Saturday, April 16, 2011

In Service on Relaying Current Transformer

Instrument Transformer and Power Management (P1) Course
Chapter (6) : Current Transformers
6.8 In Service on Relaying Current Transformer
6.8.1 Introduction : 
        Secondary winding turn to turn short circuits may develop at saturation voltage or voltages approaching this value.
        Such faults may not show up at lower test voltage levels.
       In order that these winding defects may be identified, stringent tests must be carried out on current transformers as a final step, before being placed in service, but after they have been installed in their permanent physical positions.
       These comments apply particularly to free -standing CT's , but the same practice should be adopted on CT's which are integral with other equipment, ie, bushing CT's.

6.8.2 Ratio tests
       Whenever practical, ratio tests should be performed by passing current through the primary winding. However, in cases where this procedure is not convenient or practical, a ratio meter may be used.
       If a ratio meter is used, all other cores on the same primary should be shorted during the primary to secondary ratio tests.

       Because of the high ratio, the accuracy of the primary test will probably be relatively poor. The secondary winding should therefore, be checked separately as an autotransformer to establish the correct taps

6.8.3 Saturating tests
       All current transformers must be tested to the point where the core becomes saturated. To perform tests to this level will require the use of a step up potential transformer using a variac as a means of controlling the voltage.
       The rating of a suitable transformer would be: 1000 V A 120/240 rated volts, Class "8" insulation.
       The tapped low voltage winding would permit the use of either a 115 or 230 volt variac rated at 10 and 5 amperes respectively.

6.3.4 Test period and scope :
       The foregoing procedure specifically relates to new or changed installations. In addition, these procedures should be used for any existing installations which may not have been tested and also to identify any which may have deteriorated since the last testing. All installations ( 138 KV and above) should be tested every periodically at the time of " major maintenance" or if the C. T operation is suspect.

6.3.5 Safety : 
       Normal safety procedures should be adopted while testing at these high saturation voltage levels.
      When conducting tests from indoors , it is recommended that voltages be kept below 1000 volts.
       If the CT can not be driven into saturation at this 1000 volt maximum it will be necessary to check it at its physical location on a lower ratio tap.

6.3.6 Practice for testing a current transformer:
Basic concepts  : 
       Before placing a current transformer in service, we should record any name plate data and perform the following tests:
1- A  visual inspection of the transformer.
2- A  insulation test.
3- A   polarity test.
4- A  ratio test.
5- A  saturation test A visual inspection test  :
       In this test we depend on our eyes and any abnormal notes we can see on the transformer and its windings insulation test :
       Obtain an insulation tester (Megger) from the instructor, and test the insulation between windings. and from each winding to the core (case). The tester that you use should have a test voltage of at least 500 volts, do not use the 250 volts tester .
Record your results :
a) Between the primary and secondary windings

b) Between the primary and the case ( with the secondary shorted to the case).
c) Between the secondary and the case ( with the primary shorted to the case ). A polarity test
       If at any instant, current is entering the primary from HI the current should be leave secondary from terminal marked XI. We have to use a d.c meter like voltmeter and a d.c source: For the fig. shown we can determine if the polarity markings are correct or not. When the key is pressed current enters the primary through terminal HI, If the voltmeter connect as shown, it should read positive.
Procedure :

       Connect the circuit as shown in fig. and answer the following questions:
1- When a d.c source is applied, did the meter pointer deflect up-scale or down?
2- When the d.c source is removed, did the meter pointer deflect up-scale or down?
-If the deflection of the meter was in the positive direction when voltage was applied then, HI terminal is in phase with XI. If not, the Ct polarity  is reversed. Turns ratio test
       In this test we need 2 -Ammeters , A.c source
        We know that :
Ip / IS = Ns / Np
       Then, if Ns increase, it leads to increase in Ip.
 Procedure :

       Connect the circuit as shown, record the following currents.
X1 - X2          Give : Ip1 = ........           Is1 = .....
X1 - X3         Give  : Ip2 = ........           Is2 = .....
X1 - X4          Give : Ip3 = ........           Is3 = ......

        Then , we can calculate ct ratios:
X1 - X2         Ratio =           Ip1/Is1
X1 - X3          Ratio =           Ip2/Is2
X1 - X4          Ratio =           Ip3/Is3 A Saturation test
       The final test to be performed on a CT is always the excitation test. This is the test in which the secondary voltage is increased with the primary open circuited until saturation of core is evident.
       This saturation erases any magnetizing that you did when performing polarity tests ………….. etc.
       By reducing the a.c test voltage very slowly the core will be left demagnetized.
       This test serves two other purposes :
(1) It can be used to verify the manufactures data, so you can sure thatthe Ct will operate correctly with the connected burden.
(2) t may be the only way of finding secondary turn -turn short circuits.These may not appear at the very low normal operating voltages butlikely appear when stressed by this test.
Note :
This test is looking for the knee point of the excitation curve only with the transformer unloaded. Never allow the current to exceed about IA during this test.
Lab procedure :
We will need in this test :
                                      Voltmeter , ammeter and variable a.c source.
I -Connect the CT to a low voltage variable a.c supply as shown in Fig.

Make sure that. The variable a.c supply is turned to zero and then turn the power on. Slowly increase the voltage until the ammeter reads about 500 mA. Record the voltage at this point.
V = …………………. V            1 = ……………………..mA
2- Decrease the voltage back to zero ( very slowly ). The recorded voltage is the maximum you should go to for this test. Use this maximum to select 10 appropriate tests and fill in the chart shown.
3-Repeat the two previous steps using XI -X3 winding and complete the chart , and so on

4- Plot the results of both tests on one graph. Put the current on the horizontal axis and voltage on the vertical axis.

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