Test transformer

A test transformer is a special type of transformer that is used in high-voltage laboratories as an alternating voltage source for high voltage . It is used to carry out high-voltage tests , material tests , insulation measurements and the quality test of high-voltage components used in electrical power engineering , such as insulators , power switches or high-voltage switches.

General

1.2 MV test transformer at the Leipzig trade fair

Compared to power transformers, test transformers have lower power ratings, are typically single-phase and have a significantly higher transformation ratio between the primary and secondary side, which means that test voltages of over 1  MV can be achieved. The test voltages, which are significantly higher than the nominal voltages, are necessary in order to measure limits. The characteristic design differences between power transformation and test transformers are:

feature	Power transformer	Test transformer
task	to maximize energy transfer	for generating high test voltages to test borderline cases
construction	Three-phase transformer	Single-phase transformer
rated capacity	up to over 1 GW	up to a few MW
Gear ratio	between 5 and 50	between 400 and 1500
Leakage inductance	under 15%	15% to 25%
Iron core	Weight savings	distortion-free and linear transmission of the magnetic flux
business	Continuous operation, mostly under nominal load	short test interval, shock load, overload possible

Due to the high transmission ratio, the electrical insulation plays a particularly important role. These transformers are usually of an oil-filled design. The transformer core is either earthed , which requires a corresponding, complex insulation of the high-voltage winding from the core, or the core is at half the high-voltage potential, which requires a corresponding, insulating installation on stand insulators in the test hall.

Construction of a cascade from 2 ("cascaded") test transformers

In order to reduce the problems of isolation and to be able to obtain technically manageable alternating voltages of up to a few megavolts, test transformers are cascaded . For this purpose, up to three test transformers are interconnected as shown in the adjacent sketch. Each test transformer deposited green, consists essentially of three windings: The excitation winding E (primary winding) which is the power supply, the high voltage winding H (secondary winding) and the coupling coil K . The coupling and excitation windings have a transformation ratio of 1: 1.

The second test transformer TR2 is mounted on a platform that is electrically isolated from earth and is at the high-voltage potential of the first test transformer TR1 . To avoid unwanted peak discharges in the area of ​​the coupling feed line, the line is routed within the high-voltage line, which has a large outer diameter. Since the high-voltage windings are connected in series, in this case with two test transformers the sum of the two high voltages is twice the high voltage of a test transformer. The disadvantage of this structure is the higher thermal load on the lower stages: In this example, TR1 must be designed for twice the power of TR2 .

Further application

In addition to their function as a source of high alternating voltage, test transformers are also used to feed high-voltage cascades that supply high direct voltages .

Test transformers are also used to feed so-called series resonance systems. Series resonance systems avoid the complex cascading and allow more compact designs. For this, depending on the method, high-voltage tests must be carried out on frequencies other than the mains frequency in order to be able to meet the resonance condition.

Typically, test transformers are capacitively loaded by the device under test. By connecting a lossy choke in series with the test capacitance C , a series resonant circuit with damping resistor R can be formed on the high-voltage side , which can be tuned to resonance by adjusting the inductance L of the high-voltage choke or by changing the frequency . The case of resonance is at the angular frequency ω ₀ :

${\displaystyle \omega _{0}={\frac {1}{\sqrt {L\cdot C}}}}$

in front. The voltage increase η with the test voltage U and the excitation voltage U _E supplied by the test transformer is then:

${\displaystyle \eta ={\frac {U}{U_{E}}}={\frac {1}{\omega _{0}\cdot C\cdot R}}}$

Test transformers in series resonance systems are built for voltages up to 2.1 MV.

literature

• Andreas Küchler: High voltage technology . 2nd Edition. Springer, 2005, ISBN 3-540-21411-9 .

Web links

• Cascaded test transformers in the former SibNIIE open-air test site, near Novosibirsk .

Individual evidence

1. ↑ Andreas Küchler: High voltage technology . 2nd Edition. Springer, 2005, ISBN 3-540-21411-9 , pp. 319 to 321 .
2. ^ P. Mohaupt, M. Pasquier, R. Gleyvod, G. Voigt: A 2100 kV - 90 MVA Resonant Test System , High Voltage Engineering, ISH Graz, 1995, article 4550