Then to summarise, the current transformer, ct is a type of instrument transformer used to convert a primary current into a secondary current through a magnetic medium. Pdf saturation of current transformers and its impact on digital. The instrument transformers isolate measurement or protection circuits from the high voltage of the primary system. Protection current transformers instrument transformers limited. Selecting current transformers janitza electronics. Protection its require linearity in a wide range of voltages and currents. Current transformer basics and the current transformer.
Transformer protection for low voltage applications 600 volts and below generally speaking, it is helpful to look at the different types. In this design of instrument transformers, the tank housing the cores is kept at the system voltage. Depending of the characteristics of the particular ct, this voltage may be several hundred volts if the primary current is high enough. Ass relays power transformer protection ago35005 e page 6 the current in the shortcircuited turns may be 50100 times the rated current. A highvoltage current transformer may contain several cores, each with a secondary winding, for different purposes such as metering circuits, control, or protection. Calculation of the current transformer accuracy limit factor. Current transformer protection units under normal working or fault conditions, with the burden connected, the varistor is subjected to an applied voltage. National grid current transformers for pr otection and general use technical specification ts 3. Typically, the single straight line considers as primary winding. Protection core a protection core is designed to transform a distortionfree signal even well into the overcurrent range. First, the calculation of the actual burden of the ct, including connection wires and protection relay impedance, is presented. The 67n relay offers an alternative to the 87n function. Current transformer ct construction and working principle. They are defined by their ratio, power and accuracy class.
Current transformers principle of operation of ct a current transformer is defined as as an instrument transformer in which the secondary current is substantially proportional to the primary current under normal conditions of operation and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. Mohammed tawfeeq 26 principles of current transformers the initial problem was how to connect a lowvoltage device to the high. Dimensioning of current transformers for pr otection applications ge multilin 3 voltage on ct secondaries during faults description testing and experience have shown that the performance of many relays can be adversely affected by moderate degrees of ct saturation. Pvd relays with current transformers ctos of different ratios. It is assumed with these schemes that a certain degree of ct saturation is possible under throughfault conditions. Pdf sizing current transformers for line protection. Current transformers technical datasheet schneider electric is the global specialist in energy management with the most complete power monitoring product line. The considerations for a transformer protection vary with the application and importance of the power transformer. Potential transformers consist of two separate windings on a common magnetic steel core. If a new feeder were to be added to this existing bus.
Current transformers occupy a vital part in the measurement and protection scheme of any electric installation. Similarly, current transformers take a high current and reduce it to typically 5 amps on the secondary winding so that it can be used with a watthour meter, ammeter, or protection relay. Their class accuracy as a function of ct load and of overcurrent is chosen according to the application. Cts are categorized as protection ct, special protection ct and measuring ct. Pdf current transformer application with digital ground. It is normal for a modern relay to provide all of the required protection functions in a single package, in contrast to electromechanical types that would require several relays complete with interconnections and higher overall ct burdens. Hence, it is imperative that the choice of ct is made with full knowledge of the application and the number of relays and meters which are to be connected to it.
Principles of differential relaying my protection guide. Pdf saturation of current transformers and its impact on. The terms rated and base current in this paper and referenced. If the secondary of a ct carrying primary current is open circuited, a high voltage can be developed across the ct terminals. Learn how to specify current transformers photo credit. For example, an existing bus having four 600ampere breakers may be protected with pvd relays. Principles of differential relaying current balance high impedance also known as unbiased differential protection only one actuating relay quantity current required for operation. Home download center electrical engineering books and technical guides electrical engineering guides current transformers selection guide electrical characteristics primary circuits the primary circuits of the current transformer must withstand the constraints related to the medium voltage network to which it is connected. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. 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. Saturation of current transformers and its impact on digital overcurrent relays.
Current transformers used for relaying under high current conditions must have a relay rating. Current transformers instrumentation and metering transformers dimensional drawing 2rl 56rl 76rl donut style 2sft 56sft 76sft foot mounted with brass terminals donut style foot mounted description littelfuse offers a wide array of instrument rated current transformers in inch diameter opening. The classification of protection current transformers follows the following simple formula. Influence of current transformer saturation on line. A current transformer with a wrong burden rating read more current transformers burden and classification. The number represents the voltage output at the ct terminals with 20 times normal secondary current flowing 100a for a typical 5 a.
Protection classes are 5p and 10p according to iec 185. Conventional magnetic current and voltage transformers are still in wide use in power systems. This enables the protective relays to measure the fault current value accurately, even in very high current conditions. Introduction transformers are a critical and expensive component of the power system. The degree of protection is very much improved by the application of restricted earth fault protection or ref protection.
Contactors and protection relays enclosures and accessories human machine interfaces hmi industrial communication interface, safety and control relays measurement and instrumentation motion control and robotics motor starters and protection components plc, pac and dedicated controllers power supplies, power protection and transformers. Additional cores for transient behavior are described in iec 446. Mohammed tawfeeq alzuhairi 321 differential protection differential protection is the best technique in protection. Protection current transformers instrument transformers. Similarly, current transformers take a high current and reduce it to typically 5 amps. These transformers form part of sadtems comprehensive product offering, which includes indoor and outdoor current and voltage transformers, along with bushing and combination model types. Current transformer protection opencircuit conditions. Its secondary winding then provides a much reduced current which can be used for detecting overcurrent, undercurrent, peak current, or average current conditions. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device. Scope this document describes the calculation of the actual accuracy limit factor f a for protection type p current transformers ct.
Setting procedures are only discussed in a general nature in the material to follow. The current transformer presents a negligible load to the primary circuit. As protection engineers we consider the rated or base current as the transformer rated line current. Current transformers for protection endeavour energy. Current transformers cts can be used for monitoring current. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit. Whilst measurement current transformers are intended to reach saturation point as quickly as possible once they exceed their operational current range expressed by the overcurrent factor fs in order to avoid an increase in the secondary current with a fault e. For circuits with transformers, requirements for conductor protection per articles 240 and 310 and for panelboards per article 408, must be. When the magnetic circuit of a conventional protection current transformer ct enters saturation, the wave. Dimensioning of current transformers for protection applications ge multilin 3 voltage on ct secondaries during faults description testing and experience have shown that the performance of many relays can be. Wound current transformer the transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. It acts as a passive load and draws little current, thus preventing inaccuracies in the measurement of the ct. Relays are required to perform in fault current type situations moderate accuracy over a wider range.
In practice, other components must be considered in applying circuit overcurrent protection. National grid current transformers for protection and general use technical specification ts 3. A number of cores can be placed in each current transformer. Due to the long lead time for repair of and replacement of transformers, a major goal of transformer protection is limiting the damage to a faulted transformer. These replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit in the event of an. Pdf the effect of inrush current on transformer protection. Excessive heating due to overloading or inadequate cooling. That means local overheating, arcing, decomposition of oil and release of gas. Current transformers are commonly used in metering and protective relays in the electrical power industry. Transformers 600v or less equipment protection the requirements of 450. Current transformers play an important role in the monitoring and protection of electrical power systems.
Prevents dangerous voltages developing during ct open circuit conditions. To discuss the relevant technical issues and requirements associated with the selection of current transformers for pr otection. To discuss the relevant technical issues and requirements associated with the selection of current transformers for protection. An expert in protection current transformer systems. While the latter configuration would not be used today, one may be required to make this calculation on an existing system. They are tested to iec 600441 200302 and are read more. Calculation of the current transformer 1mrs 755481 accuracy limit factor 1. These current transformers are used along with the protection equipments such as trip coils, relays, etc.
The primary of the current transformers is considered as the line or cable or busbar of the load circuit. The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the meter or relay. Transformer protection application guide this guide focuses primarily on application of protective relays for the protection of power transformers, with an emphasis on the most prevalent protection schemes and transformers. This is a unit protection scheme for one winding of the transformer. A neutral current transformer is used as earth fault protection to measure any fault current flowing through the neutral line from the wye neutral point of a transformer. Back to the basics current transformer ct testing as test equipment becomes more sophisticated with better features and accuracy, we risk. Within electrical systems, current transformers are essential to ensure the correct functioning. The magnitude of the secondary current is dependent on the turns ratio of the transformer. The classification of protection current transformers follows the following. Overcurrent protection possible primary protection for small transformers backup of primary protection 87 and 63 backup protection for faults in adjacent protection zones trip transformer before it is damaged transformer damage curves infrequent fault incident curve fewer than 5 faults in life of transformer.
Current transformers are essential components designed to be used with schneider electrics extensive power monitoring product portfolio. All current transformers are subtractive polarity and will have the following designations to guide proper installation. This rating is expressed as the letter c or the letter t followed by a number. Current transformers should never be operated with an open secondary circuit. The inrush current of transformers often causes the inadvertent operation of the circuits over current and differential protection systems 4, 5. Overlapping zones of protection are provided so that no part of power system. In this type of protection the electrical quantities entering and leaving the protected zone or area are compared by current transformers c. 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. In such applications the 6005 ctos associated with these breakers would all be used on the 6005 tap.
With respect to ansi cts, some of the major details that one would look for in the ct are. Instrument transformer basic technical information and. Transformers are a critical and expensive component of the power system. A current transformer with a wrong burden rating read more.
Pdf ground differential protection relays device 87g have been used to protect the windings of resistance grounded power transformers. The secondary of the current transformer is portable. Saturation of current transformers and its impact on digital overcurrent relays conference paper pdf available september 2006 with 7,895 reads how we measure reads. Toroidal current transformer these do not contain a primary winding. As such, it is necessary to define how much a ct must be oversized to avoid saturation due to the asymmetrical component of the fault current the dc offset or. Pdf improper selections of current transformers will result in cts saturation during fault conditions which can affect protection relays. It buffers the meter from experiencing excessive torques that might be. This permits computation of current on all sides of a transformer for a through fault e. Sadtem is a seasoned and experienced producer of protection current transformer solutions. The basics of current transformers nk technologies. A gas detector relay therefore detects a turntoturn fault. Transformer performs a great job in the power system to change voltage and current level so proper protection for the transformer is important to maintain reliability in the system.
Current transformers ct are used in high voltage hv and medium voltage mv 1 installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary. Cts are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments. It can be noted that the bushing of this ct is prone to damages in transit as its center of gravity is at large height. Protection current transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. Response of a ct in saturated state experiments wiring p. Protection ct, current transformer, nsw,act,qld,wa,tas protection pvc the ses range of pvc tape insulated protection cts are designed and manufactured to cater for wide range of applications. This protects connected metering devices from overloading in the presence of fault level current flows. The performance of line current differential protection algorithms during current transformer ct saturation is analysed. Dimensioning of current transformers for protection applications. Sizing current transformers for line protection applications. Current transformer accuracy is specified by the standards ieee std c57. Preventative measures are strongly recommended by ieee in order to protect cts. Protective current transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit.
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