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Instrument and Control Transformers: Applications and Selection

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15/529 Instrument and control transformers: applications and 15 selection Contents Section IV: Non-conventional methods of current measurement 15/569 15.1 Introduction 15/531 15.11 Current Sensors 15/569 15.2 Types of transformers 15/531 15.11.1 Resistive shunts 15/569 15.2.1 Voltage transformers (VTs) 15/531 15.11.2 Hall effect current sensors 15/570 15.2.2 Current transformers (CTs) 15/531 15.11.3 Faraday effect optical sensors 15/570 15.3 Common features of a voltage and a current transformer 15/531 15.11.4 Zero flux current sensors 15/571 15.3.1 Design parameters 15/531 15.11.5 Rogowski coil current transducers 15/571 15.11.6 Digital optical instrument transformers 15/573 Section I: Voltage transformers 15/531 Relevant Standards 15/574 15.4 General specifications and design considerations for voltage transformers (VTs) 15/531 List of formulae used 15/574 15.4.1 Instrument voltage transformers 15/531 Further Reading 15/575 15.4.2 Electromagnetic voltage transformers 15/534 15.4.3 Residual voltage transformers (RVTs) 15/534 15.4.4 Capacitor voltage transformers (CVTs) 15/538 15.4.5 Control transformers 15/540 15.4.6 Summary of specifications of a VT 15/543 15.5 Precautions to be observed while installing a voltage transformer 15/543 Section II: Current transformers 15/544 15.6 Current transformers (CTs) 15/544 15.6.1 General specifications and design considerations for current transformers 15/545 15.6.2 Measuring current transformers 15/549 15.6.3 Interposing current transformers 15/550 15.6.4 Summation current transformers 15/550 15.6.5 Protection current transformers 15/551 15.6.6 Special-purpose current transformers, type ‘PS’ 15/553 15.6.7 Core-balanced current transformers (CBCTs) 15/563 15.7 Short-time rating and effect of momentary peak or dynamic currents 15/563 15.8 Summary of specifications of a CT 15/564 15.9 Precautions to be observed when connecting a CT 15/564 Section III: Testing of instrument and control transformers 15/566 15.10 Test requirements 15/566 15.10.1 Voltage transformers 15/566 15.10.2 Current transformers 15/568 Instrument and control transformers: applications and selection 15/531 15.1 Introduction meet the requirements for a switchgear assembly, except for small variations in the test requirements. For more details refer to the following publications: Transformers are used in an auxiliary circuit, linked to a 1 For voltage transformers power circuit, to indicate, measure and control its voltages IEC 60044-2 (for two-winding transformers such as and currents. They find application in a switchgear or a CVTs) controlgear assembly and a switchyard. It would be 2 For current transformers impracticable to produce indicating and measuring IEC 60044-1 and IEC 60044-6 instruments or protective devices to operate at high to very high voltages or currents. The universal practice, therefore, is to transform the high voltages, say, 415 V and above, and currents above 50 A to reasonably low SECTION I: VOLTAGE values, as discussed later, for these applications. Indicating TRANSFORMERS and measuring instruments and protective devices are designed for these reduced values. The transformers used to transform voltages are known as voltage transformers* 15.4 General specifications and and those to transform currents as current transformers. design considerations for Below we discuss their classifications, basic requirements voltage transformers (VTs) and design parameters. These transformers develop a voltage on the secondary, substantially proportional to the voltage on the primary 15.2 Types of transformers (there being no knee point saturation, as is sometimes required in CTs (Section 15.6.1(viii)). 15.2.1 Voltage transformers (VTs) These may be classified as follows: 15.4.1 Instrument voltage transformers 1 Instrument voltage transformers 1 Rated primary voltage (i) Conventional two-winding, electromagnetic voltage transformers This will generally be the nominal system voltage, except (ii) Residual voltage transformers (RVTs) and for transformers connected between a phase and the ground (iii) Capacitor voltage transformers (CVTs). These or between the neutral and the ground, when the primary may be used for metering or protection, with voltage will be considered as 1/ 3 times the nominal very little difference between the two as noted systems voltage (Vr). later. 2 Control transformers 2 Rated secondary voltage In Europe and Asian nations this is generally 110 or 110/ 15.2.2 Current transformers (CTs) 3 V, (63.5 V) for phase-to-phase or phase-to-ground auxiliary circuits respectively. In the USA and Canada These may be classified as: these voltages are 120 or 120/ 3 V for distribution 1 Instrument current transformers systems and 115 or 115/ 3 V for transmission systems. (i) Measuring current transformers (ii) Protection current transformers and 3 Rated frequency (iii) Special-purpose current transformers, class ‘PS’. This may be 50 or 60 Hz as the system may require. The 2 Interposing current transformers permissible variation may be considered as ±2% for 3 Summation current transformers measuring as well as protection VTs. These limits are 4 Core balance current transformers (CBCTs) based on the recommended variations applicable for a switchgear assembly (IEC 60439-1) or an electric motor (Section 1.6.2). 15.3 Common features of a voltage and a current transformer 4 Insulation systems These transformers may be PVC taped, thermoplastic 15.3.1 Design parameters (service conditions (polypropylene) moulded, fibreglass taped, polyester resin and likely deratings) cast or epoxy resin cast depending upon the system voltage and the surroundings. HV indoor transformers, for instance, These are similar to parameters for a switchgear assembly are generally polyester or epoxy resin cast, and are as discussed in Section 13.4. Since they are directly economical with good dielectric properties. They are resistant associated with the same power system and interrupting to humid, chemically contaminated and hazardous areas. devices as a switchgear assembly, they should generally Outdoor HV transformers, however, may be epoxy resin cast, oil or SF6 insulated and oil or SF6 cooled. Epoxy insulation provides better mechanical and constructional * Potential transformer (PT) is not the appropriate word to identify qualities. They are resistant to humid, contaminated and an instrument voltage transformer. corrosive atmospheres and are suitable for all HV 15/532 Electrical Power Engineering Reference & Applications Handbook systems. They are mechanically strong and can bear 9 Short-time rating shocks and impacts. This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays 5 Creepage distances will carry any inrush current during a switching operation For outdoor installations the recommended minimum or a fault. No short-time rating is thus assigned to such creep distances for all types of voltage or current transformers. transformers are given in Table 15.1, according to IEC The electromagnetic unit, however, as used in a residual 60044-1 or IEC 60044-2. VT (Section 15.4.3) or a capacitor VT (Section 15.4.4) should be suitable for carrying the heavy discharge or 6 Tappings inrush currents during a capacitor discharge or switching respectively. Tappings are generally not necessary, as a transformer is designed for a particular voltage system. If and when such a need arises (as in a control transformer (Section 10 Accuracy class 15.4.5)) they can be provided on the primary side of the The accuracy of a VT depends upon its leakage reactance transformer. and the winding resistance. It determines the voltage and the phase errors of a transformer and varies with the 7 Rated output VA on the secondary side. With the use of better core material (for permeability) (Section 1.9) and better heat The standard ratings, at 0.8 p.f. lagging, may be 10, 15, dissipation, one can limit the excitation current and reduce 25, 30, 50, 75, 100, 150, 200, 300, 400 or 500 VA or as the error. A better core lamination can reduce the core the auxiliary circuit may demand. The procedure to size and improve heat dissipation. determine the total VA burden of a circuit is described in Section 15.4.5. Typical values of VA burden for a few • Measuring voltage transformers Standard instruments are given in Table 15.2 from data provided accuracy class may be one of 0.1, 0.2, 0.5, 1 or 3. by the manufacturers. The recommended class of accuracy will depend upon the type of metering and generally as noted in 8 Rated burden Table 15.3. This is the maximum burden the transformer may have • Protection voltage transformers Generally, a to feed at a time. The preferred values will follow series measuring voltage transformer may also be used for R-10 of ISO-3 (IEC 60059) and as noted in Section the purpose of protection. A protection transformer, 13.4.1(4). however, is assigned an accuracy class of 3 or 6, which defines the highest permissible percentage voltage error at any voltage between 5% of the rated voltage up to the voltage obtained by multiplying the rated voltage Table 15.1 Recommended values of minimum creepage by the rated voltage factor of 1.2, 1.5 or 1.9. And distances for a VT or a CT when the secondary has a burden between 25% and 100% of the rated burden at a p.f. of 0.8 lagging. This Pollution level Minimum creepage distance between phase accuracy class is followed by a letter ‘P’ such as 3P and ground mm per kV (r.m.s.) and 6P etc. The voltage and phase displacement errors (phase to phase) should not exceed the values noted in Table 15.6.
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