Polarizing Current
- Thread starter mull982
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72.5kv
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Its a reference current that directional elements use to determine direction of power flow.
T-line
-----X1-----Breaker1-------------X2----------------Breaker2------X3------
The directional relays at breaker1 and breaker2 need to have some sort of reference to determine what direction a fault is whether at X1, X2, or X3.
T-line
-----X1-----Breaker1-------------X2----------------Breaker2------X3------
The directional relays at breaker1 and breaker2 need to have some sort of reference to determine what direction a fault is whether at X1, X2, or X3.
How in theory is this reference current used and how does it determine direction?
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
For a protective relay, or meter for that matter, to determine the direction of power flow it must have a current & voltage to compare or two currents to compare with each other. If the directional relay uses a voltage reference ( PTs ) it is voltage polarized. If it uses a current reference ( a fourth CT ) then it is current polarized.
In the electromechanical relays the polar unit would restrain the overcurrent unit ( induction disc ) from operating unless power was flowing at a specified direction.
In the electromechanical relays the polar unit would restrain the overcurrent unit ( induction disc ) from operating unless power was flowing at a specified direction.
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
Voltage Polarized is generally used for phase protection & current polarized is generally used for ground protection. The fourth CT measuring zero sequence currents. The current polarized units may use residual current also, I left that out before. It depends on which directional overcurrent relay you are using.
In either case when the polar unit determines the angle is in a specific range it allows the overcurrent unit to operate.
In either case when the polar unit determines the angle is in a specific range it allows the overcurrent unit to operate.
broadgage
Senior Member
- Location
- London, England
"polarised relays" in the UK are sometimes understood to mean relays with a DC coil, and THREE contact positions.
Any standard relay can only have two contact positions, with the coil energised, or with coil not energised.
Polarised relays have three contact positions, with coil energised with one polarity, with the coil energised with the oposite polarity, and with the coil de-energised.
They are used in railway signalling, and have 2 main advantages.
Firstly fewer control wires are needed, a matter of some importance if these are miles long, and secondly the circuit may be arranged to fail safe.
For example the system may be designed thus
Coil energised with one polarity-------- green signal shows (proceed)
Coil energised with opposite polarity----yellow signal shows(caution)
Coil de-energised ---------------------red signal shows (stop danger)
Therefore the burning out of the relay coil, the failure of the control supply, or the cutting of the control cable, will result in the red lamp lighting, which is desireable.
Any standard relay can only have two contact positions, with the coil energised, or with coil not energised.
Polarised relays have three contact positions, with coil energised with one polarity, with the coil energised with the oposite polarity, and with the coil de-energised.
They are used in railway signalling, and have 2 main advantages.
Firstly fewer control wires are needed, a matter of some importance if these are miles long, and secondly the circuit may be arranged to fail safe.
For example the system may be designed thus
Coil energised with one polarity-------- green signal shows (proceed)
Coil energised with opposite polarity----yellow signal shows(caution)
Coil de-energised ---------------------red signal shows (stop danger)
Therefore the burning out of the relay coil, the failure of the control supply, or the cutting of the control cable, will result in the red lamp lighting, which is desireable.
I read up on this a bit and have a little better understanding.
As was stated the polarizing element is usually the reference element. Lets take the simple case of a Delta-Wye transformer where the polarizing element is placed on the secondary neutral. How is this reference then used?
Is the reference used during normal operation with small current flows, or does it only become active when there is ground fault or 3IO current flowing in the neutral. How does the reference element then take this 3IO current and use it as a reference against other line currents to determine fault direction.
I also saw that PT's can be used as a polarizing reference in a broken delta PT connection. How is this voltage reference used in comparison with other line values? Does it compare other line voltages or currents against this value to determine direction?
As was stated the polarizing element is usually the reference element. Lets take the simple case of a Delta-Wye transformer where the polarizing element is placed on the secondary neutral. How is this reference then used?
Is the reference used during normal operation with small current flows, or does it only become active when there is ground fault or 3IO current flowing in the neutral. How does the reference element then take this 3IO current and use it as a reference against other line currents to determine fault direction.
I also saw that PT's can be used as a polarizing reference in a broken delta PT connection. How is this voltage reference used in comparison with other line values? Does it compare other line voltages or currents against this value to determine direction?
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
Here is some more reading for you. The first two pages covers the theory & operation of a electromechanical directional relay, the Westinghouse (ABB) CR & CRC. The CRC is current polarized version. Typical external connections are shown starting on page 12.
http://www05.abb.com/global/scot/scot229.nsf/veritydisplay/483515962c74815b85256ead00692c39/$File/41-285R.pdf
Looks like you will have to copy & paste the address in to the address bar manually.
The directional unit operates under fault conditions. A broken delta only produces a voltage output during unbalenced voltage conditions on it's input side which would be caused by a fault.
Notice on page 3 Figure 5 that point 13 has a + symbol next to it. This indicates that when current flows into point 13 contact D will close allowing overcurrent to flow through the CO coils,
which cause an induction disc to rotate
& close the contact named CO
which allows tripping current to flow through CS
which seals itself in
a breaker "a" contact opens the CS circuit when the breaker opens ( trips ).
If overcurrent is flowing into the load the D contact remains open. These type relays are usually applied to Main breakers.
http://www05.abb.com/global/scot/scot229.nsf/veritydisplay/483515962c74815b85256ead00692c39/$File/41-285R.pdf
Looks like you will have to copy & paste the address in to the address bar manually.
The directional unit operates under fault conditions. A broken delta only produces a voltage output during unbalenced voltage conditions on it's input side which would be caused by a fault.
Notice on page 3 Figure 5 that point 13 has a + symbol next to it. This indicates that when current flows into point 13 contact D will close allowing overcurrent to flow through the CO coils,
which cause an induction disc to rotate
& close the contact named CO
which allows tripping current to flow through CS
which seals itself in
a breaker "a" contact opens the CS circuit when the breaker opens ( trips ).
If overcurrent is flowing into the load the D contact remains open. These type relays are usually applied to Main breakers.
Thanks for this information it helped!
I understand the concept of using a CT on the neutral for polarizing current however I'm having trouble understanding the polarizing voltage concept.
First, why must a broken delta be used for polarizing current to calculate zero sequence voltage? Cant the relay just add the three phasors from the delta or wye connected PT to calculate the zero sequence voltage?
How is the fault current then compared to the polarizing voltage? Does it compare the phase angles of the two? How is a direction determined form this angle?
I understand the concept of using a CT on the neutral for polarizing current however I'm having trouble understanding the polarizing voltage concept.
First, why must a broken delta be used for polarizing current to calculate zero sequence voltage? Cant the relay just add the three phasors from the delta or wye connected PT to calculate the zero sequence voltage?
How is the fault current then compared to the polarizing voltage? Does it compare the phase angles of the two? How is a direction determined form this angle?
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