Table of Contents
What is ref CT?
Restricted earth fault (REF) protection is a sensitive way to protect a zone between two measuring points against earth faults. The CT secondaries are wired to cancel each other’s currents during through faults and to drive all secondary current to the relay when the fault is inside the protected zone.
What is class PS CT?
PS Class CT is also known as PX Class or simply Class X Current Transformer. It is special type of current transformer meant for protection purpose. Deviation in characteristics of CTs may lead to spill current in the secondary and even lead to actuation of protection element even under through fault condition.
Which class of CT is used for differential protection?
The PS class CTs are used for differential protection. The differential protection is also called the zone or unit protection.
What is 5P class in CT?
Limit of Error for Accuracy Classes 5P, 10P and 15P: This marking means that, the burden of CT is 30 VA and the maximum composite error at 10 times of rated current is 5%.
What is ref relay?
This relay protects the delta or unearthed star winding of the power transformer against the fault current. The connection of earth fault relay with the star or delta winding of the transformer is shown in the figure below.
Why we use Ref relay?
Fault detection is confined to the zone between the two CTs hence the name ‘Restricted Earth Fault’. REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs. If CTs are located on the transformer terminals only the winding is protected.
What is 0.2 S Class CT?
An 0.2s accuracy class means that the measurement has an error rate of 0.2% over a range of 20 to 120% of the nominal current (In) and at a specific accuracy above 1% of In.
What is class1 CT?
The class designation is an approximate measure of the CT’s accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important, especially in power measuring circuits.
What is RCT in CT?
Lm is the saturable magnetisation inductance, L is secondary of an ideal current transformer, RCT is resistance of the CT secondary winding, RW is resistance of wiring and RL is the burden i.e. the protection relay.
What is neutral CT?
Neutral Current Transformers (NCT) are common components in Electrical Distribution Systems. Resolution: A Neutral Current Transformer encircles the neutral conductor (or is placed in-line on a bus); required on circuit breakers with Ground Fault Protection when applied on a grounded system.
What is CT class 5P20?
Class 5P20: The letter ‘P’ indicates it is a protection CT. The number 5 indicates the accuracy of the CT. Most common. accuracy numbers are 5 and 10.
What is 5P10 class CT?
If you see the specification or name plate of a protection class CT, you will find that it is given like 5P10. This CT can be interpreted as a protection class CT having an accuracy of 5% over a current range of 10 times of normal primary current rating.
What’s the difference between A ref and a CT?
CTs are the eyes of a relay. A relay monitors the system condition through the CT. As mentioned in the article ” Restricted Earth Fault Protection Philosophy ” REF is basically a differential protection.
What should line CT ratio be for ref protection?
This requirement does not create any problem if the neutral point is solidly earthed. As per the picture beside say, a 11/0.433 kV 2.5 MVA transformer needs REF protection, having a solidly earthed Neutral. So line CT ratio shall be 4000/1A or 4000/5A.
What should line CT ratio be for solidly earthed transformer?
As per the picture beside say, a 11/0.433 kV 2.5 MVA transformer needs REF protection, having a solidly earthed Neutral. So line CT ratio shall be 4000/1A or 4000/5A. Since the neutral is solidly earthed earth fault current will be very high (fault impedance not considered), nearly to phase fault current.
Why do I need an extra relay for ref?
Only reason to use an extra relay is if the funcion is not incorporated (or there is some technical limitiation). I was told that I need to have a 3pole ( and not 4 pole) main incomer on LV switchboard to incorporate REF.