POWER TRANSFORMER TERMS GLOSSARY

The following technical terms apply to transformers.


BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.

The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.

Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.

Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.

Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.

Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers. Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.

Isolating transformer: See insulating transformer. Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.

Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.

Temperature rise: The incremental temperature rise of the windings and insulation above the ambient temperature.

Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.

Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.

POWER TRANSFORMER DIELECTRIC TESTS TYPES BASIC INFORMATION

Low-Frequency Tests

There are two low-frequency tests:

1. Low-frequency wet-withstand voltage test

2. Low-frequency dry-withstand voltage test

Low-Frequency Wet-Withstand Voltage Test — The low-frequency wet-withstand voltage test is applied on bushings rated 242 kV and below while a waterfall at a particular precipitation rate and conductivity is applied. The values of precipitation rate, water resistivity, and the time of application vary in different countries.

American standard practice is a precipitation rate of 5 mm/min, a resistivity of 178 ohm-m, and a test duration of 10 sec, whereas European practice is 3 mm/min, 100 ohm��m, and 60 sec, respectively.

If the bushing flashes over externally during the test, it is allowed that the test be applied one additional time. If this attempt also flashes over, then the test fails and something must be done to modify the bushing design or test setup so that the capability can be established.

Low-Frequency Dry-Withstand Voltage Test — The low-frequency dry-withstand test was, until recently, made for a 1-min duration without the aid of partial-discharge measurements to detect incipient failures, but standards currently specify a one-hour duration for the design test, in addition to partial-discharge measurements.

The present test procedure is:

Partial discharge (either radio-influence voltage or apparent charge) shall be measured at 1.5 times the maximum line-ground voltage. Maximum limits for partial discharge vary for different bushing constructions and range from 10 to 100 ��V or pC.

A 1-min test at the dry-withstand level, approximately 1.7 times the maximum line-ground voltage, is applied. If an external flashover occurs, it is allowed to make another attempt, but if this one also fails, the bushing fails the test. No partial-discharge tests are required for this test.

Partial-discharge measurements are repeated every 5 min during the one-hour test duration at 1.5 times maximum line-ground voltage required for the design test. Routine tests specify only a measurement of partial discharge at 1.5 times maximum line-ground voltage, after which the test is considered complete.

Bushing standards were changed in the early 1990s to align with the transformer practice, which started to use the one-hour test with partial-discharge measurements in the late 1970s. Experience with this new approach has been good in that incipient failures were uncovered in the factory test laboratory, rather than in service, and it was decided to add this procedure to the bushing test procedure.

Also from a more practical standpoint, bushings are applied to every transformer, and transformer manufacturers require that these tests be applied to the bushings prior to application so as to reduce the number of bushing failures during the transformer tests.

TRANSFORMER INRUSH EFFECTS STUDIES DOWNLOAD LINK

DOWNLOAD LINK OF STUDIES ON TRANSFORMER INRUSH EFFECTS
Transformer Inrush Studies Link


Introduction
In a typical UK wind farm a series of radial 33kV collector circuits run from the main switchboard and link together individual wind turbine generator (WTG) transformers. At the design stage it is necessary to determine the maximum number of WTG transformers that can be energised simultaneously from the 33kV system.

One of the factors to be considered is the voltage dip experienced at the point of common coupling (PCC) or interface between the electrical system of the wind farm and the utility company. The UK standard applied is the Electricity Council’s Engineering Recommendation P28, which allows a 3% voltage dip. This article describes wind farm transformer inrush analysis studies the Glasgow based power systems consultants Mott MacDonald have undertaken using PSCAD to demonstrate compliance with P28.

Transformer inrush
When a transformer is energised, it may draw a high magnitude transient current from the supply causing a temporary voltage dip. This current, characterised as being almost entirely unidirectional, rises abruptly to its maximum value in the first half-cycle and then decays until the normal steady-state magnetizing conditions are reached. The magnitude and duration of the inrush current depends upon the following all of which can be represented using a PSCAD model:

the point on the voltage wave at the instant the transformer is energised (i.e. switching angle);
the impedance of the supply circuit;
the value and sign of the residual flux linkage in the core;
the non-linear magnetic saturation characteristic of the core.

VIEW THE ENTIRE DOCUMENT HERE!!!