Research and Application of Variable Frequency Resonant Technology in On-Site AC Withstand Voltage Test of SF 6 Insulated Electric Device

Kai Zhang,  Guangyan Guo
HIMALAYAL - SHANGHAI - CHINA

Abstract: The field test should also observe the principle of high voltage test and must simulate operation filed strength condition. Therefore, AC voltage test can locate breakdowns correctly while the above principle of high-voltage test is not suitable for DC test. The voltage applied in the high voltage system of SF 6 insulated electric devices (such as GIS)is high and the power is large. Because of heavy weight, traditional test transformers or variable inductance resonant power source is not suitable for on-site usage. The solution is to adopt variable frequency resonant power source, which can reduce weight and volume. This article will discuss the principle, characteristics and application of variable frequency resonant system used on site. We will make the following conclusion: the variable frequency resonant system has many advantages and is easy to carry, which represents the future of on-site test power source.

Key Words: SF 6 insulated electric device, DC voltage, Variable frequency resonant

1. Principle of On-Site High Voltage Test
A general principle of insulation-related and high-voltage test technologies is that field strength of test voltage applied on test objects must simulate operating condition of high-voltage electric devices; all conclusions drawn from high-voltage tests need to illustrate whether weaknesses of high-voltage electric devices can bring harm to operations in future. That means fault mechanism in the test should have the same physical process as operation mechanism in the electric device operation. In order to accelerate this physical process, test voltage needs to be higher than operation voltage.

Based on the above principle, the most typical applications in practice are AC withstand voltage test of high voltage electric device, lightening impulse test and switching impulse test. This principle has been fully applied in labs while it is difficult to apply it to on-site test. The main difficulty is that it is not convenient to carry traditional test equipments when used in test objects with large capacity. After an electric device passing both type and delivery test arrives at the site, acceptance test is conducted to check whether the device is damaged during the transportation and installation. Whether field test can be conducted on site according to the above principle depends on whether there are suitable high voltage equipments.

The traditional field test adopts DC power source, characteristics of which are light weight, good movability and low capacity. However, it is not suitable for GIS. Therefore, power frequency test power source is the best choice for field test. According to Guide for Withstand Voltage and Insulated Test of Gas-insulated Metal-Enclosure Switchgear on Site – DL/555-94 (IEC517, IEEE P469-1D5 GIS 1981), on-site GIS test can adopt higher frequency ranging from 30 to 300Hz.