The adhering to points highlight the 2 major kinds of power transformers. The forms are: 1. Current Transformers (CTs) 2. Potential Transformers (PTs).

Type # 1. Current Transformers (CTs):

Current transformers are connected in ac power circuits to feed the present coils of indicating and metering instruments (ammeters, wattmeter’s, and also watt-hour meters) and protective relays. Hence the CTs broaden the borders of measurements and preserve a watch over the curleas flowing in the circuits and also over the power tons. In high voltage installations CTs in enhancement to over, likewise isolate the indicating and also metering tools and also protective relays from high voltage.

The present transprevious (CT) basically consists of an iron core on which are wound a major and also one or 2 additional windings. The main winding of the CT is associated in series via the fill and carries the actual power device present (normal or fault) while the secondary is connected to the measuring circuit or the relay. The primary winding is generally single rotate winding and the number of turns on the additional winding depend upon the existing to be brought by the power circuit.

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The bigger the current to be carried by the power circuit, more the number of transforms on the second. The ratio of primary existing to the secondary existing is recognized as transdevelopment proportion of the CT. The present ratio of a CT is generally high. The secondary existing ratings are of the order of 5 A, 1 A and also 0.1 A, the last being supplied for static relays. Primary existing ratings vary from 10 A to 3,000 A or more.

The current transprevious in procedure slightly differs from the power transprevious. In case of a current transprevious as the load impedance or “burden” on the secondary is exceptionally tiny, so the present transprevious operates on short-circuit conditions. Also the present in the secondary winding is not governed by the fill impedance on the second yet depends upon the current flowing in the primary winding (i.e., power circuit current).

The amount of power which the CT handles is tiny. The product of voltage and also existing on the secondary side when the CT is offering the instrument or relay through its maximum rated worth of existing, is well-known as rated burden and is measured in volt-amperes (VA). The volt-ampere rating of CTs is low (5 – 150 V A) as compared with that of power transformers (a couple of kVA to several MVA).

All kinds of CTs are employed for protective relaying purposes. The bushing CT is the majority of common for hv circuits as it is less expensive than the various other forms. However, it is not employed for circuits below 5,000 V or in metal clad tools.


The bushing kind CT consists only of an angular shaped core via a secondary winding. Such CT is developed into equipment such as circuit breakers, power transformers, generators or switchgear, the core being arranged to encircle an insulating bushing via which a power conductor passes.

Tbelow is just one major rotate and the inner diameter of the bushing CT core is huge. Hence, the intend length of turn of the magnetic course is higher than those in various other CTs. The cross- section of the core is made bigger. The saturation in the core is less and also a bushing CT is more precise than other CTs at high multiples of the main current rating. At low currental fees, a bushing CT is commonly less specific as a result of its big interesting present.

Ratio and Phase Angle Errors of CTs:

CTs introduces two errors—ratio error and a phase angle error.


Current proportion error is largely as a result of power component of excitation current and also is offered as:


wbelow Ip is the main current. KT is the turn-proportion and also IS is the second current.

Phase angle error is presented on account of magnetising component of excitation current and a phase angle error δ is given in minutes of angle of leave of the additional existing from precise opplace to the major current. For relaying purposes, the phase angle error has negligible impact on the accuracy of the CT. The load on the secondary of the CT is normally very inductive so that the second current is almost in phase through the interesting present.


The CTs are rated for rated voltage of the installation, the rated curleas of the major and also additional windings and also the accuracy class. The accuracy course shows the limit of the error in percent of the rated turn-proportion of the provided CT.

Type # 2. Potential Transformers (PTs):

The potential transformers are employed for voltages above 380 V to feed the potential coils of indicating and also metering instruments (voltmeters, wattmeter’s, and also watt-hour meters) and relays. These transformers make the plain low voltage tools suitable for measurement of high voltage and isolate them for high voltage.

The major winding of the potential transprevious is linked directly to the power circuits either between 2 phases or in between a phase and ground depending on the rating of the transformer and also its application. To the second windings, assorted indicating and metering tools and also relays are linked.

The main winding has a large variety of turns and also the secondary winding, which has a much smaller number of transforms, is coupled magnetically with the magnetic circuit to the major winding. The primaries of PT are rated from 400 V to a number of thousand volts and secondaries always for 110 V (occasionally for 110 √3 V). The ratio of the rated primary voltage to the rated secondary voltage is well-known as rotate or transformation proportion.

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The ends of the windings of potential transformers are marked according to the same rules as offered for power transformers to encertain correct connection of the windings to each various other, to the high-voltage power circuit and also to assorted indicating and also metering tools and relays they serve to supply.

The theory of operation of a potential transprevious (PT) is essentially the exact same as that of the power transprevious. The primary distinction between a PT and a power transformer is because of the truth that the load present of the previous is of the order of its amazing present and additional impedance of the PT is largely resistive being comprised of the potential circuit of the instrument.

The PT is rated in terms of the maximum burden (volt-ampere output) it delivers without exceeding specified limits of error, whereas the power transprevious is rated by the secondary output it delivers without exceeding a stated temperature climb. The output of PTs is typically restricted to a couple of hundred volt-amperes while the output of a power transformer may be of the order of several MVA.

Ratio and Phase Angle Errors of Potential Transformers:

Ideally a PT need to offer second voltage specifically proportional to the main voltage and also specifically in phase opposition. This cannot obviously be accomplished in exercise as a result of major and secondary voltage drops. Thus both the proportion and also phase angle errors are introduced by a PT.

The voltage ratio error is mainly expressed in regards to the measured voltage and is offered as:


wright here KN is the nominal ratio—the ratio of rated primary voltage and rated secondary voltage.

The phase angle error δ is commonly given in minutes and suggests by what little angle δ the second voltage VS decomponents from exact opposition to the applied major voltage VP.

With the rise in the variety of tools and relays associated to the second of a PT, i.e., through the rise in burden of the additional circuit, both of the errors will certainly boost.

The ‘burden’ is the complete outside volt-ampere load on the second at rated additional voltage. Wbelow a number of tons (tools and relays) are linked in parallel, it is usually enough to include their individual volt-amperes arithmetically to identify the total volt-ampere burden.

The proportion and also phase angle errors are also influenced by the main voltage. Operation in excess of 10% overvoltage may reason enhanced errors and excessive heating. However, if a PT has actually acceptable accuracy at its rated voltage, it is suitable over the range from zero to 110% of rated voltage.

The ratio and also phase angle errors of any kind of conventional ASA PT are so tiny that they might be neglected for the protective relaying purpose if the burden is within the ‘thermal’ volt-ampere rating of the PT. This thermal VA rating synchronizes to the full-load rating of the power transprevious.

The rated burden of a PT is the VA burden which have to not be exceeded if the transprevious is to operate through its rated accuracy (this burden is suggested on the name plate and in the information sheet).

The limiting or maximum burden is the best VA load at which the PT will certainly run continuously without overheating its windings beyond the permissible limits. This burden is a number of times better than the rated burden.

In practice, usage is made of single phase and three-phase potential transformers, the last being of the two-winding or three-winding type.

Tright here are two forms of potential transformers; the traditional wound kind (or electromagnetic type) and also the capacitor voltage (potential) transprevious. For voltages exceeding 100 kV (phase) the standard form of potential transformer becomes very expensive owing to the insulation demands. For voltperiods over 100 kV a capacitor potential transprevious, a combination of a capacitor potential divider and a magnetic potential transprevious (well-known as the intermediate transformer) of fairly little ratio is supplied.

The circuit diagram is presented in Fig. 16.18. A stack of high-voltage capacitors develops the potential divider, the capacitors of the 2 sections being C1 and also C2 respectively and also Z is the burden. The voltage used to the primary of the intermediate transformer T is typically around 10 kV. Both the potential divider and the intermediate transprevious will have actually ratios and also insulation demands which are suitable for economical building and construction. The intermediate transprevious need to be of exceptionally little proportion error and phase angle to offer satisfactory performance of the complete unit.