EE448-Electrical Power Engineering-lecture 13.pptx

electric power protection

1. EE448-Electrical Power Engineering Dr. Mona Ibrahim & Eng. Ghada Elboudy

2. Lecture (13) –current transformers

3. contents

4. Introduction to system protection Protection systems have three basic components: Instrument transformers Current transformers Potential or voltage transformers Relays Circuit Breakers (oil, air vacuum and SF6) SF 6 110 kV Current Transformer SF6 insulated Inductive Voltage Transformers

5. Introduction to system protection Protection systems have three basic components : Instrument transformers Current transformers Potential or voltage transformers Relays Circuit Breakers (oil, air vacuum and SF6) Relay A 110 kV CT SF6 circuit breakers

6. Introduction to system protection The figure shows a simple overcurrent protection schematic with: One type of instrument transformer—the current transformer (CT) An overcurrent relay (OC) A circuit breaker (CB) for a single-phase line . The function of the relay is to discriminate between normal operation and fault conditions. The OC relay has an operating coil, which is connected to the CT secondary winding, and a set of contacts . When I’ exceeds a specified ‘‘pickup’’ value, the operating coil causes the normally open contacts to close. When the relay contacts close, the trip coil of the circuit breaker is energized, which then causes the circuit breaker to open. Note that the circuit breaker does not open until its operating coil is energized , either manually or by relay operation. Based on information from instrument transformers, a decision is made and ‘‘relayed’’ to the trip coil of the breaker, which actually opens the power circuit—hence the name relay .

7. Instrument transformers There are two basic types of instrument transformers: Voltage transformers (VTs), formerly called potential transformers (PTs) Current transformers (CTs). The transformer primary is connected to or into the power system and is insulated for the power system voltage. The VT reduces the primary voltage and the CT reduces the primary current to much lower, standardized levels suitable for operation of relays.

8. current transformers The primary winding of a current transformer usually consists of a single turn, obtained by running the power system’s primary conductor through the CT core. The normal current rating of CT secondaries is standardized at 5 A in the United States, whereas 1 A is standard in Europe and some other regions. Currents of 10 to 20 times (or greater) normal rating often occur in CT windings for a few cycles during short circuits .

9. current transformers Standard CT ratios are given in Table

10. current transformers Ideally, the CT secondary is connected to a current-sensing device with zero impedance, such that the entire CT secondary current flows through the sensing device. In practice, the secondary current divides, with most flowing through the low-impedance sensing device and some flowing through the CT shunt excitation impedance. CT excitation impedance is kept high in order to minimize excitation current.

11. current transformers equivalent circuit An approximate equivalent circuit of a CT is shown in Figure Where: Z’ = CT secondary leakage impedance Xe = ( Saturable ) CT excitation reactance Z B =Impedance of terminating device ( relay including leads)

12. current transformers equivalent circuit The total impedance ZB of the terminating device is called the burden and is typically expressed in values of less than an ohm. The burden on a CT may also be expressed as volt-amperes at a specified current.

13. current transformers equivalent circuit Associated with the CT equivalent circuit is an excitation curve that determines the relationship between the CT secondary voltage E’ and excitation current Ie .