What is differential CT?
A differential protection monitors an area limited by CTs which measure incoming and outgoing currents. If the outgoing currents are not consistent with the incoming currents, this is normally because a fault has occurred in the protected area.
What class of CT is preferable for differential protection?
PS class CTs
The PS class CTs are used for differential protection. The differential protection is also called the zone or unit protection.
What is 64R protection?
Rotor earth fault relay 64R which is used to protect the generator or alternator rotor against earth fault (the short circuit between rotor’s winding to ground). It senses the alternator’s rotor earth leakage current. The ANSI code for rotor earth fault relay is 64R.
What is 59N protection?
The 59N relay will detect and protect the generator for earth faults over about 95% of the generator winding. The chosen 59N relay needs to be chosen so as to not react to third harmonic voltage generated during normal service.
How do I choose CT for differential protection?
These CTs are selected based on the amount of current they are expected to measure up to certain value of current, during fault conditions, in which they can measure without loss of accuracy. In their application to transformer differential protection, CTs are usually selected based on the full load rating of circuit.
What is difference between PS and PX class of 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. Various protection class CTs as described in IS 2705 part-3 is only useful for overcurrent protection. You may refer Accuracy Class of CT for detail.
Why do we use neutral CT in a transformer?
A neutral current transformer is used as earth fault protection to measure any fault current flowing through the neutral line from the wye neutral point of a transformer.
Why NGT is used in generator?
NG cubicle is used to install Neutral Grounding Transformer (NGT) and Neutral Grounding Resister (NGR) for grounding/earthing of neutral current during unbalanced three phase load of a generator, essentially required for protection of the generator winding.
How do you size a CT?
Consider a CT with the following specification and protection requirements:
- CT: 600/1 5P20 15 VA, Rct = 4 Ω
- CT Leads: 6 mm2, 50 m long. – use R=2 ρ l /a to calculate = 0.0179 Ω/m.
- Relay: Siemens 7SJ45, Ktd = 1.
- Short circuit current, Iscc max = 30 kA.
What is PS class CT?
The PS class CT is special protection class CT. Both the PS class CTs used for differential protection have identical saturation characteristics and their knee point voltage is the same.
What is burden of CT and PT?
Burden of CT can be specified as Volt-Ampere absorbed at certain Power Factor i.e the VA that can be consumed by the load. The burden can also be expressed as total Impedance in terms of ohms connected on secondary of CT i.e. pilot conductor and instrument burden (I2 x R=VA).