of the second stage is turned on. The remaining mechanisms connected to the protection circuits are disconnected. Only those units remain whose shutdown can lead to an emergency when the boiler stops. For example, in order to avoid the explosion of coal dust in the boiler furnace, it is unacceptable to turn off the smoke exhaust.
Conclusion. The principle of operation of the second stage of protection aims to bring the boiler into safe extinguishing and shutdown mode.
Conclusion
It follows from the foregoing that the principle of operation of the protection, which responds to the minimum voltage, is closely related to the functioning of the technological equipment to which it is attached. Protective equipment is located at a substation that supplies power to electrical installations of technological equipment. Thus, to finally understand what protection is needed for, you can only get at least a minimal idea of how the entire technological complex works.
Finally, we recommend watching a useful video in which an overview of the protective devices that are used today is provided:
So we examined the purpose and principle of operation of the minimum voltage protection. We hope the information provided was useful and interesting for you!
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