Instrument transformer pdf

Electrical instrument transformers transform high currents and voltages to standardized low and easily measurable values that are isolated from the high voltage.

When used for metering purposes, instrument transformers provide voltage or current signals that are very accurate representations of the transmission line values in both magnitude and phase.

These signals allow accurate determination of revenue billing. When used for protection purposes, the instrument transformer outputs must accurately represent the transmission line values during both steady-state and transient conditions. These critical signals provide the basis for circuit-breaker operation under fault conditions, and as such are fundamental to network reliability and securit. Instrument transformers used for network control supply important information for determining the state of the operating conditions of the network.

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instrument transformer pdf

Instrument Transformers. More than 0. Get your offer now.ABB's website uses cookies. By staying here you are agreeing to our use of cookies. Learn more. Search now. There was a problem with your request.

instrument transformer pdf

Please fill in required fields. Sign up. All units are tailored for specific applications, either to operate accurately at steady state conditions or to maintain reasonable accuracy reading up to extreme fault level conditions. These products consist of high dielectric strength oil-filled designs using paper and foil shields impregnated with refined dehydrated and degassed transformer oil, or stress-controlled, shield-graded SF 6 gas-insulated designs.

This ensures conservative reliable performance for decades of service in demanding global outdoor environments. ABB is an innovator in instrument transformer technology and has the most comprehensive product line in the market. Access to the latest technology, global sourcing and up-to-date processes ensures long life and meets the most stringent needs defined by the industry.

instrument transformer pdf

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Instrument transformers and sensors

Learn more I agree. Navigate Search Login layouts-flyoutmenu-cart. Search Search now. Login to myABB There was a problem with your request. Rate this page General impression.The specific performance characteristics of current and voltage instrument transformers are easily determined from the equivalent circuit.

Figure 1 works well for most instrument transformers. For current transformers, the value of the reactance X is determined in a special way so that it represents the leakage flux. The flux flows in the part of the core represented by the left-hand exciting branch of the equivalent circuit shown in Figure 1.

An additional winding or windings placed over the outer leg s of the core and connected back in parallel with the secondary winding, as shown in Figure 4, can keep the leakage flux out of the core. The leakage reactance is then effectively connected ahead of both exciting branches as shown in Figure 1.

Voltage transformers are designed so the through impedance R SR, X Pand X is as low as possible, while current transformers are designed so the excitation impedance Z o and Z i is as high as possible.

The leakage flux is shown entering the outer part of the core and is represented by reactance X. The reactance develops voltage applied to the exciting branch Zo, which represents the outer side of the core.

The loss of current in transformation is due to current by-passed by the exciting branches, Z o and Z i. Current transformers are specially designed to keep these by-pass exciting impedances as high as possible.

Leakage flux enters the core even though the winding is uniformly wound over a ring core. The equivalent circuit is the same as for Figure A.

The parallel auxiliary winding effectively keeps the leakage flux out of the core s o that the leakage reactance in the equivalent circuit is effectively ahead of the exciting branches.

This simplifies the calculation of the current by-passed through Z o and Z i. This resembles the transformer in B but has only negligible leakage flux in the core because the return conductor is far away. This transformer still has a good deal of leakage reactance, but the leakage flux does not enter the core in significant amount.

The reactance is ahead of the by-pass branches Z o and Z i so that the performance as a current transformer can be easily calculated. Search for:. Premium Membership. More Information. Electrical engineering guides and research studies. Equivalent circuits The specific performance characteristics of current and voltage instrument transformers are easily determined from the equivalent circuit.

This difference is important for current transformers because leakage flux in the core affects current ratio. It also improves the performance of current transformers and subjects their performance to simple calculation. The voltage transformers are carefully designed to keep this impedance as low as possible.

Related EEP's content with sponsored links. It helps you to shape up your technical skills in your everyday life as an electrical engineer. Leave a Comment Cancel Reply. Subscribe to Weekly Digest Get email alert whenever we publish new electrical guides and articles. The content is copyrighted to EEP and may not be reproduced on other websites. Facebook Linkedin Linkedin Twitter.There are various types of transformer used in the electrical power system for different purposes, like generation, distribution and transmission and utilization of electrical power.

The different types of transformer are Step up and Step down Transformer, Power Transformer, Distribution Transformer, Instrument transformer comprising current and Potential Transformer, Single phase and Three phase transformer, Auto transformer, etc.

This type of transformer is categorized on the basis of a number of turns in the primary and secondary windings and the induced emf. Step up transformer transforms a low voltage, high current AC into a high voltage, low current AC system In this type of transformer the number of turns in the secondary winding is greater than the number of turns in the primary winding.

Step down transformer converts a high primary voltage associated with the low current into a low voltage, high current. With this type of transformer, the number of turns in the primary winding is greater than the number of turns in the secondary winding.

What is Instrument Transformer - What is CT Ratio- What is CT and PT-

The power transformers are used in the transmission networks of higher voltages. They are mainly rated above MVA. Mainly installed at the generating stations and transmission substations.

They are larger in size as compared to distribution transformer. At a very high voltage, the power cannot be distributed to the consumer directly, so the power is stepped down to the desired level with the help of step-down power transformer.

The transformer is not loaded fully hence the core loss takes place for the whole day, but the copper loss is based on the load cycle of the distribution network. If the power transformer is connected in the transmission network, the load fluctuation will be very less as they are not connected at the consumer end directly, but if connected to the distribution network there will be fluctuations in the load.

The transformer is loaded for 24 hours at transmission station, thus, the core and copper loss will occur for the whole day. The power transformer is cost effective when the power is generated at low voltage levels. If the level of voltage is raised, then the current of the power transformer is reduced, resulting in I 2 R losses and the voltage regulation is also increased.

This type of transformer has lower ratings like 11 KV, 6. They are rated less than MVA and used in the distribution network to provide voltage transformation in the power system by stepping down the voltage level where the electrical energy is distributed and utilized at the consumer end. The primary coil of the distribution transformer is wound by enamel coated copper or aluminum wire.

A thick ribbon of aluminum and copper is used to make secondary of the transformer which is high current, low voltage winding. Resin impregnated paper and oil is used for the insulation purpose.

The various types of the distribution transformer are categorized on the following basis and is shown in the figure below. The distribution transformer less than 33 KV is used in industries andV is used for the domestic purpose. It is smaller in size, easy to install and has low magnetic losses and is not always loaded fully.

As it does not work for constant load through out 24 hours as in the daytime its load is at its peak, and during the night hours it is very lightly loaded thus the efficiency depends on load cycle and is calculated as All Day Efficiency.

The current and potential transformer is explained below in detail. Where, I p โ€” primary current I s โ€” secondary or reversal current I 0 โ€” excitation current K T โ€” turn ratio.

The potential transformer is also called as the voltage transformer.

instrument transformer pdf

The primary winding is connected across the High voltage line whose voltage is to be measured, and all the measuring instruments and meters are connected to the secondary side of the transformer. The main function of the Potential transformer is to step down the voltage level to a safe limit or value. The primary winding of the potential transformer is earthed or grounded as a safety point. For example the voltage ratio primary to secondary is given asit means the output voltage is of V when the V is applied to the primary.

The different types of potential transformer are shown below in the figure.ABB's website uses cookies. By staying here you are agreeing to our use of cookies. Learn more. Search now. There was a problem with your request. Please fill in required fields. Sign up. Submit your inquiry and we will contact you.

Quickly find an ABB channel partner. Learn more I agree. Navigate Search Login layouts-flyoutmenu-cart. Search Search now. Login to myABB There was a problem with your request.

Rate this page General impression. Positive Negative. Your cart Learn more about shopping on ABB. Instrument transformers and sensors. Current and voltage measurements from 0. The range of electric values in power supply systems is very extensive. This is why it is necessary to match the respective currents and voltages to the values appropriate to connected measuring, protection, and control instruments.

Are you looking for support or purchase information? Contact us. Our offering. Indoor application. Outdoor application. ANSI indoor instrument transformers. Indoor sensors. IEC indoor instrument transformers. ANSI outdoor instrument transformers. Outdoor sensors. IEC outdoor instrument transformers. Indoor current transformer AT.InstrumentTransformers A Vital Part of the Protection and Meteringg System y Instrument Transformer transforms the high current or high voltage connected to their primary windings to the standard low values in the secondary within the required accuracy limits which feed the metering and protection t ti apparatus t Provide insulation against High voltage isolation Protect personnel and apparatus from high voltages Provide possibilities of standardizing the relays and instruments 6.

Current Transformer Theory When alternating current flows in the primary winding, that current creates a MMF which results in a alternating flux in the core, which in turn induces an EMF in the primary winding and in any other winding wound on, or linked with, the core. How Current Transformer is connected IIn current operation i or series i mode, the primary winding is connected in series with the power system whose relatively high impedance determines the magnitude of primary winding current which is independent of the secondary winding load The current transformer has assigned rated output termed as burden in VA which are invariably small as against the high outputs in KVA or MVA of power transformers Current Transformer Theory The error in the reproduction will appear both in amplitude and phase.

The error in amplitude is called currentt or ratio ti error and d the th error in i phase h i called is ll d phase error or phase displacement. The culprit is core loss and magnetizing current, which introduces ratio error as well as phase error What is inaccuracy?

The secondary current which we get is not true reflection of its p primaryy current. Current Transformer Theory Th The exciting i i current Ie introduces i d ratio i error, which hi h is i defined d fi d as the difference in magnitude of the primary and secondary current expressed as percentage of primary current.

Under steadystate conditions, the r. Kn istheratedtransformationratio Ip isther. Burden of Current Transformer Burden The external load e. D i transducer, d meter, relay l etc Burden B d Furnished F i h d by b the manufacturer 2. Burden of Interconnecting Leads can be calculated by using the above formula, use conductor resistance total to the device and back for Z 3.

Internal Burden of CT Windings This is so small that it can generally be ignored or specified by manufacturer Magnetization Curve TheexcitationofCT dependon a Crosssectionalarea b Lengthofmagneticpath ofcore c Numberofturnsinthe winding d Magneticcharacteristics ofthecore f h.

A component of primary current excites the core to the flux density necessary to induce in the secondary winding an e. Current Transformer Theory At constant burden, A b d core flux fl density d i varies i directly di l as the h secondary current, hence, as the primary and consequently secondary current increases, a point is reached when core material start saturating and exciting current becomes excessive, thus resulting in excessive current error Bm isdecidedbytherequirederror.

Instrument Transformers

LowerB is decided by the required error Lower Bm forbetter for better accuracy. Effect of Secondary Open Circuiting E. Current Transformer Accuracy Measuring CTs are required to be accurate over normal working range of current, while protective CTs required to maintain the accuracy up to several times of the rated current Metering if we want to measure current for metering purpose, we desire that whatever current we measure, that should be very accurate as the metered data may be used for tariff purpose.

Accuracy Class A designation assigned to a current transformer, the errors of which remains within specified limits under prescribed condition of use Instrument Security factor ISF Th The instruments i connected d to the h secondary d off a CT should h ld be b protected from getting damaged during primary fault condition, when primary current is many times higher than the rated value, the core should get saturated For this purposes, Instrument Security Factor ISF for Metering CTs has been defined The CT cores should be such that it saturates at its instrument security factor ISF for safeguarding the instrument from getting d damaged d under d fault f lt currentt condition diti.

ISF is defined as the ratio of rated instrument security primary current to rated primary current ISF is expressed as 3,5,7 or 10 it shall be chosen as small as possible Protection Current Transformer Protection Class During fault condition, value of primary current may be 10 to 20 times the rated primary current Here, main requirement is ability of CT to faithfully transform f the h primary i current during d i fault f l condition di i At such high level of primary current, if CT is not properly l designed, d i d it may saturate t t and d relay l will ill receive very less current and, therefore, would not make right decision.

The errors are specified at rated current and ALF times rated current with rated burden connected Accuracy Limiting Factor ALF U Unlike lik measuring i CTs, CT which hi h are required i d to be b accurate over the normal working range of currents, protective CTs are usually required to maintain their ratio up to several times the rated primary current At some value of primary current above the rated value, core commence to saturate, saturate resulting in increase in secondary current error Protection Class CTs cores should not get saturated below its A Accuracy Li iti Factor Limiting F t ALF up to t which hi h the th primary i currentt should be faithfully transformed to the secondary side, maintaining the specified accuracy.

ALF is defined as the ratio of the rated accuracy limit primary current to the rated primary current Protection Current Transformer Designation off Protection CTs for f speciall applications l For protection like circulating current differential, restricted earth fault etc.

Why Voltage Transformer is Required Systemhastwobasic h b requirements t i ofenergy metering f sourcedorconsumed protection ofthe of the electricalsystemfrom faults and faultsand disturbances.To browse Academia. Skip to main content. Log In Sign Up. Adrian M Rodriguez. Instrument transformers are designed specifically for use with electrical equipment falling into the broad category of devices commonly called instruments such as voltmeters, ammeters, wattmeters, watt-hour meters, protection relays, etc.

Figure 1 shows some of the most basic uses for instrument transformers. Voltage transformers are most commonly used Figure 1 โ€” Common uses of instrument transformers to lower the high line voltages down to typically volts on the secondary to be connected to a voltmeter, watthour meter, or protection relay.

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. Construction Features Potential transformers consist of two separate windings on a common magnetic steel core. One winding consists of fewer turns of heavier wire on the steel core and is called the secondary winding.

The other winding consists of a relatively large number of turns of fine wire, wound on top of the secondary, and is called the primary winding. Current transformers are constructed in various ways. One method is quite similar to that of the potential transformer in that there are two separate windings on a magnetic steel core. This is called the wound type due to its wound primary coil.

The performance characteristics of the two base plate at ground potentials. For outdoor types the hardware constructions are essentially the same, but the physical appearance must be of the non-corrosive type and the insulation must be of and hardware are different.

The outdoor unit must be protected the non-arc-tracking type. One other feature that differentiates the for possible contaminated environments while indoor units are indoor from the outdoor is the orientation of the primary terminals.

The indoor types must be compatible for connection to bus type Thus most outdoor units will have larger spacing between line and electrical construction as opposed to the outdoor types that are ground, which is achieved by the addition of skirts on the design. Thus amperes X 2 turns is The number is dependent on the primary to secondary current ampere turns on the primary.

To equalize this on the secondary of transformation desired. Thus we have modified a standard current rating made by the manufacturer was to 5 amperes. Had we chosen through the window twice, we can make an effective ampere to back off the 4 secondary turns instead of adding, we would have unit out of a ampere unit. Smaller increments of current had a ampere CT.

Refer to instruction for using a variable-ratio change can be achieved by adding or backing off secondary turns current transformer. The primary amperes turns must equal 4 www. Figure 2 depicts the most basic magnetic circuit of an ideal instrument transformer.

As a current passes through the primary winding it induces a magnetic flux in the steel core. The flux flows through the core and induces a current on the secondary winding proportional to the ratio of turns on the primary to the secondary. Instrument transformers are not a perfect device and incur losses from resistance and stray inductance of the copper winding and core. The two biggest losses are due to the copper windings that carry the current and the magnetic core that carries the flux.


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