High voltage test
The high voltage test is used in the field of electrical engineering to ensure the insulation resistance of electrical equipment and electrical installations . In this area of application, it represents part of the insulation coordination test, which is carried out with high voltage and requires special safety precautions. It must not be confused with the measurement of the insulation resistance during the insulation measurement , which is sometimes carried out with a voltage higher than the rated voltage of the equipment.
The area of high-voltage tests also includes procedures using surge voltage generators to locate faults in high-voltage cables that are laid in the ground.
Testing of low voltage components
In the low-voltage devices of the used protection class I and II is checked so that whether the insulation a prescribed according to standard withstand voltage has. This determines whether the insulation of the current-carrying conductors and the safety distance to the housing are correct. In principle, this high-voltage test is carried out at the same connection points as for the insulation resistance test. The test is carried out with higher test voltages than the operating voltages occurring during normal operation. The test voltage can be both an AC and a DC voltage and is typically in the range of a few kilovolts .
When testing with AC voltage, both the current through the ohmic insulation resistance and the capacitive insulation resistance are recorded. A total current from the ohmic and capacitive component is determined. The actual insulation measurement also includes further tests and measurements that are not carried out with high voltage.
Tests of high voltage components
High-voltage components that are used in electrical power engineering and that are also operated with high voltage in normal operation, such as circuit breakers , power transformers or even insulators , are tested for the guaranteed limit values in high-voltage test fields and sometimes in high-voltage laboratories as part of a high-voltage test before final installation and final acceptance .
Test voltages in the range of 1 MV and just above are used, which can also be above the permissible operating voltages to determine the limits. As a rule, the outputs are much smaller than the nominal outputs in order to avoid damage to the test item. Special test transformers in combination with voltage multipliers such as the high-voltage cascade for generating high DC voltages serve as high-voltage sources . To generate high AC voltage , several test transformers can be connected in series as a cascade. Other high-voltage sources for testing purposes are the series resonance systems, which are used, among other things, in mobile testing systems. Marx generators are used to generate high, defined pulse voltages as part of the test.
The high-voltage test takes place in a cordoned-off area, which is also known as a test field or high-voltage laboratory. The area, for example a hall or part of it, may not be entered during the high-voltage test for safety reasons and is separated from the neighboring control rooms by electrically conductive walls or grids. The entrances to the high-voltage test field must be secured by opening switches on the doors. In the case of larger parts of the system, parts of the factory hall are also blocked off by means of erectable barriers and conductive fences or walls, warning lights, signal tones and information signs.
In the case of high-voltage tests, particular attention must be paid to avoiding the "spreading" of the high voltage via measuring lines into areas outside the high-voltage area, as this can damage measuring devices and control devices. For this purpose, all cables leading out of or into the high-voltage area must be routed through earthed shields and overvoltage protection devices.
After the high-voltage test has been carried out and the test system has been switched off, strictly regulated procedures apply in order to avoid accidents, since charged system parts such as high-voltage capacitors pose a risk of electrical accidents even after they have been switched off . Behaviors such as the five safety rules include, for example, that after disconnection with earthing rods, permanently visible connections must be established before work can continue on the system components.
Testing the withstand voltage of electrical machines
DIN EN 60034-1 defines the test of the withstand voltage (commonly referred to as the high-voltage test) as one of the mandatory tests for series testing of electrical machines in order to identify faults in the insulation system. The test voltage is applied between the winding and the housing. All windings or sensors not included in the test are connected to the housing. If the individual winding ends in multi-phase machines over 1 kV can be reached individually (e.g. separable star point), this test must be carried out separately for each winding phase.
The test voltage has a mains frequency and is as sinusoidal as possible. In exceptional cases, machines over 6 kV may be tested with DC voltage that corresponds to the peak value of the line-frequency test voltage. The test voltage for machines with rated voltages between 100 V and 24 kV generally has a level of 1000 V + double the rated voltage, at least 1500 V. This is to be applied for a duration of 1 min with defined rise and fall times of the voltage. For other voltage levels, special windings or operating modes, other, mostly higher test voltages are also prescribed. No carbon copy may take place during the test. Apart from the voltage curve, no further measurement results are recorded.
The test is carried out once on new machines in full. Since it places great strain on the winding insulation due to the voltage level and the duration of the test, it is only repeated for all subsequent tests (e.g. acceptance tests and repairs) with reduced values for the test voltage and on freshly cleaned and dried machines. An operational test of the insulation (e.g. during commissioning and maintenance) is generally only carried out by measuring the insulation , measuring the polarization index or, in special cases, measuring the partial discharges .
As a result of the very close and therefore strong, capacitive coupling between the winding and the laminated core, as well as line-frequency and sometimes high test voltages, considerable test currents occur in large high-voltage machines, so that test transformers with several 100 kVA power may be required.
literature
- Andreas Küchler: High voltage technology . 2nd Edition. Springer, ISBN 3-540-21411-9 .
Web links
- Press photo of an 800 kV ultra-high voltage converter transformer in the test field for high voltage measurement. On the right is the high-voltage cascade, on the bottom right in the background the test transformer for supply.