WHAT IS A PROTECTION ? Why it is required?

1. WHAT IS A PROTECTION ?Why it is required? Humans need protection from external disease , sun and whether. All the internal parts of the human beings are protected naturally. Heart and lungs are protected by ribs bones from external impact. Brain is protected by skull.Eyes are protected by eyelids and by closing eyes. We need cloths to protect us from external environment conditions. Similarly electrical equipment needs protection from any external or internal faults which may produce a detrimental effect on it.

2. What is protection? • The process of keeping (something or someone) safe and Secure • Electrical equipment are protected from abnormal conditions.

3. Different parameter to measure the fault • Voltage • Current • Power • Frequency • Capacitance • Inductance • Resistance

4. Any abnormalities of these parameter is fault Protective relays finds, calculates and locate the fault and display it and isolates the faulty circuit.To prevent any further damage to equipment.

5. Different protection is used to locate the fault • For voltage – Over voltage/under voltage protection • For current – Differential , over current and fuse • For power – Overload • For lines- Distance relay

7. PRIMARY RELAYING Generator Protection Low voltage SG Protection Power Trans. Protection High voltage SG Protection Transmission line Prot. High voltage SG Protection

8. A. TO MAINTAIN PROPER OPERATION IN ELECTRICAL POWER SYSTEM WE MUST HOLD TOGETHER THE VARIOUS AREAS OF THE POWER SYSTEM. DUE TO GEOGRAPHIC LOCATION OF GENERATING STATIONS PRACTICALLY ALL PRODUCED ELECTRIC ENERGY IS TRANSMITTED OVER TRANSMISSION & SUB TRANSMISSION LINES & THIS IS THE REASON WHY OPERATIONAL RELIABILITY OF THE TRANSMISSION NETWORKS IS OF VITAL IMPORTANCE. THE TRANSMISSION LINES ARE THE MOST WIDELY SPREAD PART OF THE POWER SYSTEM AND OVERHEAD LINES ARE THE LEAST PROTECTED FROM ENVIRONMENTAL INFLUENCES. THE NUMBER OF LINE FAULTS IS CONSEQUENTLY VERY HIGH COMPARED TO THE NUMBER OF FAULTS ON THE OTHER ELEMENTS OF THE POWER SYSTEM. THE RELIABILITY & SECURITY OF POWER TRANSMISSION LINE LARGELY DEPENDS ON PROTECTION SYSTEM. PHILOSOPHY OF PROTECTION

10. FUNCTIONALCHARACTERISTICS OF PROTECTIVE RELAYING • SENSITIVITY • SELECTIVITY • SPEED • REPEATABILITY • ECONOMY Any relaying equipment must be sufficiently sensitive so that it will operate reliably when required under actual condition that produce the least operating tendency.

11. FUNDAMENTAL PRINCIPLES OF PROTECTIVE RELAYING 1.Two groups of relaying is necessary a. Primary relaying – First line of defense b. back up relaying – functions only when primary relaying fails. 2. Primary relaying may fail because of failure in any of the following a. Current or voltage supply to the relays b. DC Supply c. Protective relaying d. Tripping circuit or circuit breaker mechanism e. Circuit breaker

13. Primary Relaying Generator Protection Circuit Breaker Low voltage SG Protection Power Trans. Protection High voltage SG Protection Transmission line Prot. High voltage SG Protection

14. NEED OF TL PROTECTION DUE TO GEOGRAPHIC LOCATION OF GENERATING STATIONS PRACTICALLY ALL PRODUCED ELECTRIC ENERGY IS TRANSMITTED OVER TRANSMISSION & SUB TRANSMISSION LINES & THIS IS THE REASON WHY OPERATIONAL REALIABILITY OF THE TRANSMISSION NETWORKS IS OF VITAL IMPORTANCE. THE TRANSMISSION LINES ARE THE MOST WIDELY SPREAD PART OF THE POWER SYSTEM AND OVER HEAD LINES ARE THE LEAST PROTECTED FROM ENVIRONMENTAL INFLUENCES.

15. Objective of Relay Protection • Protect persons and equipment in the surrounding of the power system • Protect apparatus in the power system • Separate faulty parts from the rest of the power system to facilitate the operation of the healthy part of the system

16. Electrical faults in the power system • Transmission lines 85% • Busbar 12% • Transformer/ Generator 3%

17. Fault statistics • Single phase to earth 80% • Two phases to earth 10% • Phase to phase faults 5% • Three phase faults 5% The probability of line faults caused by lightnings are 0 - 2.3 faults/ 100 km and year

18. Fault types • Transient faults – are common on transmission lines, approximately 80 - 85% – lightnings are the most common reason – can also be caused by birds, falling trees, swinging lines etc. – will disappear after a short dead interval • Persistent faults – can be caused by a broken conductor fallen down – can be a tree falling on a line – must be located and repaired before normal service

19. REQUIREMENTS OF PROTECTIVE RELAYS SENSITIVITY : The relay shall be sensitive to operate for minimum quantity of operating parameter. SELECTIVITY: The relay/scheme should be able to select the faulty section and isolate. SPEED : The relay should operate faster so that fault is isolated as fast as possible. RELIABILITY : The relay/scheme should operate for all types of faults with repeatability and reliability. COST :The relay/scheme should be economical .

20. Measuring principles • Overcurrent protection • Differential protection • Phase comparison • Distance protection

21. Ohms Law: • Voltage / Current = Resistance • In AC, this constant is called IMPEDANCE • Z = V / I • We had studied this in School. We thought Impedance is Constant. But, to understand the basics of protection, let us examine Impedance in detail.

22. Think of a sub station from where one transmission line is taken out to another sub station. Once the line is charged, we can monitor the voltage and current on this line through instrument transformers. V/I gives impedance. Can this be constant?

23. A transmission line is made up of aluminium conductors with a fixed resistance per km (R). Once the line is strung, the reactance (X) is also fixed (depends on the tower configuration, height from ground etc.). Impedance is R+jX . So this should be constant. So ….

24. No. How to measure this impedance? V/I gives only the Impedance seen from the sub station. This contains the load effect also. Hence, depending on the load variation, the impedance seen is going to vary. • Then why we should monitor V/I? • Due to load variation, V/I will vary in a limited Range.

25. During fault, like a tree touching the conductor, the Current will shoot up and voltage will collapse. Thus V/I will change rapidly. • Impedance Relays or Distance Relays work on this principle. When the impedance seen by the relay is reduced within the set limits, the relay operates and trip the Circuit Breaker. • E.g. YTG, RAZFE, Micro Mho, LZ96, Numerical Relays

26. For Protecting a transmission line, the best method is Impedance Protection. We can also use a current comparison method. In this method, the currents at both ends of the line are continuously compared at both ends (other end information through Power Line Carrier Communication), and in case of non-match, a trip is generated. (e.g. P40)

27. For protecting a transformer, we can compare the currents on High Voltage Side and Low Voltage side. It should match as per the transformer ratio. This comparison is used in Differential Protection. • In a shunt reactor, the currents on the HV side and Neutral ends are Compared. • E.g. RADSB

28. To protect a transformer or reactor, another important protection is “Restricted Earth Fault” (REF). The sum of current entering and leaving a transformer or reactor is zero. After paralleling all CT’s, the relay is connected across to monitor the spill current which occurs if there is earth fault within the protected area.

29. Transformer is also protected with Directional Over Current Relays where the power flow direction is also measured by monitoring current and voltage. • Reactor is also protected with Impedance Relay (Backup Impedance) where the winding impedance is monitored like a Line Distance Protection.

30. Another protection of transformer is Over Fluxing: Here the V/f ratio is monitored from the Bus Voltage. • In addition to these, transformer and reactors are having mechanical protections like : Pressure Release Device, Winding Temperature Indicator, Oil Temperature Indicator, Oil Surge Relay, Gas Operated Relay (Buchholz). A contact from these mechanical relays operate the trip relay to trip the circuit breaker.

31. Transformers are also having Alarms for Buchholz, WTI, OTI, Magnetic Oil Gauge. If the transformer is provided with “OF” (Oil Forced) ie., pumps, and “AF” (Air Forced) ie., fans, they are automatically switched ON and OFF through WTI contacts. The WTI is also wired for control room indication.

32. All electrical protections require Voltage or Current or Both. The current is taken from the Bay CT or Bushing CT or Line CT. To sum up the current, the main and tie bay CT secondary will be paralleled. • The Voltage for transformer protection is taken from Bus CVT. For the line, it is from Line CVT.

33. A “Zone of Protection” will be decided by the CT location. • 400KV CT’s are having 5 cores. 2 cores are used for Bus Bar Protection, 2 cores are used for Line / Transformer Protection and the centre one ie., Core 3 is used for metering. • CVT is having 3 cores : 2 for protection and 1 for metering.

34. The sub station has 2 sets of 220 Volts batteries. They produce DC-1 and DC-2. This supply is used for powering the relays. The relays generally work at 48 Volts. For this purpose, it uses DC-DC converters to step down from 220V to 48V. • 1 or 2 sets of 48V batteries are used to power up PLCC and similar equipment. • The entire protection and operation system covering circuit breaker Trip Coil 1, 2 , Closing Coil, Isolator opening and closing contactor coils etc., are permanently connected to ‘-’ ve of the DC

35. The ‘+’ ve for these are provided in a controlled way to get the desired operation. • E.g. When we hand trip a breaker, + is extended to the TC-1 and TC-2 which will operate the breaker. • When a protection operates, + is extended to correct breakers, PLCC panel etc.

36. The wiring in the sub station is most important to have correct trippings, avoiding open circuit of CT. • The protection system consists of Instrument Transformers – Wiring Scheme – Relays – Wiring Scheme – Breaker. • Out of these, CT is provided with 2 wires per secondary to avoid failure. In case of failure of Line CVT, Bus CVT voltage can be used.

37. The relays are mostly duplicated and working parallelly and independently. For line they are called Main-I and Main-II. • The circuit breaker trip coils are continuously monitored for its continuity. The breaker is having 2 trip coils which are acting parallelly and independently. The breaker readiness is further ensured by monitoring the air pressure, gas pressure etc.

38. Still if a breaker fails to operate due to any problem like mechanical etc., the Local Breaker Backup (LBB) or Breaker Failure Relay (BFR) will operate to trip other related breakers to clear the fault. The LBB monitors the circuit current for 200 milli second after a trip command to the breaker, and if the circuit current is persisting, LBB trips all connected breakers. This can lead to tripping of Bus Bar, other end of line etc.

39. To trip a line, it is required to trip Main breaker, tie breaker and other end Main and tie breakers. The other end tripping is achieved through PLCC codes. While tripping the second local breaker or during protection operation, PLCC panel will get triggering to generate the code and transmit to other end. Once the other end PLCC panel get the code, it will trip those breakers.

40. The distance relay require voltage and current inputs. Since the relay has got some processing delay, it require some memory to store the voltage before it can collapse. But if the line is charged to a fault, voltage may not build up at all. The distance relay may not get any voltage. There is one feature in distance relay : Switch On To Fault (SOTF). If the relay gets current and no voltage from idle condition, SOTF will trip the breaker.

41. The line is divided into Zone 1, Zone 2, Zone 3 for the distance protection. If the impedance set in the relay corresponds to 80% of the line, it is Z1. It will trip immediately. Z2 covers 60% of next line also. Z2 will operate after 0.5 sec. Z3 covers further lines and operate after 1.5 sec. • While tripping one end in Z1, it will send a PLCC code to other end. If the other end is picked up in Z2, on receipt of this code, it will trip immediately without waiting up to 0.5 sec.

42. A slow reduction in Impedance can happen during load changes due to fault in other lines. If this happens, it is called “Power Swing”. The distance relay can block its operation if it finds a power swing. This will allow temporary overloading of the line to save other systems.

43. What happens if the fuse of CVT fails? • The voltage to the distance relay collapses and still current is there. It can trip the line. To save the line, Fuse Failure Relay is used. FFR gets 2 sets of voltage from the CVT. One set is given to the distance relay also. If only one side voltage is absent in the FFR, it will block the operation of the distance relay.

44. REQUIREMENTS OF PROTECTIVE RELAYS SENSITIVITY : The relay shall be sensitive to operate for minimum quantity of operating parameter. SELECTIVITY: The relay/scheme should be able to select the faulty section and isolate. SPEED : The relay should operate faster so that fault is isolated as fast as possible. RELIABILITY: The relay/scheme should operate for all types of faults with repeatability and reliability. COST: The relay/scheme should be economical .

45. BUS BAR PROTECTION Bus bar is subdivided into sections, each of which is separately protected. A fault in one section does not involve the tripping of the complete station. Important load can be supplied from the healthy sections, and will not suffer any interruption of supply if one section is tripped. Bus bar protection is stable and free from thro’ faults.

46. SUMMARY ON SELECTION OF PROTECTION The protection system for transmission lines depends on several factors such as: • Network earthing • System requirements on -- speed of fault clearing -- Selectivity -- Dependability -- Security • Existing protection system in the network • Requirement of additional functions as -- fault location -- event recording -- Fault recording -- remote communication with the protection • Type of communication scheme

47. SYNCHRONIZING SCHEME CHECK SYNCHRONIZING RELAY: Generally used in auto re closing sequencing of inter connector along with auto re closing relay. The relay contacts are connected in series with the circuit breaker closing circuit and ensures that the differences in phase voltage and frequency are with in pres selected limit before closing up two section of a power system .

48. V.T. SUPERVISION: The Ac voltage connection and circuits of distance relays are normally protected by fuses. The voltage transformer supervision unit uses filters o derive the NPS (negative phase sequence) components of the line voltage and line currents, with level detectors to determine whether these components are above the set levels. Under healthy system conditions, both NPS voltage and NPS current levels are below the set levels and the VTS unit does not operate. An unbalanced fault on the primary transmission systems causes both NPS voltage and NPS current to be above threshold and again the VTS unit does not operate. Only an unbalanced fault in the voltage transformer circuit causes NPS voltage above setting without detectable NPS current and cause VTS to operate. In case of all the three fuses getting opened, VTS will not operate. TO overcome this eventuality, a separate voltage sensitive circuit is connected across one of the fuses.

49. AUTOMATIC SWITCH ON TO FAULT PROTECTION The switch on to fault system detection is activated by using an external breaker closing signal. The breaker close signal is used in applications where the voltage transformers are placed on the bus side of the line circuit breaker. When activated the switch on to fault protection will stay activated for some fixed duration (say 1 sec). Faults occurring during this period will be tripped instantaneously. Faults occurring after this fixed time period will be measured in the normal way.

50. STUB PROTECTION When a line is supplied via two circuit breakers in a 1-½ breaker bus arrangement, the line protection includes the area between the two CT’s. However, when the line isolator is open, the line CVT’s for the distance protection are connected to the line and cannot provide the correct voltage for the stub end, the area between the line isolator and CT’s. To provide protection for a fault in this area, stub protection is provided which gives an over current trip if the line isolator is open and the current exceeds the set value in each phase. Separate DC logic is provided for connection of line isolator auxiliary contact.