CHAPTER THREE instrument transformers. instrument transformers are a special type of transformers, used for the measurement of voltage and current. As the name suggests, these transformers are used in conjunction with the relevant instruments such as amateurs volt meters, wide meters, energy meters as well as protection relays. Such transformers are of two types. There are current transformers that are used when the magnitude of the AC current exceeds the safe value of the current measuring devices or the instruments and they are designed for working at various voltages Levels potential Transformers or voltage transformers are used for a similar purpose and that is when the voltage of the AC circuit exceeds 750 volts as it is not possible to provide adequate insulation on measuring instruments for that voltage level, then the bolt potential or voltage transformer is introduced into the circuit So, that the the voltage to the instrument can be at a safe level.

And just as there are various voltage ratings on a current transformer there's also various voltage ratings on a potential transformer. These are examples of CTS or current transformers that are found in a power transformer. They are known as donut CTS for their similar look to a doughnut. The picture on the right is not a CT but a typical transformer bushing which would be mounted through the center of a doughnut CT. The secondary winding is wrapped concentrically around a toroid, which is usually made up of laminated iron or steel. The primary is a single conductor, usually a bushing mounted through the center of the toroid.

The doughnut fits over the primary conductor which constitutes one primary turn. The secondary is wound around the toroid core which is usually made up of laminated iron that concentrates the magnetic flux and forces it through the secondary turns. Some donut CTS come with the primary conductor incorporated in the CT as you see here. If the toroid is wound with 240 secondary turns, then the ratio of the CP is 240 to one or 1200 to five amp. The five amp designates the continuous reading of the secondary winding and is normally five amps in North America and one amp in or point five amp in many other parts of the world. This type of donut CP is almost or most commonly used in circuit breakers and transformers.

The CT fits into the bushing turret and the bushing fits through the center of the doughnut CTS. Up to four CTS of this type can be installed around each bushing instrument transformers are used for the following reasons. One to insulate high voltage circuits from the measuring circuit in order to protect the measuring instrument from damage to to make it possible to measure high voltage with low range volt meters and high current with low range and meters. These instrument transformers are also used in controlling and protecting circuits to operate relays, circuit breakers etc. voltage and current phasers can be summed easily on the secondary circuits of these Transformers without causing short circuit as as what happened if you were trying to say some two currents on the primary side so that a lot of times when you want to start Various current say in in in a circuit and measure them you can do this quite safely on the secondaries of current transformers.

Now, let's take a look at current transformer ratios in an ideal transformer with simple load on the secondary and an AC voltage on the primary, the secondary voltage V subscript S is determined by the turns ratio such that the s all over VP is equal to the same ratio as the turns of the secondary of the transformer over the terms of the primary of that transformer. The secondary current is is determined by the turns ratio such that i p times and p is equal to is times and s Or the secondary current is equal to the primary current by multiplied by the ratio of NP over an S. If NP are the primary turns ratio is one, then I S is equal to IP divided by n s, the number of terms of the secondary. A current transformer differs from a voltage transformer in that the secondary current is determined entirely by the load on the primary system and not its own secondary load.

In other words, the secondary current is determined by the current in the primary conductor. The voltage across the secondary load or burden will rise and fall depending on the current integration Primary and secondary, both primary and secondary, both the primary and secondary of CTS have relatively few turns of heavy wire and thus low impedance subsequently, a current is readily induced into the secondary portion of the primary current. As we have seen CTS behave according to the mathematical relationship where I asked is given by the turns ratio times i P. And if the n p or the primary turns is one, then I S equals IP divided by the number of terms of the secondary the transformer ratio of a CT is defined by primary current divided by the secondary current for a given CT, where the primary current is 100 amps and the secondary current is say five amps then the transformer ratio or current ratio is 100 to five or 100 to five in this fashion or you could say it was 20 to one for the same C key what would happen if the primary was loop back then fed through the CP one more time the CP would see a total of 100 times two or 200 amps flowing in the primary therefore, the secondary would be 10 amps, the transformer ratio is 100 to 10 or 10 to one.

This is sometimes used as the primary current is too low for a meter to be able to read, you simply loop the primary conductor through another time. And you could actually loop it through a couple of times if you want and that would increase the amount of current flowing in the primary that the CP would see. Looking at it another way the CTE sees two primary wires and sees a total of 200 amps flowing in the primary. Therefore the secondary would 10 amps, the transformer ratio then would be 100 to 10 or 10 to one designating the polarity of a CP is very important and it's usually done by the manufacturer to indicate the polarity of the secondary with respect to the polarity of the primary. And the primaries and secondaries are marked to indicate which direction the current will flow during each half cycle of current.

So in this case, we've got a spot on the facing side of the CT. And we have a spot on the right hand terminal of the secondary of that CT in this picture the I triple Use a different standard, but it means the same thing. The spot side of the primary is h1 and the spot side of the secondary is x one. And in terms of the IEC or International Electrotechnical Commission, they use p one and s one to designate the same thing as h one and x one that the I triple E do suffice it to say we now have a terminal mark on the secondary and we have a side of the CP mark with a spot of this particular donut CP. So, what does this mean? This indicates that when the primary current is positive during the sinusoidal half cycle or the current is flowing into the spot on the primary, the secondary current will be out Have the spot on the secondary okay.

So, the primary current is positive during the sinusoidal half cycle or current is flowing into the spot and the current will be flowing out of the spot on the secondary. And what this also indicate is that when the primary current is negative during the sinusoidal half cycle or the current is out of the polarity spot on the primary, the secondary current will be out of the non polarity spot of the secondary. So, the primary current is negative during the sinusoidal half cycle current out of the polarity spot, the secondary current will be out of the non polarity spot This is sometimes shortened to primary current flowing into a spot results in secondary current flowing out of the spot. Potential transformers. In general, a secondary voltage is proportional to the ratio of the turns and opposite in phase to the primary voltage. This relationship is not completely exact for the following reasons.

The exciting current that is necessary to magnetize the iron core causes the impedance drop in the causes of new pistons drop in the primary winding. The load current that is drawn by the burden causes the impedance drop in both the primary and secondary windings. Both of these produce an overall voltage drop in the transformer and Introduce errors in the ratio and phase angle. The net result is that the secondary voltage is slightly less than the ratio of the turns would indicate and there's a slight difference in the phase relationship. These two errors are called ratio errors and phase angle errors and may be represented by the transformer equivalent circuit. Now, I say this and I mentioned it not that it's necessarily typical of only potential transformers, all Transformers have these errors introduced into them because of non linearity and magnetization current.

And so anytime you're using a transformer configuration you should be looking at the exact transformer representative With all its losses, however, in most cases you can ignore these losses when calculating the secondary voltage in terms of the primary voltage. I mentioned this only in passing so that it is in the back of our minds if we do get into very close measuring circuits, we have to take these into account. Okay, designating the polarity of a PT is very important and done much the same as in the case of a current transformer a CT biomarker marking the PTT, primary and secondary terminals and or indicating it on the nameplate. Again, this is especially important when PPS are used for relaying or revenue meters. As in the case of the CP polarity, the primary and secondary terminals are marked in such a way that when The primary current is positive during the sinusoidal half cycle current into a polarity, the secondary current will be out of the polarity spot or in terms of voltage, voltage rise on the h1 terminal gives a voltage rise on the x one terminal.

On schematic drawings PTs and CTS with their polarity markings may look something like this. This is an actual standard drawing using PPS and CTS with their polarity marks and connected to give the right measurements for a kilowatt hour meter. You can see the PTS on the bottom left the Spot markings of the PP look like axes on the coils of the transformer. And in the case of the CTS, the spot markings are indeed or look like square or rectangular boxes on the sides that are the mark sides of the CT is another example this time on a protection circuit for a transformer differential protection circuit. You can see that the markings for the PTS and the CTS are designated with circles. And in the case of the actual power transformer itself, the polarity markings are indicated in the same manner.

This is the end of chapter three