Instrument transformers are special types of transformers intended to measure cur rents and voltages. These devices use standard transformer theory with 23 windings one primary with one or two secondary windings with a magnetic connection via an iron core. 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. Mar 10, 2016 to start with, both are instrument transformers basically used for measurement and protection purpose in power systems. Any single phase transformer can be used on a three phase source by connecting the. 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. Explain and interpret information found on transformer name plates. Standard current transformer multiratio taps 214 burden standards 214 5 kv 36 kv indoor voltage transformers primary current class abb type geiti equivalent abb product bulletin page 2400 to 4800 5 kv indoor viy60 ptw3160 and ptw3260 1vap429511db 148149 2400 to 72007200 8. 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.
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. High voltage connections are made with elbow cable connectors to bushing inserts installed in wells shown on top of the transformer. Three phase transformer connections phasor diagrams. It is used to transforms 2 kv grid voltage into 27.
A transformer which has only one winding per phase, part of which is common to both the primary and secondary circuits. Pdf voltage transformer operation in connection with the power. Selecting a voltage transformer vt if the ratio, burden, class and optionally voltage factor are selected, the voltage transformer vt or potential transformer pt has been specified basically. Single phase power transformers in north america could be additive or subtractive depending on the kva and the voltage class. The primary terminal of the potential transformer is connected to the line for measuring the line voltage.
This is a simplest form of potential transformer definition. Taps are provided on some transformers on the high voltage winding to. The company will make final connections to the instrument transformer secondary terminals. The basic principle of operation of a transformer is explained in chap. Connections to the bottom of the meter connection block shall be made by the customer. Threephase voltage and current ratios for rectifier circuits. Generally, the primary winding of a transformer is connected to the input voltage supply and converts or. An optical voltage transformer exploits the faraday effect, rotating polarized light, in optical materials.
However, the line current is also the current in the transformer winding and, if i is the rated current of each of the two transformers and v the rated voltage, then the rated 3phase power is vi. The capacitor voltage transformer cvt uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic pt. Thepoten tialcircuitofthewattmetermaynowbeusedasavoltmeterto. Easy understanding of 3phase transformer connections deltadelta, wyewye, deltawye and wyedelta on photo. Easy understanding of 3phase transformer connections. Banked two or more single phase transformers connected together to supply a three phase load.
Mutual induction is the process by which a coil of wire magnetically. The behavior of a 3phase transformer bank is analyzed in the same way as for a singlephase unit assume that both the primary and secondary windings are connected in wye configuration consider only one transformer the primary voltage is the linetoneutral voltage, vh1, n the secondary voltage is the linetoneutral voltage. Potential transformers pts or voltage transformers vts pts or vts are the most common device used. The deltawye connection is the most commonly used threephase transformer connection. Acme transformers can be used at frequencies above 60 hz up through 400 hz with no limitations provided nameplate voltages are not exceeded. Key differences between current and potential transformer. 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.
Thus we see that open delta connection, secondary line voltages form a balanced system and balanced three phase load can be supplied. A threephase transformer is built for a specific connection and voltage transformation and the unit will have a nameplate with the internal connections shown. A transformer operates on the principals of electromagnetic induction, in the form of mutual induction. Each transformer is rated as 10 kva current rating is 10 a and voltage. However, 60 hz transformers will have less voltage regulation at 400 hz than 60 hz. 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. Capacitor voltage transformer cvt or capacitor coupled voltage transformer ccvt is a switchgear device used to convert high transmission class voltage into easily measurable values, which are used for metering, protection, and control of high voltage systems additionally, a cvtccvt used as coupling capacitors for coupling highfrequency power line carrier signals to the transmission line. The secondary voltage bears a fixed relation with the. For example, the potential transformer mentioned above having a rating of 480. A schematic drawing of a singlephase distribution transformer appears in fig. Potential transformers pt or voltage transformers vt can be defined as devices used to step down voltage from high voltage typically used in transmission and distribution to a low voltage typically between 50150v. Potential transformers used primarily in a step down environment to monitor voltage. A transformer can not act as a phase changing device and change singlephase into threephase or threephase into single phase.
In the case of three phase transformer windings, three forms of connection are possible. Basics, maintenance, and diagnostics vii contents continued page 4. If the hv side of the transformer primary side was connected wyegrounded similar to figure 6 and we had a 3leg core type transformer connection, the primary winding with the lost phase will have an induced voltage equal to the voltage before the open phase condition. They are designed for connection linetoline or linetoneutral in the same manner as ordinary voltmeters.
The figure below shows the connections for testing a potential transformer using a resistance potential divider. Lv transformer electrical substation consists, therefore, of the set of devices dedicated to the transformation of the voltage supplied by the distribution network at medium voltage e. Here we will cover six important aspects of using instrument transformer in the power system. To do so can result in very high voltages in the secondary systems. These potential transformers will be used as busbar p.
Threephase transformer connections there are various methods available for transforming 3phase voltages to higher or lower 3phase voltages i. Actually they are protective type voltage transformer. Identify transformer polarity using dot and conventional labeling. Working of potential transformer the electrical portal. The singlephase distribution transformer consists of a primary winding and a secondary winding wound on a laminated steel core. Voltage transformer or potential transformer theory. Capacitor voltage transformer cvt or ccvt construction. Potential transformer voltage transformer is used for protection and measurement. Electrical substations can also be divided into public and. When a single unit or bank of three is used, there are four types of connections. Instrument transformers is a generic term which refers to either current transformer ct or voltage potential transformer vtpt as compared to power transformers.
Cts current transformers pts potential transformers. Abb oem instrument transformer reference guide april 2014. For a short circuited transformer the equation 1 can be used. If there is an ul tap changer, its wiring must also be checked. The above figure shows the connection diagram of an open delta system. The ratio of transformation between primary and secondary line voltage is exactly the same as that of each. This connection is economical for large, low voltage transformers in which insulation problem is not so urgent, because it increases the number of turnsphase.
The transformer does this by linking together two or more electrical circuits using a common oscillating magnetic circuit which is produced by the transformer itself. Pdf conductive disturbances in the supplying voltage may affect the root mean square voltage and shape of the voltage curve on the mains which reflect. Figure 9 voltage transformer connections for ungrounded system it is of particular interest to compare figure 7b and figure 8 above. Of course additional requirements like type, frequency, environmental conditions should also be specified. There are three basic styles used to transformer system voltages to control voltages. Applies to these magnelab ac potential transformers. V l is the linetoline voltage, and v p is the phasetoneutral voltage. But voltage transformers can also be constructed for connection to not only one single phase, but for twophases, threephases, sixphases and even elaborate. Singlephase transformer singlephase transformers are used as a conventional traction power supply system in railway applications.
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. 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. The potential transformer reduced the high value of voltage into the small value which can easily be measured by the voltmeter or meter. The potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. Ultracompact equipment specifically built for customers complex mechanical and electrical. List characteristics on these devices and explain how. In this diagram, a threephase unity pf load resistive load is supplied by two transformers. These are connected across or parallel to the line which is to be monitored. The wyeconnected secondary allows singlephase load to be distributed among the three phases to neutral instead of being placed all on one winding as with a fourwire delta secondary. They are designed for connection linetoline or linetoneutral. Instrument transformer basic technical information and. Figure 31 shows a basic schematic of a single phase traction transformer and its phasor diagram. The transformer connections and voltage triangles are shown in fig. Medium voltage transformers schneider electric global.
Transformer rfq form to request a specific part to meet your requirements. Threephase transformers with the neutral points of the high voltage and low voltage windings connected together internally and brought out through an hoxo bushing should not be operated with the hoxo bushing ungrounded floating. Three phase rectifier circuit data deltadelta full wave, figure 1211 item primary va secondary vi leg secondary ileg secondary va ripple voltage % ripple frequency factor 1. Adding potential transformers has the effect of reducing the measured line voltage by the pt ratio lets say 35. Y, and the first symbol indicates the connection of the primary, and the second symbol is the. The potential transformer is mainly classified into two types, i. The standard current transformer secondary winding is rated at 5a as per ansi standards. Adjustment capacitors are provided in the device for connecting in parallel with the burden on one secondary winding to correct the totalburden power factor to.
A potential transformer must be capable of operating at 110% above rating voltage continuously provided the secondary burden in volt amperes at this voltage does not exceed the thermal rating. Bil basic impulse level is a means to express the ability of the insulation system to withstand high voltage surges. This is characteristic of balanced 3phase systems, and is true only. Read through the different types we supply below and use our instrument transformer rfq form to request a specific part to meet your requirements. Jun 01, 2015 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. To make the transformer connections compatible with threephase supplies we need to connect them together in a particular way to form a three phase transformer configuration. The electromagnetic potential transformer is a wirewound transformer. In case of bus ct, there must be always connected ct shielding to the primary bar.
Metering connection arrangements for voltage potential transformer starconnection of 3 transformers. Indentify the schematic symbols of potential and current transformer. Medium voltage products technical guide the mvlv transformer. Open delta or vconnection of transformer electrical. Basic connection of voltage potential transformer phasephase.
Among the materials consulted while preparing this book are. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. If the transformer leakage impedances are neglected, and their ratio is a, then the secondary voltages are but from kirchhoffs law from which showing that the secondary voltages v ab, v bc, and v ca are equal in magnitude and displaced from each other by an angle of 120. Designed to create the hv network neutral point and to limit the fault current in the phaseearth connection. The rating of two identical transformers operating deltadelta is 1 of the rating of three similar transformers, but connected deltadelta. To start with, both are instrument transformers basically used for measurement and protection purpose in power systems. Line to ground voltage monitoring on ungrounded and. Potential transformer accuracy voltage disturbance. Using potential transformers continental control systems. By using a pt, voltage is reduced that can be safely handled by meter or relay circuits.
Instrument transformer theory and testing steve hudson, pe. It would seem that this rating should be 23 instead of 1. Maximum allowed torques for screw connections of current transformers maximum allowed torque for screw connection of voltage transformer is 20 nm. Instrument transformers reduce the line current and voltage being provided to the. Up to 33kv, these are of electromagnetic type single and three phase voltage transformers. A comparative study of different transformer connections for.
Brooks testingpotentialtransformers 421 needstobemeasuredwithonlymoderateaccuracy. The emergency rating of potential transformers is defined at one minute of operation, thus enough time for protective equipment to operate. Voltage transformers are in most situations connected between phase. A wide variety of potential transformer connections options are available to you, such as shape, application, and connection. Potential transformers pts pts step down high voltages to the voltage needed by the meter usually 120v occasionally 67v.
Metering connection arrangements for voltage potential transformer star connection of 3 transformers. Instrument transformers reduce the line current and voltage being provided to the customers load to safe levels for input to solid state meters. Some of the most commonly occurring failures of the transformer and their causes are listed below. 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. A potential transformer is a conventional transformer having primary and secondary windings. The main purpose of a current transformer is to translate the primary current in a high voltage power system to a signal level that can be handled by delicate electromechanical or electronic devices.
For the instrument transformers the typical engineering laws that are also used for power transformers can be applied. The primary winding is connected directly to the power circuit either between two phases or between one phase and ground, depending on the rating of the transformer and on the requirements of the application. Jefferson electric transformer while threephase devices are usually the more costeffective option, the singlephase option provides more versatility and can be attractive from a reliability and maintenance standpoint. Compare and contrast the performance of three phase transformer connections. Assume idea operation of both cts and pts potential transformer schematic symbol schematic symbols should indicate polarity mark can. The ratio of an instrument transformer is the relationship of its primary rating to its secondary rating. The primary and secondary windings of a transformer can be connected in different configuration as shown to meet practically any requirement. The burden with which the transformer is to be tested is connected across the secondary winding, and the normal primary voltage, at the. Setting procedures are only discussed in a general nature in the material to follow. Testing of potential transformers electrical engineering info.
1408 725 628 460 414 342 1200 974 1111 354 1426 512 1459 741 852 1488 1533 759 259 956 361 1160 25 704 837 37 816 958 1304 131 839 142 272 1436 1613 2 1465 177 981 1262 789 1153 150 947 252 1374 1037 1061