CURRENT TRANSFORMER TESTING

TEST	EQUIPMENTS USED
1.    General Visual Check	a.    5KV Insulation Tester (Make: Megger)        b.    CT Analyzer (Make: Omicron)        c.    CPC 100 (Make: Omicron)
2.    Insulation Resistance Measurement
3.    Polarity Check
4.    CT Winding Resistance
5.    Knee Point Voltage Measurement
6.    Ratio Test

*General Visual Checks Includes

·         Name Plate Details

·         HV & LV Terminal Marking

·         Checking for tightness

·         Position of P1 towards Bus (as per KHARAMA)

·         CT Wiring with ring type lug and as per schematic diagram

CT Secondary Winding Impedance

As a protection CT may be required to deliver high values of secondary current, the secondary winding resistance must be made as low as practicable. Secondary leakage reactance also occurs, particularly in wound primary current transformers, although its precise measurement is difficult. The  non-linear  nature  of  the  CT  magnetic circuit  makes  it  difficult  to  assess  the definite ohmic value representing secondary leakage reactance. It is, however, normally accepted that a current transformer is of the low reactance type provided that the following conditions prevail:

a. The core is of the joint less ring type (including spirally wound cores)

b. The secondary turns are substantially evenly distributed along the whole length of the magnetic circuit

c. The  primary  conductor(s)  passes  through  the approximate  centre  of  the  core  aperture  or, if wound, is approximately evenly distributed along the whole length of the magnetic circuit

d. Flux equalizing windings, where fitted to the requirements of the design, consist of at least four parallel-connected  coils, evenly distributed along the whole length of the magnetic circuit, each coil occupying one quadrant

Alternatively, when a current transformer does not obviously comply with all of the above requirements, it may be proved to be of low-reactance where:

e. The composite error, as measured in the accepted way, does not exceed by a factor of 1.3 that error obtained  directly  from  the  V-I  excitation characteristic of the secondary winding

Anti-remanence CT’s

This  is  a  variation  of  the  over dimensioned  current transformer and has small gap(s) in the core magnetic circuit,  thus  reducing  the  possible  remanent  flux  from approximately 90% of saturation value to approximately 10%.  These gap(s) are quite small, for example 0.12mm total,  and  so  the  excitation  characteristic  is  not significantly  changed  by  their  presence.    However,  the resulting  decrease  in  possible  remanent  core  flux confines  any  subsequent  d.c.  flux  excursion,  resulting from  primary  current  asymmetry,  to  within  the  core saturation  limits.    Errors in current transformation are therefore significantly reduced when compared with those with the gapless type of core. Transient protection current transformers are included in IEC 60044-6 as types TPX, TPY and TPZ and this specification gives good guidance to their application and use.

Ohms Law

The most fundamental law in electricity is Ohm’s law or V=IR. The V is for voltage, which means the potential difference between two charges. In other words, it is a measurement of the work required to move a unit charge between two points. When we see a value such as 10 Volts, it is a measurement of the potential difference between two reference points. Normally the two points will be +10V and 0V (also known as ground), but it can also be the difference between +5V and -5V, +20V and +10V, etc. In the field, you might hear the term “common grounds” which refers to each device in a system using the same zero-point reference (or ground) to ensure the same potential difference ( or voltage) is applied throughout the system. The next component of Ohm’s law is current, the units of which are Amperes; in the formula, current is represented by the very logical choice of the letter I. As mentioned previously, current is the measurement of the flow of charge in a circuit. This leaves us with the letter R which represents Resistance. Electrical resistance, measured in Ohms, is the measure of the amount of current repulsion in a circuit. Simply, resistance resists current flow. When electrons flow against the opposition offered by resistance in the circuit, friction occurs and heat is produced. The most common application for resistance in a circuit is the light bulb. The light bulb introduces enough resistance in a circuit to heat up the filament inside, causing light to be emitted. Resistance in a circuit can also be helpful when needing to alter voltage levels, current paths, etc. Resistors are self-contained packages of resistance that can be added to a circuit and are commonly used to divide voltage levels.

Basic Electrical Concepts & Terms

• Electrical voltage
• Electrical current
• Electrical resistance
• Electric power
• Electric charge
• Power efficiency
• Power factor

ELECTRICAL TESTING & COMMISSIONING INTERVIEW QUESTIONS

ELECTRICAL TESTING & COMMISSIONING INTERVIEW QUESTIONS

1.      C.T – MOS for Testing?

2.      What is CT Knee point voltage? What is the purpose of doing Knee point test?

3.      Explain about knee point curve using plot?

4.      How can you do polarity test in CT?

5.      Explain about IR, Polarity, Winding resistance with diagram?

6.      CT & VT Magnetizing current testing method

7.      V.T – MOS for Testing?

8.      What is the purpose of doing magnetizing current test in VT?

9.      Operating Principle of Transformer?

10.  What is meant by Magnetic flux?

11.  What do you mean by % Impedance?

12.  How to calculate % Impedance?

13.  What is the value of IR while testing Transformer at site?

14.  What is value of tolerance in short circuit impedance?

15.  What is the test procedure to test magnetic balance in transformer?

16.  What are tests to carry out in oil?

17.  How transformer differential stability will be carried out?

18.  What is the difference between no load current and magnetizing current in transformer?

19.  What is the procedure for doing Zero sequence Impedance test?

20.  a. What is trip time / Closing time of circuit breaker

       b. Why tripping time is lesser than closing time?

21.  Draw and explain IDMT characteristics?

22.  What are the curves in Relays?

23.  Relays time calculation formula.

24.  Explain the operation of Directional over current relay with diagram?

25.  What is the formula for relay timing test ( 2times)

26.  Explain TCS (pre-closed & post-closed) circuit with diagram.

27.  Backup Relay –Method of Testing

28.  How can you check synchronizing check

29.  Inter trip logics- Operations

30.  Explain about line protection differential scheme?

31.  a. Explain how load transfer to be done for double bus bar scheme.

b. During load transfer, If there is bus fault, bus coupler CB will trip (or) not.

32.  What is the initial charging current and final charging current for 1320AH battery of DC scheme?

33.  How do you check Synchronizing Scheme?

34.  How much distance between spark gap contacts?