Guide for Insulation Test on Site AC voltage withstanding test
1 Scope
This Standard specifies some technical details of the test connection, test equipment, test method and cautions etc. involved in AC voltage withstanding test on high-voltage electrical equipment.
This Standard is applicable to the AC voltage withstanding test on high-voltage electrical equipment in the condition of power plant, substation field, repair workshop and laboratory etc.
2 Normative References
The following standards contain provisions which, through reference in this text, constitute provisions of this standard. For dated reference, subsequent amendments to (excluding correction to), or revisions of, any of these publications do not apply. However, the parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the standards. For undated references, the latest edition of the normative document referred to applies.
GB 1094.3—2003 Power transformer Part 3: Insulation level, Insulation Test and External Insulation Air Gap (mod IEC 60076-3:2000)
GB/T 1692.7.2—1997 High Voltage Test Techniques Part 2: Measurement System
DL/T 596—1996 Preventive Test Code for Electric Power Equipment
DL/T 1015 Guide for Application of Voltage Measuring System in DC and AC High Voltage Tests on Site
3 Terms
3.1 Peak value
It refers to the maximum value of AC voltage with no view of the minute high frequency oscillation arising from nondestructive discharge.
3.2 Root-mean-square value (rms)
It refers to the square root of the mean value of the squared voltage in a complete cycle.
3.3 Test voltage
It refers to the value of its peak value divided by
4 The Requirements to AC Test Voltage
4.1 Voltage Frequency and Waveform
Test voltage usually shall be AC voltage of 45 to 65Hz and is generally called power-frequency test voltage. According to the requirements of relevant equipment standards, some special tests may require the frequency far below or above that scope. For example, AC withstand voltage of 0.1 Hz or AC withstand voltage between 10 Hz and 300 Hz can be applied in cross-linked PE cable and AC withstand voltage between 10 Hz and 300 Hz can be applied in GIS.
The result of high voltage test will be considered free from the effects of the waveform distortion if the waveform of test voltage is approximately sine wave of two half identical waves and the ratio of the peak value to the root-mean-square shall be within
Bigger distortion is permitted for some test circuits. We should pay attention that the tested object, especially the one with the characteristic of nonlinear impedance, may trigger apparent distortion to the waveform.
Note: it will be considered the requirements of the above-mentioned voltage waveform are met if the root-mean-square value of the subharmonics isn't larger than 5%of that of the fundamental wave.
4.2 Allowable Deviation
If relevant equipment standards have no other regulations, the measured value of the test voltage shall be kept within of the assigned voltage value throughout the test process. If the test duration exceeds sixty seconds, the measured value of the test voltage shall be kept within± 3% of the assigned voltage value throughout the test process.
Note: allowable deviation refers to the disparity between the specified value and the measured value. It's different from measuring error which refers to the disparity between the measured value and the real value.
5 Generation of AC Test Voltage
5.1 The Way of the Production of AC Test Voltage
Power-frequency high voltage is usually generated by applying HT testing transformer. For the tested objects with larger capacity, voltage resonance circuit can be used to generate high voltage. For the tested objects with windings such as power transformer, potential transformer etc., high voltage can be generated by applying intermediate frequency power supply of to excite magnetism in the winding on the low voltage side and induce on the high voltage winding.
5.2 HT Testing Transformer Circuit
The connection of AC voltage withstanding test is determined by the voltage, capacity of the tested object and the equipment of the actual test on site. In general, the test transformer is an assembly. It's a typical test connection in Figure 1.
Test transformer often can't meet the test requirements when we do AC voltage withstanding test on the tested object with larger capacity such as transformer, capacitor etc. So a parallel reactor is often used to make up the deficit. If the references are proper and the capacitive reactance and inductance of the two parallel branches are equal, the circuit will be in the state of parallel resonance and the test transformer load the minimum. The test transformer shall be installed with overcurrent breaker when parallel resonant circuit is adopted. Because when the tested object comes into a breakdown, the test transformer has the risk of overcurrent.
Ty—voltage regulator T—test transformer; r—current-limiting resistor r—sphere gap protective resistor G—sphere gap
Gx—capacity C1, C2—high and low-voltage arms of the capacitive voltage divider PV—voltmeter
Figure 1 The Principal Wiring Diagram of Power-Frequency Withstand Voltage Test
5.3 Series Resonant Circuit
Test equipment with large capacity is needed in the AC voltage withstanding test on the object with large capacity and high test voltage such as GIS, generator and transformer, crosslinking cable, high-voltage breaker etc. Voltage resonance test device can be adopted for it is able to test the object with large capacity and high test voltage with less capacity. The circuit is made up by the capacitance the tested object loads and its serial reactor and the power supply. As shown in Figure 2
When the frequency of power supply (f), inductance L and the capacitance of the tested object meet Formula (1), the circuit will be in the state of voltage resonance. In this case,
Contents
Foreword i
1 Scope
2 Normative References
3 Terms
4 The Requirements to AC Test Voltage
5 Generation of AC Test Voltage
6 Test Equipment
7 Measurement of Test Voltage
8 Test Methods
9 Cautions in AC Voltage Withstanding Test.
