2 INTRODUCTIONFunction of the protection relaysInformation to the protection relays have to be accurate and reliableCurrents and voltagesHow to overcome this difficulty
3 WHAT DOES CURRENT AND VOLTAGE TRANSFORMERS DO… Insulate secondary circuits from primaryProvide currents/voltages proportional to the primary
4 ROLE OF CT, PT IN POWER SYSTEM PROTECTION Protection systems depend heavily on information providedCTs and PTs a constituting part of the protection systemNo great difference between a measuring voltage transformer and a protective voltage transformerHowever, Current transformer is different
5 BASIC PRINCIPLES OF OPERATION Transformer can be operated inShunt modeSeries mode
6 SHUNT MODE Voltage applied across the primary terminals When secondary is on open circuit, Induces an EMF equal to the voltage appliedCurrent flowing will be that required to excite the coreWhen secondary is loaded Ampere turns of the primary winding will exceed that of secondaryDifference is the flux needed to excite the core.
7 SERIES MODE Primary winding is connected in series with a circuit. Its current is determined by the sources and loads.Its current is determined by its sources/load conditionsA component of this current is used to excite the core sufficient to induce an EMF to drive a current in the secondary
8 COMPARISON-SHUNT MODE AND SERIES MODE Voltage applied across the primary terminalsWhen secondary is on open circuit, Induces an EMF equal to the voltage appliedCurrent flowing will be that required to excite the coreWhen secondary is loaded Ampere turns of the primary winding will exceed that of secondary– Difference is the flux needed to excite the coreSeries modePrimary winding is connected in seriesIts current is determined by its sources/load conditionsA component of this current is used to excite the core sufficient to induce an EMF to drive a current in the secondary
10 CT ACCURACY If the CT is ideal Primary AT = Secondary AT Secondary current ∞ Primary currentEvery CT requires a certain AT to induce in it the magnetic fluxWith secondary impedance fixed exciting AT form a larger proportion at smaller primary currentsExciting current required to magnetize the core is the cause for th errors in CTs
11 AC PERFORMANCE OF A CT ES = 4.44 f A B 10-8 Volts ES = Secondary induced voltsf = Frequency in cycles/secondN = Secondary number of turnsA = Core cross sectional area in sq.cmB = Flux density in lines per sqcm.ES = Is( Zb + Zs + Zl)
12 CT MAGNETIZATION CURVE Exciting current is the source of errorsValue of exciting current depends on the core material and the amount of flux.Ankle point and knee point,
14 OPEN CIRCUITING OF CTIf the secondary of a CT is kept open with a current flowing in the primary, thenNo secondary currentNo MMF to oppose the fluxCore driven to saturationHigh rate of change of fluxHigh voltages
15 CT ERRORS Phase angle error Composite error Ratio error (KnI s ‑ Ip ) x 100/ IpKn = Nominal ratioI s = Actual secondary currentIp = Actual primary currentPhase angle errorAngle by which the secondary current vector when reversed differs from the primary current.Composite errorr.m.s. value of the difference between the ideal secondary current and the actual secondary current. Includes the current and phase angle errors
17 CT RATINGS Accuracy limits Limit up to which the specified accuracy could be retained. Accuracy Limit FactorRated short time currentr.m.s. value of the a.c. component of the current which a CT is capable of carrying for the rated time without being damaged by thermal or dynamic effectsCT Burdenthe load applied to the secondary of the CT. Normal practice is to express the burden in terms of VA and power factor, the VA being what would be consumed in the burden impedance at rated secondary current
18 CHOICE OF CT RATIO Secondary current 5 Amps or 1 Amp Primary current to be equal to or greater than the normal full load current of the protected circuitMaximum ratio of a CT is made about 3000/1 and secondary CTs of 20/1 are used.
19 TRANSIENT RESPONSE OF CT Transient conditions, prevail within the first few cycles of a fault.System conditions are very different to that under steady state conditions.Power systems expansions increases fault currents and time constants of dc transient components.It is mandatory for the protective equipment/circuit breakers to clear the faults within few cyclesThis makes CT duties more onerousHence, behaviour of CTs during transient conditions is of great importance.
20 TRANSIENT CONDITIONS IN A POWER SYSTEM Sudden application of a sinusoidal voltage to a series R/L circuit will cause a current to flow in the circuit.This is equivalent to a fault condition of a power system,