Testing of Transformer

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A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors?the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp), and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:

Vs/Vp = Ns/Np

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np.
In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.
Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical.

High Voltage Transformer
High voltage transformers convert votages from one level or phase configuration to another, usually from higher to lower. They can include features for electrical isolation, power distribution, and control and instrumentation applications. High voltage transformers usually depend on the principle of magnetic induction between coils to convert voltage and/or current levels.
High voltage transformers can be configured as either a single-phase primary configuration or a three-phase configuration. The size and cost of a transformer increases when you move down the listing of primary windings. Single-phase primary configurations include single, dual, quad (2+2), 5-lead, and ladder. A 5-Lead primary requires more copper than a Quad (2+2) primary. A Ladder is the least economical primary configuration. Three-phase transformers are connected in delta or wye configurations. A wye-delta transformer has its primary winding connected in a wye and its secondary winding connected in a delta. A delta-wye transformer has its primary winding connected in delta and its secondary winding connected in a wye. Three phase configuration choices include delta - delta, delta - wye (Y), wye (Y) ? wye (Y), wye (Y) ? delta, wye (Y) ? single-phase, delta ? single phase, and international. Primary frequencies of incoming voltage signal to primaries available for power transformers include 50 Hz, 60 Hz, and 400 Hz. 50 Hz is common for European power. 60 Hz is common in North American power. 400 Hz is most widely used in aerospace applications. The maximum primary voltage rating is another important parameter to consider. A transformer should be provided with more than one primary winding if it is to be used for several nominal voltages.
Other important specifications to consider when searching for high voltage transformers include maximum secondary voltage rating, maximum secondary current rating, maximum power rating, and output type. A transformer may provide more than one secondary voltage value. The Rated Power of the transformer is the sum of the VA (Volts x Amps) for all of the secondary windings. Output choices include AC or DC. For Alternating Current waveform output, voltage the values are typically given in RMS values. Consult manufacturer for waveform options. For direct current secondary voltage output, consult manufacturer for type of rectification.
High voltage transformers can be constructed as either a toroidal or laminated transformer. Toroidal transformers typically have copper wire wrapped around a cylindrical core so the magnetic flux, which occurs within the coil, doesn't leak out, the coil efficiency is good, and the magnetic flux has little influence on other components. Laminated transformers contain laminated-steel cores; they are also called E-I transformers. These steel laminations are insulated with a nonconducting material, such as varnish, and then formed into a core that reduce electrical losses. Power transformers can be one of many types. These include autotransformer, control transformer, current transformer, distribution transformer, general-purpose transformer, instrument transformer, isolation transformer, potential (voltage) transformer, power transformer, step-up transformer, and step-down transformer. Mountings available for high voltage transformers include chassis mount, dish or disk mount, enclosure or free standing, h frame, and PCB mount

Testing Of Transformer
As regards complex electrical equipment such as high voltage power transformers, internal insulation is subject to defects due to several reasons associated to bad material, design, manufacturing processesor resulting from shipment.
On-site electrical tests are for the test voltage to simulate on the transformer under testing the
equivalent stresses which may be established during service condition.
Basically, electrical tests on power transformers are grouped in type and routine tests. The goal of a routine test is to check correct manufacture of HV insulation while the goal of a type test is to confirm correct design of HV insulation.
In addition, the application of on-site tests may be able to be separated in:
? commissioning tests: as part of the on-site equipment commissioning procedure in order to
demonstrate that shipment and erection have not caused any new defects to HV insulation;
? on-site repair or refurbishment: as part of the repair or refurbishment procedure in order to
demonstrate that repair or refurbishment have been successfully completed and HV insulation is free of dangerous defect; ?and
? diagnosis: as part of a diagnostic procedure in order to provide reference values to further tests or to confirm results obtained from other types of test.
Up to date, on-site high voltage withstand tests including partial discharge monitoring and
measurements are the most significant tests in order to quantify HV insulation quality. The use of a separate HV source is more informative than measurement at normal operation voltage, as it allows investigation of the HV insulation performance with voltage.
Alternating voltages are most important for on-site tests . Other voltage shapes for simulation of overvoltages have been used; however, they are strongly dependent on availability of on-site testing systems.
The application of HV on-site tests has been a good practice in South America. Since 1992, on-site HV tests have been performed in more than 110 power transformers ranging from 30MVA to 550MVA, 115kV to 765kV (AC) and 600kV (DC). Large electric power utilities and industrial plants are the main customers to this technology.

 

?HV ON-SITE TEST SETUP
To perform HV on-site tests, a complete set of mobile testing equipment is made available at field,
including:
? variable frequency 60-240Hz motor-generator group. There are three motor-generator groups
available: 300kVA, 850kVA and 2MVA. The proper group is selected according to transformer
power and voltage;
? step-up and regulating transformers;
? reactive power compensating capacitors and reactors;
? no-load and load measuring system; and
? partial discharges measuring and monitoring system as per IEC60076-3 and IEC60270.

 

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