Directional Overcurrent Relay
- Thread starter sdilucca
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- Wisconsin
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- PE (Retired) - Power Systems
Re: Directional Overcurrent Relay
Directional ovecurrent realys are often wired to measure the current going into a transformer and the current going out of the transformer. directional relays are very useful for quickly detecting short circuits while not reacting to normal overloads.
Under normal conditions the phase angle between the primary and secondary currents relatively constant and the relay does not operate. A short circuit on the secondary side will cause a sudden and large phase shift (usually lagging). The directional relay will operate on this phase angle difference even though the ratio of the currents has remained fairly constant.
Directional ovecurrent realys are often wired to measure the current going into a transformer and the current going out of the transformer. directional relays are very useful for quickly detecting short circuits while not reacting to normal overloads.
Under normal conditions the phase angle between the primary and secondary currents relatively constant and the relay does not operate. A short circuit on the secondary side will cause a sudden and large phase shift (usually lagging). The directional relay will operate on this phase angle difference even though the ratio of the currents has remained fairly constant.
zog
Senior Member
- Location
- Charlotte, NC
Re: Directional Overcurrent Relay
Jim, what you are describing sounds more like a differential relay than a directionsl relay.
A directional relay is usually used on the main breakers in a secondary selective (double ended) substaion to prevent backfeeds. A directional relay is basically a overcurrent relay that only monitors current in one direction (The bad direction).
Please correct me if I am wrong.
Jim, what you are describing sounds more like a differential relay than a directionsl relay.
A directional relay is usually used on the main breakers in a secondary selective (double ended) substaion to prevent backfeeds. A directional relay is basically a overcurrent relay that only monitors current in one direction (The bad direction).
Please correct me if I am wrong.
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Directional Overcurrent Relay
I agree with zog. I believe I've seen them used in transmission lines as well, looking in one direction for a fault.
I agree with zog. I believe I've seen them used in transmission lines as well, looking in one direction for a fault.
rcwilson
Senior Member
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- Redmond, WA
Re: Directional Overcurrent Relay
Directional overcurrent relays are useful in a loop circuit or any circuit where power can flow in both directions. Overcurrent relay settings are coordinated so the device closest to the fault trips first. The relay furthest from the fault, at the source, is set to trip last. This isolates the fault quickly and minimizes the extent of the outage.
But if the system is a loop or can have power flow in both directions, the same relay has to trip fast for one power flow direction and trip slow for the opposite. An individual overcurrent relay can't be set for different tripping times and it can't tell the fault direction to decide to trip fast or slow.
A directional overcurrent relay senses the direction of power flow and only trips if the fault is on the correct side of its location. Two directional relays at the same location can give complete discrimination and provide coordination with protection in both directions. One is set to coordinate with the relays on the left, the other to coordinate with relays on the right.
Directional overcurrent relays are useful in a loop circuit or any circuit where power can flow in both directions. Overcurrent relay settings are coordinated so the device closest to the fault trips first. The relay furthest from the fault, at the source, is set to trip last. This isolates the fault quickly and minimizes the extent of the outage.
But if the system is a loop or can have power flow in both directions, the same relay has to trip fast for one power flow direction and trip slow for the opposite. An individual overcurrent relay can't be set for different tripping times and it can't tell the fault direction to decide to trip fast or slow.
A directional overcurrent relay senses the direction of power flow and only trips if the fault is on the correct side of its location. Two directional relays at the same location can give complete discrimination and provide coordination with protection in both directions. One is set to coordinate with the relays on the left, the other to coordinate with relays on the right.
Re: Directional Overcurrent Relay
thanks... my understanding from your posts and personal research is that one uses directional relaying when there are multiple sources/feeders which are interconnected downstream. Directional relays allow you to establish different time coordinations based on the direction of current. Correct me if I'm wrong.
also... i'm intrigued the differential relaying that was mentioned... An application was mentioned, but what is the BASIC function of diff. relays?
thanks... my understanding from your posts and personal research is that one uses directional relaying when there are multiple sources/feeders which are interconnected downstream. Directional relays allow you to establish different time coordinations based on the direction of current. Correct me if I'm wrong.
also... i'm intrigued the differential relaying that was mentioned... An application was mentioned, but what is the BASIC function of diff. relays?
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Re: Directional Overcurrent Relay
My description of a directional relay was based on information taken from page 55 of "The Art and Science of Protective Relaying" by C. Russell Mason, copyright 1956.
A differential relay is used to see if all of the current going into a transformer is properly related to the current leaving a transformer. A directional relay is used to see that the primary and secondary current have the correct phase relationship. A subtle but important difference.
Very simplistically, the directional relay can be used to see a line-line fault while the differential relay sees a line to ground fault.
edit: typos and phrasing
[ September 07, 2005, 01:03 PM: Message edited by: jim dungar ]
My description of a directional relay was based on information taken from page 55 of "The Art and Science of Protective Relaying" by C. Russell Mason, copyright 1956.
A differential relay is used to see if all of the current going into a transformer is properly related to the current leaving a transformer. A directional relay is used to see that the primary and secondary current have the correct phase relationship. A subtle but important difference.
Very simplistically, the directional relay can be used to see a line-line fault while the differential relay sees a line to ground fault.
edit: typos and phrasing
[ September 07, 2005, 01:03 PM: Message edited by: jim dungar ]
Re: Directional Overcurrent Relay
Directional Overcurrent IEEE device #64.
From ABB:
KH Directional Relays
KH-1 Directional Relay
The KH-1 relay is used to supervise breaker closing to insure that power flows from the source to the load bus after closing.
KH-2 Instantaneous Directional Overcurrent Relay
The type KH-2 instantaneous directional overcurrent relay is used whenever a directional unit with an adjustable pickup is desired. A typical application is for directional lockout for current balance paralleling when two or more load tap changing transformers are connected in parallel.
KH-21 Direction Overcurrent Relay
The KH-21 relay is an under power relay using two high speed single phase directional elements in an FT-21 case. This is equivalent to two single phase elements for measuring power using these elements? contacts in series. The relay utilities break contacts for an under power application. The contacts will be closed to trip in the absence of power flow.
Directional Overcurrent IEEE device #64.
From ABB:
KH Directional Relays
KH-1 Directional Relay
The KH-1 relay is used to supervise breaker closing to insure that power flows from the source to the load bus after closing.
KH-2 Instantaneous Directional Overcurrent Relay
The type KH-2 instantaneous directional overcurrent relay is used whenever a directional unit with an adjustable pickup is desired. A typical application is for directional lockout for current balance paralleling when two or more load tap changing transformers are connected in parallel.
KH-21 Direction Overcurrent Relay
The KH-21 relay is an under power relay using two high speed single phase directional elements in an FT-21 case. This is equivalent to two single phase elements for measuring power using these elements? contacts in series. The relay utilities break contacts for an under power application. The contacts will be closed to trip in the absence of power flow.
beanland
Senior Member
- Location
- Vancouver, WA
Re: Directional Overcurrent Relay
Directional relays are commonly used to limit or backfeed where a generation plant is running in parallel with the utility and the utility does not permit the generator to deliver power to the utility.
Directional relays are also used in emergency generators to detect motoring.
Directional relays are commonly used to limit or backfeed where a generation plant is running in parallel with the utility and the utility does not permit the generator to deliver power to the utility.
Directional relays are also used in emergency generators to detect motoring.
O
oliver100
Guest
Re: Directional Overcurrent Relay
Differntial relay protection protects power plant generator from outside faults.
rcwilson
Senior Member
- Location
- Redmond, WA
Re: Directional Overcurrent Relay
A typical differential relay has two sets of current transformer inputs. The area between the two CT locations is the protected zone. The relay compares current into the zone with current out of the zone and trips if the currents don't match. The relays have adjustments to account for the different current values if the CT's are at different voltage levels or differnt ratios, such as when protecting a transformer with CT?s on the high and low voltage sides.
A differential current relay can have multiple CT inputs. An example is a switchgear bus differential relay that compares the main breaker current with the currents in all of the feeder breakers. It trips if all current going in is not coming out.
Differential relays provide the fastest response to faults. But the cost of the relay and CT's required usually restricts their use to larger generators and transformers (>5 MVA) or critical bus and switchgear.
On a generator, the differential CT?s are on the neutral leads (before the three phases are joined together on the neutral) and on the load side of the generator breaker. Faults in the generator or generator cables trip the unit immediately. A power system fault or a fault external to the generator is not detected.
Directional power relays protect a generator or utility from reverse power.
Actually, differential current relays only protect for faults inside of their zone and cannot protect a generator from external faults. But, a power system with differential current relays will have fast fault clearing times and that is always better for a generator.
A typical differential relay has two sets of current transformer inputs. The area between the two CT locations is the protected zone. The relay compares current into the zone with current out of the zone and trips if the currents don't match. The relays have adjustments to account for the different current values if the CT's are at different voltage levels or differnt ratios, such as when protecting a transformer with CT?s on the high and low voltage sides.
A differential current relay can have multiple CT inputs. An example is a switchgear bus differential relay that compares the main breaker current with the currents in all of the feeder breakers. It trips if all current going in is not coming out.
Differential relays provide the fastest response to faults. But the cost of the relay and CT's required usually restricts their use to larger generators and transformers (>5 MVA) or critical bus and switchgear.
On a generator, the differential CT?s are on the neutral leads (before the three phases are joined together on the neutral) and on the load side of the generator breaker. Faults in the generator or generator cables trip the unit immediately. A power system fault or a fault external to the generator is not detected.
Directional power relays protect a generator or utility from reverse power.
O
oliver100
Guest
Re: Directional Overcurrent Relay
Absolutely correct. IT protects the zone between the CTs.
To the best of my knowledge reverse power protection relay uses power (voltage and current) in order to detect reverse power.
Absolutely correct. IT protects the zone between the CTs.
To the best of my knowledge reverse power protection relay uses power (voltage and current) in order to detect reverse power.
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