The effect of the increasing burden could eventually increase the PT output current above its operating level and so introduce inaccuracies
With most voltage transformers the burdens are expressed not in ohms but in terms of what the transformer must be able to supply in VA
Experiences John that generally the VT does provide good reproduction of primary voltage both transient and steady-state
With sufficient accuracy for protection function at this point let's take a well-earned break and then we'll come back and look at the subject which is vital to VT and CT polarity for now switch the tape off and go through this material in your workbook
In connecting up instrument transformers to realize it's obvious that we have to pay particular attention to the direction of current flow that is polarity
In discussing polarity in an AC circuit we are referring to instantaneous values of voltage and current but what is meant by polarity
As an example look at this typical transformer here the primary and secondary windings are wound on separate legs of the core
They can be one in either the same or opposing directions note that here both primary and secondary windings are in the same direction
If the windings are wound in the same direction the secondary current flows in the same direction as the primary
The polarity of the transformer is termed additive we can see that current flows out of the secondary winding at the opposite end to current entering the primary to help identify this the terminals are marked like this
By convention H refers to the primary and X the secondary H1 is current into the primary and X1 is current out of the secondary
Conversely when primary and secondary turns are wound in the opposite direction the secondary current flows in the opposite direction to the primary current
The transformer is said to have subtractive polarity
Here the secondary current flows out at the same end as the primary is entering and the terminals are marked accordingly
In instrument transformers the windings are almost always opposing that is subtracted
On schematic diagrams the CT is often shown like this
The plus sign indicates where the primary current flows in and where the secondary current flows out
You will also see CT polarity markings shown as X’s dots or even squares
This identification enables us to connect the secondary windings to the relays in the correct relationship
The same polarity rules apply for potential coils the voltage transformer is marked on both the primary and secondary
Before connecting a relay to a CT it is advisable to check that the polarity markings are correct
One of the most common methods used to determine polarity is the kickback method
In this method DC moving coil voltmeter is connected across the CT primary winding that is the high-voltage side with the positive lead connected to the terminal marked H1 and the negative lead connected to H2
The positive terminal of a dry cell battery typically a one-and-a-half for 9 volt is then connected to the secondary terminal marked X1 the negative terminal of the battery is connected to X2 with an open switch in the line
Beware the current applies in this test must be limited to a very low value
That is why a high resistance dry cell is recommended to prevent the core from becoming magnetized
Now close the switch when it makes you should see an upscale kick of the meter if the polarity markings are correct
Take care not to apply the voltage to the high-voltage terminals primarily in the case of the CT otherwise an extremely high voltage will be induced into the secondary
Voltage transformers can be checked in the same manner but another check can also be used by applying 120 volts AC on primary
As this is a 10 to 1 step-down transformer we should read 12 volts across the secondary terminals
Jumper a primary and a secondary terminal on one side of the transformer connect a voltmeter between the two terminals on the other side of the transformer
Because of the jumper the voltmeter will feel the effects of both the source voltage and the induced voltage
In this VT the windings are connected in a subtractive configuration so the induced voltage is in Phase with the source voltage
The voltmeter responds to the difference of the two in phase voltages
Therefore on our meter if the terminal polarity is correct we should read 120 minus 12 equals 108 volts
For an additive transformer the induced voltage is out of phase with the source voltage and therefore we should read 120 plus 12 equals 132 volt
You can see now where the terms additive and subtractive come from and referring to polarity of course there are other ways of checking polarity and some equipment that’s available also check ratios