Digital power system protection basic lecture

Digital power system protection

1. Digital Protection of Power System 1

2. Why Electrical Energy? Electrical Energy is neither available directly nor utilize directly. Even though several forms of energies exist, we prefer electrical energy over othe r form of energy Electric Energy is the most versatile form of Energy. Flexible Operation and control Efficient Transmission Convertible Clean Energy 2

3. Typical Power System 3

4. Current Flow in the Undesired Path 4

5. F aults P ossibilities - current flows in unintended or undesirable path or breaking of conductors. Undesired path - very less impedance - compared - impedance - supplying the load - heavy current flow through that unintended path, which we call it as power system fault or opening of conductors. The fault is an abnormal condition – result in increased current or voltage depending on the fault. A fault/fay-free power system is not technically feasible 5

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7. Fault Statistics Percentage distribution of Faults in various elements of a power system 7 Element Percentage of Faults Overhead Lines 50 Underground Cables 9 Transformers 10 Generators 7 Switchgears 12 CTs, VTs, Relays Control Equipment etc. 12

8. Types of Faults Unsymmetrical Faults Single Line to Ground Fault Double Line Fault Double Line to Ground Fault Open Circuit Fault Winding faults – turns on the machine windings Symmetrical Faults Triple Line Fault (Three Phase Faults) Triple Line to Ground Fault Simultaneous Faults Short Circuit Fault (Shunt Fault) Open Circuit Fault (Series Fault) 8

9. Frequency of Occurrence of different types of faults on Overhead line 9 Types of Faults Percentage of Total Faults Line to Ground 85 Line to Line 8 Double Line to Ground 5 Three Phase 2

10. R easons for f aults The two general reasons are Breaking of Conductors Failure of Insulation – overvoltage due to lighting or switching surges – mainly occurs in transmission and distribution lines. Other reasons are Mechanical failure Accidents – due to weather conditions, falling of trees, animal intervention Excessive internal stresses Excessive external stresses 10

11. F aults Transient (Non-Permanent Fault) Flashover – across line insulator Arc – between two phases Die out in shorter duration i.e. within very few cycles Permanent Fault Remains in the system for longer duration Permanently damages the electrical equipment 11

12. E ffects of f aults Equipment damage – heat generated – I 2 R t Fire - Unsafe to human Interruption to continuity of Power Supply Unstable Reduced Voltage Magnitude – unbalance in voltage 12

13. W hy P rotection? Fault is inevitable Essential to protect the power system from the adverse effects of faults Power system Protection – deals with detecting and isolating the faulty section from the healthy sections. Protective system comprises of Transducers (Current Transformers and Potential Transformers) Circuit Breaker Protective relays 13