R.P.P. Smeets

Analysis of Very Fast Transients in Layer-Type Transformer Windings

This paper deals with the measurement, modeling, and simulation of very fast transient overvoltages in layer-type distribution transformer windings. Measurements were performed by applying a step impulse with 50-ns rise time on a single-phase test transformer equipped with measuring points along the winding. Voltages along the transformer windings were computed by applying multiconductor transmission-line theory for transformer layers and turns. Interturn voltage analysis has also been performed. Computations are performed by applying an inductance matrix determined in two different ways; by making use of the inverse capacitance matrix and by making use of the well known Maxwell formulas. The modeling of the transformer and the computations are verified by measurements

Design of test-circuits for HVDC circuit breakers

DC circuit breakers are essential components in the protection of multi-terminal and meshed DC grids. So far, no practical application of HVDC circuit breakers is known and intense research is being carried out regarding various concepts, practical realization and ultimately testing of such devices. In the present contribution, several breaker concepts, based on the hybrid method of DC fault current interruption, described in (patent-) literature, are modelled in order to investigate their interaction with a DC grid. For this purpose, the CIGRE B4 test-system, a combination of DC and AC grids, consisting of multiple in-feeds and having lines and cables, is modelled. The impact of a fault on the breaker is considered. Since industrial concepts of hybrid HVDC breakers are modelled including their key elements, the electrical stresses on various parts of the breakers are analyzed in detail. These stresses are used as a guide to design high-power test circuits that can be used to test DC switchgear under realistic conditions. Two classes of DC test circuits are considered and investigated based on high-power test circuits, supplied by AC generators: · One class can produce pseudo-DC currents of short duration that can be adequate to test DC equipment (including switchgear) under normal and fault conditions. Three basic circuits will be demonstrated and compared. · Another class is a set of basic test-circuits having capability of creating similar stresses on HVDC circuit breakers as in service. Four basic circuits are investigated and compared to an ideal DC circuit: discharge of a charged high-current reactor, discharge of a charged high-voltage capacitor bank and high-power high-voltage AC circuits of 16.7 and 50 Hz. Comparison of each of these circuits shows that low frequency AC circuits are possible candidates to represent the interaction of HVDC circuit breaker with a DC circuit adequately; regarding fault current rise, interruption performance, counter voltage creation and energy management.

Magnetic saturation at short-circuit tests on power transformers

The post-set and pre-set short-circuit test methods for power transformers are discussed. KEMAs testing experience with special attention to the pre-set method, inrush currents and premagnetization, are highlighted. By means of premagnetization inrush currents can be avoided to a large extend The saturation of the supply side transformers leads to a distortion of the supply side voltage, thus preventing saturation of the test objects. The different saturation modes of the transformer under test are briefly mentioned in relation to the dynamic stresses on the windings.

Recent standardization developments and test-experiences in switching inductive load current

In recent years, the demand for testing of inductive load current switching duties has significantly increased. Parallel to this, a range of changes to the IEC standard 62271-110 have become effective. This contribution is intended to give an overview on some recent test experiences in inductive load switching with circuit breakers in a voltage range 12-550 kV, showing practical examples, changes and interpretations of the latest IEC standard. In the first part, an introduction will be given on the inductive switching duty, its typical features and its standard, IEC 62271-110. In the second part, vacuum circuit breakers are considered. Compared to SF6 circuit breakers, the re-ignition pattern differs significantly, given the inherent capability of vacuum circuit breakers to interrupt high-frequency current. This was identified by CIGRE WG A3.27 on high-voltage vacuum circuit breakers. Test-examples of vacuum breakers up to 84 kV regarding shunt-reactor switching are shown and explained in its Technical Brochure 589 from 2014. Also, test-examples of HV motor switching, and the inherent overvoltages due to multiple reignition and virtual current chopping are highlighted and explained quantitatively. In the third part, shunt reactor switching with EHV circuit breakers is considered. Test examples will be highlighted of 550 kV circuit breakers. Recent publications have shown that more and more air-core shunt reactors are applied at EHV level. It will be demonstrated that in such cases, TRV requirements, in excess of those, as specified in the IEC standard may be met.

Novel Black-Box Arc Model Validated by High-Voltage Circuit Breaker Testing

In this paper, we present a new black-box arc model that was validated using tests with short-line fault interruption of high-voltage circuit breakers. This new arc model shows superior performance compared with four types of existing black-box arc models. Black-box arc models are widely used to simulate current and voltage waveforms during the interruption process. Applied together with arc parameters, they determine arc conductance and interruption behavior. In this study, the arc parameters of each arc model are derived from an optimization approach in a defined time interval to minimize the difference between the measured and simulated arc voltages. The black-box arc models with optimized arc parameters are applied to a simplified circuit for short-line fault simulation, and the model accuracy of the interruption prediction and waveform fitting is quantitatively scored.

Climatic, environmental and fire behaviour class verification on dry-type transformers; KEMA laboratories testing

Transformers belong to the crucial components in distribution and transmission networks. Reliable use for many years needs to be guaranteed. Besides quality assurance of the manufacturer type testing is the most common way to prove this reliability under normal and extreme conditions. In addition, for dry-type transformers, special tests to prove climatic (C), environmental (E) and fire behaviour (F) class are defined in the international standard IEC 60076-11. End users, e.g. utilities and wind turbine manufacturers, are demanding proof of the claimed qualities of dry-type transformers increasingly. The independent KEMA laboratories, owned by DNV-GL, gained large testing experience for C and E class verification. In 2013 also verification for F class was added to KEMA laboratories portfolio, where now experience has been built-up. For C class verification, a thermal shock test is to be performed. The most common failure is the appearance of cracks, found during visual inspection after the test. The most important part for E2 class verification is the performance of a condensation test, where the transformer is energized while salt condensation is to be present on the surfaces of the object. Failures are flashovers causing voltage breakdowns and serious tracking degrading the transformers dielectric qualities. As part of the F class verification, a complete assembled phase of a transformer is set on fire in a test chamber equipped with air inlet and chimney. Temperatures in the chimney above the allowed limits will lead to unsuccessful results. In the last five years, many (> 40) dry-type distribution transformers (power range up to 3,5 MVA) were tested for verification of one or more of these classes. Often (> 50%) the requirements as per IEC 60076-11 and/or clients specifications could not be met. In some cases design changes and retests were required to meet the requirements, in other cases test programs were stopped due to failure of the object. This paper will explain the importance of the above mentioned tests. Also explanation will be given on the testing methods and procedures required for reliable and reproducible tests, results and conclusions. Climatic, environmental and fire behaviour tests are important special tests proving utilities and other end users of dry-type transformers the quality of the product.

Testing of 800 and 1200 kV class circuit breakers

The most critical transient a circuit breaker has to endure during its operation is the transient recovery voltage (TRV), initiated by the electric power system as a natural reaction on current interruption. For circuit breakers intended to operate in ultra-high voltage systems (with rated voltage above 800 kV), the realization of realistic transient recovery voltages in testing of these circuit breakers becomes a real challenge. In this contribution a new and proven method is described of creating adequate TRVs by a double stage synthetic test circuit. Due to the extremely high voltages to be applied immediately after interruption of very high fault current, a two-stage approach is necessary. This is the only way to perform full-pole testing, the realistic laboratory simulation of service conditions during fault current interruption.

Why cable type testing is essential

In this paper the results of 20 years of type testing MV, HV and EHV cables, accessories and cable systems are presented. This survey is an update of a previous publication [1] and confirms the previously presented data. It shows that still 20 % to 50 % of all type tests on accessories result in a change in design or it results in stopping the type test. These results show the manufacturer the necessity of thoroughly testing new designs of cables and accessories. For the user of cable systems, these results indicate the value of purchasing type tested components or even systems. Interfacial problems show the importance of testing the combination of cable and accessories that will be used. Individually type tested components are not a guarantee that the combination will pass the type test. Especially for large cable projects, it is advisable to type test the desired combination of cable and accessories before installation commences.