1. CURRENT TRANSFORMERS AND VOLTAGE TRANSFORMERS

2. INTRODUCTION
Function of the protection relays
Information to the protection relays have to be accurate and reliable
Currents and voltages
How to overcome this difficulty

3. WHAT DOES CURRENT AND VOLTAGE TRANSFORMERS DO…
Insulate secondary circuits from primary
Provide currents/voltages proportional to the primary

4. ROLE OF CT, PT IN POWER SYSTEM PROTECTION
Protection systems depend heavily on information provided
CTs and PTs a constituting part of the protection system
No great difference between a measuring voltage transformer and a protective voltage transformer
However, Current transformer is different

5. BASIC PRINCIPLES OF OPERATION
Transformer can be operated in
Shunt mode
Series mode

6. SHUNT MODE Voltage applied across the primary terminals
When secondary is on open circuit, Induces an EMF equal to the voltage applied
Current flowing will be that required to excite the core
When secondary is loaded Ampere turns of the primary winding will exceed that of secondary
Difference is the flux needed to excite the core.

7. SERIES MODE Primary winding is connected in series with a circuit.
Its current is determined by the sources and loads.
Its current is determined by its sources/load conditions
A component of this current is used to excite the core sufficient to induce an EMF to drive a current in the secondary

8. COMPARISON-SHUNT MODE AND SERIES MODE
Voltage applied across the primary terminals
When secondary is on open circuit, Induces an EMF equal to the voltage applied
Current flowing will be that required to excite the core
When secondary is loaded Ampere turns of the primary winding will exceed that of secondary
– Difference is the flux needed to excite the core
Series mode
Primary winding is connected in series
Its current is determined by its sources/load conditions
A component of this current is used to excite the core sufficient to induce an EMF to drive a current in the secondary

9. STEADY STATE THEORY OF CTS

10. CT ACCURACY If the CT is ideal Primary AT = Secondary AT
Secondary current ∞ Primary current
Every CT requires a certain AT to induce in it the magnetic flux
With secondary impedance fixed exciting AT form a larger proportion at smaller primary currents
Exciting current required to magnetize the core is the cause for th errors in CTs

11. AC PERFORMANCE OF A CT ES = 4.44 f A B 10-8 Volts
ES = Secondary induced volts
f = Frequency in cycles/second
N = Secondary number of turns
A = Core cross sectional area in sq.cm
B = Flux density in lines per sqcm.
ES = Is( Zb + Zs + Zl)

12. CT MAGNETIZATION CURVE
Exciting current is the source of errors
Value of exciting current depends on the core material and the amount of flux.
Ankle point and knee point,

13. CT EQUIVALENT CIRCUIT

14. OPEN CIRCUITING OF CT
If the secondary of a CT is kept open with a current flowing in the primary, then
No secondary current
No MMF to oppose the flux
Core driven to saturation
High rate of change of flux
High voltages

15. CT ERRORS Phase angle error Composite error Ratio error
(KnI s ‑ Ip ) x 100/ Ip
Kn = Nominal ratio
I s = Actual secondary current
Ip = Actual primary current
Phase angle error
Angle by which the secondary current vector when reversed differs from the primary current.
Composite error
r.m.s. value of the difference between the ideal secondary current and the actual secondary current. Includes the current and phase angle errors

16. LIMITS OF ERRORS
Metering CTs
Protection CTs

17. CT RATINGS Accuracy limits
Limit up to which the specified accuracy could be retained. Accuracy Limit Factor
Rated short time current
r.m.s. value of the a.c. component of the current which a CT is capable of carrying for the rated time without being damaged by thermal or dynamic effects
CT Burden
the load applied to the secondary of the CT. Normal practice is to express the burden in terms of VA and power factor, the VA being what would be consumed in the burden impedance at rated secondary current

18. CHOICE OF CT RATIO Secondary current 5 Amps or 1 Amp
Primary current to be equal to or greater than the normal full load current of the protected circuit
Maximum ratio of a CT is made about 3000/1 and secondary CTs of 20/1 are used.

19. TRANSIENT RESPONSE OF CT
Transient conditions, prevail within the first few cycles of a fault.
System conditions are very different to that under steady state conditions.
Power systems expansions increases fault currents and time constants of dc transient components.
It is mandatory for the protective equipment/circuit breakers to clear the faults within few cycles
This makes CT duties more onerous
Hence, behaviour of CTs during transient conditions is of great importance.

20. TRANSIENT CONDITIONS IN A POWER SYSTEM
Sudden application of a sinusoidal voltage to a series R/L circuit will cause a current to flow in the circuit.
This is equivalent to a fault condition of a power system,

28. Equivalent circuit