Meter & Pilot Lead Burden

Burden is the load imposed on the secondary of the CT at rated current and is measured in VA (product of volts and amps). The accuracy class applies only to loads at rated VA and below, down to one quarter VA .The burden on the secondary of a CT includes the effect of pilot leads, connections etc , as well as the instrument burden itself.

In situations where the meter is remote from the current transformer, the resistance of the pilot wires may exceed the meter impedance many times in these cases it is often economical to use 1 amp meters and CTs.

The diagram shows the burden imposed on the CT due to a run of pilot wire. It will be seen that a pilot loop of 2.5mm2 wire, 60 metres long (30 metres distance) has a load of 12.5 VA on a 5 amp CT but only 0.5VA on a 1 amp CT.

Wound Primary CTs

Thus, current transformers for 80 amps and
below frequently require more than 1 turn to achieve the desired accuracy class. Considering again the previous example.

Using the same core and by winding 200 secondary turns and 4 primary turns a 50/1 ratio is achieved. The magnetising ampere turns
remains at 2 as before, however the magnetising current becomes 2 divided by 4 turns or 0.5A and the percentage error is reduced to 1% (approx.).

It is therefore possible to achieve accuracy requirements, without using expensive core materials, by constructing a wound primary transformer. Of course, the cost of the primary
winding with its insulation and terminations must be weighed against the cost of the more expensive core which would be required to achieve a 1% accuracy for a ring CT at a 50/1 ratio.

Ampere Turns Rule

An ideal transformer is based on the Amperre Turns Rule, i.e. Primary Ampere Turns = Secondary Ampere Turns or: IpTp = IsTs (Ts/Tp=Ip/Is)

Thus the current transformation ins in INVERSE proportion to turns whereas voltage transformersion is in DIRECT proportion to turns ie Ts/Tp=Vs/Vp

Design Considerations

As in all transformers, errors arise due to a proportion of the primary input current being used to magnetise the core and not transferred to the secondary winding. The proportion of the primary current used for this purpose determines the amount of error.

The essence of good design of measuring current transformers is to ensure that the magnetising current is low enough to ensure that the error specified for the accuracy class is not exceeded.

This is achieved by selecting suitable core materials and the appropriate cross-sectional area of core. Frequently in measuring currents of 50A and upwards, it is convenient and technically sound for the primary winding of a CT to have one turn only.

In these most common cases the CT is supplied with a secondary winding only, the primary being the cable or busbar of the main conductor which is passed through the CT aperture in the case of ring CTs (i .e. single primary turn) it should be noted that the lower the rated primary current the more difficult it is (and the more
expensive it is) to achieve a given accuracy.

Considering a core of certain fixed dimensions and magnetic materials with a secondary
winding of say 200 turns (current ratio 200/1 turns ratio 1/200) and say it takes 2 amperes of the 200A primary current to magnetise the core, the error is therefore only 1% approximately. However considering a 50/1 CT with 50
secondary turns on the same core it still takes
2 amperes to magnetise to core. The error is then 4% approximately. To obtain a 1%
accuracy on the 50/1 ring CT a much larger core and/or expensive core material is required.

Saturation

Magnetic materials are such that when the magnetic flux reaches a certain value the core will saturate. At this point a large proportion of the primary current is required to magnetise the core Increasing the primary current in the saturation region will therefore cause only a marginal increase in secondary current. It is obvious that the CT is completely inaccurate when saturated Saturation can occur if the actual burden exceeds the rated burden, or if heavy overcurrents occur.

This phenomenon can be used to protect an instrument against damage due to heavy overcurrent and a Saturation Factor is sometimes specified. For example, if a Instrument Sensitivity Factor (Fs) of less than 5 is specified, the CT must be designed to ensure that, at the rated burden, the core is well into the saturation region (defined point) at 51 times the rated primary
current.

It is critical that the actual burden is established to ensure the saturation factor is complied with. Fs is the ration of instrument limit primary current to the rated primary current. The lower the factor the higher the degree of safety. However it is not always practical to achieve a high accuracy class with an extremely low instrument security factor.

Open Circuit Current Transformers

It is important to ensure that the secondary of any CT is not left disconnected while the primary supply is on. In this condition, high voltage spikes are produced in the transformer secondary, often thousands of volts, sufficient to break down the transformer insulation.

Dual or Multi-Ratio Transformers

Frequently when a new plant is commissioned, it is planned for further extension and consequent increase in power consumption. In this event, it may be advisable to install dual ratio CTs with a tapped secondary to allow alterations to the metering without the expense and disruption of replacing the CTs. In this case, the unused terminal should be left open-circuit.

Construction of Measuring Current Transformers

Measuring current transformers are available in a variety of different forms and terminations to meet the requirements of the particular installation.

Taped Ring CTs
(R Range)

Suitable for Indoor use, are probably the most common type of CT Installed in LV switchgear and control gear Circular cores wound in clock spring fashion provide a near ideal magnetic circuit, free of air gaps and having very low leakage or stray fields. After applying suitable robust insulation to these cores, the windings are applied evenly around the cores by toroidal winding
machines. Taped ring CTs are a so used extensively in HV switchgear and power transformers where the insulation is provided by the HV bushings. Where CTs are to be installed under hot oil, insulation materials have to be selected to avoid pollution of the oil. Rectangular core CTs are also available to suit applications where space is very restricted.

Enclosed Plastic Moulded Case
Current Transformers (M Range)

Similar to the ring CTs but enclosed in tough injection moulded shells to provide a clean and uniform appearance. Mounting brackets and facilities for busbar clamps and other accessories can be incorporated in the mould design thereby resulting in a lower cost unit. The speed and simplicity with which moulded case CTs can be clamped to busbars is an important feature of the unit.

Resin Cast Current Transformer

Produced by casting the transformer in liquid resin which is then cured to the solid state. These transformers are expensive, but they are robust and immune to difficult climatic conditions. Special resins are now available to make the transformers suitable for outdoor use. Resin Cast CTs are commonly used at high voltages, up to 33kV. In these applications, highly controlled casting techniques are required to avoid air voids where corona discharge could effect the quality of insulation.