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Instrument Transformers Chapter 3 Instrument Transformers Introduction Protective relays in ac power system are connected in the secondary circuits of Current Transformers and Potential Transformers. The design and use of these transformers are quite different from that of well - known Power Transformers. Both current transformers (CTs) and Potential Transformers (PTs or VTs) come under the title “Instrument Transformers”. A Transformer intended to supply measuring instruments, meters, relays and other similar apparatus are called Instrument Transformers. 1. Current Transformers The current transformers are manufactured as per IS – 2705 Part I, Part II, Part III and Part IV. Part I deals with general requirement of CT s, Part II deals with the specification of Metering CT s, Part III gives the specification of Protection CTs and Part IV gives the specification of special protection ( PS Class ) CTs. a) Definitions (1) A Current Transformer is defined as an instrument Transformer in which the secondary current, in the normal conditions of use is proportional to the primary current and differs in phase from it by an angle, which is approximately zero for an appropriate direction of the connections. (2) Primary Current : The primary current of the CT depends on the load on the circuit at which CTs is connected. The standard values are 10, 15, 20, 30, 50, 75, 100, 150, 200, 300, 400, 600, 800, 1000 and 2000A. The selection should be based on the load in the circuit. (3) Secondary Current : The standard values of rated secondary current shall be 1A and 5 A. For distribution application 5 A is the Standard Secondary Current. For sub station application 11 kV metering CT is having 5A secondary current. For 66 kV to 400 kV metering 1A secondary current is used. (4) Actual Transformation Ratio: The ratio of the actual primary current to the actual secondary current. (5) Rated Transformation Ratio: The ratio of the rated primary current to the rated secondary current. (6) Phase Displacement: The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually expressed in minutes. 50 (7) Saturation factor is defined as the ratio of current at which the core saturate to the normal rated current. (8) Instrument Security Factor (ISF): The ratio of the rated instrument security current to the rated primary current. (9) Rated Instrument Security Current: The value of the current assigned by the manufacturer, as the lowest r m s primary current at which the r m s value of the secondary current multiplied by the rated transformation ratio does not exceed 0.9 times the value of the primary current. (10) Accuracy Class: A designation assigned to a current transformer the errors of which remain within specified limits under prescribed conditions of use. (11) Burden: The impedance of the secondary circuit in ohms at a specified power factor. The burden is usually expressed as the apparent power in volt-amperes absorbed at a specified power-factor and at the rated secondary current. (12) Short time thermal current: The r m s value of the primary current which the current trans- former will withstand for a rated time with the secondary winding short-circuited without suffering harmful effect. The values of the rated time shall be 0.5,1.0 and 2.0 seconds (13) Rated Dynamic current: The peak value of the primary current which a current transformer will withstand, without being damaged electrically or mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited. Normally this current should be more than 2.5 times short time thermal current. (14) Ratio and phase angle error: Ratio Error (Current Error) is the error which a transformer introduces into the measurement of a current arid which arises from the fact that the actual transformation ration is not equal to the rated transformation ratio. The current error expressed in percent is given by the formula. Current Error Percent = (KnIs – Ip) x 100 Ip Where Kn is the rated transformation ratio; Ip is the actual primary current; and Is is the actual secondary current when Ip is flowing, under the conditions of measurement. (15) Temperature Rise: The temperature rise of a current transformer winding when carrying a Primary current equal to the rated continuous thermal current, at a rated frequency and with rated burden shall not exceed 55˚C in winding and 45˚C in the oil. 51 (16) Insulation Level Nominal Highest Equipment One Minute power Impulse system Voltage Frequency withstand withstand Voltage Voltage Voltage KV KV KV KVP 11 12 28 75 33 36 70 170 66 72 140 325 110 145 275/230 650/550 220 245 460/395 1050/950 400 420 630 1425 2. Factors affecting the design of CTs The following are the major factors which influences the design of a Current Transformer: a. System Voltage and frequency b. Current Ratio c. Number of cores d. Burden, accuracy class, Instrument Security Factor, Accuracy Limit Factor, Knee Point Voltage, Secondary winding resistance, Composite Error and Secondary limiting voltage. e. Short time current rating f. Temperature rise limits g. Rated Insulation levels h. Atmospheric pollution levels. a) Design Considerations: For a current transformer the flux density in the core varying with its primary current, therefore the eddy current loss and hysteresis loss also varying with its primary current. The primary current is not controlled by the secondary circuit conditions. The secondary circuit of a CT should not be kept open when the primary is charged. No fuses should be installed in the secondary circuit of C T s. The ammeters, watt meters, energy meters and relays are connected across the secondary terminals of the CT. b) Constructional Aspects: The CT cores are manufactured with Cold Rolled Grain Oriented Silicon Steel ( CRGO ) or Mumetal (Nickel alloy). These are laminated silicon steel sheets having a thickness of 0.3 m and are wound spirally to form the round core. The secondary windings are having large number of turns and are wound over the insulated ring core. Normally double cotton covered copper conductor (d.c.c) are used for oil immersed CTs and enamelled copper conductor for dry type C Ts. The copper conductor primary winding passes through the secondary wound ring core. Normally three to five secondary cores are provided for EHV class C Ts for different applications. 52 c) Errors of C Ts: A part of the primary ampere - turn is utilised for exciting the core. The exciting current is having two components known as ‘magnetising component’ and ‘Iron loss Component’. The current ratio error is proportional to Iron loss Component and phase angle error is proportional to Magnetising Component. Similarly errors will vary with value of burden and power factor of Burden. Finally it can be concluded that the errors of a Current Transformer is not a constant value under all conditions of load and it can be changed with primary current, value of burden and power factor of the burden. 3. Measuring Current Transformer IS 2705 Part II covers specific requirements for measuring Current Transformers used in measuring such as indicating instruments, integrating meters and similar equipments. a) Accuracy class for measuring Current Transformer: It shall be designated by the highest permissible percentage current error at rated current for the accuracy class concerned. The standard accuracy class shall be 0.1, 0.2, 0.5, 1, 3 and 5. 0.1 and 0.2 class C Ts are used for Laboratory application. For LT metering, HT metering and EHT metering 0.5 class C Ts are used in KSEB. Class 1.0 C Ts were used in 66 kV and 110 kV sub stations. For 220 kV and 400 kV sub station metering 0.5 class C Ts are using. b) Burden: The standard values of rated burden used in KSEB are 15 VA, 30 VA and 60 VA and 100 VA. For distribution metering 15 VA is the standard VA rating of measuring CTs. For 110 kV and 66 kV sub station metering normally 30 VA C Ts are used. For 11 KV sub station metering 15 VA C Ts are used. For 220 kV and 400 kV sub stations 60 VA and 100 VA Burdens are normally used. c) Instrument Security Factor: The purpose of specifying the instrument security factor is to protect the meters under heavy fault condition. This is achieved by saturating the metering core at heavy fault currents. Normally the maximum value is 5. If ISF is 5 then the maximum value of secondary current coming at any value of primary current is less than 4.5 A for a CT with 1A rated secondary current. The measuring current Transformers are characterised by Accuracy Class, Burden, and Instrument security factor. 4. Protective Current Transformers IS 2705 Part III gives the specific requirements and tests for protective Current Transformers used with electrical protective devices. 53 Protective Current Transformers are characterised by Accuracy Class, composite error, Accuracy limit factor, secondary limiting voltage, Exciting current at SLV, secondary winding resistance and Burden. The protective Current Transformers are marked as 30 / 5P10, where 30 indicate out put in VA, 5P indicates accuracy class, 5 indicates composite error and 10 ALF. (1) Accuracy class: The accuracy class shall be designated by the highest permissible percentage composite error at the rated accuracy limit primary current prescribed for the accuracy class concerned followed by the letter “P” (meaning protection). The standard accuracy classes for protective current Transformers shall be 5P,10P, and 15 P.
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