0.1Hz ultra-low frequency sine wave withstand voltage test technology and its application-EEWORLD

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0.1Hz ultra-low frequency sine wave withstand voltage test technology and its application

Abstract This paper focuses on the 0.1Hz ultra-low frequency sine wave withstand voltage test technology commonly used internationally and the withstand voltage test equipment using this technology. The 90kV withstand voltage tester newly launched by the American High Voltage Company solves the problem that there is no suitable withstand voltage test equipment for 35kV cross-linked polyethylene insulated cables in China. The test equipment directly generates a true sine wave without adding additional technical difficulties. This not only reduces weight and saves costs, but also simplifies its application in diagnostic tests, such as dielectric loss test, partial discharge test and on-site fault location. This paper also introduces the core content of the IEEE P400.2/D1 standard newly released by the American Institute of Electronics and Electrical Engineers in August 2003. This standard can be used to guide the withstand voltage and diagnostic tests of on-site shielded power cable systems. Keywords

Ultra -low frequency sine wave 90kV withstand voltage test discharge resistance

I. Introduction

One of the important measures to ensure power supply reliability is to conduct a withstand voltage test (preventive test) on (buried) power cables, that is, to check the insulation condition of power cables.

Since the 1970s, polyethylene/cross-linked polyethylene cables have been widely used and gradually replaced traditional oil-paper cables, especially medium and low voltage power cables. The DC withstand voltage test method is no longer applicable to this type of cable. The reason is that under the action of DC voltage, the space charge effect is serious, and the DC withstand voltage test endangers the dielectric strength and life of the cross-linked polyethylene cable. The use of AC test equipment in field power frequency testing also has its own problems. The test equipment is often large and bulky, and the price is also expensive. Ultra-low frequency detection equipment has been used for many years to detect large rotating motors, such as large hydroelectric generators. However, this technology has not been used for cables. The equipment that was available in the past usually used a voltage with a frequency of 0.1 cycle (Hz) and a variety of different waveforms, such as rectangular waves and triangular waves. The equipment has reasonable cost and weight factors in some form, and does not make up for the lack of technical data for testing cables with these non-standard waveforms.

It can be seen that the old method that everyone knows and works: DC testing has been proven to be ineffective, and in some cases it will amplify damage that cannot be found during testing. AC test is effective, but because the cable is a capacitive load, a large test capacity is required (S=ωCUS2=2πfCUS2 kVA)

where: C—Test cable capacitance μf/km

US—Test voltage kV

f—Power frequency, which is 50Hz in China.

Traditional ultra-low frequency technology is not suitable for cables, so a new cable detection method needs to be studied.

2. 0.1Hz ultra-low frequency sine wave technology

As early as the 1970s, the American High Voltage Company was committed to the research of ultra-low frequency cable detection methods. They adopted a new method and produced cable detection equipment that can generate real high-voltage sine waves. The equipment is very light and the cost is close to that of the DC test system. Practice has also confirmed that the breakdown voltage of solid insulated cables using ultra-low frequency and high voltage is equivalent to the voltage value obtained using AC power frequency.

The new design method allows the test load of cables far exceeding 5 microfarads at the usual withstand voltage level, and only uses a power supply with a voltage of 120 volts and a frequency of 60Hz (or 220 volts 50Hz). Moreover, this design can also be extended to some lower frequency operations of 0.05, 0.02, and 0.01Hz (the frequency test of 0.01Hz is for ultra-long cables).

1. System Design Principle

The basic idea of this design is to generate an ultra-low frequency waveform like a sine wave. Use ultra-low frequency to charge the test piece to a high voltage with a low charging current and a relatively long time interval. The ultra-low frequency waveform is the key here, and the sine wave is particularly suitable because it avoids the high-frequency harmonics that may be generated by other waveforms, which will produce standing waves or harmful voltage mutations on the test target. The basic concept of the new design has been shown in Figure 1. The input power required by the system is obtained from a normal 120V 60Hz or 220V 50Hz power supply. The amplitude of the output voltage is automatically adjustable.

Figure 1

The output of the transformer is controlled by a modulating transformer, represented by T1 in Figure 1. The output of the transformer is modulated with the required ultra-low frequency, such as 0.1Hz. This process is represented by T2 in Figure 1. The output of T2 increases or decreases periodically in a sinusoidal mode, with a frequency that is twice the output frequency, which will produce a 60 (or 50Hz) voltage, and the amplitude variation is shown in Figure 2 (a). The modulated power frequency voltage is gradually stepped up by the high-voltage transformer, that is, T3 in Figure 1. The output of the high-voltage transformer is rectified by a full-wave rectifier that can produce a unipolar voltage, as shown in Figure 2 (b). Finally, a polarity switch between the rectifier and the terminal reverses the polarity of the rectified voltage every half cycle. The output cable and the capacitor of the test product will provide sufficient filtering to reduce the 120Hz fluctuations to an acceptable level, and the final waveform is a high-voltage ultra-low frequency sine wave shown in Figure 2 (c). The waveform shown in Figure 2 does not take into account the energy storage of the test product. Ultra-low frequency detection equipment is mainly used for high-voltage testing of large power equipment components. This type of DUT has a large capacitance. When the phase angle of the applied voltage is between 90º-180º or 270º-360º, the capacitance must be discharged.

Figure 2 Output waveforms of the system at each stage

2. Discharge of the Capacitor of the Test Item

When a large-capacitance capacitive test item is applied with AC, it must be discharged at each half-wave. Traditional power frequency AC testing requires a large energy flow between the test item and the power supply, which requires large transformers, regulators, etc. In ultra-low frequency systems, the power required is very low, 500 times smaller in a 0.1Hz system than in a 50Hz system. As a result, this energy is easily exchanged within the test device itself.

This chapter discusses a patented technology for discharging the test item that ensures that the high-voltage output is a true sine wave. The system continuously connects a series of resistive loads to the output circuit to discharge the test item capacitor. Under normal circumstances, a resistor is connected in parallel with the capacitor to cause the capacitor voltage to decay exponentially. The resistor is selected so that the voltage drops below the standard sine wave through the RC mixed discharge circuit. Therefore, when the resistor is connected to the circuit, the sinusoidal voltage of the system high-voltage transformer is maintained in a sinusoidal shape by compensating for the current required by the discharge resistor. The initial RC circuit exponential decay rate is very high and decreases over time. Finally, the voltage from the high-voltage transformer reaches equilibrium with the load voltage. At this point, the second discharge resistor is quickly connected, reducing the RC time constant. Repeating this process as needed can maintain the sinusoidal shape of the output voltage. Figure 3 illustrates this technique of using three discharge resistors connected to the output loop. The exponential decay curve of the first resistor and the load capacitance is represented by RC1. The exponential curve and the applied sine wave are in phase at 2.41 radians. At this time, the second resistor is added, and the decay curve is represented by RC2. At a phase of about 2.86 radians, the third resistor is added to the loop, and the exponential curve decay is represented by RC3.

Figure 3 Load discharge characteristics

The power supply of the high-voltage transformer compensates the space (energy difference) between the sine wave and the exponential decay curve. An important factor in this design is that the discharge resistor can be selected based on the time constant required when the load changes the most. Whether it is light or heavy load, a satisfactory waveform can be produced. The choice of resistor is independent of the load.

The current of the discharge resistor is very low, and the power loss is easily solved. Three discharge resistors fully meet the requirements of 40-60kV output voltage. The number of resistors can also be increased, and the same principle can be applied to systems with higher voltage levels such as 90 and 120kV.

3. Frequency

The operating frequency of the system can be obtained by adjusting T2 in Figure 1 through the technology discussed earlier. At present, the 0.1Hz VLF test is commonly used. Adjusting T2 can obtain different operating frequencies such as 0.05, 0.02, and 0.01Hz. Lower frequencies can test ultra-long cables.

Three. Characteristics and application of 0.1Hz ultra-low frequency sine wave withstand voltage test equipment

As mentioned above, DC withstand voltage test is not suitable for polyethylene and cross-linked polyethylene insulated cables, because the space charge formed under the DC electric field will be stored in the cable insulation layer. When the test is completed and put back into operation, the remaining space charge (electric field) will be superimposed with the electric field of the operating voltage, causing breakdown of the cable that can still operate in practice. AC power frequency withstand voltage test is the most effective, but due to the large capacitance of power cables. Large-capacity test equipment is required, and the equipment is bulky and difficult to conduct preventive withstand voltage tests (field tests) on polyethylene/cross-linked polyethylene cables. In response to the on-site withstand voltage test of medium-voltage polyethylene/cross-linked polyethylene cables, the American High Voltage Company, which has always been in a leading position in VLF sine wave theory research and equipment development, has been committed to the research of this technology as early as the 1970s. The 0.1Hz ultra-low frequency sine wave withstand voltage test equipment has the following significant characteristics. It is precisely because of these characteristics that this withstand voltage test equipment has been widely used.

a) The power consumption

of 0.1Hz ultra-low frequency sine wave withstand voltage test equipment is small, which is 1/500 of that of 50Hz withstand voltage test equipment. b) The output voltage of 0.1Hz ultra-low frequency sine wave withstand voltage test equipment is relatively high, and there are 250kV AC output test equipment for higher voltage systems.

c) Due to the low input power, the size and weight of the equipment are also small, and they are all two-piece or three-piece portable designs.

d) 0.1Hz ultra-low frequency sine wave withstand voltage test equipment can test longer cables (larger capacitance), and the maximum VLF test equipment can reach 50 microfarads, which is ten times that of similar foreign products.

e) It has a capacitance test circuit, which can measure the capacitance of the cable under test and estimate the cable length.

In addition to these features, the sine output of the 0.1Hz ultra-low frequency sine wave withstand voltage test equipment can also be used for diagnostic testing. The traditional on-site test only checks whether the withstand voltage passes or not. Now the more scientific method is diagnostic testing - partial discharge test and dielectric loss test, which are used to detect water trees in insulation and comprehensively evaluate the insulation condition of the cable. Sine waves can be effectively used for partial discharge test and dielectric loss test. The equipment also has a rapid resistance reduction and burn-through function, automatically limits the arc current, and can be used to quickly locate and repair faults, with significant cost-effectiveness.

VLF-90CMF is the latest AC withstand voltage test equipment launched by American High Voltage Company, suitable for XLPE, PE, EPR, PILC cables and their connectors with medium voltage levels of 10kV and 35kV. The equipment can continuously output 0-90kV voltage, and can obtain sine waves of different frequencies such as 0.1, 0.05, and 0.02Hz. The waveform is independent of the capacitive load and can test 7500 meters of cable. The two-piece portable design can be installed on the cable test vehicle or equipped with a trolley for easy movement.

The main technical parameters are as follows:

Input: 230V, 50/60Hz, maximum 6A, average 2.5A

Output: sine wave, 0-90kV AC peak voltage (0-63.6kV AC RMS voltage), 0.1Hz/0.05Hz/0.02Hz continuous operation, shielded output cable

Measurement: 3.5” meter, 0-90kV peak voltage; 3.5” meter, 0-100mA peak current, 0-6µF capacitance

Load: 0.55 µF @0.1Hz 1.1 µF @0.05Hz 2.75 µF @0.02Hz Dimensions: (w length; h height; d width) Controller: 635mm w × 406mm h × 330mm d High voltage: 185mm w × 685mm h × 330mm d Weight: Controller: 34kg High-voltage device: 82kg In addition to 90kV, the company also produces 40kV, 60kV, 120 kV, 180 kV, 250kV and other series of withstand voltage test equipment, which meets the demand for such products in China to the greatest extent. For cables of 10kV and below in China, 40kV withstand voltage test equipment can be selected. The products of American High Voltage Company have been recognized by the power systems and enterprises in many countries in the world. They have been used in Benxi Iron and Steel Group, Lingyuan Iron and Steel Group, Xianyang Airport Power Supply Station and other units in China, showing the superior performance of 0.1Hz ultra-low frequency sine wave withstand voltage test equipment. 4. Power cable withstand voltage test specification After the 0.1Hz ultra-low frequency sine wave withstand voltage test technology was widely used, in order to standardize the withstand voltage test method for cross-linked polyethylene cables, oil-impregnated paper cables and hybrid cables, the American Institute of Electronics and Electrical Engineers issued the latest IEEE P400.2/D1 standard in August 2003. The standard stipulates that the test voltage of cables with a voltage level of 15-35kV should be 3U0 during installation and acceptance tests; for cables with a voltage level of 5-8kV, the test voltage should be greater than 3U0; the test voltage during maintenance is 80% of the acceptance test voltage. If multiple cycle tests are performed, the test voltage can be reduced by another 20%. Table 1 lists the cable rated voltages for the sine wave withstand voltage test of shielded power cables. Although only 5-35kV cables are listed, VLF can still be used for power cables with higher voltages. The test voltage given in the table can be used as both an effective value and a peak value. If used as a peak value, the test time needs to be doubled. The reason is that the effective value is 0.707 times the peak value.

Ultra low frequency sine wave test voltage 1
Cable grade relative to	Installation 2 relative to ground	Acceptance 2 relative to ground	Maintenance 3 relative
kV (effective value)	kV (effective value)	kV (effective value)	kV (effective value)
5	12	14	10
8	16	18	14
15	25	28	twenty two
25	38	44	33
35	55	62	47

1 The choice of effective value or peak value depends on the possible mechanical damage. If this voltage is taken as the peak value, the test time is doubled. 2 The recommended test time is 15 to 60 minutes. The actual test time depends on the supplier and the user and depends on the test conditions, cable system, installation conditions, frequency of testing and the selected test method. When the test is interrupted, the timer is reset to zero and the test is restarted. 3 The recommended maintenance test time is 15 minutes. The frequency range of the VLF test method is 0.01 Hz to 0.1 Hz. The most commonly used is 0.1Hz, and other available frequencies range from 0.0001 Hz to 0.1 Hz. If the frequency is lower than 0.1Hz, the corresponding test time is extended. Test points · It is necessary to prepare a wiring diagram so that the personnel are familiar with the cable being tested, the location of the disconnection point, where it is easier to access the cable or connection point and the type of structure of the cable being tested. · If a fault is found during the withstand voltage test, the fault location should be carried out. · In the diagnostic test, if the test cable is severely aged, it will cause a breakdown before the test is completed. · When the VLF test is finished or interrupted, the test piece should be immediately grounded and discharged. 5. Discussion on the application of 0.1Hz ultra-low frequency sine wave withstand voltage test technology in China After nearly 30 years of research, development and application, the 0.1Hz ultra-low frequency sine wave withstand voltage test technology has become a relatively complete withstand voltage test technology. It not only provides a reliable and effective method for the withstand voltage test of polyethylene/cross-linked polyethylene cables, but can also be used for the withstand voltage test of oil-paper cables. The design of the ultra-low frequency system provides high voltage output by a portable, lightweight and economical device. The output waveform is a sine wave, which allows it to be used in many diagnostic technologies, both to evaluate the degree of cable aging and to detect local defects in the cable. Therefore, the 0.1Hz ultra-low frequency withstand voltage test technology has been widely used in power systems and enterprises in many countries in the world. For example, the IEEE standard of the Institute of Electronics and Electrical Engineers of the United States, the 0.1Hz withstand voltage test standard for cross-linked polyethylene cables formulated by the German Electrical Committee, and the cable test standard of the North China Electric Power Group Corporation have also included the 0.1Hz ultra-low frequency withstand voltage test in the test standard. In particular, the latest 90kV AC high-voltage test equipment launched by the American High Voltage Company for the Chinese market fills the gap of China's lack of suitable withstand voltage test equipment for 35kV cables. The 40kV withstand voltage test equipment first used by Benxi Steel and Linggang in China has outstanding features and stable operation, and has been well received by industry insiders. In summary, the 0.1Hz ultra-low frequency sine wave withstand voltage test technology will inevitably become a popular technology in China.

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