Instrument Transformers are of two types :
1.Current Transformer (or Series Transformer)
2.Potential Transformer (or Parallel Transformer)
Current transformers are used when magnitude of AC currents exceeds safe value of current of measuring instruments.
Potential transformers are used where voltage of an AC circuit exceeds 750 V as it is not possible to provide adequate insulation on measuring instruments for voltage more than this.
CURRENT TRANSFORMER (CT) Definition
1. CTs are used for reducing / stepping down ac current from higher value to lower value for measurement / protection / control. Typical secondary current is 5 A rms
2. CT secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.
Working principle
1.A CT functions with same basic working principle of electrical power transformer but here is some difference.
If a electrical power transformer or other general purpose transformer, primary current varies with load or secondary current.
2.In case of CT, primary current is system current and this primary current or system current transforms to CT secondary, hence secondary current or burden current depends upon primary current of current transformer
Causes of error in CT
In ideal CT
Actual transformation ratio Kc=turn ratio Kt and phase angle would be zero. but due to physical limitations inherent in electric and magnetic ckt , errors are caused.
REASONS:
1.CT draws magnetising current Im to provide exciting mmf required by primary winding to produce flux
2.CT draws energy current to supply core losses (eddy current and hysterisis losses) +electrical I2R losses. Thus energy loss component
Ie is required to supply losses associated with flux and associated culoss in winding due to exciting current Io in primary winding
3.Flux density in core is not linear function of magnetising force . Thus ct core gets saturated.
4.There is always a magnetic leakage
How to Reduce Error in Current Transformer
1. Using a core of high permeability and low hysteresis loss magnetic materials.
2. Keeping the rated burden to the nearer value of the actual burden.
3. Ensuring minimum length of flux path and increasing cross-sectional area of core, minimizing joint of core.
4. Lowering secondary internal impedance
Tests on current transformers
Type tests as per IS
The following tests are type tests
a) verification of terminal markings and polarities
b) short-time current tests
c) temperature rise test
d) impulse voltage test on primary
e) power frequency voltage withstand test on primary
f) error measurement
g) overvoltage inter-turn test
Routine tests as per IS
The following tests apply to each individual CT
a) verification of terminal markings and polarities
b) high voltage power-frequency voltage withstand test on secondary
c) inter-turn overvoltage test
d) determination of errors and accuracy class
--- Order of tests is not standardized, but determination of errors shall be performed after other tests
Special tests
Following tests are performed upon agreement between
manufacturer and purchaser:
a) chopped lightning impulse test
b) measurement of capacitance and dielectric dissipation factor
c) multiple chopped impulse test on primary winding
d) mechanical tests
e) measurement of transmitted over-voltages
Wuhan huayi electric power technology is developing and manufacuring of test equipment for all kinds of current transformers. We provide nice products and proffional solution for you. Protect and maintain your CTs.