Why CT is used in substation?

A Current Transformer (CT) is used to measure the current of another circuit. CTs are used worldwide to monitor high-voltage lines across national power grids. A CT is designed to produce an alternating current in its secondary winding that is proportional to the current that it is measuring in its primary.

Why CT and PT are used in substation?

A CT lowers the current signals for measurement purposes, while a PT lowers high voltage values into lower ones. The transformers are designed to measure whether power systems are both accurate and safe. In addition, the CT and PT transformer reduces the current and voltage from high to low value.

Which CT is used in substation?

Outdoor Current Transformers

Outdoor type transformers are used in high voltage circuits like substations & switchyards. These are available in two types namely oil-filled & SF6 gas insulation.

Why CT is placed after breaker?

Because it is designed as a current transformer and acts a current source. It has a high output impedance, which means that the output current is directly proportional to the current in the primary (often a single turn - a conductor passing through the toroid).

Why CT ratio is important?

CT Ratio. The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of 300:5 is rated for 300 primary amps at full load and will produce 5 amps of secondary current when 300 amps flow through the primary.

What is Current Transformer(CT)| Practical Knowledge in substation| How does CT work|

Why are CT circuits grounded?

The grounding of current transformers is important to both safety and the proper operation of the protective relays. To assure the safe and reliable operation, the neutral of the current transformer secondary should have a single ground location for each circuit.

Why CT S2 is earthed?

To prevent the secondary circuits from attaining dangerously high potential to ground, these circuits have to be grounded. Connect either the S1 terminal or the S2 terminal to ground.

What is CT ratio?

The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of 300:5 is rated for 300 primary amps at full load and will produce 5 amps of secondary current when 300 amps flow through the primary.

Why CT secondary is shorted?

The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.

Why is PT connected in parallel?

Why does potential transformer is connected in parallel? Because it is used to measure potential between the lines it is connected to. It's as simple as te working of voltmeter in parallel . It has very high impedance and so draws less current thereby giving you the voltage between the lines.

What is the work of CT?

A computerized tomography (CT) scan combines a series of X-ray images taken from different angles around your body and uses computer processing to create cross-sectional images (slices) of the bones, blood vessels and soft tissues inside your body. CT scan images provide more-detailed information than plain X-rays do.

How does CT measure current?

Handheld Current Transformers

Clamp meters open and close around a current carrying conductor and measure its current by determining the magnetic field around it, providing a quick measurement reading usually on a digital display without disconnecting or opening the circuit.

Why CT secondary is 1A or 5A?

The standard CT secondary current ratings are 1A & 5A,The selection is based on the lead burden used for connecting the CT to meters/Relays. 5A CT can be used where Current Transformer & protectives device are located within same Switchgear Panel. 1A CT is preferred if CT leads goes out of the Switchgear.

What is purpose of Buchholz relay?

The Buchholz relay purpose is to give the protection to a transformer from the different faults happening in the transformer like the Short circuit, inter-turn, core, incipient, etc. This relay will sense these faults and shuts the alarm circuit. The Buchholz relay diagram is shown below.

What happens if CT is open circuited?

If CT is kept open-circuited, no current will flow in the secondary, and consequently, no secondary flux will be set up in the core. In absence of secondary current, the net flux will be higher than the flux generated in the core when CT is connected to load.

Why CT is step up transformer?

A CT will have very few turns ( in very high power circuits only one turn) on the primary and a large no of turns on the secondary. In principle it is a step up transformer.

What happens if CT polarity is reversed?

Otherwise (polarity reversed), the devices starts malfunctioning, i.e energy meter shows negative readings, relay may read negative current hence relay may activate the trip circuit, reverse power relay activation is alternator or generator circuits etc.

Why is PT open circuited?

Since, the power (P=VI) in a transformer (input and output) is same, the current rises to a very high level. Thus, a very high resistance is maintained at the secondary terminal to limit the current (which appears as open circuit)... Short circuiting the secondary would burn out the windings.

What is the output of CT?

The output voltage of the CT should be 333 mVac when the full-scale current rating of the CT is flowing in the conductor.

What is S1 and S2 in CT?

Terminals S1/S2 (k/l)

The connections of the primary winding are designated "K" and "L" or "P1" and "P2", and the connections of the secondary winding are designated "k" and "l" or "S1" and "S2". The polarity must be established such that the "flow direction of the energy" runs from K to L.

How is CT accuracy calculated?

In the above-given data, CT is 5P20 with CTR 200/1. The ALF is 20. CT accuracy class is 5. It means if 20 times of the rated primary current(4000 amperes) flows in the primary at the time of fault, the CT reads the current with 5 % accuracy.

Why fuse is not used in CT?

For CT primary is just a current carrying straight conductor passed through coil which is secondary. Secondary generates feeble voltage so don't require fuse protection.

Why CT secondary is star connected?

In potential transformer the primary and secondary windings are star connected, because in star configuration, the line voltage is root 3 times of phase voltage, hence the voltage distribution to each winding is reduced or divided by 1.732 times. Due to that the insulation cost on the primary winding comes lesser.

What is P1 & P2 in CT?

CTs are marked with P1 and P2 to indicate which way they should be fitted around the cable or buss-bar. The side marked P1 must point towards the generator, and P2 must point towards the load. If an arrow is printed on the CT it must point towards the load.