What are blocks used in static differential relay?
The rectifier bridge type comparator is generally used in the static differential relay. The block diagram is shown in the Fig. 1. In normal conditions, the two quantities balance each other and the comparator output is zero and the relay us inoperative.
Why the harmonic restraint scheme is required in case of transformer?
The secondary current is zero and the primary current is high at the time of charging of the transformer. This difference in current between primary and secondary may lead to operation of differential protection relay. This requirement is fulfilled by harmonic restraining in differential protection of the transformer.
How does a transformer differential relay work?
The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.
What is harmonic restraint in transformer?
This current, since it flows in only one winding of the transformer, causes a differential current to flow which, to an ordinary relay, “looks” the same as the current due to an internal fault. …
What are the advantage of static relay?
Advantages of Static Relays
- Low power consumption: This relay provides less burden on CTS and PTS compared to conventional relays.
- No moving contacts: As there are no moving contact, there is no problem of contact bounce, arcing, erosion, replacement of contacts etc.
Which is not advantages of static relay?
The working of the relay depends on the electrical components. The relay has less overloading capacity. The static relay is more costly as compared to the electromagnetic relay. The construction of the relay is easily affected by the surrounding interference.
What is the reason behind providing a second harmonic restraint relay?
To avoid unnecessary trip caused by the magnetizing inrush current, the second harmonic component is used to block the differential relays on the power transformers.
How a harmonic restraint relay is different from percentage bias differential relay?
The only difference between them is the restraining coil. The percentage differential relay consists restraining coil for overcoming the trouble arising out of differences in the current ratio for the high value of an external short circuit current.
Why differential relay is used in transformer?
Similar to bus protections, transformers are protected by differential relays. Inter-winding faults (short circuits) and ground faults within power transformers can be detected by this protection scheme. Failure to detect these faults and quickly isolate the transformer may cause serious damage to the device.
What is harmonic blocking?
Harmonic blocking method has been introduced in many modern microprocessor relays. When the ratio of the second harmonic component to the fundamental is greater than second harmonic set point, inrush condition is announced and the differential operation is blocked.
What are the advantages and disadvantages of static relays?
Low power consumption: This relay provides less burden on CTS and PTS compared to conventional relays. So power consumption in measuring circuit of static relay is lower. No moving contacts: As there are no moving contact, there is no problem of contact bounce, arcing, erosion, replacement of contacts etc.
How are harmonics used in a differential relay?
These early relays used all the harmonics to restrain. Sharp and Glassburn introduced the idea of harmonic blocking instead of restraining [9] with a relay that used only the second harmonic to block. Many modern transformer differential relays employ either harmonic restraint or blocking methods. These methods
Why are differential relays prone to undesired operation?
I. INTRODUCTION Transformer differential relays are prone to undesired operation in the presence of transformer inrush currents. Transformer energization is a typical cause of inrush currents, but any transient in the transformer circuit may generate these currents.
What causes inrush currents in a differential relay?
Transformer differential relays are prone to undesired operation in the presence of transformer inrush currents. Transformer energization is a typical cause of inrush currents, but any transient in the transformer circuit may generate these currents. Other causes include voltage recovery after the
Why do I need an overcurrent relay Element?
A substantial difference indicates a fault in the device or between the current transformers (CTs) located around the device. A simple overcurrent relay element could provide basic differential protection, provided the CTs could be sized and connected to perfectly match the secondary current presented to the relay.