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Instrument Transformer Basics

Instrument Transformers

Current Transformers (CTs) and Voltage Transformers (PTs or VTs) are the frontline guardians of any AC power supply system.

The purpose of instrument transformer is to reproduce as accurately as possible the voltage (of PT) or current (of CT) in their primary circuit to their secondary circuit in order to be of any use to protection devices connected to them.

1. The parameters of Instrument are grouped just as is in other types of transformer but differ slightly when described with their inherent parameters. Exciting branch of potential transformers are usually ignored but not in current transformers.

2. The term "burden" is used to express the load of instruments transformers which is the impedance connected to its secondary. Burden is simply the volt-amperes delivered to the load. Thus, when an instrument transformer delivers 5amps to a resistive load of 0.1 ohm the burden is 2.5VA at 5amps (P= I2 x R)

3. Unlike power transformers, instrument transformers are designed to carry the calculated design load or peak load of the circuit.

4. Current transformer secondary winding currents have been standardized at 5amps in the U.S.; in Europe, 1amp is common.

5. Electrical manufacturers following ANSI-IEEE standards design multi-ratio instrument transformers at fixed ratios.

6. Relays and watt-meters can be an instrument transformers load, but it is advantageous for a utility company to have a separate instrument transformers for protection and metering.

Current Transformers

Selection of a current transformer must be based on its capability to accurately generate the primary current in its secondary winding in terms of wave shape and its magnitude at maximum and minimum current levels. It must have an accuracy at a burden rating equal to or greater than the load connected to its secondary for both the maximum and minimum current; and be able to withstand 10 up to 20 times normal load current during a fault in order to perform its critical function in protective relaying applications. Instrument transformers can be connected wye or delta in series or in parallel. They have a high and a low side, and also have specific phase sequence and polarity.

Phase relationship in sensed current in the primary of instrument transformers may not always be in phase with the load current, however they can be configured to shift the angle by as much as 30o.

Proper study of Instrument Transformers is in order, external PDF files included in this section will give you quick access to valuable information you might need.

Current Transformer Types

Current transformers are chosen according to its type and they are:

- Big High Voltage CTs for high voltage substations.

- The Bushing Type (Window or Donut) used in low to medium voltage switchgears and circuit breakers.

- Wound Type, which is called the "Choicest CT". It is the preferred CT for low voltage application up to 600Volts, because of its excellent performance under a wide variety of operating conditions.

- Bar Type which are offered in higher insulation level.

Current transformer Burden

The burden of CTs are not computed cumulatively but individually since a single phase to ground fault will involve only one CT to trip a breaker. For a three phase fault, however, all three CTs will pick up the fault current and the resistance of all device leads must be considered in the calculations.

CT burden is now quantified in terms of impedance of the load (lead wires) with its corresponding resistance and reactance components, since this is more accurate. CT burden must be distributed, equally as much as possible, between all the three phases of the system. Add all the burden of of the CT in series in the secondary of the CT; In case smaller wires will increase the burden of the CT as they cannot be made as short as possible, use larger wire size.

Current Transformer Saturation

At very high current occurring during a fault, the magnetic circuits of CTs starts to saturate and the impedance of the burden decreases as the secondary increases. For this reason, the performance of CTs below C100 rating must be thoroughly evaluated. It may be essential for you to perform calculations for the CT's burden for every value of secondary current to determine the CT’s actual accuracy.


Current Transformer Accuracy

Accuracy class of current transformer must be high enough in order to generate a secondary current as closely as possible with that circulating in the primary winding in terms of its wave shape, current magnitude at both the maximum and minimum level and with a burden rating equal or greater than the connected load. The CT must also be capable of withstanding 10 to 20 times normal current values.

Current in the primary of a power transformer is determined by Ipri = Isec /Transformer Ratio. It is not so in CT primary load (burden), but by the series lead wire resistance at the CT primary to a circuit whose load current is being measured or monitored for metering device/s or protection relays.

Ratio Correction Factor (RCF) is the term used to identify the accuracy of a current transformer. The Accuracy Class of current transformer for a particular application must be determined using ANSI C57.13-1993 or latest that is available, which shows a table of standard percent error, standard metering burdens at 60Hz at 90% lagging power factor. To determine the minimum to maximum range the CT will operate under normal condition, the accuracy of the CT must be determined at 10% and 100% load current.

Proper study of CTs is in order, PDF files included in this section gives you quick access to valuable CT selection information that simply cannot all be explained in this space.

Voltage Transformers

- Voltage Transformer (VT) or Potential Transformer (PT) have both a primary and secondary winding. Voltage transformers are connected to phases in either line-to-line or line to ground as dictated by the system requirements of a power supply. VT or PT is similar to the conventional transformers.

- Capacitor Voltage Transformers (CVTs) are capacitors connected in series to divide the applied voltage in its terminal. CVTs are used in high voltage system and are connected line to ground only. CVTs or CCPDs are use in both metering and Power Line Carrier System applications

Safety Considerations

Safety and continuity of service of any electrical installation is the electrical engineer's primordial concern. To achieve safety and continuity of service, however the protective relays must also be well coordinated. The term “coordination” simply mean the proper calibration of protective relays, both in their tripping time and the current magnitude to which they should actuate and trip corresponding circuit breaker/s in either the upstream or downstream level of the power supply system.

A caveat, safety and coordination are one, they simply cannot be treated as separate items: safety is compromised without proper coordination. To achieve this, the electrical engineer must be comfortable employing the symmetrical components, which is the great tool for any power protection specialist to guarantee the appropriate functioning of all systems and sub-systems. His familiarity with applicable standards and manufacturer’s device/s documentations is likewise of equal importance, too.


Relay Coordination Studies

Voltage transformers or potential transformers (PTs) carry very small current under normal operating parameters, thus they have small magnetic core cross-sectional area and wound with very fine wire. The primary of voltage transformers must be fused since they are prone to damage by transients than a power transformer. Select a fuse sized at 300% of the potential transformer primary full-load current, if it is not available, select the next standard larger size. Relay application dictates the voltage transformer's connection configuration that is, wye-wye or wye-delta. The ratio and phase angle inaccuracies of voltage transformers can be neglected since they are not within significant values if the burden is not within its thermal volt-rating danger point, i.e. so long as its thermal volts-ampere rating is not exceeded.

Typical potential transformer have emergency rating of about 60 seconds and magnetizing inrush current labeled 10X or 12X. If a 01 seconds set time is used as an assumption, the emergency overvoltage of 125% of its nameplate will allow the protective relays to operate.

7.2kV Ring Main Unit

7.2kV Ring Main Unit PTs and CTs

7.2kV Ring Main Unit PTs and CTs including the protection relays for the system are shown in this partial image capture. The RMU also have two panels for interconnecting adjacent light railway transit system RMUs for emergency power supply in case of supply outage in one of the adjacent stations.

View clear image here

The Art and Science of Protective Relaying

Protective relaying is an art, with the cost of protection always being balanced with system reliability and safety. How the protection system is configured and set up all depends on you.

Yesteryear's protection specialists concerned with providing reliable electrical service have done their jobs so well by using just the longhand computation method. Protection specialists of today, on the other hand, have a less tedious process and that is by investing in advanced software that can fairly predict the outcome of a design protection configuration parameters. You could easily understand and use these advance software programs if you are familiar with - again - symmetrical components. Without this very essential tool, your learning curve in the use of the software would be long and tedious. Remember, a sharp knife does more job that a dull one.

A brief rationalization in the implementation of a well-designed protection system that will both yield safety and coordination simply means everything is in order with the system capable of coping the forecasted peak and off peak loads with a comforting knowledge that serious power outages would be minimized in case of a major fault. However, even a well-designed and properly coordinated protection system would not operate as calculated if proper periodic maintenance/system checks are not carried out according to schedule or, to unseen operating circumstances not earlier predicted.

Electromechanical relays still in service in old geothermal power plants, in my opinion, is one major area of concern if the building housing them is not well insulated against H2S gas. Electromechanical relays can be affected since they are not hermetically sealed, or with weakened or defective seals as age sets in. This happened to one 50MW unit of geothermal plant complex. The 50MW turbine generator blew up sending a heavy turbine component flying and tearing off the roof and landing more than 50 meters away from the power house.


34.5 kV Switchgear

34.5kV swithchgear for railway power system

34.5kV switchgear partial image. It supplies 1500KVA 34.5kV/6.6kV Auxiliary Transformer and 2-2150KVA 34.5kV/6.6kV Rectifier Transformers. The partial SLD image also show the CTs and PTs including the protection relays for the system. The switchgear also have a metering cubicle for incoming 34.5kV feeder.

View clear image here

A team, to which I was a member, was formed to investigate the cause. The Team found that a fault occurred just after the GCB that failed to trip causing the power cables between the generator and the GCB including the GCB contacts itself to instantly melt and weld. With the generator/turbine suddenly losing its load as the circuit opened due to the disintegration of the power cables, the generator/turbine spun at a very high speed because the emergency steam vent failed to work, too. Operations personnel tried to open it manually, to no avail; and ran as fast as they could as the whole power house started shaking violently. Luckily no one was injured in the incident.

The Sequence of Events Recorder indicated the GCB protection relay actuated, but failed to establish a connection, since its contacts, after investigations were conducted, revealed a contaminated contact points caused by H2S gas. The steam emergency vent did not open, too, and manual operation did not work because of heavy corrosion caused by the same gas.

Instrument transformer equation

Determination of Proper Relay Tap

The Basic CT Equation based on the minimum relay closing current.

Using equation: Relay Pickup = CT Ratio X Relay Tap

@10X the minimum relay closing current::

Assume a 1MVA load, a 4.16kV feeder protected by a 5kV circuit breaker with a 50/5 CT ratio:

I = 1000KVA/4.16 X 31/2 = 138Amps (The exponent "1/2" is in lieu of the square root symbol)

A 50/5 CT ratio is: 50/5 =10

138/10 = 13.8Amps. This is the current that the CT delivers to the relay. The relay must be programmed according to your calculations. If your calculations say it must act at 10 times its minimum closing current, select a relay tap of: 13.8Amps/10 = 1.38. Choose the relay tap closest to 1.38.

@5X the minimum relay closing current:

13.8Amps/5 = 2.76. Choose the relay tap closest to 2.76. Electromechanical relay taps ranges from 0.5 to 2, 1.5 to 6, and 4 to 16 Amps. This is indicated in the CT nameplate.


- Programmable solid state relay taps may need different computations from the "basic" computation above as may be recommended by its manufacturer.

- Solid State relays uses smaller lead wires due to the small current used by its electronic circuitry. Always use the current transformer manufacturer's recommendation to determine the size of the CT based on your calculated CT burdens.


Electric Power System Protection and Coordination by: Michael Anthony - University of Michigan

Instrument Transformer Basics by: Kent Jones, P.E. Line Power Manufacturing

ABB Calculation of the Current Transformer Accuracy Limit Factor

Current transformers: how to specify them by: Schneider Electric

Instrument Transformer Theory and Testing by: Steve Hudson, PE Engineering Manager