High Voltage Test Solutions
As part of the commissioning, maintenance or diagnostic procedures, it is typically used on field a high voltage test for condition assessment of the electrical equipment; there are many high voltage test techniques for transformers, breakers, generators, power cables, and other devices designed to operate under medium or high voltage conditions; typical high voltage test applications are for example the insulation resistance testing, dielectric testing, withstand testing, etc.
Consequently, we can find different high voltage test equipment, which are designed accordingly to the specific requirements and different test signals range and type for each device; a high voltage test may be based in AC or DC voltage injection, with different objective sought and techniques to be applied; the following SMC equipment are products which use a high voltage test for different test and measurement purposes.
SMC products suitable for high voltage test applications in transformers, motors and generators
It is typical the use of high voltage tests on transformers to confirm their performance, which leads to get different parameters such as insulation resistance, degradation of the insulation, dielectric rigidity, voltage ratio, magnetizing characteristic, leakage currents, etc.
The following SMC equipment are suitable for high voltage test in transformers, among other applications:
The CV-05 configuration is the minimum required to perform a high voltage test with the Raptor System, and it is formed by the Raptor Master unit and the high voltage test unit, the Raptor HV.
A typical high voltage test performed in all kind of CTs, VTs and PTs is the dielectric rigidity test, or withstand voltage test, through which is checked that the leakage current is below a preset safety limit for the test voltage signal; it is a non destructive high voltage test which verify the insulation integrity of the equipment and checks if the transformer dielectric strength is in suitable condition for the service, capable of withstanding overvoltages during operation.
The Raptor includes a template for determining the transformer turns ratio in voltage transformers, as well as the polarity, phase angle between primary and secondary, and the connected burden, for which just the Master unit can be used, but the use of higher voltage values in the ratio minimize the errors, so that it is more advisable the high voltage test provided by the use of the Raptor HV unit; similarly, the use of the CV-05 provides lower errors in the measured values in the corresponding test templates for PT voltage ratio test and PT short-circuited test included in the Raptor System.
The CT magnetization automatic test is included in the Master unit up to 200 V, which is sufficient for measuring CTs, but not for protection CTs which may eventually need more than 1000 V to saturate; therefore, for protection CTs it is used the high voltage test capacity up to 2000 V provided by the HV unit, which also requires a much simple connection, just connecting the voltage injection cables in the transformer secondary, since the excitation curve and knee point measurements during the high voltage test are performed by the built-in ammeter and voltmeter of the HV unit.
It is also needed to highlight that the power of high voltage injection of the Raptor HV unit allows for performing Step & Touch voltage measurements in the protective earthing installations of substations and other electrical facilities, and also for performing a high voltage test in voltage sensors and converters which requires that suitable voltage range.
This high voltage test equipment is part of the ETP system for predictive power transformer maintenance, so that it can cross results with the other units to provide a complete diagnostic; the ETP-2 provides a fully guided and automatic high voltage test for insulation resistance and recovery voltage analysis in power transformers of any type with oil-paper dielectric.
Through a high voltage test of different cycles with DC voltage levels up to 2000 Vdc, the unit get parameters such as: recovery voltage, insulation resistance (temperature normalized), time constant and polarization index; these parameters helps to detect problems in the transformer such as contaminated insulation, paper and oil dielectric degradation or anomalous aging, and evaluates the overall condition of the oil/paper system.
The high voltage test provided by the ETP-2 not only completes the range of other insulation diagnosis methods that use other kind of high voltage test, but it also enables for trending tracking of each transformer thanks to expert trending and diagnosis software, and the analysis of all the results in the database from successive tests performed in the same transformer, becoming in this way a powerful decision-making tool.
It is also typical the use of a high voltage test on motors, generators, and any electrical rotary machine, for the evaluation and diagnosis of their insulation condition; this is the case of the EDA-III System, a high voltage test equipment for insulation analysis in motors and generators.
This system provides a high voltage test which uses appropriate DC voltage levels up to 6000 Vdc with minimum risk for the machine; the EDA is a type of high voltage test used by many utilities, workshops, manufacturers and testing companies worldwide, with an experience of more than 20 years on field.
For analyzing the condition of the stator winding insulation in rotary machines, the EDA III performs a high voltage test with two different voltages, each one with two cycles, a charging cycle (or absorption cycle) of 30 minutes, and a discharge cycle with no voltage (or reabsorption cycle) of 2 minutes, during which the charge and reabsorption currents are measured, and several parameters are also calculated and displayed numerically and graphically; for each high voltage test, the EDA system gets parameters such as Insulation resistance (corrected in temperature), polarization index, capacitance ratio, absorption ratio, time constant, reabsorption current, etc; the correlation of these parameters allows to detect problems such as internal or superficial contamination, humidity, anomalous ageing, dielectric degradation, etc.
As required by some customers, it is lately included in the EDA III the chance to also make a PDCA test (Polarization-Depolarization Current Analysis), which is a high voltage test that basically features different changes in the charge and discharge time compared to the normal times in EDA.
The EDA III provides a high voltage test with maximum information in minimum time, software guided, and integrated into a diagnostic and trending analysis system with expert software, for detecting when a machine enters a risk zone, and scheduling the most appropriate maintenance actions.
For more information, please check the related SMC products or contact us.