Most commonly used voltage transformers have 110V as the secondary voltage and primary voltages in the range of 11KV and above
In all general purposes, single phase Potential Transformers (PTs) are used even for the three phase circuits.
Typical description of a Potential transformer is like: 33000V/ √ 3 / 110V/ √ 3, 15VA, Class 0.5 Oil cooled PT.
Here 33000V is the system voltage for which the PT is uses and root 3 is for calculating the per phase voltage. 110V is the voltage at the secondary terminals and root 3 is to calculate the per phase secondary voltage
VA indicates the total burden. Burden on an instrument transformer is the maximum load that can be connected to the PT. Burden is expressed in Volt-ampere (VA). Power factor depends on the nature of load (meters, relays etc) connected across the secondary terminals of the PT
And Class 0.5 indicates the accuracy class of the PT. There are various accuracy classes in PTs. A 0.5 class PT will show a maximum error in its measurement upto 0.5%. Refer IS 3156 for more details.
IS/IEC standard specifications mention the preferred values of impedance and accuracy classes
Tests on the Potential Transformers
The tests are categorized into 3 parts. The reference standard for testing of PT is IS3156.
Type tests on Potential Transformers are:
This test simulates a condition equivalent to a lightning. An impulse similar to that of a lightning is generated in the laboratory and is applied across the windings. While the impulse is applied on the Primary side, the secondary along with the tank will be solidly earthed. The applied impulse form is observed on an oscilloscope. The peak value of voltage that needs to be applied across the windings is determined from the rated voltage of the PT. Impulse values according to each rated voltage is prescribed in the IS/IEC standards. No test is done on the secondary side as the rated voltage on secondary side is too small. The test is considered passed, if, even after repeated application of impulses as prescribed in the standards, the wave shapes observed on the oscilloscope is similar and having no distortions. External flashovers if any may be ignored
The PT shall withstand the appropriate full wave lightning impulse voltage specified in ISS / part – 1 according to the highest system voltage and the specified insulation. Example for 11 KV & 33 KV System Highest system Power frequency Lightning impulse voltage voltage withstand voltage withstand voltage KV rms KV rms KV rms KV peak
11 12 28 60 or 75 33 36 70 145 or 170
Temperature Rise Test :
Every electrical equipment that carry load shall have limitations in carrying the load and since insulation is the heart of an electrical equipment, every care must be taken to avoid any stress on it and eventual deterioration. Like maximum voltage limit, equipment will have a temperature rise limit also. This is the maximum temperature that can develop in the instrument over and above the ambient temperature.
This test is done on the potential transformer by connecting the secondary of the instrument with the rated burden while the primary carries the rated voltage. The load, ie the rated burden (equilent resistance in Ohms) is kept in the circuit till a steady temperature is achieved. The voltage applied across the primary shall be 1.2 times the rated voltage
Temperature rise limit at 1.2 times rated voltage, rated frequency and at rated burden or at higher burden if there are several rated burdens at any power factor ( optionally 0.8 to unity) shall not exceed the appropriate value specified below for a type of insulation. ( In general )
Routine and Acceptance tests:
Routine and acceptance tests are same on the potential transformers. Routine tests are carried out by the manufacturer's quality department, whereas the Acceptance tests are carried out by the manufacturer in presence of the Buyer’s representative. Routine tests are done on the 100% quantity and acceptance tests are done on sample quantity.
This is the test that determines the accuracy of the instrument. Here, the rated voltage of the primary is applied and the output is taken from the secondary. The rated burden as specified by the manufacturer is connected in parallel with the secondary. The same primary voltage is also applied to the primary of a standard potential transformer. This standard PT shall have an accuracy class much better than that of the PT under test. No burden is connected across the standard PT
Now the secondary voltages from both PTs are fed to an accuracy meter where the percentage error is calculated internally. The error meter shows the error in ratio and phase angle in percentage. This is the percentage of error present in the PT while the burden is connected in parallel. If the error is measured as + 0.2%, this means the PT will measure a voltage of 100V as 100.2V. Normally tests are done y connecting burden in parallel with the PT whose values range between 120% and 20% of the rated burden
2) High voltage test at Power frequency
This is the test that determines the healthiness of insulation. A high voltage at the power frequency is applied on one of the windings with the other being shorted together and earthed along with the tank. for example, if the test is done on the 33KV side of a PT, 70KV is applied on the winding with the secondary shorted and connected to earth. This voltage is maintained at the terminals for one minutes, during which there shall not be any flash over or collapse of voltage
There are two types of PTs used depending on the system in which they are used. One is that used with earthed system and the other with un -earthed system. in earthed system, one end of the primary of the PT is connected to the earth through an earth link, where as in the unearthed system, primary is not earthed.
In case of earthed system, the high voltage applied on the primary side will be only 3KV irrespective of its rated voltage as the insulation is graded.
Insulation resistance values: are measured on Primary and Secondary sides using an insulation tester before and after the HV test
Induced over voltage test: This is the test that determines the soundness of the insulation between the adjacent turns. Here, a voltage, double the rated secondary voltage is applied on the secondary side. Obviously double the rated Primary voltage is induced on the primary winding. During this time, there will be a voltage equal to double the voltage that may appear between the turns in normal operation. In case of any inter turn insulation problem, there will be high circulating current in the windings and this n turn trips the circuit. While double the voltage is applied, in order to avoid saturation of core, the frequency of the supply has also been doubled
Induced overvoltage test is also used as a substitute to the power frequency high voltage test on the PT that is used in an earthed system. Secondary voltage applied on the secondary side will be such that a voltage equivalent to the voltage equal to the power frequency voltage that has to be applied in case of an unearthed system. This induced high voltage ensures any insulation problem or any clearance problem.
3) Polarity check
Ensuring the correct polarity of the secondary and primary windings is important. Starting of the primary and that of secondary shall be marked and again the ending of the primary and secondary are marked. Generally beginning of the primary is marked as P1 and end as P2 and secondary are marked as S1 and S2. While instruments are connected to Potential transformers, care shall be taken to ensure correct polarity
As the name indicates, this instrument transformer is used to step down current to a value that is easy to handle. High currents are always difficult to handle as the conductor that has to carry the current will be very big. An ammeter or a over current relay is not expected to carry high currents as these instruments are to be made as small as possible for easy handling. Here comes the role of Current transformers.
Unlike Potential transformer, current transformer is not following the conventional transformer’s working principle. The step down from a high current on the primary side to a very low current on the secondary side is made possible using the Ampere Turn matching. There shall be a balance between Ampere turns on the primary and secondary side.
For example, if the rated primary current is 40 Amperes and the rated secondary current is 1 ampere, product of primary current and the number of turns shall be equal to the product of secondary current and number of turns on the secondary side
i.e. 40 x N1 = 1 x N2.
Due to this step down, no induction of voltage is happening on the secondary. The voltage that will appear on the primary and secondary windings will be the product of current and the resistance of the windings. This will be a very low value to the tune of a few voltages
Current transformers are of three types: Metering CT, Protection CT and PS class CT
Metering CTs are used for measuring purposes like in ammeters, energy meters, and watt meters. Protection CTs are used in protection circuit such as over current protection, earth fault protection. PS class CTs are Special class CTs that are used in sensitive protection CTs like in differential protection
A metering CT is specified as for example: 40/1A, 10VA, Class 0.5: Whereas 40 is the rated primary current, 1 is the rated secondary current, 10VA is the total burden that can be connected in series with the CT and class 0.5 is the percentage error allowed.
A protection CT is specified as, for example: 40/1A, 10VA, 5P10, whereas 40/1 is the secondary and primary currents, 10VA is the total burden that can be connected in series with the secondary winding of the CT and 5P10 is the class of accuracy. Here 5 indicate the percentage of error allowed and 10 indicates the accuracy limiting factor (ALF). Reference to the standard may be made to know the percentage of error corresponding to the denomination 5. P, here indicates that this CT is used for protection. A special class CT is specified as, for example for example 40/1A, class PS. Vkp >300 Volts , Iex < style="font-weight: bold;">Tests on Current transformers
Type tests :
Effective Dynamic Current shall be 2.50 X I thermal (short time) = 65.5 KA peak in this case.
Determination of ratio errors and Phase angle displacements : This is also a routine test as explained in the routine test section
Visual examination is done to ensure that the CTs are manufactured with good workmanship. Positioning of various accessories like primary and secondary terminals, oil level indicators in case of oil filled CTs, pressure relief device if any, terminal markers, earthing studs and so on. Visual and dimensional examinations are to be carried out with the help of customer approved drawings and specifications
Composite error : This is an error that is attached to protection class CT. In simple terms, this is an error that can be limited to a value as specified in the standards, when an accuracy limiting current is flowing through the windings. Accuracy limiting current is the highest current that can flow through the CT without affecting the accuracy of the CT. This current is influenced by the accuracy limiting factor. Composite error gives an indication of the combined errors of ratio and phase angle. Composite error gives a value always more than the vector sum of ratio and phase angle error
A CT with an ALF of 10 can carry an accuracy limiting current of 10 times the rated current. This means, if the rated current is 40/1A, the accuracy limiting current on primary will be 10 x 40 = 400Amps and on secondary it will be 1 x 10 = 10 Amps - Error at this current shall be measured and recorded. The value that is measured is called composite error
The major disadvantage in measuring the composite error this way is practically difficult as the primary current that we have to drive is 400 Amps- There is an indirect method to measure the composite error
Here, first the excitation voltage that is needed to drive 10 times (ALF) of the rated secondary current is calculated. This voltage is called excitation voltage. For this, first the resistance of the secondary winding is measured. Now this value is converted to a value at 75 degree celcius. Now an addition of this resistance Rct and rated burden ie Rct + VA is calculated. Voltage that will be developed in the secondary will be = Rated current x (Rct+VA) and again the voltage that will be developed across the secondary if 10 times (ALF) will be 10 x rated current x (Rct +VA)
This calculated voltage, otherwise called excitation voltage is applied across the secondary side of the CT with the primary open circuited. Now, with the primary open, only a current that is called excitation current will be induced in the secondary winding. This current is measured with an ammeter.
This excitation current is the current that is induced when an accuracy limiting voltage is applied on the secondary winding. This excitation current is expressed in a percentage of the rated current
For example, here the calculation is : excitation current x 100/( ALF x rated secondary current) % - This is the composite error.
Knee point voltage: Important factor for a PS class CT is that there shall not be an over saturation till the rated excitation voltage is passed through the windings. Here, to measure the knee point voltage, first as described in clause about composite error, excitation voltage at rated current is measured using the value Rct and rated burden. This voltage is applied across the secondary winding, keeping the primary winding open circuited. An ammeter is connected in series with the circuit to measure the excitation current. Till the voltage reaches the calculated excitation voltage, there will not be any disproportionate increase in current. Once the applied voltage exceeded calculated excitation voltage, the CT core starts getting saturated. As the voltage is further increased, the current starts increasing faster. At a particular stage, it can be noted that with the increase of 10% of voltage, there will be an increase of more than 50% of current. This voltage is called Knee point voltage
As described in case of PTs, here also it is very important to maintain the correct polarity. As in PT, CT primary is also marked as P1 and P2 and secondary is marked as S1 and S2
There are CTs with multiple cores. These cores are used to make number of secondary windings. A single CT can be used for measuring and Protection purpose by using a number of secondary windings. each secondary core will have a core that carries the flux and a winding that suits the requirement, for example a measuring core, two protection cores and one PS class. This is an example. Any number of cores can be included. A typical example of a multi core CT is : 40/1-1-1-1 A, 10VA class 0.5 (for a metering CT ), again 10VA class 0.5 for another metering CT, 10VA 5P15 for a protection CT and 10VA PS for a special protection CT. Here the number of denominations used to mention secondary current is 4. This means, this CT is a 4 core CT with different specifications
Instruments are connected in series with the secondary windings of CTs and in case of PTs they are connected in parallel. While the burden for a CT is generally of a lower value, to the tune of upto 15 VA (for example) burden on PT will have higher values like 50VA to 200VA. As an instrument that is connected to a current circuit is connected in series, the impedance value will be very low, a virtual short circuit. Instruments that are connected to PTs are connected in parallel and hence they will have higher impedance. This is the reason; CT's rated burden is a low value whereas PT's burden is of a higher value.
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Saturday, April 4, 2009 by chalu ·
Labels: Instrument Transformer Testing
There are two main types of instrument transformers used in the electrical systems. They are Current Transformers and Voltage transformers. While, as the name indicates, Current transformers are associated with the current in the circuit, voltage transformers are with Voltage.
Voltage Transformers: These transformers are working on the same principle as a normal transformer. Voltage that needs to be measured is stepped down to a very low voltage using a potential transformer. This voltage is made available at the secondary terminals.
Type tests are the tests that are carried out on the PT to verify the design parameters. These tests are done on one piece of each design.
Impulse voltage tests:
Temperature of the cooling medium is measured at regular intervals and a steady state is assumed to have achieved if the rate of rise in every one hour is less than 1ͦ K. Temperature rise of the winding then shall be measured with resistance method. Resistance of the winding shall be measured using resistance meter after this and noted as R2 at the temperature of the winding at shut down (T2). Resistance at ambient temperature T1 shall be measured before the test and noted as R1. - Now the Ratio : R2/ R1 = (T2+234.5) / (T1+234.5) OR : T2 = R2/R1(T1+234.5) -234.5 - Ie, the winding temperature at the time steady state is achieved, T2 can be calculated in this method. The temperature rise will be T2 ( –)ambient temperature at the time of shut down
1) Ratio and phase angle error measurement
Shorttime current test : Testing of withstand capacity of CT for particular short time current rating (i.e. depending on breaking capacity of breaker for fault level in the system) This test is to analyze the compressive and axial forces developed during system fault current. For example For 11 KV system , if the fault level of the circuit is 500 MVA/ 1.0 Sec , the Short time current on 11 KV CT ( any ratio ) will be - STC: (500x1000)/ (3x11000) = 26.2 KA for 1.0 sec. ie, CT should withstand the fault current or thermal current of 26200 A for 1.0 sec.
Routine & Acceptance tests :
Dimensional and Visual examinations:
Ratio and phase angle measurement determines the actual accuracy of the CT. Primary and secondary amperes are fed to a ratio error meter. Burden is connected in series with the CT. Burden that is connected to CT varies from 120% and 25% of the rated burden for metering CT and 100% for Protection and PS class CTs. Error values are observed at various burdens and various values of current. Required error limit shall be matched with the value specified in the standards