Multiple relays sharing CTs consideration

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Ingenieur

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mbrooke said:
But say you just want it to trip the relay on a certain voltage? A simple resistor (sized correctly of course) will do?
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If what you are saying can you convert the ct current to a corresponding voltage, yes
2 ohm 0-5 A = 0-10 V
but why? Get a current sensing relay

if you mean convert ct current to the measured ckts voltage to trip on under/over voltage, no way
 
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GoldDigger

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Ingenieur said:
If what you are saying can you convert the ct current to a corresponding voltage, yes
2 ohm 0-5 A = 0-10 V
Click to expand...
And that had better be a 50W resistor or larger with free air circulation!!!

More likely a .01 ohm resistor and a 0 to .05V output. Then you could use a 1/4W resistor. But the meter needs to be very sensitive.

Are you starting to see the point, mbrooke? :)
 
mbrooke

mbrooke

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Ingenieur said:
If what you are saying can you convert the ct current to a corresponding voltage, yes
2 ohm 0-5 A = 0-10 V
but why? Get a current sensing relay

if you mean convert ct current to the measured ckts voltage to trip on under/over voltage, no way
Click to expand...

Current relays are low impedance, thus heavy fault currents can lead to CT saturation. The high impedance method has been proven to help in such scenarios.
 
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GoldDigger

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mbrooke said:
This says 2,000 ohms :D


https://selinc.com/products/587Z/
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Two voltage-based setting levels are available. A high-value setting can be used for a standard trip, while a low-value setting can be used as an open-circuited CT alarm. The differential element’s internal 2000 ohm resistor allows high security to be maintained during heavy through faults or during CT saturation issues.
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And with a typical CT you would be running the CT to a parallel current meter or have a low impedance external shunt.
Note that one use is to detect an open CT, which is just a voltage going essentially to zero.

The 2000 ohm impedance protects against excessive fault current when the input voltage is too high and the protection devices on the electronics start to conduct.
Use high-impedance differential elements for fast tripping for in-zone faults, while provid-
ing security during heavy through faults and CT saturation. Use familiar high-impedance equations to
calculate the voltage-based differential element settings. Save time, money, and panel space because
the relay includes the resistors and metal oxide varistors (MOVs) required for high-impedance differ-
ential protection.
Click to expand...

The "familiar high-impedance equations" describe how to use a CT for voltage output. So that mode of operation is possible, but AFAIK uncommon except for substations, etc.

JMO
 
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mbrooke

mbrooke

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GoldDigger said:
And with a typical CT you would be running the CT to a parallel current meter or have a low impedance external shunt.
Note that one use is to detect an open CT, which is just a voltage going essentially to zero.

The 2000 ohm impedance protects against excessive fault current when the input voltage is too high and the protection devices on the electronics start to conduct.


The "familiar high-impedance equations" describe how to use a CT for voltage output. So that mode of operation is possible, but AFAIK uncommon except for substations, etc.

JMO
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Let me digest all this... but how is open CT protection normally accomplished?
 
SG-1

SG-1

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mbrooke said:
Let me digest all this... but how is open CT protection normally accomplished?
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GE Thyrite Protectors have been used for open CT protection in the past. I have not see any used for years now.

High Impedance Differential relays such as Westinghouse (ABB) KABs or now the SEL 587Z are high speed devices. An 86 is normally used to trip every breaker on the bus & short circuit the CTs.
 
mbrooke

mbrooke

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SG-1 said:
GE Thyrite Protectors have been used for open CT protection in the past. I have not see any used for years now.

High Impedance Differential relays such as Westinghouse (ABB) KABs or now the SEL 587Z are high speed devices. An 86 is normally used to trip every breaker on the bus & short circuit the CTs.
Click to expand...

Do new substations have any forum of open VT protection? Im thinking that for low impedance relays I should at least add some high power resistors in the breaker compartment. Either that trip the breaker for open CT saturation?
 
SG-1

SG-1

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mbrooke said:
Do new substations have any forum of open VT protection? Im thinking that for low impedance relays I should at least add some high power resistors in the breaker compartment. Either that trip the breaker for open CT saturation?
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I am in medium voltage metal clad & metal enclosed switchgear assemblies, not normally used in substations. They are generally used as service entrance equipment. I do not know the latest trend in substations.

Many SEL relays can detect a saturated CT & take appropriate action.

Not sure what you are asking, Generally, CTs saturate under fault conditions, not open circuit. Are you thinking about placing resistors in parallel with the protective relays ? I have never seen this before.

Open VT ?, open delta ? Are you asking about ferroresonance protection ?
 
mbrooke

mbrooke

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SG-1 said:
I am in medium voltage metal clad & metal enclosed switchgear assemblies, not normally used in substations. They are generally used as service entrance equipment. I do not know the latest trend in substations.

Many SEL relays can detect a saturated CT & take appropriate action.

Not sure what you are asking, Generally, CTs saturate under fault conditions, not open circuit. Are you thinking about placing resistors in parallel with the protective relays ? I have never seen this before.

Open VT ?, open delta ? Are you asking about ferroresonance protection ?
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Open circuit where they can develop very high voltages.
 
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MMT77002

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Multiple shorting blocks on relay circuit

Multiple shorting blocks on relay circuit

mbrooke said:
I would but it needs to be so the relay can be taken out of service will keeping the other on line. Any idea how much impedance one relay holds? Or where I can find it?
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Correct me if I am wrong, but shorting the CT circuit at any location would not allow for a redundant relay scheme because there would be no CT input once the circuit is shorted. Redundant relay schemes to me always employ separate CT circuits...
 
mbrooke

mbrooke

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MMT77002 said:
Correct me if I am wrong, but shorting the CT circuit at any location would not allow for a redundant relay scheme because there would be no CT input once the circuit is shorted. Redundant relay schemes to me always employ separate CT circuits...
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There would be no CT input on the shorted relay, that being the one taken out of service. The other still gets input as its still looping through, the shorting is done near the relay.
 
mbrooke

mbrooke

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MMT77002 said:
Redundant relay schemes to me always employ separate CT circuits...
Click to expand...


In a sense you are correct here. Where genuine redundant relaying is required separate CTs are optimal as is the norm with 345 and 115kv bulk power systems. However, with things like medium voltage its not uncommon to have only one protection CT.
 
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GoldDigger

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MMT77002 said:
Correct me if I am wrong, but shorting the CT circuit at any location would not allow for a redundant relay scheme because there would be no CT input once the circuit is shorted. Redundant relay schemes to me always employ separate CT circuits...
Click to expand...
We have discussed two cases above. When low impedance (current sensing) meters are placed in series, shorting one of the meters will leave current flowing through the others. If one meter failed open all meters would lose their input signal.
When high impedance (voltage sending) relays are used they will be in parallel and no shorting should be done. If the last parallel meter is removed there should still be a protector in the circuit. If one meter fails open the others will continue to read.
 
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