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Instrument TransformerS

Technical Section

Introduction

Instrument transformers, as the name suggests, are used with measurement and protection instruments in order to measure electrical parameters such as current and voltage or to use these parameters to activate protection schemes. The secondary side of the instrument transformer is connected to meters for measurement of current and voltage, and to protective relays for tripping circuit breakers in the event of faults.

Their role in electrical systems is of primary importance as they are a means of stepping down, accurately, the higher system current and voltage values to a measurable and standardized value of current and voltage that can be used to operate instruments. Extremely important, too, is the isolating functionof the instrument transformer.No matter how high the system voltage, the secondary circuit need be insulated only for a low voltage, thus offering ease of operation and safety to personnel.

Principle of operation

An ideal instrument transformer would produce, in the secondary winding, a current (CT) or voltage (VT) value that is exactly proportionate and in precise phase opposition to the value in the primary winding.

However, practically, the primary and secondary ampere- turns are not equal. Some of the primary ampere-turns is drawn to magnetize the core and to supply its losses, so the transformation ampere-turns are lower than the primary ampere-turns, and this is what gets converted into secondary ampere-turns in precise phase opposition. So, both a ratio error and a phase error get introduced in the operation of an instrument transformer.

In the case of a voltage transformer, in addition, resistance and leakage reactance drops in the primary and secondary windings play a significant part in determining the errors. Secondary leakage reactance affects the errors of the CT especially during overcurrent.

Kappa CT s and VT s are designed deliver the required accuracy at rated or extended burdens without compromising on the other performance requirements such as the short time rating ( for CT) or voltage factor (for VT)requirement.

Definitions

Rated primary current or voltage: The value of current or voltage which is to be transformed to a lower value

Rated secondary current or voltage: The required current or voltage in the secondary circuit, on which the performance of the CT or VT is based. Typical values of secondary current are 1 A or 5A. In the case of transformer differential protection, secondary currents of 1/ 3 A and 5/ 3 A are also specified. Typical values of secondary voltage are 110 V or 100V, or 110V/rt3 or 100 V/rt3.

Rated burden: The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current or voltage and at a specific power factor. The burden of the circuit includes the burden of the instrument and the leads to and from the instrument.

Short time rating, STR: 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. The dynamic rating is usually 2.5 times the thermal current. The short time rating must be complied with for the protection scheme to be meaningful.

Unearthed VT s: These VT s are intended to be connected between lines and no primary side winding can be earthed in service.Examples are double pole and three phase three limb VT s.

Earthed VT s: One end of the primary winding is earthed in service in the case of single pole VT s. The primary neutral is earthed in service to extract the residual voltage of the system in the case of three phase five limb VT s.

Rated voltage factor, Fv: Multiplying factor to be applied to the rated primary voltage to determine the maximum voltage at which the VT must meet thermal and performance requirements. Typically, 1.2 cont., 1.5/ 30 sec or 1.9/ 30 sec or 1.9/ 8 hours. The voltage factor depends on the system earthing condition. 1.2 is used for connection between phases, 1.5 for solidly earthed system, 1.9/30 secs for non-effectively earthed system with automatic fault tripping, 1.9/ 8 hrs for resonant earthed without automatic fault tripping or isolated systems.

Thermal limiting output: The apparent power that the secondary can deliver at rated voltage without exceeding temperature rise limits, as given in Table-1.

Temperature Category: The ambient operating temperature categories are defined as in Table-2

Rating factor(ANSI): A multiplying factor specified for the continuous rating of the instrument transformer upto to which it has to maintain accuracy and stay within temperature rise limits. Typical values are 1, 1.33, 1.5, 2, 3 and 4.

Extended burden for CT s: If the max burden is limited to 15VA, an extended burden with the same performance requirements can be specified down to a burden of 1 VA, with UPF over the entire range.

Extended current for measurement CT s: The CT has to maintain accuracy upto the extended current, usually 150% or 200% of the rated current. The temperature rise limits should also be maintained.

Measuring current or voltage transformer: A CT or VT that is intended to transmit an information signal to measuring instruments and meters.

Protection CT s or VT s: Those that transmit an information signal to electrical protective and control devices

Rated insulation level: Specifies the highest continuous voltage withstand (HSV), the one-minute power frequency withstand voltage (PFW) and the lightning impulse withstand voltage (kVpeak). Table-3 below is extracted from IEC 61869-1.

Table 1- Limits of temperature rise for various parts, materials and dielectrics of instrument transformers

Table 2- Temperature categories

Table 3- Rated insulation levels for transformers primary windings having highest voltage for equipment Um<300 kV

Latest versions of Product standards

  • IEC 60044 and IEC/ BS EN 61869 series
  • IS 16227 series
  • ANSI IEEE C57.13 series
  • Australian standard AS 60044 series

Polarity and Terminal marking

Polarity is a means of designating the relative instantaneous directions of the currents/ Voltages in the primary and secondary circuits.

With reference to the diagram below, when primary current flows from P1 to P2, the secondary current will flow from S1 to S2 if the polarity markings are correct.

Winding diagram Is explained with an example

Core 1 : 600/5 A and Core 2 600-300/5A

1S1 and 1S2 relate to Core 1 – 600/5A

2S1 and 2S2 are the terminals of the 300/5A winding, Core 2

2S1 and 2S3 are the terminals of the 600/5A winding, Core 2

With reference to the winding diagram below, all terminals marked A or a have the same instantaneous direction, and all terminals marked N or n have the same instantaneous direction

11 kV/rt3//110V/rt3//110V/rt3 – 110V/3

A – N – 11 kV/rt3
Secondary 1 – 1a to 1n – 110V/rt3
Secondary 2 – 2a1 to 2n – 110V/rt3
Secondary 2 – 2a2 to 2n – 110V/3

Product Tests

A number of routine and type tests have to be conducted on IT s before they can meet the standards specified above. The tests can be classified as:

  1. Verification of terminal markings and Polarity test
  2. Accuracy tests to determine whether the errors of the CT/ VT are within specified limits over the specified range. This includes ratio and phase angle errors and, for CT s, the determination of excitation characteristics such as knee point voltage, exciting current and winding resistances.
  3. Loading of VT secondaries depending on customer requirement while conducting accuracy tests
  4. Dielectric insulation tests such as
  • Separate source withstand
  • Induced overvoltage (for VT s)
  • Power frequency withstand voltage test on primary and secondary windings for one minute
  • Inter-turn insulation test at power frequency voltage
  • Impulse tests with 1.2/50μ wave (type test)
  • Partial discharge tests to determine whether the discharge is below the specified limits.

All these tests determine the healthiness of the insulation.

5.Temperature rise tests, as type test

6. Short time current tests, as type test

Kappa conducts routine tests on each and every unit produced and all designs are periodically type tested. Factory test reports are furnished for all supplies made.

Application notes

The following points need to be taken into consideration in order to purchase an economical CT that can deliver the required performance.
  1. Specifying a CT whose ratio is as close as possible to the current that needs to be measured. Remember that the accuracy of the CT is greatest at the rated current, and is measured only upto 5% or 1% of the rated current, where it is much less accurate.
  1. Specifying a burden that is as close as possible to the actual burden: The performance of the CT is tested at the rated burden and at 25% of the rated burden, as per IEC requirements. If the actual burden is lower than 25% of the rated burden or higher than 100% rated burden, the accuracy of the CT is not known, and it may deliver worse errors. The same applies to VT s as well. In addition, lightly loaded VT s are prone to ferro-resonance and it is essential to specify a burden as close as possible to the actual burden.
  1. The dimensions, burden, accuracy and short time rating of the CT are inter-related, and sometimes there must be a compromise on one or more of the parameters to achieve the others. Typically, there cannot be a compromise on the short time rating. That said, low ratio CT s of a few primary amps can be rated for a short circuit of 100 times the primary only.
  1. While specifying a class PS/ PX/ X CT, it is better to specify the knee point voltage in terms of a formula such as KPV >= Kd * Is * (RCT + 2RL), where Kd is a dimensioning factor (see definitions) and 2RL the secondary circuit resistance. The choice of RCT is best left to the manufacturer. A wrong selection of RCT by the user will result in an unnecessarily large and expensive CT.
  1. Specifying the secondary current of the CT: The lead burden in proportionate to the square of the secondary current. So the burden of a 1A secondary will be 1/25th of the burden of a 5A secondary. However, the 1A will also develop 5 times the voltage of a 5A secondary.
  1. ISF (FS) of a CT: If the actual burden is lower than the rated burden, the ISF of the CT at actual burden will be much more than the specified ISF, which can be achieved at the rated burden only. Thus, there are chances of the instruments in the secondary circuit being damaged.
  1. The voltage factor of the VT must be specified taking into account the power system in which the VT is to be installed. See Definitions: Rated voltage Factor, Fv.
  1. The errors of the VT / CT are measured at its terminals and hence the secondary circuit must not impose a greater than specified burden. Where VT s are supplying both high accuracy metering and protection devices, separation of higher burden protection devices is recommended so that the metering winding can deliver the required accuracy of class 0.2 or class 0.3.

Precautions

  1. Never leave the CT secondary open: This can result in large and dangerous over-voltages being developed across the secondary. In the case of dual core CT s, every unused core must be shorted. In the case of dual ratio CTs, it is sufficient to short any one ratio.
  1. Magnetization of the core: Measurement of winding resistance and polarity checks can leave the core in a magnetized state, which will give erroneous performance. Demagnetization of the core then has to be carried out prior to use. This can be done by applying a slowly increasing voltage to the secondary of the CT until saturation is reached. The voltage is then slowly decreased and the cycle repeated a few times.
  1. Power frequency withstand test: This HV test should be conducted at 80% of the rated withstand voltage, as the manufacturer would already have tested at rated voltage. Repeated tests at higher values can damage the insulation of the units.
  1. Never short circuit the secondary of any VT. Any unused secondaries must be left open.
  1. Any part of the IT frame marked with an earth symbol should be earthed in service.
  1. In both CT s and VT s, one (the same) end of the secondary winding should be earthed. Multiple earthing should be avoided.
  1. In the three phase three limb VT, the primary neutral is not accessible, and should not be earthed in service. No primary terminal earthing should be done for single phase double pole units.
  1. In the three phase five limb VT, the primary neutral is brought out and should be earthed when detecting earth fault and should be connected to the capacitor bank neutral when used for capacitor bank protection.
  1. Use lifting lugs to move the unit. Where lifting lugs are not provided, lift the unit from the base. Do not try to move the unit using any projections from the resin body or any cables such as neutrals.
  1. Operating instructions for Draw-out voltage transformers available on request.