Current and Voltage Transformers

1 INTRODUCTION

Current and voltage transformers are required to transform high currents and voltages into more manageable quantities for measurement, protection and control. This section describes the properties of current transformers (CTs) and voltage transformers (VTs) and how to specify them for particular applications.

2 CURRENT TRANSFORMERS

2.1 Introduction

A current transformer is used to transform a primary current quantity in terms of its magnitude and phase to a secondary value such that in normal conditions the secondary value is substantially proportional to the primary value. IEC 60044 (instrument transformers) covers CTs and VTs, superseding IEC 60185 (current transformers for measuring and protective applications) and its predecessor IEC 185.

2.2 Protection CT Classifications

Protection CTs, unlike measuring CTs, may be required to operate at many times full load current. Linearity under these conditions is not of great importance. The essential point is that saturation must be high enough to drive the magnetizing current and the secondary current under fault conditions.

2.2.1 5P or 10P Classification

Several terms are used in connection with CTs and these are described below:

Rated primary (or secondary) current: This value, marked on the rating plate of the CT, is the primary or secondary current upon which the performance of the transformer is based.

Rated transformation ratio: The rated transformation ratio is the ratio of rated primary current to rated secondary current and is not necessarily exactly equal to the turns ratio.

The magnetizing current depends upon the magnitude of the primary volt age which in turn depends upon the magnitude and power factor of the bur den. It’s possible partially to compensate for the magnetizing current ratio error in CT designs by slightly reducing the number of turns on the secondary. However, no similar compensation is available for small phase errors.

The standards to which the CTs are specified may not detail a continuous overload rating. It’s therefore prudent to choose a primary current rating at least equal to the circuit rating. An accuracy class of 5P (P stands for protection) is usually specified for large systems where accurate grading of several stages of inverse definite time lag (IDMTL) overcurrent relay protection is required. An accuracy class of 10P is also often acceptable and certainly satisfactory for thermal overload relays on motor circuits. These accuracy classes correspond to 5% or 10% composite error with rated secondary burden connected at all currents up to the primary current corresponding to the rated accuracy limit factor.

Composite error --- Under steady-state conditions the rms value of the difference between the instantaneous values of the primary current and the actual secondary current multiplied by the rated transformation ratio.

Rated output at rated secondary current --- The value, marked on the rating plate, of the apparent power in VA that the transformer is intended to supply to the secondary circuit at the rated secondary current.

The rated-VA should be specified to correspond to the relay and connecting lead burden at rated CT secondary current. If relays are mounted on the switchgear adjacent to the CTs then the lead burden can often be neglected.

It’s best to allow a margin for greater than anticipated burden but this should be included in the specification for the rated accuracy limit factor.

Rated accuracy limit factor (RALF) --- The primary current up to which the CT is required to maintain its specified accuracy with rated secondary burden connected, expressed as a multiple of rated primary current.

Ideally the RALF current should not be less than the maximum fault current of the circuit up to which IDMTL relay grading is required, and should be based upon transient reactance fault calculations. If a switchboard is likely to have future additional fault-in-feeds, then it’s sensible to specify an RALF corresponding to the switchgear fault-breaking capacity. Rated outputs higher than 15 VA and rated accuracy limit factors higher than 10 are not recommended for general purposes. It’s possible to make a trade-off between RALF and rated output but when the product exceeds 150 the CT becomes uneconomic with large physical dimensions. An RALF of 25 is an economic maximum figure. A reduction in RALF is not always possible and therefore the following measures should be considered:

_ Use the highest possible CT ratio.

_ Investigate relays with a lower burden. Solid-state relays have burdens of 0.5 VA or less and don’t change with tap setting.

_ At lower system voltage levels (15 kV and below) consider the use of fuses on circuits of low rating but high fault level.

Typical electromagnetic protection relays have a burden of about 3 VA at the setting current. The burden increases on the minimum plug setting (50% for a typical overcurrent relay). Precautions are therefore taken in protection relay designs to ensure that the increase in burden does not exceed half the nominal value as the tap setting is changed. In addition to the relay burden the CT leads and connecting cables must be taken into account. A 100 m length of typical 2.5 mm^2 cable would have a burden of about 0.74 ohm per core or 0.74 VA for a 1 A secondary rating and 18.5 VA for a 5 A rating.

Hence the advantage of using 1 A secondary CTs for substations with long distances between relays and CTs.

A typical marking on a protection CT would be 15 VA Class 5P 10, where 15 VA is the VA output at rated secondary current, Class 5P indicates that this is a protection (P) CT with a composite error of ,5% at rated accuracy limit primary current and 10 is the rated accuracy limit factor (RALF) for the CT, i.e. overcurrent=10^3 rated normal current.

2.2.2 Knee Point

FIG. 1 Typical magnetizing characteristic.

For protective purposes, current transformer specifications may be defined in terms of the 'knee point'. This is the voltage applied to the secondary terminals of the CT with all other windings being open circuited, which, when increased by 10%, causes the exciting current to be increased by 50%. A typical CT magnetizing characteristic is shown in FIG. 1. Older standards (BS3938) catered for the specification of such 'Class X' CTs in terms of:

_ Rated primary current.

_ Turns ratio.

_ Rated knee point emf.

_ Maximum exciting current at a stated percentage of rated knee point emf.

_ maximum resistance of secondary winding.

In addition the CT must be of the low reactance type and the turns ratio error must not exceed 0.25%. Bar-type CTs with jointless ring cores and evenly distributed secondary windings will provide negligible secondary winding leakage reactance and will usually satisfy this reactance requirement. For Class X CTs turns compensation is not permitted and a 400/1 Class X CT should have exactly 400 turns. Such carefully controlled CTs are used in pilot wire and balanced differential protection schemes and the manufacturer usually provides an excitation curve at the design stage which may be later confirmed by routine testing and site tests. Such CTs could be specified for use with IDMTL relays but this is not usual.

2.2.3 Other Standards

American Standards designate CTs with negligible secondary leakage reactance as Class C and the performance may be calculated in a similar manner to the now obsolete BS3938 Class X CTs. Class T CTs have some leakage and tests are called for in the ANSI Standards to establish relay performance.

In addition to the leakage reactance classification, the CTs are specified with a permissible burden in ohms equivalent to 25, 50, 100 or 200 VA for 5-A rated CTs. The secondary terminal voltage rating is the voltage the trans former will deliver to a standard burden at 20 times rated secondary current without exceeding 10% ratio correction. This is not exactly equivalent to the Class X CT knee point voltage since the terminal voltage will be of a lower value due to losses in the secondary winding resistance.

BS3938, referred to above, remained current until 1999 because it referred to class X CTs. IEC 60044-1 and its European equivalent EN 60044-1 (and the UK national BSEN60044-1) deal with class PX and completely replace class X. Hence the eventual withdrawal of BS3938.

2.3 Metering CTs

For non-protection purposes metering CTs need to perform very accurately but only over the normal range of load up to, say, 120% full load current.

Metering CTs are specified in terms of:

_ ratio,

_ rated-VA secondary burden,

_ accuracy class.

Accuracy classes recognized by IEC 60044 are 0.1, 0.2, 0.5 and 1. Accuracy classes 3 and 5 are also available from manufacturers. For each class the ratio and phase angle error must be within specified limits at 5%, 20%, 100% and 120% of rated current. A class 0.2 metering CT means that at 100_120% of the rated current, the percentage ratio error will be 60.2; i.e. for a class 0.2 CT with a rated secondary current of 5 A the actual secondary current would be 5 6 0.01 A. Phase displacement error is also specified in the IEC standard. For special applications an extended current range up to 200% may be specified. Above these ranges accuracy is considered to be unimportant since these conditions will only occur under abnormal fault conditions. There is an advantage in the CT being designed to saturate under fault conditions so that the connected metering equipment will have a lower short-time thermal withstand requirement. It’s preferable not to use common CTs to supply both protection and metering equipment. If, For example only one set of protection CTs is available then it’s good practice to separate the measuring instrumentation from the protection relays by means of saturable interposing CT or by adding saturable shunt reactors. This has the advantage of protecting the instrumentation and reducing the overall burden under fault conditions. A typical marking on a metering CT would be 15 VA Class 0.2 120%.

_ The VA output at rated secondary current is 15 VA.

_ The percentage error is 60.2 at rated current.

_ The extended current rating is 120% of rated secondary current.

2.4 Design and Construction Considerations

2.4.1 General

There are a number of points which the power system design engineer should appreciate with regard to CT design. The most important are covered in the following sections.

2.4.2 Core Materials

_ Non-oriented silicon steel is usually the least expensive.

_ Grain-oriented cold-rolled silicon steel gives a higher knee point voltage and lower magnetizing current.

_ Mumetal may be used for high accuracy metering CTs having a very low magnetizing current and low knee point voltage.

_ Special cores with air gaps may be used for linear output.

2.4.3 Knee Point

The knee point of a CT is directly proportional to the cross-sectional area of the core. The magnetization current of a CT at a particular voltage is directly proportional to the length of the magnetic core around its mean circumference.

2.4.4 Secondary Winding

The knee point voltage is directly proportional to the number of secondary turns which are usually determined by the turns ratio. High voltages can appear across the open circuit secondary terminals of CTs. Therefore switching contact arrangements must be added to protection schemes such that when relays are withdrawn from service (e.g. for maintenance) their associated secondary CT terminals are automatically short circuited.

2.4.5 Space Considerations

The design of a CT is based upon the best compromise between choosing maximum core cross-section for the highest knee point voltage and choosing maximum cross-section of copper for the secondary winding to achieve the lowest winding resistance.

2.4.6 Transient Behavior

The transition from steady-state current to fault current conditions is accompanied by a direct current component. The magnitude of the DC component depends upon the point on the wave at which the fault occurs. The DC component will then decay with an exponential time constant proportional to the ratio of resistance to inductance in the circuit. While the DC component is changing a unidirectional flux is built up in the CT core in addition to the AC working flux. If the protection scheme requires a constant transformation ratio without significant saturation under all possible fault conditions then the DC time constant must be allowed for in the knee point derivation formula.

Some high impedance relay protection schemes are designed to operate correctly under saturated CT conditions. Distance relays would tend to operate more slowly if the CTs are not designed to avoid transient saturation.

Low impedance-biased differential protection, pilot wire protection and phase comparison schemes would tend to be unstable and operate under out of-zone fault conditions if the CTs are allowed to saturate.

Some typical CT knee point requirements, all based on 5-A secondary CTs, for different types of protection are detailed below:

Distance impedance measuring schemes

Phase comparison scheme

Pilot wire differential scheme

Electromagnetic overcurrent relay scheme

15 VA 5P 20. (Note that the rated accuracy limit factor (RALF) is dependent on the maximum fault level, CT ratio and type of relay.) Solid-state overcurrent relay scheme

5 VA 5P 20.

High impedance relay scheme

…where…

Vkp =CT knee point voltage

Rct =CT secondary wiring resistance (75_)

I=CT, and relay, secondary rating (5 A assumed)

If =maximum symmetrical fault level divided by the CT ratio (for distance protection relays use If at the end of zone 1, otherwise use the maximum through fault level)

Rt 5resistance per phase of CT connections and leads.

2.5 Terminal Markings

The terminals of a CT should be marked as indicated in the diagrams shown in FIG. 2. The primary current flows from P1 to P2 and it’s conventional to put the P1 terminal nearest the circuit breaker. The secondary current flows from S1 to S2 through the connected leads and relay burden. Typical ring CTs are shown in Figs. 5.3 and 5.6 and an example of terminal marking is shown in FIG. 5. Checking the correct polarity of CTs is essential for differential protection schemes and a simple method is explained in Section 19.

FIG. 2 CT terminal markings.

FIG. 3 Ring CT. Insulation Iron area Primary Secondary

2.6 Specifications

TBL. 1 gives a typical format for setting out CT requirements on a substation circuit-by-circuit basis. Open terminal substation CTs will also require insulator details (creepage, arcing horns, impulse withstand, etc.) to be specified (see Section 6).

TBL. 1 Current Transformers (to IEC60044-1)

3 VOLTAGE TRANSFORMERS

3.1 Introduction

IEC 60044 applies to both electromagnetic (inductive) and capacitor type voltage transformers, superseding IEC 60186 and its predecessor IEC186.

For protection purposes VTs are required to maintain specified accuracy limits down to 2% of rated voltage.

_ Class 3P may have 3% voltage error at 5% rated voltage and 6% voltage error at 2% rated voltage.

_ Class 6P may have 6% voltage error at 5% rated voltage and 12% voltage error at 2% rated voltage.

3.2 Electromagnetic VTs

These are also referred to as inductive voltage transformers and are fundamentally similar in principle to power transformers but with rated outputs in VA rather than kVA or MVA. It’s usual to use this type of voltage trans former up to system rated voltages of 36 kV. Above this voltage level capacitor VTs become cost effective and are more frequently used. The accuracy depends upon the control of leakage reactance and winding resistance, which determines how the phase and voltage errors vary with burden. Permeability and core losses affect the magnetizing current and the errors at low burdens.

Therefore electromagnetic measurement VTs normally operate at lower flux densities than power transformers. The derivation of residual voltages for earth fault protection using open delta tertiary windings and five limb or three single-phase VTs is explained in Section 10.

It’s usual to provide fuse protection on the HV side of electromagnetic VTs up to 36 kV, although some utilities prefer to dispense with these at voltages below 15 kV, on the basis that fuse failure is much more likely than VT failure in modern equipment at these voltages. In addition fuses or miniature circuit breakers (MCBs) are used on the secondary side to grade with the HV protection and to prevent damage from secondary wiring faults.

FIG. 4 Capacitor voltage transformer arrangement. Reactor 12 kV (Typical) Intermediate voltage Phase voltage Electromagnetic transformer Power line carrier coupling option Relays, etc.

110/v3 V (Typical)

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TBL. 2 Coupling Transformers and Capacitor Voltage Transformers

Manufacturer Type Intermediate phase-to-earth voltage (kV) Total capacitance at 100 kHz (pF) 1-min power frequency withstand (kV) Impulse withstand 1.2/50 µs (kV) Insulating medium Dielectric power factor at kHz Choose frequency to suit power line carrier system Weight (kg) CVTs Rated burden per class (VA) Temperature coefficient of ratio per _ C Maximum errors with 5% primary voltage ratio (%) Phase angle (minutes) Intermediate voltage (kV) Secondary output voltage and electromagnetic transformer tapping range (V, 6V)

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3.3 Capacitor VTs

Capacitor voltage transformers (CVTs) use a series string of capacitors to provide a voltage divider network. They are the most common form of volt age transformers at rated voltages of 72 kV and higher. A compensating device is connected between the divider tap point and the secondary burden in order to minimize phase and voltage errors. In addition a small conventional voltage transformer is used to isolate the burden from the capacitor chain. Tapping connections are added to this wound isolating transformer in order to compensate for manufacturing tolerances in the capacitor chain and to improve the overall accuracy of the finished CVT unit. Coupling transformers may also be added to allow power line carrier signaling frequencies to be superimposed upon the power network. A typical arrangement is shown in FIG. 4. In addition to the accuracy class limits described for electromagnetic transformers CVTs must be specified to avoid the production of over voltages due to ferroresonant effects during transient system disturbances.

FIG. 5 LVAC switchboard busbars and ring CTs. Note the clearly displayed CT terminal markings P1, P2 and S1, S2.

3.4 Specifications

Capacitor voltage transformers and coupling capacitors may be specified in the format shown in TBL. 2 for open terminal 145 kV-rated voltage equipment.

FIG. 6 Transformer neutral ring CT with insulator support.

FIG. 7 Optical CT HV installation ( ABB).

4 FUTURE TRENDS

In conjunction with the development of complete substation automation systems (see Section 10), future trends include optical data communication to 'optical' CTs and VTs. IEC standard 61858 covers this optical communication from the process side.

An optical CT HV installation with optic communication to the relay is pictured in FIG. 7.