The secondary consists of a large number of turns of fine wire. Testing current transformers burden test only if we cannot get to the primary side of the ct we can check that the output is. Ct is essentially tested as a voltage transformer by applying. The complete guide to electrical insulation testing. Instrument transformer basic technical information and. There are various test required on transformer to conform performance of transformer. The test code covers measurement and calculation of ratio and phase angle, demagnetization, impedance and excitation measurements, polarity determination, resistance measurements, shorttime characteristics, temperature rise tests, dielectric tests, and measurement of opencircuit voltage of current transformers. Ieee standard requirements for instrument transformers. Magnetization and primary injection test of current transformer for protective relaying jacob tsado 8 current transformers. Basics, maintenance, and diagnostics xi contents continued page 9. With our testing system, you can use a single device to determine all of these parameters in an easy way. The ct burden is the amount of impedance ac resistance connected to the ct secondaries and is usually rated in ohms or volts. Optical measurements of the arc movement may be difficult to.
The resulting overloads of grid sections can lead to farreaching supply and production failures. All cts will list their ra o which is the amplitude of the primary current in rela onship to the amplitude of secondary current. Transformer tests eep electrical engineering portal. Generally, the primary winding of a transformer is connected to the input voltage supply and converts or transforms the electrical power into a magnetic field. Transformer winding resistance measurements are obtained by passing a known dc current through the winding under test and measuring the voltage drop across each terminal ohms law. Power transformer testing transformers may be tested using ac or dc voltage. A very lowfrequency test voltage is applied across the equipment whose insulation to be tested. In this book we have laid down important aspects on transformer. The purpose of this standard work practice swp is to standardise and prescribe the method for testing current. Transformer manufacturers perform two main types of transformer testing type test of transformer and routine test of transformer. This small amount of current would not, of course, harm good insulation but would be a problem if the insulation has deteriorated. Common electrical tests on power transformers omicron.
In this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer. A routine series of diagnostics and testing for your current transformer should include ct saturation testing to identify the knee point against ieee or iec standards, ct ratio tests to identify that the voltage ratio is within expected parameters and current transformer resistance testing for both the transformer windings and ct insulation. The basics of current transformers nk technologies. Some transformer tests are also carried out at the consumer site before commissioning and also periodically in regular and emergency basis throughout its service life. The arcroot velocity is an important parameter in switchgear devices using magnetically driven arcs for current interruption. Testing of power transformers seems like a similar experience. The current twelfth edition has been prepared by martin j. During secondary testing of numerical transformer differential relays, the appropriate secondary currents i. Back to the basics current transformer ct testing as test equipment becomes more sophisticated with better features and accuracy, we risk turning our field personnel into test set operators instead of skilled field service technicians. The methods used during tests and measurements of the power transformers, test and measurement circuits, calculations and evaluation criterias are included in this manual.
A distinction has to be made between a metering class and a protection class current transformer. Transformer testing type tests and routine tests of. Circuits used in direct test of a current transformer by the. Figure 5 shows the result of the curve fitting for core 1, considering the y axis to be the current and x axis to be the. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Equuipment for testing current transformers nist page. On theother hand, when j is large, then theuse of a detector of highimpedance may shift theload from transformer of higher. Ensure the ct is connected to system properly in all respect primary and secondary.
Ct analyzer opens the black box through secondary injection with variable frequency and a patented lowvoltage, lowfrequency method for. Transformer testing has developed considerably over the past years. Testing at higher voltages can yield additional and more useful information about the condition of the test objects insulation, but using a voltage thats too high for a particular test object to withstand can seriously damage it. It evolved from the simple gonogo verdict into a sophisticated segment within transformer manufacturing. Transformer book is still in demand is a tribute to the publishers and to the. When the regulation began, the current was essentially constant in spite of the inductance of the windings and fluctuation of the battery voltage or lead resistance.
Testing and protection of current transformer practical experiences. Procedure for current transformer factory and site acceptance tests 765 765 1800 0 2100 1425 2400 1550 method 2. A fault leading to an open secondary could cause the primary current to overwhelm the. The typical excitation current test data pattern for a threelegged core transformer is two similar current readings and one lower current reading. Transformers regulate the voltage in circuits, but in some cases these can go bad and cause a circuit to not.
Current transformer they generally used to step down current in a very predictable fashion with respect to current and phase. Insulation resistance, ratio and polarity, excitation curve and winding resistance. Tan delta test loss angle test dissipation factor test. Metering accuracy cts are used where a high degree of accuracy is required from low load values up to fullload of a system.
National grid current transformers for protection and general use technical specification ts 3. The current transformer shall be subjected to the appropriate specified powerfrequency pre stress voltage given in table 2b for 1o seconds. Transformers are an electrical component that transmit electrical energy between at least two circuits. The need for a comprehensive current transformer testing regime is evident when the alternative is considered. The following tests are routinely conducted in the field on the transformer. In this paper we will discuss how the cpc 100 of omicron is. Ezct10 and can also be used to run ct tests from the. Abb field engineering services instrument transformer.
Guidance for impulse testing of power transformers is given in iec 60722. Before applying the current transformer in oum bouaghi substation 6030 kv in the algerian network,the. Excitation tests are performed by applying an ac voltage to the secondary winding of the ct and increasing the voltage in steps until the ct is in. The cable, winding, current transformer, potential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. Magnetization and primary injection test of current. Instrument transformer theory and testing steve hudson, pe engineering manager 10737 lexington drive. Back to the basics current transformer ct testing valence. Testing current transformers burden test only if we cannot get to the primary side of the ct we can check that the output is constant as burden changes.
Power transformers are the central hub in the field of energy distribution and transmission. We also offer solutions for testing bushingtype current transformers for. In this example we have chosen a ct with a ra o of 20. As such, their condition is critical for reliable, faultfree operation. Current transformer testing on site presentation calmet. Test sets are now available that will allow testing to be carried out at voltages up to 15 kv. Current transformers are defined by accuracy classes depending on the application. Ac voltage is preferable to dc voltage for transformer testing because ac voltage simulates the internal stress that the transformers face during operating conditions. Tabulated test records can be exported in pdf, excel, and xml formats for further.
The voltage shall then be reduced to the corresponding specified partial discharge test. When the circuit burden gets too large, the ct secondary voltage becomes distorted. C electrical power from one voltage to another voltage keeping the frequency same by electromagnetic induction discover. The current or series transformer has a primary coil of one or more turns of thick wire connected in series with the line whose current is to be measured. Current transformer white paper the basics of current transformers ratio the ct ratio is the ratio of primary current input to secondary current output at full load. In addition some other tests are also carried out by the consumer at site before commissioning and also. Modern test equipment for this purposes utilizes a kelvin bridge to achieve results.
Mainly two types of transformer are done by manufacturer before dispatching the transformer mainly 1 type test of transformer and 2 routine test. Current transformer testing swp check this is the latest process zone version before use. The testing times have been greatly reduced using modern microprocessor based test equipment. Cts should be tested before they are mounted on the transformer. In other words, for a transformer there is no direct electrical connection between the two coil windings, thereby giving it the name also of an isolation transformer. Transformer bushing cts should be tested using the current ratio test method before the transformer has been completely assembled. Testing power transformers highvoltage transformers are some of the most important and expensive pieces of. To confirm the physical condition and electrical characteristics of current transformer installed in the installation. Information concerning loadability of power transformers is given in iec 60354, for oilimmersed transformers, and iec 60905, for drytype transformers. Once you have completed the testing, you can also demagnetize the transformer. Specialized testing for ct continuous current capability. Turnsratio test results should not deviate more than onehalf percent from either the adjacent coils or the calculated ratio. A test set operator connects the leads, pushes the buttons, and records the. The instrument current transformers supply, and isolate, the secondary relays, meters and other instruments, providing the suitable and manageable secondary quantities which are proportional to those in the primary.
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