High voltage sf6 current transformers image source. Verify that all required grounding and shorting connections provide contact. A test set operator connects the leads, pushes the buttons, and records the. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. The specific performance characteristics of instrument transformers are easily determined from the equivalent circuit. The international electrical testing association neta is an accredited standards developer for the american national standards institute ansi. Current transformer basics and the current transformer. Current transformers testing and commissioning guidelines. Magnetization and primary injection test of current transformer for protective relaying jacob tsado 8 current transformers. Background temperature rise testing techniques on current transformers can be divided in two major groups, according to their technology. A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and. Dec 03, 2010 this video explains how to perform tests on ct current transformer with ratio and polarity voltage and current mode.
Current transformers cts are used to convert high level currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment. Current transformer testing swp check this is the latest process zone version before use. Diagnostic testing and monitoring of power transformers. For current transformers, the value of the reactance x is determined in a special way so that it represents the leakage flux. Instrument transformer basic technical information and. Current transformer with winding or secondary wiring. General description campbell scientifics cs10 and cs15 detect and measure the ac current along an electrical wire using the magnetic field that is generated by that current.
A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. Back to the basics current testing valence electrical. Generally, aircore transformers are used when the voltage source has a high frequency above 20 khz. Effects of a current transformers magnetizing current on. Each of these materials is suitable for certain applications. The composition of a transformer core depends on voltage, current, and frequency. The basics of current transformers ratio the ct ratio is the ratio of primary current input to secondary current output at full load. The time specified would depend on the protection in the circuit. Verify that adequate clearances exist between primary and secondary circuit wiring. Megger cter mct1605 user manual basics of current transformers nk technologies testing and commissioning of current. Cts are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments.
This video explains how to perform tests on ct current transformer with ratio and polarity voltage and current mode. The primary current of minimum of 25% rated primary current to be injected on primary side of ct with secondaries shorted and the secondary current can be measured and recorded for all cores. A fault leading to an open secondary could cause the primary current to overwhelm the insulation, causing the winding to short circuit. Current transformer burdens are typically expressed in va. This white paper will discuss in detail basic ra ngs listed on instrument current transformers and help clarify. Pdf conversion of current transformer errors at unity. Verify correct connection of transformers with system requirements page 8 4. This protects connected metering devices from overloading in the presence of fault level current flows. Testing and protection of current transformer practical experiences.
Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8. On bushings, the voltage is applied to the main conductor while measuring the current on the measurement tap. Current transformers cts are a link between transmission and distribution systems, and instruments that monitor the currents and other electrical values in the system. Circuits used in direct test of a current transformer by the. The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating. Basics, maintenance, and diagnostics xi contents continued page 9. The need for a comprehensive current transformer testing regime is evident when the alternative is considered. Abb field engineering services instrument transformer. Transformers used for voltage potential measurement steps down the voltage to the level of voltmeter. A burden test is typically performed at full rated secondary current value ex. On subtractive polarity transformers the h1 primary lead and the x1 secondary lead will be on the same. 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 voltage, the following expression give the standard. It buffers the meter from experiencing excessive torques that might be.
Current transformers play an important role in the monitoring and protection of electrical power systems. The basics of current transformers nk technologies. Instrument transformers as a standalone test object, and instrument transformers enclosed in a gis. Current transformer accuracy is specified by the standards ieee std c57.
Check for level, colour of oil in the sight glass for outdoor current transformers. Apr 18, 2020 suppose, you have one current transformer with current ratio 4001 a and its protection core is situated at 500 a. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. What is the working principle of a current transformer. A current transformer ct is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree ct accuracy class or current transformer class. Singlephase transformers connected into a 3phase bank retain all basic singlephase properties individual voltage and current transformations are based on the singlephase turns ratio phase shift between primary and secondary is zero 3phase transformer banks can introduce a phase shift between the. Cs10l and cs15l current transformers specifications measurement ranges. Secondary current will be within 10% accuracy at 120 x rated current with burden levels of 3 ohms or less. Conversion of current transformer errors at unity power factor burden, to 0. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. 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. Turns ratio tests optional test this test is to ensure the turns ratio of ct at all taps. Purpose and scope the purpose of this standard work practice swp is to.
Ring core cts are available for measuring currents from 50 to 5000 amps, with windows power conductor opening size from 1 to 8 diameter. Current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. All merlin gerin instrument transformers are in conformity with iec standards. A fault leading to an open secondary could cause the primary current to overwhelm the. Use small jumper to short the ct secondary from ct box, the measurement will be gone at panel. Within electrical systems, current transformers are essential to ensure the correct functioning. In high frequency transformers eddy current losses are reduced by using a core made of a ceramic. Used in ac system for the measurement of current, voltage, power and energy. A regular inspection should ensure that instrument transformers, together with the connected relays, react quickly and correctly in the event of a fault and guarantee the highest possible security of. Testing and commissioning instrumenttransformers free download as powerpoint presentation. Primary bar assembly for type jado current transformer. Effects of a current transformer s magnetizing current on the driving voltage in selfoscillating converters january 2014 turkish journal of electrical. 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. Commonly used core materials are air, soft iron, and steel.
Equuipment for testing current transformers nist page. 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. Figure 4 works well for most instrument transformers. Current transformers current transformers cts provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. Before applying the current transformer in oum bouaghi substation. The short time current must never be less than that corresponding to the breaking capacity of the associated circuit breakers. A distinction has to be made between a metering class and a protection class current transformer. Scribd is the worlds largest social reading and publishing site. Effects of a current transformer s magnetizing current on the driving voltage in selfoscillating converters january 2014 turkish journal of electrical engineering and computer sciences 221.
On theother hand, when j is large, then theuse of a detector of highimpedance may shift theload from transformer of higher. 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. Basic understanding on current transformer free download as powerpoint presentation. Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way the transformer leads are brought out of the case. Unique to a current transformer the current transformer function is different than that of a voltage transformer. For example, the potential transformer mentioned above having a rating of 480. If the primary current of the current transformer becomes a the secondary current will still be 1. Current transformer testing 1 dvd alexander publications. Magnetization and primary injection test of current. Cts current transformers pts potential transformers. Basic understanding on current transformer transformer. Page 1 of 9 standard work practice sp0507 ver 5 ergon energy corporation limited abn 50 087 646 062 ergon energy queensland pty ltd abn 11 121 177 802 1. This behavior is used to calculate the value of the burden resistor for an optimum currenttovoltage relationship.
Instrument transformers for protection purposes supply protection relays and must also function accurately at a multiple of the nominal current. Within electrical systems, current transformers are essential to ensure the correct functioning and control of equipment and for providing. The circuit connection shall be made as shown figure 4. Courtesy siemens current transformers cts are used to convert highlevel currents to a smaller more reasonable level for use as inputs to protection relays and metering equipment. Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. The burden test is performed to verify that the ct is capable of supplying a known current into a known burden while maintaining its stated accuracy. Pdf effects of a current transformers magnetizing current. Chapter 16 current transformer design university of north. Neta defines the standards by which electrical equipment is deemed safe and reliable. Instructions types jkw150 to jkw350 super bute current transformers. The name instrument transformer is a general classification applied to current and. The primary current of minimum of 25% rated primary current to be injected on primary side of ct with secondaries shorted and the secondary current can be measured and recorded for all cores limits. If return points match curve a the core was magnetized.
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