In power systems, electrical impedance relays are used for various protection schemes, including generator out-of-step protection. Out-of-step protection is essential for maintaining the stability of the power system when a generator is at risk of falling out of synchronization with the rest of the system due to severe disturbances or faults.
The basic principle behind an electrical impedance relay is to measure the impedance (or the apparent impedance) of the power system and compare it to a predefined characteristic curve. In the case of generator out-of-step protection, the relay is designed to monitor the impedance between the generator and the rest of the power system.
Here's how an electrical impedance relay operates in generator out-of-step protection schemes:
Impedance Measurement: The relay continuously measures the impedance of the generator's stator winding or the impedance between the generator terminals. This impedance measurement can be done using voltage and current transformers to obtain voltage and current signals.
Characteristic Curve: The relay is programmed with a characteristic impedance versus time curve. This curve represents the allowable impedance for the generator under normal operating conditions. The characteristic curve is usually determined based on the generator's capability to stay in synchronism with the system during disturbances.
Comparison: The relay continuously compares the real-time measured impedance with the characteristic curve. If the measured impedance moves outside the allowable region defined by the characteristic curve, it indicates that the generator is experiencing an abnormal condition.
Out-of-Step Detection: If the measured impedance exceeds the limits set by the characteristic curve, the relay detects an out-of-step condition. An out-of-step condition typically means that the generator is losing synchronism with the rest of the system due to a disturbance or fault.
Tripping Signal: Upon detecting an out-of-step condition, the impedance relay sends a tripping signal to the generator's circuit breaker. The circuit breaker opens, disconnecting the generator from the power system. This action prevents the generator from destabilizing the entire power system and causing more significant damage.
Blocking and Time Delay: Out-of-step protection should be carefully coordinated with other protective schemes in the power system. To avoid false tripping, out-of-step protection is often blocked during certain conditions, such as during startup or shutdown of the generator. Additionally, a time delay may be introduced to allow the generator to correct minor disturbances before initiating a trip.
In summary, an electrical impedance relay in generator out-of-step protection schemes continuously monitors the impedance of the generator and trips the generator offline if it detects an out-of-step condition, helping to maintain the stability of the overall power system.