Basics of CT and PT

CT

Current Transformer (CT) (CT)

used to measure or monitor the current in transmission lines and to isolate the metering equipment and relay connected to secondary side

CT is

Voltage Transformer or Potential Transformer (PT) Voltage PT is

used to measure or monitor the voltage on transmission lines and to isolate the metering equipment from the lines

• For safety purpose the secondary should be completely insulated from the high voltage primary and should be in addition grounded

CT Precaution • Ammeter resistance is very low ,the current transformer  normally works short circuited. • If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short circuited with the help of short circuit switch . • If this is not done, then due to high m.m.f. will set up high flux in the core and it will produces excessive core loss which produce heat and high voltage across the secondary terminals Hence the secondary of current transformer is never left open.

Measuring CT and protective CT

• Two major differences: • 1. Accuracy class: Metering needs high accuracy at load current while Protection need not have that high accuracy. • 2. Safety Factor: It determines the multiplicity of  load current at which the CT saturates. Metering CT should have lesser value so that it saturates before the meters can get damaged due to some abnormality. Protection should have high value since its main purpose desires the CT to withstand high fault currents to be sensed back to the relays.

Measuring CT and protective CT • There is difference between the protection CT and metering CT in operating conditions The CT using for protection will have to carry the fault currents which are 10 times the normal full load current It is designed at much bellow the saturation point (knee) in order to avoid saturation. • The CT used for metering will have to carry only full load current • it is designed near to knee point. as per accuracy concern the metering CT should be more accurate under normal operating condition. i.e under normal operating condition the protection CT accuracy not important

CT / PT • A CT will typically have a toroidal core and evenly distributed secondary windings so as to minimize leakage reactance. The primary is typically the main power line conductor, which passes directly through the toroidal core. • This type of transformer is specifically for the purpose of measuring current values, and the secondary windings cannot be left open-circuited, or a large voltage will be produce, resulting in dielectric failure (and often an explosion). • If a device is not connected to the CT, its secondary must be short-circuited.

CT / PT • A PT is connected between the main conductor  c onductor  and ground and can be either wound in the normal way, or  • the voltage can be taken from a subsection of a string of capacitors (this is called a Capacitive Voltage Transformer or CVT, and is usually cheaper than the wound type, but is typically not as accurate). • This type of transformer measures voltage values, and the secondary winding cannot be shortcircuited, as this will produce excessively high currents, resulting in the failure of the PT or the wires it is connected to. • A PT can be left open-circuited

Current transformer  • P is designation for protection. 5P10 means that the CT will have max of 5% inaccuracy when the current of 10 times rated current of CT is experienced. M is designation for metering. 0.5M means that the CT max inaccuracy is 0.5% • The CT accuracy class for 5P10 transformers per IEC Std 60185 is as follow: 5 = max. permissible total error, P= stand for protection and 10= Accuracy limit factor (ALF).  In

addition to the American instrument transformer  tr ansformer  standard, ANSI C.57.13, enclose are a few popular  standards used for CT :

Composite error 

Protective CT • Large Cross sectional area (should not saturated at Max fault current) • Saturation flux density is high • Cold rolled grain oriented silicon steel use as magnetic material • CT Core have High Knee point Voltage( B-H curve ) • Faithful reproduction of primary current right up to Max fault current • Accuracy is not important as metering CT • Rarely necessary to determine phase angle error for protective CT • Errors 10% are tolerable for IDMT over current relay • Errors 3% to 5% are tolerable for differential relay

Measuring CT • Error of measuring CT should be 0.5% to 1% • CT must not faithfully transform the fault current ( otherwise measuring instrument will be damaged ) • Small cross sectional area • Nickel iron core • Saturation flux density is less