InstrumentTransformers CONTENTS Introduction Theory of Current Transformer Selection of Current Transformers Theory and Selection of Voltage Transformer Conclusion Introduction Ears. Nose & Skin CTs VTs CTs.Protection System Analogy Brain Relay Eyes. VTs Hands & Legs g Circuit Breakers 4 . Protection System Analogy Fault in the Power System Fault in the Power System Sensed by Instrument Transformers & communicated to Relay Relay Issues Trip Command To Breaker Breaker Trips & Clears Fault 5 . Instrument Transformers • A Vital Part of the Protection and Meteringg System y • Instrument Transformer transforms the high current or high voltage connected to their primary windings to the standard low values in the secondary within the required accuracy limits which feed the metering and protection t ti apparatus t • Provide insulation against High voltage (isolation) • Protect personnel and apparatus from high voltages • Provide possibilities of standardizing the relays and instruments 6 . Classification of Instrument Transformers • Based Based on application‐ on application‐ • Metering • Protection • Based on use‐ • Indoor I d • Outdoor • Types of Instrument Transformer T fI T f • Current Transformer (CT) • Voltage Transformer (VT) • Electromagnetic Voltage Transformer (EVT) • Capacitive Voltage Transformer (CVT) 7 . Theory of y Current Transformers . e. such as. 1000A/1A (CT ratio) i. transforms currentt from f th level the l l off 1000A into i t currentt off 1A level l l 9 .What is Current Transformer • Direct use of high current (in the tune of 100A or more) is not possible as protective relays and metering devices are not designed to handle such huge amount of current • Current Transformer is an instrument transformer which transforms current from one level to another level. Why Current Transformer is required • SSystem t h two has t b i requirements basic i t ¾metering of energy sourced or consumed ¾Protection of the electrical system from faults and disturbances • Types of Current Transformer (CT) • Measuring CTs • Protection CTs p pp • Protection CTs for special applications 10 . that current creates a MMF which results in a alternating flux in the core. or linked with. which in turn induces an EMF in the primary winding and in any other winding wound on. the core 11 .Current Transformer Theory • When alternating current flows in the primary winding. How Current Transformer is connected • It has a primary winding and one or more secondary windings wound on core of magnetic material • Metering and Protection devices are connected to the secondaries of the CT • Primary winding connected in series and transforms the line current to the standard 1A or 5A suitable for the meter or relay P R IM ARY W IN DI N G O R B U S H IN G M AG N E T IC CO RE S E C O N D AR Y W IN D IN G 12 . the primary winding is connected in series with the power system whose relatively high impedance determines the magnitude of primary winding current which is independent of the secondary winding load • The current transformer has assigned rated output termed as burden in VA which are invariably small as against the high outputs in KVA or MVA of power transformers 13 .How Current Transformer is connected • IIn current operation i or series i mode. Current Transformer Theory • FFor a short‐circuited h t i it d transformer t f th following the f ll i relation holds good ‐ Primary Ampere turns (I1N1) = Secondary Ampere Turns (I2N2) I1 N2 = I2 N1 • An ideal current transformer is a short‐circuited transformer where the secondary terminal voltage is zero and the magnetizing current is negligible • The voltage across the secondary is very small. It is minimum when the secondary is short circuited and maximum when open circuited . Current Transformer Theory • Simplified CT equivalent circuit Is ≠ Ip Ip Zp Zs Ie Es Vp Im Iw Vs ZB∠φ . Current Transformer Theory Phasor Diagram 16 . Current Transformer Theory • Primary current has two components. first is secondary current which is transformed in inverse of the turns pp the eddyy & ratio and an excitingg current which supplies hysteresis losses and magnetize the core • Exciting current is not transformed and causes errors • The exciting current determines the maximum accuracy that can be achieved with a current transformer Ip = Ie + Is . or Is = Ip ‐ Ie . The error in amplitude is called currentt or ratio ti error and d the th error in i phase h i called is ll d phase error or phase displacement 18 .Current Transformer Theory The error in the reproduction will appear both in amplitude and phase. Is Secondary Is Kn = Ip Is 19 .Current Transformer Theory Ip Primary Ip Kn δ: Phase error Kn. 4 minutes) • the current error is positive if the secondary current is too high. and the phase error is positive if the secondary current is leading the primary 20 .Current Transformer Theory • Since δ is a very small angle.572 dgree = 34. angle the current error ε and the phase error δ could be directly read in percent on the axis ( δ = 1% = 1 centiradian = 0. if we get 0.99‐1)/1 x 100= ‐ 1% and phase error of 15 minutes 21 . for example.99 amps in secondary leading primary current by 15 minutes (0. p for a CT with CT ratio of 1000/1 amps. which introduces ratio error as well as phase error • What is inaccuracy? • The secondary current which we get is not true reflection of its p primaryy current.Current Transformer Accuracy Why h at allll CTs are inaccurate?? The culprit is core loss and magnetizing current.25 degree) for primary current of 1000 amps. amps so the CT has ratio error of (0. Current Transformer Theory • Th The exciting i i current Ie introduces i d ratio i error. which hi h is i defined d fi d as the difference in magnitude of the primary and secondary current expressed as percentage of primary current Current ( Ratio) Error = K n .I s − I p Ip × 100 Kn= Rated transformation ratio Ip = Actual A t l primary i currentt Is = Actual secondary current • The Phase angle error is the phase angle difference between the primary current and the reversed secondary current vectors . m. value of the difference between the instantaneous values of the actual primary current.Current Transformer Theory • Composite C it error • • Under steady‐state conditions.s. and the instantaneous values of the actual secondary current multiplied by the rated transformation ratio.m.m. value of the primary current ip is the instantaneous value or the primary current is is the instantaneous value of the secondary current T is the duration of one cycle 23 . integrated over one cycle including the effects of phase displacement and harmonics of excitation current C Composite it error is i generally ll expressed d as a percentage t off r. value l off primary current according to the formula 100 1 2 (Knis − ip ) dt εc = ∫ Ip T 0 T Kn is the rated transformation ratio Ip is the r.s. the r.s. produces an increase of 50% in the exciting current 24 .f.Knee Point Voltage (KPV) • Knee Point Voltage ‐ The point on magnetizing characteristic (plot between secondary applied voltage and the corresponding magnetizing current) at which an increase of 10% in exciting e.m. use conductor resistance ((total to the device and back) for Z 3.Burden of Current Transformer • Burden The external load (e.. relay l etc)) Burden B d ‐ Furnished F i h d by b the manufacturer 2. relays etc) connected to the secondary of a CT is called the burden The burden can be expressed p in volt‐amperes p or in ohms VA = I2 x Z Z = Total CT secondary impedance I = Secondary current (Generally 1A or 5A) • Total burden is the sum of 1 Device 1.g. Burden of Interconnecting Leads ‐ can be calculated by using the above formula. D i (transducer. transducers. ( d meter. meters. Internal Burden of CT Windings ‐ This is so small that it can generally be ignored or specified by manufacturer 25 . Magnetization Curve • The excitation of CT depend on a) Cross‐sectional area b) Length of magnetic path of core c) Number of turns in the winding d) Magnetic characteristics of the core f h 26 . IIp. e m f sufficient to drive the secondary current through total impedance of secondary circuit Hence core flux density is dependent on the magnitude of primary current and the impedance of secondary circuit Es is decided by the total burden Es= Total burden (VA + lead burden + sec.Current Transformer Theory Basic Induced Voltage equation Basic Induced Voltage equation Es = 4. IIs Ns= NpIp/Is 27 .f. N i d id d b th ti i Np.m. winding burden) * Is Ns is decided by the ratio i.e.44 * Bm * Aeff * f * Ns where Bm ‐ flux density = ϕ where B flux density = ϕm/Aeff Aeff ‐ Core effective area f ‐ frequency Ns ‐ secondary turns secondary turns Es ‐ Induced voltage in the secondary • • A component of primary current excites the core to the flux density necessary to induce in the secondary winding an e. Current Transformer Theory At constant burden.44 * Bm * f * Ns) Higher core area is required for • better accuracy (lower Bm . ) • and higher burden (higher Es) 28 . lower Ie‐ Excitation current ). thus resulting in excessive current error Bm is decided by the required error. Lower the Bm lower is the excitation current yielding better accuracy but larger core area b tl Rewriting the equation Aeff= Es/( /(4. a point is reached when core material start saturating and exciting current becomes excessive. • lower amp‐turns (lower N lower amp‐turns (lower Ns). hence. Lower B is decided by the required error Lower Bm for better for better accuracy. A b d core flux fl density d i varies i directly di l as the h secondary current. as the primary and consequently secondary current increases. ..f.m.f.m.f. Es of high peak value in the secondary winding 29 . (ampere turns) • With secondary open circuited. p which drive core into saturation on each half wave of the current • This Thi high hi h rate t off change h off flux fl in i the th region i off primary i currentt zero induces an e.F.f. induced in secondary winding is that required to drive secondary current through total impedance of secondary circuit. is provided by a small difference between primary and secondary m.m.m. there are no secondary ampere turns to oppose those due to primary current and whole of primaryy m. act on the core as an excessive excitingg force.M. and core flux inducing this e.Effect of Secondary Open Circuiting • E. but also to life 30 . even higher voltages would be induced and not only constitute hazard to i l ti off CT itself insulation it lf and d connected t d instruments. i t t relays l and d associated wiring. peak value may be as low as few hundred volts in small measuring CT with 5A secondary winding. say / p protective CT with a large g core section 2000/1A • With system fault currents flowing in primary.Effect of Secondary Open Circuiting • With rated primary current. in the case of. but it might reach many kilovolts. Selection of Current Transformers . g. (e g indoor switchgear cubicles with closely located relays) • Preferred where primary current ratings are very high • Comparatively low peak voltage when secondary gets open • Fine turns ratio adjustment j is not p possible when p primaryy rating is low – 1A Secondary •P Preferred f d when h CTs CT are outdoor td and d lead l d burden b d are high hi h • Comparatively high peak voltage when secondary is open • Fine turns ratio adjustment possible 32 .Current Transformer Secondary Rating • Choice of CT secondary rating – 5A Secondary • Preferred where lead burden is insignificant (e. the errors of which remains within specified limits under prescribed condition of use 33 . we desire that ¾ whatever current we measure. while protective CTs required to maintain the accuracy up to several times of the rated current • Metering if we want to measure current for metering purpose. that should be very accurate as the metered data may be used for tariff purpose • Accuracy Class A designation assigned to a current transformer.Current Transformer Accuracy • Measuring CTs are required to be accurate over normal working range of current. 10P.2 class : Laboratory class 0 5 class : industrial metering 0.0 class : First grade indicating wattmeter 3.5 class : industrial metering 1.Classification of Current Transformer • Metering Class CTs 0.0 & 5.0 class : For general use/WTI l l / • Protection Class CTs – 5P.1 class : High precision testing 0. 15P – PS class PS class 34 . i ) ISF (Instrument Security Factor) Example: 2000/1.2. 0.0. 5. 3. A Accuracy class. 2000/1 Class 0. 1.2. 20VA ISF – 5 • Standard Error Class – 0. 0.1. 0 2 20VA.0 • The errors are specified between 5‐120% of rated current and 25‐100% of rated burden connected • Higher errors are permitted at lower currents 35 .5.0.Measuring Current Transformer • Designation D i ti off Metering M t i CTs CT Metering CTs are specified in terms of – R i Ratio. l B d Burden (VA rating). 5 36 .35 02 0.2 0 75 0.5 0.75 0 35 0.Current Transformer Accuracy Limits Metering Cores • IEC 60044 60044‐1 1 Limits of error for accuracy Class of metering cores Class 5% of rated t dI 20% of rated t dI 100% of rated t dI 120% of rated t dI 02 0.2 02 0.2 0.5 0.5 1.75 0. Current Transformer Accuracy Limits • IEC60044‐1 has laid down standards on this 37 . when primary current is many times higher than the rated value. the core should get saturated • For this purposes. Instrument Security Factor (ISF) for Metering CTs has been defined • The CT cores should be such that it saturates at its instrument security factor (ISF) for safeguarding the instrument from getting d damaged d under d fault f lt currentt condition diti • ISF is defined as the ratio of rated instrument security primary current to rated primary current • ISF is expressed as 3.7 or 10 (it shall be chosen as small as possible) 38 .5.Instrument Security factor (ISF) • Th The instruments i connected d to the h secondary d off a CT should h ld be b protected from getting damaged during primary fault condition. Protection Current Transformer • Protection Class • During fault condition. if CT is not properly l designed. value of primary current may be 10 to 20 times the rated primary current • Here. would not make right decision 39 . therefore. d i d it may saturate t t and d relay l will ill receive very less current and. main requirement is ability of CT to faithfully transform f the h primary i current during d i fault f l condition di i • At such high level of primary current. 30 – VA rating 5. 10P. 10. 20. 5P20 10VA • Standard Error Class/ALF/VA rating – Error Class 5P. 15P – ALF 5. 15. 25.Protection Current Transformer • Designation D i ti off Protection P t ti CTs CT Protection CT are specified in terms of – R i Accuracy Ratio. 200/1 5P20. 10. 30 • The errors are specified at rated current and ALF times rated current with rated burden connected 40 . Burden i ) ALF (Accuracy (A Limit Factor) Example: 200/1. A class. 15. l B d (VA rating). .Protection Current Transformer . 8 18 centiradians ±5% 5% 10P ±3% 3% - - ±10% 10% 42 .Current Transformer Accuracy Limits Protection Cores • BS 3938:1973 Limits of error for accuracy Class 5P and 10P Accuracy Class Current Error at rated Primary Current Phase displacement Composite Error error at rated at rated accuracy Primary Current limit (ALF) Primary Current 5P P ±1% 1% ±60 60 min i ±1. saturate resulting in increase in secondary current error • Protection Class CTs cores should not get saturated below its A Accuracy Li iti Factor Limiting F t (ALF) up to t which hi h the th primary i currentt should be faithfully transformed to the secondary side. protective CTs are usually required to maintain their ratio up to several times the rated primary current • At some value of primary current above the rated value. core commence to saturate. maintaining the specified accuracy • ALF is defined as the ratio of the rated accuracy limit primary current to the rated primary current 43 . CT which hi h are required i d to be b accurate over the normal working range of currents.Accuracy Limiting Factor (ALF) • U Unlike lik measuring i CTs. Protection Current Transformer • For a given CT. Phase fault protection etc. Inverse relay. for example. earth fault protection. • While selecting 5P10 class CT for IDMT O/C or Earth fault relays – CT should have optimum ALF/VA rating. if connected burden is less than rated then ALF would increase • Applications of this CT are Over current relay. VA and ALF are inversely related. so that they do not saturate up to at least 20 times current rating (either by selecting low burden relays or by selecting a ratio of appropriate high value) – Over rated CTs having high VA rating and ALF may produce high secondary currents during severe faults (in excess of 20 times setting) that may cause thermal stressing of relay current coils and eventual failures 44 . restricted earth fault etc. PS Class.25% which helps in protection system y duringg maintainingg balance between the p maximum through fault condition 45 . Vk > 200V. RCT < 2.Protection Current Transformer • Designation off Protection CTs for f speciall applications l For protection like circulating current differential. etc where balanced of current/turns is required between associated CTs with close tolerance Special class Protection CT of are specified in terms of – 1) 2) 3) 4)) 5) Ratio R ti Accuracy class Knee Point Voltage (Vk) CT Secondary d winding i di resistance i ( CT) corrected (R d to75OC Excitation current (Ie) usually at Knee Point Voltage or a stated percentage thereof Example ‐ 200/1.0 ohms. Ie < 30mA at Vk/4 • The turn ratio error are limited to +0. Outdoor Current Transformer • Outdoor CTs are basically of 3 types of Construction – Dead Tank with U (Hair Pin) shaped primary – Dead Tank with Eye Bolt primary – Live Tank or Inverted primary CT Live Tank or Inverted primary CT 46 . Dead Tank Current Transformer Bus P Feeder P CB Insulator Primary winding Core Secondary winding S S Terminal Box 47 . Dead Tank Current Transformer 1) Eye Bolt Type 2) Hair Pin Type 48 . Dead Tank Current Transformer P1 P2 CO RE 1 CO RE 5 CO RE 2 CO RE 4 CO RE 3 49 . Outdoor Current Transformer 50 . Manufacturing of Current Transformer CT Secondary in Progress . Live Tank Current Transformer 52 . Live Tank Current Transformer 53 . 420 kV dead Tank CT k d d k (Hair Pin Design) 420 kV Live Tank CT 54 . Theory of y Voltage Transformers g . What is Voltage Transformer • Voltage Transformer is an instrument transformer which transforms voltage f from one level l l to t another th level l l such h as 400KV/√3:110V/√3 (VT ratio) i.e.3kV or more) is not possible as devices used for measurement of voltage are not designed to handle such high level of voltage lt 56 . transforms voltage from the level of 400KV/√3 into voltage of 110V/√3 level • Direct measurement of high voltage (in the tune of 3. Why Voltage Transformer is Required • System has two basic h b requirements ¾ t i of energy ¾metering f sourced or consumed ¾protection of the of the electrical system from faults and faults and disturbances 57 . U/F and over fluxing protections are also configured from VTs • Voltage signal also used for synchronizing. Disturbance recorders and event logs 58 . we can in a way compute power or impedance of system alongg with its direction • Moreover O/V. Therefore when voltage is also measured along with current during faults.Why Voltage Transformer is Required • Faults can be of many kinds. some faults such as O/C can be detected solely on current measurement. but currentt does d nott provide id discretion di ti about b t nature t and d location of the fault • Therefore. O/F. U/V. How Voltage Transformer is connected • VT has h a primary and d one or more secondary d windings • Metering M t i and d Protection P t ti d i devices are connected t d to t the secondaries of the VT • In voltage operation or shunt mode.5 volts suitable for the meter or relay 59 . mode the primary winding is connected in parallel with the power system to transform the phase voltage to usually 63. Voltage Transformer Theory • For a transformer f in no load l d the h following f ll is valid ld Voltage transformation is proportional to the ratio of primary and secondary turns E1 N1 = E2 N2 • An ideal voltage transformer is a transformer under no‐load conditions where the load current is zero and the voltage drop is only caused by the magnetizing current and is thus negligible . Voltage Transformer Theory • Simplified VT equivalent circuit Is ≠ Ip Ip Zp Zs Ie Es Vp Im Iw Vs ZB∠φ . Voltage Transformer Theory Vp IpRp IsRs V′p Es Vs IeRp θ Ip Is Ie 0 φ Phasor diagram with referance to voltage error 62 . Voltage Transformer Theory • Ratio error. which h h is defined d f d as the h difference d ff in magnitude of the primary and secondary voltage expressed as percentage of primary voltage Vs .K n − V p Voltage g ( Ratio) Error = ×100 Vp Kn= Rated transformation ratio Vp = Actual primary voltage Vs = Actual secondary voltage • Phase Angle error is the difference between the reversed secondary and the primary voltage vectors . primary winding System Non‐earthed Effectively or non‐effectively earthed 64 . which in turn dependent on the system and voltage transformer earthing conditions VT used VTs d in i non‐effectively ff ti l earthed th d system t h have hi h voltage high lt f t since factor i i in the event of an earthed fault in one of the phases.T. the healthy phase voltage may rise to phase to phase value Voltage Factor VF Duration Earthing conditions 1.5 30 s Earthed Effectively earthed 1.Voltage Factor • • Voltage V lt f t determines factor d t i th maximum the i operating ti voltage lt f voltage for lt transformers expressed in per unit of rated voltage.9 8 h Earthed Isolated neutral or resonant earthed without automatic E/F tripping V.9 30 s Earthed Non‐effectively earthed with automatic E/F tripping 1.2 Continuous 1. Protection of Voltage Transformer • Protection of EVT from accidental overloads and short circuit across its secondary terminal is achieved by incorporating fuses or MCB in secondary circuit located near to transformer as possible • Normal secondary current is not more than 5A and short circuit current in the range of 100A, 100A simple fuses can be employed • Short circuit on secondary winding gives only a few amperes in primary winding and is not sufficient to rupture a high voltage fuse at primary side (HRC fuses on primary side up to 66kV) • Hence high voltage fuse on primary side do not protect transformer they protect only network in case of any short transformer, circuit on the primary side • CVT invariably solidly connected to the system so that there is no primary protection 65 Voltage Transformer Accuracy • As stated for CT, we need it for ¾Metering voltage measurement, energy, power measurement ¾Protection for distance protection, O/V, U/V, O/F and U/F protections, protections field failure, failure over‐fluxing etc • For metering VTs we need high accuracy in the voltage measurement duringg stable conditions i.e. 80% to 120% of nominal system voltage with burdens from 25% to 100% of rated burden at power factor of 0.8 lagging • Combination of magnitude and phase error depends on the power factor of the burden 66 Voltage Transformer Accuracy • IEC 60044‐2 and 60044‐5 defines this as 67 8 pf lagging 68 . 1.2.Voltage Transformer Accuracy • For Protection VTs we need faithfulness of voltage measurement in the higher range of voltage such as from value as low as 2% of nominal voltage to the rated voltage multiplied by rated voltage factors such as 1.5. 1.9 with burden of 25% to 100% of rated burden at 0. Voltage Transformer Accuracy • IEC 60044‐2 and 60044‐5 defines this as d d f h 69 . Voltage Transformer Connections • There are three types of connections – V‐V connection – Star/Star connection – Star/Open delta connection • V‐V V V connection ti – Used for measurement and for those protections which do not require phase to neutral voltage input (2 VTs are used) – Primary of VTs is connected in V (one VT primary across R‐Y phase and other across Y‐B phase) with identical V connection for the secondary – In this connection zero sequence voltage can not be produced 70 . Voltage Transformer Connections • Star/Star connection – Either 3 separate single phase VT or a single VTs i l 3 phase. h 3 limb li b VT is used – Both p primaryy and secondaries are connected in star with both star neutrals solidly grounded – Each E h primary i phase h li b is limb i thus th connected between phase to earth of the supply circuit and replicate similar phase to earth voltage on the secondary 71 . Voltage Transformer Connections • Star/Open S /O D l connection Delta i – Primary windings are connected in star with star neutral solidlyy grounded and the secondaries are connected in series to form an open p delta connection – This type of connection is called residual connection and require either 3 single phase VTs or a single 3 phase 5 limb VT – This residual connection is used for polarising l ii di directional i l earth h fault f l relays or for earth fault detection in non‐effectively grounded or isolated neutral system 72 . Types of Voltage Transformer • Types of Voltage Transformer (VT) f l f ( ) • Electromagnetic Voltage Transformer (EVT) • Capacitive Voltage Transformer (CVT) C iti V lt T f (CVT) M P M P P P INDUCTIVE VOLTAGE TRANSFORMER CAPACITIVE VOLTAGE TRANSFORMER 73 . Types of Voltage Transformer • El Electromagnetic t ti Voltage V lt T Transformers f similar i il to t a smallll power transformer and differs only in details of design that control ratio accuracy over the specified range of output. but may be inferior in transient performance • Capacitors C i allow ll the h injection i j i off high hi h frequency f signals i l onto the power line conductor to provide end‐to‐end communications between substations for distance relays. due to prohibitive cost of insulation. CVT may be more economical than EVT particularly when the high voltage capacitors can serve also for carrier current coupling (PLCC). hence. cooling ( t t nott more than (output th 200‐300 200 300 VA). telemetry/supervisory l / i and d voice i communication i i 74 . at 132 kV and higher voltages. VA) insulation i l ti (designed (d i d for f system impulse voltage level) and mechanical aspects • At high system voltages the cost of conventional potential transformer is high. Terminal Box 75 .Capacitive Voltage Transformer Primary Terminal Capacitor Part Electromagnetic Unit HF Terminal Sec. CVT Internal Components Tank PT F R C k t. Resistor Capacitor FR Choke HV Choke 76 . Capacitive Voltage Transformer . Power Line Carrier (PLC) equipment C1 Wave Trap L1 Carrier oscillator C3 >500KHZ NOISE PICKUP <30KHZ-HARMONIC LIGHTENING.CORONA L3 C4 Coupling capacitor C2 L2 Matching Transformer VT Transmitter and receiver fa = 30kHz to 500 kHz 78 . 79 .