Instrument transformers

Instrument transformers Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. Their role in electrical systems is of primary importance as they are a means of "stepping down" the current or voltage of a system to measurable values, such as 5A or 1A in the case of a current transformers or 110V or 100V in the case of a voltage transformer. ·         Voltage transformers ·         Current transformers Voltage transformers Principle of operation Definitions Standards Tests Typical Specifications Principle of operation The standards define a voltage transformer as one in which "the secondary voltage is substantially proportional to the primary voltage and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections." In a "practical" transformer, errors are introduced because some current is drawn for the magnetization of the core and because of drops in the primary and secondary windings due to leakage reactance and winding resistance.  The best grades of cold rolled grain oriented electrical steels which enables operation at optimum levels of magnetic induction. Terminology   voltage transformer (VT) or potential transformer (PT) Rated primary voltage: This is the rated voltage of the system whose voltage is required to be stepped down for measurement and protective purposes. Rated secondary voltage: This is the voltage at which the meters and protective devices connected to the secondary circuit of the voltage transformer operate. Rated burden: This is the load in terms of volt-amperes (VA) posed by the devices in the secondary circuit on the VT. This includes the burden imposed by the connecting leads. The VT is required to be accurate at both the rated burden and 25% of the rated burden. Accuracy class required: The transformation errors that are permissible, including voltage (ratio) error and phase angle error. Phase error is specified in minutes. Typical accuracy classes are Class 0.5, Class 1 and Class 3. Both metering and protection classes of accuracy are specified. In a metering VT, the VT is required to be within the specified errors from 80% to 120% of the rated voltage. In a protection VT, the VT is required to be accurate from 5% upto the rated voltage factor times the rated voltage. Rated voltage factor: Depending on the system in which the VT is to be used, the rated voltage factors to be specified are different. The table below is adopted from Indian and International standards. Rated voltage factor Rated time Method of connecting primary winding in system 1.2 Continuous Between phases in any network Between transformer star-point and earth in any network 1.2 1.5 Continuous for 30 seconds Between phase and earth in an effectively earthed neutral system 1.2 1.9 Continuous for 30 seconds Between phase and earth in a non-effectively earthed neutral system with automatic fault tripping 1.2 1.9 Continuous for 8 hours Between phase and earth in an isolated neutral system without automatic fault tripping or in a resonant earthed system without automatic fault tripping Temperature class of insulation: The permissible temperature rise over the specified ambient temperature. Typically, classes E, B and F. Residual voltage transformer (RVT): RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is no voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. When using a three phase RVT, the primary neutral should be earthed, as otherwise third harmonic voltages will appear across the residual winding. 3 phase RVTs typically have 5 limb constructions. Metering Units: 11kV metering units consist of one 3 phases VT and 2 CT's connected together in a single housing. This can be used for three phase monitoring of energy parameters. It is used with trivector meters and energy meters. Standards Standard Standard Number Year Indian 3156 1992 British BS EN 60044-2 1997 British BS 7729 1994 International Electro technical Commission (IEC) IEC 60044-2 1997 Tests A number of routine and type tests have to be conducted on VT s before they can meet the standards specified above. The tests can be classified as: Accuracy tests to determine whether the errors of the VT are within specified limits Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, induced over-voltage test , impuse tests with 1.2u/50u wave, and partial discharge tests (for voltage>=6.6 kV) to determine whether the discharge is below the specified limits. Temperature rise tests Short circuit tests Verification of terminal markings and polarity Typical specification for a 11 kV VT System voltage: 11 kV Insulation level voltage (ILV) : 12 /28/75 kV Number of phases: Three Vector Group: Star / Star Ratio: 11 kV/ 110 V Burden: 100 VA Accuracy: Class 0.5 Voltage Factor: 1.2 continuous and 1.5 for 30 seconds With provision for fuse Current transformers Principle of operation Definitions Standards Tests Typical Specifications Principle of operation A current transformer is defined as "as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. The current transformer works on the principle of variable flux. In the "ideal" current transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and opposite to the primary current. But, as in the voltage transformer, some of the primary current or the primary ampere-turns is utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to be "transformed" into the secondary ampere-turns. This naturally introduces an error in the transformation. The error is classified into a) the current or ratio error and b) the phase error. Terminology   Rated primary current: The value of current which is to be transformed to a lower value. In CT parlance, the "load" of the CT refers to the primary current. Rated secondary current: The current in the secondary circuit and on which the performance of the CT is based. Typical values of secondary current a)  are 1 A for metering and 5 A for protection. In the case of transformer differential protection, secondary currents of 1/ √3 A and 5/ √3 A are also specified. Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards) Accuracy class: In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor. In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT. Composite error: The rms value of the difference between the instantaneous primary current and the instantaneous secondary current multiplied by the turns ratio, under steady state conditions. Accuracy limit factor: The value of primary current upto which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current. Short time rating: The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds. Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished. Interposing CT's (ICT's) : Interposing CT's are used when the ratio of transformation is very high. It is also used to correct for phase displacement for differential protection of transformers. Standards Standard Standard Number Year Indian 2705 1992 British BS EN 60044-1 1999 International Electro technical Commission (IEC) IEC 60044-1 2000 Tests Accuracy tests to determine whether the errors of the CT are within specified limits. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute, inter-turn insulation test at power frequency voltage, impulse tests with 1.2u/50 wave, and partial discharge tests (for voltage >=6.6kv) to determine whether the discharge is below the specified limits. Temperature rise tests. Short time current tests. Verification of terminal markings and polarity. Typical specification for a 11 kV CT System voltage:11 kV Insulation level voltage (ILV) : 12/28/75 kV Ratio: 200/1 - 1 - 0.577 A Core 1: 1A, metering, 15 VA/class 1, ISF<10 Core 2: 5A, protection, 15 VA/5P10 Short time rating:20 kA for 1 second