Testing of current transformer and potential transformer pdf

If a current transformer is not properly sized based upon secondary loop burden, it may result in a decrease in ct secondary current. This means that when 480 volts is applied to the primary winding, 120 volts will be induced on the secondary. Byleswarnerpcavalenceand chriswerstiukvalenceelectricaltrainingservicesllc potential transformers pts are necessary to a power system for metering and protective relaying to convert higher system voltages to lower control. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watthour meters, etc. Lb36w3 36kv precision current transformer ct is oil immesed post type outdoor high voltage current transformer purpose it is outdoor,single phase and oil immersed harpin type current transformer, which is used for metering and relay protection of 33kv,50 or 60 hz system. The magnetized iron core induces a voltage in the secondary coils. Potential transformer pt definition the potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. Field testing swp sp0506, particular safety risks applicable to. Ensure the ct is connected to system properly in all respect primary and secondary. Since the meter sees 120 vac, many of the measurements it reports will be low by a factor of 35 unless they are scaled up by 35. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. Use caution when conducting a ct ratio test and do not apply a voltage high enough that would cause the transformer to saturate. 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.

One of the major difference between them is that the current transformer convert the high value of current into low value whereas the potential or voltage transformer convert the high value of voltages into low voltage. The burden of a current transformer can be defined as the total impedance in ohms on the secondary output terminals. Nov 26, 2017 instrument transformers potential transformer, current transformers for high voltage and current measurement. 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. Neil dwyer, manager technical services ergon energy corporation limited abn 50 087 646 062 ergon energy queensland pty ltd abn 11 121 177 802 risk score task no. The figure below shows the connections for testing a potential transformer using a resistance potential divider. It buffers the meter from experiencing excessive torques that might be. Inductive current and voltage transformers ct and vt and capacitive voltage transformers cvt can develop ratio and phase deviations. Contact with high voltage at ct primary connections. Voltage transformer or potential transformer theory. Industry standards have established 120 volts as the secondary rating of potential transformers having primary ratings up to 24,000 volts and 115 volts as the secondary rating of pts having ratings above 24,000 volts. Ac voltage is preferable to dc voltage for transformer testing because ac voltage simulates the internal stress that the transformers face during operating conditions.

Testing potential transformers relay testing training. Current transformer insulation resistance measurement. Potential transformers pts pts step down high voltages to the voltage needed by the meter usually. Measuring a transformer s dc resistance from one external terminal to another can reveal a great deal of information about the transformer. A test set operator connects the leads, pushes the buttons, and records the. What are a potential transformer and a current transformer. Markings on current transformers ct and potential transformers pt have been occasionally misapplied from the factory. Power transformer testing transformers may be tested using ac or dc voltage. Commercially available relays and meters used for protection and metering, are designed for low voltage. 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. Abb field engineering services instrument transformer. Equuipment for testing current transformers nvlpubsnistgov. The instrument transformers like voltage transformer and current transformer are used for connecting the electrical instruments to the measuring instruments.

A noninductive potential divider is connected across the primary winding. The following tests are routinely conducted in the field on the transformer. This protects connected metering devices from overloading in the presence of fault level current flows. Lets say, for instance, that you have a 100kv grid and want to know how much powerenergy you. Using potential transformers continental control systems. Cts are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments the importance of instrument transformer tests is often underestimated. The burden with which the transformer is to be tested is connected across the secondary winding, and the normal primary voltage, at the. Authors paul price, nate myers, jim dehaan bureau of reclamation hydropower technical services group denver, colorado 5d. The potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. Testing of potential transformers electrical engineering info. Job safety analysis power transformer testing check this is the latest process zone version before use.

One end of the secondary winding is connected to one end of the primary winding. Testing current transformers burden test only if we cannot get to the primary side of the ct we can. Back to the basics current testing valence electrical. Back to the basics current transformer ct testing valence. Instrument transformers are special types of transformers intended to measure cur rents and voltages. Some other differences between the current and the potential transformer are explained below in the the comparison chart. The basics of current transformers nk technologies. Transformer device itself a winding and secondary windings fixed in the body, winding. Chapter 1 introduction the transformer is one of the most important components in all of ac circuitry. Operation, maintenance, and field test procedures for.

After all my body strength, hermetic tests and xray flaw detection. Page 3 of 8 reference sp0504r01 ver 1 reference owner. Potential transformers pts are necessary to a power system for metering and protective relaying to convert higher system voltages to lower control voltages that are more practical from an equipment, operation, and safety perspective. To confirm the physical condition and electrical characteristics of current transformer installed in the installation. Cts are expected to provide the secondary output current based upon their accuracy class. Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used in phase shifting without affecting other primary connected devices. Testing and protection of current transformer practical experiences.

Potential transformer is a voltage stepdown transformer which reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. Back to the basics current testing relay testing training. Each current transformer has a secondary burden when connected in a relay or metering circuit. This is step up transformer because when we step up, the voltage increases and current decrease. Major differences between the current transformer ct and potential transformer pt are listed below. The terminal of the transformer should be designed so that the variation of the voltage ratio with load is minimum and the phase shift between the input and output voltage is also minimum.

When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. Dec 30, 2018 potential transformer or voltage transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Current transformers play an important role in the monitoring and protection of electrical power systems. Instrument transformer testing instrument transformers for metering purposes must have a high degree of accuracy up to class 0. Y, supply transformers for lowcurrent primary circuit. If the secondary circuit is closed, a current will flow through the secondary circuit in. China current transformer, voltage transformer, potential. Adding potential transformers has the effect of reducing the measured line voltage by the pt ratio lets say 35. Instrument transformer basic technical information and. Included in the new high accuracy standard are new testing points and burdens to. Mar 10, 2016 to start with, both are instrument transformers basically used for measurement and protection purpose in power systems.

Instrument transformer theory and testing steve hudson, pe. These transformers reduce the voltage and current from high value to the low value which can be measured by conventional instruments. Mar 11, 2017 a potential transformer or voltage transformer is a special kind of transformer used to measure voltages higher than you would want to measure directly. Diagrammatic plan of connections for potential transformer testing voltage can not. Brooks testingpotentialtransformers 421 needstobemeasuredwithonlymoderateaccuracy. Thepoten tialcircuitofthewattmetermaynowbeusedasavoltmeterto. Testing current transformers ratio vs applied burden cts installations can be fully verified in the field we can verify that the ratio is correct and constant as burden changes. Sep 21, 2015 in this we know about what is the instrument transformer. The total burden is a combination of impedance offered by watthour meter coils, relay current coils, contact resistance, terminal blocks, wire resistance and test switches used in the secondary loop. Here we will cover six important aspects of using instrument transformer in the power system. There are three basic styles used to transformer system voltages to control voltages. Fist 38, operation, maintenance, and field test procedures for protective relays and associated circuits 5a.