NDXZ-B Generator Power Frequency AC Withstand Voltage Device

Product Introduction

   Currently, both internationally and domestically, XLPE (cross-linked polyethylene insulated) power cables are increasingly replacing traditional oil-filled paper-insulated power cables. However, due to the large test capacity and limitations of testing equipment, the DC withstand voltage test method is still commonly used before commissioning XLPE cables. In recent years, research from many international and domestic institutions has shown that DC testing can cause varying degrees of damage to XLPE cables. Some studies suggest that the XLPE structure has the ability to accumulate unipolar residual charges. If these residual charges are not effectively discharged after DC testing, the combination of DC residual charges and AC voltage peaks during operation may lead to cable breakdown. Some domestic research institutions believe that during DC withstand voltage testing of XLPE cables, the actual electric field strength in the insulation can be up to 11 times higher than the operating electric field strength due to space charge effects. Even if the XLPE insulated cable passes the DC test without breakdown, it may still suffer severe insulation damage. Additionally, the distribution of the DC voltage field strength differs from that of the AC voltage field strength during operation. DC testing cannot accurately simulate the overvoltage conditions that cables experience in operation, nor can it effectively identify defects in the cables, cable joints, or installation processes. Therefore, non-DC methods for withstand voltage testing of XLPE cables are gaining increasing attention. At the same time, AC withstand voltage tests for large transformers, as well as thermal and hydro generators, are also conducted periodically. These tests require high-capacity testing equipment, typically achieved through resonance methods, but must be performed under power frequency or equivalent power frequency conditions. Equivalent power frequency conditions generally use a frequency range of 45-65Hz, though many testing institutions require a 50Hz test power supply for AC withstand voltage tests on such equipment.

   Our company's series of series frequency conversion automatic resonance test devices are primarily used for frequency conversion AC withstand voltage tests on 10kV, 35kV, and 110kV cross-linked rubber and plastic power cables, as well as 66kV, 110kV, and 220kV gas-insulated switchgear (GIS). They are also used for power frequency AC withstand voltage tests on hydro and thermal generators or power transformers. The basic principle involves using an adjustable (30-300Hz) series resonance test device to resonate with the capacitance of the test object, generating an AC test voltage. Due to the large capacitance of cables, traditional power frequency test transformers are bulky, heavy, and require high-current power sources that are difficult to obtain on-site. Therefore, series resonance AC withstand voltage test devices are generally used. These devices significantly reduce the required input power capacity, are lighter in weight, and are easier to use and transport. Initially, adjustable inductance series resonance devices (50Hz) were commonly used, but they suffered from low automation and high noise. Nowadays, frequency conversion resonance is widely adopted, offering higher quality factor (Q value), automatic tuning, multiple protections, reduced noise, flexible configurations, and lighter individual components.

Product Features

☆ Significantly reduced power supply capacity requirement. The series resonance power supply uses a resonant reactor and the test object's capacitance to generate high voltage and high current. In the entire system, the power supply only needs to provide the active power consumption, so the required power is only 1/Q of the test capacity.

☆ Greatly reduced weight and volume of the equipment. The series resonance power supply eliminates the need for bulky high-power voltage regulation devices and conventional high-power power frequency test transformers. Additionally, the resonant excitation power is only 1/Q of the test capacity, reducing the system's weight and volume to typically 1/5 to 1/10 of conventional test setups.

☆ Improved output voltage waveform. The resonance power supply uses a resonant filter circuit, which reduces waveform distortion of the output voltage, providing a clean sinusoidal waveform and effectively preventing false breakdowns due to harmonic peaks.

☆ Prevention of large short-circuit currents burning the fault point. In a series resonance state, when an insulation weak point in the test object breaks down, the circuit immediately detunes, and the current rapidly drops to 1/Q of the normal test current. In contrast, during parallel resonance or test transformer-based withstand voltage tests, the breakdown current increases by tens of times. The difference in short-circuit and breakdown currents between the two methods can be hundreds of times. Thus, series resonance effectively identifies insulation weaknesses without the risk of large short-circuit currents burning the fault point.

☆ No recovery overvoltage occurs. When the test object breaks down, the high voltage disappears immediately due to the loss of resonance conditions, and the arc extinguishes instantly. The process of re-establishing the recovery voltage is long, making it easy to disconnect the power supply before reaching the flashover voltage again. This voltage recovery process is an intermittent oscillation of energy accumulation, which is prolonged and does not result in any recovery overvoltage.

Product Parameters

☆ Rated voltage: 40kV—suitable for AC withstand voltage tests on 10kV generators

☆ Output voltage waveform distortion rate: <1.0%

☆ Allowable continuous operation time: 5 minutes continuously under rated conditions

☆ Device quality factor: Q > 50

☆ Quality factor under full load during generator testing: Q > 15 (load-dependent)

☆ Input power: Single-phase 380V

☆ Frequency adjustment range: 20Hz to 300Hz

☆ System measurement accuracy: 1.5%

☆ The device features protection functions such as overvoltage, overcurrent, and zero-start protection.