Power System Protection Prepared by: Loo Pin Likh Power System Protection: Objective • To describe the principle of protection in power system. • To understand the basic components of protection system. • To define the characteristic of protection system. Power Protection System: The Structure of Power System - Generation Subsystem Transmission Subsystem Distribution Subsystem Load Subsystem - Extra High Voltage – 500KV High Voltage – 275KV and 132KV Medium Voltage – 33KV & 11KV Low Voltage – 415/230V Power Protection System: The History Power Protection System: Why are power protection systems so important? • The power system’s equipment are expensive. Ex: Transformer, Switchgear, Busbar, transmission line and etc. • A symmetrical and unsymmetrical faults carry a high current. It can spoil the equipment in very short time. • Design, Coordination, Setting and Installation of protection system become inevitably compromised. • Power system protection plays an important that is to keep power system stable by isolating the faulty component and leaving the rest of network to operate normally. Power Protection System: Basic Components(1) • Potential Transformer/Current Transformer: instrument transformer, scale down from higher to lower value. The stepped down value is a safe limit value which can be easily for measurement and accurate signal for protective relay, wattmeter, voltmeter, var-meter and etc. • Example: Potential Transformer – 11000/√3:110/√3 , CL 0.5, 50VA Current Transformer – 1000/5A, 5P10, 15VA Power Protection System: Basic Components(2) • Protective Relays – A device which to analyse the signal from CT and will give a logical instruction to circuit breaker under faulty conditions. Power Protection System: Basic Components(3) • Circuit breakers/Fuses - The characteristic of circuit breakers are used to open and close the circuit. Fuses are self-destructive device and to isolate the downstream circuit. Power Protection System: Basic Components(4) • DC Batteries – Uninterrupted Power System, is a main or emergency power supply for control and relay device. Power Protection System: Basic Components(5) • Telecommunication – Tele-protection system, is a key technology of protecting the high voltage grid and monitoring transmission line system for precise coordination. Power Protection System: Type of Protection (1) • Main Protection Scheme: Protect a specific equipment of system, it operates instantaneously and selectively when faults happened in zone; the algebraic sum of current entering and leaving the node is zero, knows as unit protection. Example: i. Line differential protection ii. Transformer differential protection iii. Restricted earth fault iv. High-impedance busbar protection v. Voltage and frequency protection (1) (1): The relay provides DMT and instantaneous operation but not included differential feature. Power Protection System: Type of Protection (2) • Backup Protection Scheme – Protect a specific areas, also detecting adjacent zone within particular outside zone. These relays will be operated when the fault is not cleared or abnormal condition not detected, the time-graded protection is required to operate tripping circuit breaker. Example: i. Overcurrent & Earth fault protection ii. Distance protection iii. Standby earth fault iv. Breaker failure v. Thermal overload Power Protection System: Related Electrical Law to Protection system (1) • Branch Law i. Kirchhoff first law – The current law (Iexciting + Ientering = 0), the term of NODE in electrical circuit generally refer to a connection or junction two or more current carrying paths. ii. Kirchhoff second law – The voltage law, Any closed loop network, the total voltage around the loop is equal to sum of all voltage drops within the same loop. iii. A single or group of components, Resistors or a source, connected between nodes. Power Protection System: Related Electrical Law to Protection system (2) • Junction Law i. A junction, connection or terminal within a circuit two or more circuit elements are connected or joined together (Σ I = 0). Power Protection System: Related Electrical Law to Protection system (3) • Mesh Law i. This refer to mesh current analysis or nodal voltage analysis (Σ E = Σ IZ). The algebraic sum of all driving voltage in any closed path(or mesh) on a network is equal to the algebraic sum of all passive voltage (products of the impedances and currents) in components branches. Power Protection System: Related Circuit Theorem to Protection system (4) • Superstition theorem A network which is having more than one source. The superstition theorem is to solve the network two or more sources are connected. • Thevenin’s theorem (Maximum Power Transfer) Any linear circuit contains several voltage and resistance can be replaced by just one single voltage in series with a single resistance connected across the load. • Kennelly Star(Wye)/delta(Δ) theorem To simplify the analysis of wye-delta transformation, 3 terminals can be replaced by delta or star impedance equivalent without disturbing external network. Power Protection System: Type of Characteristic and Requirement (1) • Dependability This is a concern when faults occur within the protected zone. A dependability shall result in the designed speed and time. • Speed Minimum operating time to clear a fault in order to avoid damage to equipment when call is up. The occurrence of fault must operate instantaneously which contact is open • Reliability The quality of designed system shall supervise the circuit for a long time before a fault occurrence. The relays must respond instantly and correctly Power Protection System: Type of Characteristic and Requirement (2) • Discrimination To identify load and fault. It is the properties of selective tripping. • Sensitivity The leakage fault current or system’s abnormalities can be detected as the relay is operating correctly This is to detect the smallest value of fault current while operating in abnormal condition • Stability To leave all healthy circuit ensuring continuity supply Remain stable within defined operating scenarios and procedure Power Protection System: Type of Characteristic and Requirement (3) • Selectivity Tripping the circuit breakers in referring to timing or scheme’s coordination. Aims to maintain the continuity supply by disconnecting the minimum section of network.
