A new wind turbine transformers protection method against high du/dt switching transients taking place during a wind turbine operation is described. High-frequency transients, characterized by a high rate of voltage rise (du/dt) and overvoltage at the wind turbine transformer high-voltage terminals may be a result of operations of the turbine switchgear typically comprising a vacuum circuit breaker. The protection concept described in this paper is a follow up of research activities on a transients suppressing method dedicated for distribution transformers. The principle of operation of the protective device proposed and parameters optimization procedures for windmill transformers are provided in this paper. The performance of the device was verified by means of Alternative Transients Program-Electromagnetic Transients Program simulations. The physical device was built and experimentally verified by full-scale functional tests reflecting a real wind turbine power network.

An investigation on internal arc fault tests in gas-insulated metal enclosed MV switchgears is described and discussed. The influence of different gases (SF 6 and air) and electrode materials (Cu, Al, and Fe) has been put into evidence. This is highly relevant based on the fact that IEC Standards and utilities technical specifications allows that internal arc fault tests are performed with air replacing SF 6 with some precautions. The experimental tests were carried out at NEFI High Power Laboratory in Skien, Norway. Full-scale test objects representative of typical gas insulated metal enclosed MV switchgear were prepared, filled with air or SF 6 and tested. The short circuit current was 16 kA rms and the duration was 1 second. Electric input energy, internal gas pressure in different locations, opening time of bursting discs and temperature were acquired. The arc was ignited at the end of the simplified, but still representative, 400 mm long bus bars that were located in the middle of the metal enclosure. The full-scale experiments were additionally analyzed by means of Finite Element Method. The comparison between experiments and simulations show that there was possible to set up an arc model and tune it in to the results. The main difference between testing of units filled with air versus SF 6 is the significant faster pressure increase in air. As a result of this and the fact that the bursting discs need a certain time to open, the pressure inside the test object will be higher at the time the bursting discs open when testing with air. The maximum pressure reached during a test with air however may be equal or lower than in SF 6 depending on dimensional parameters of bursting disc and encapsulation. Furthermore it is evident that there is a clear difference in the exhaust characteristics of SF 6 and air from an internal arc test.

Arc fault tests of medium voltage switchgear have been performed with reduced volume and arc energy. This has been done in order to investigate if small scale experiments can be used to predict the pressure build-up during full scale arc fault test. Between 40 and 50 % of the arc energy was transferred to the gas in both small scale and full scale tests. The results show that it is reasonable to assume that small scale testing down to 10 % can be used to predict the pressure rise in a full scale test with a single phase arc with about 10 % precision. However, the scaling method of the arc energy seems to be important.

SF6 is used as an insulation and interruption medium in medium-voltage (MV) gas-insulated switchgear (GIS), but has a very high global warming potential. In recent years, several environmentally-friendly (eco-efficient) alternatives have been explored, focusing on dielectric and thermal properties. The interruption of low currents in MV application is emerging as an important topic for equipment manufacturers and users. The reduced arcquenching properties of the most prominent eco-efficient alternatives may require the use of vacuum interrupters for simple load current interruptions. However, this may not be a cost-effective solution and simpler interruption principles are desirable. In this paper, we explore low-current interruption in AirPlus TM , a m i x t u r e o f d r y a i r a n d t h e C 5 F 10O fluoroketone (C5-FK) as well as mixtures of CO2 and C5-FK. We find that it is possible to achieve the E3 electrical endurance class (100 c/o) with a switch based on the puffer principle in AirPlus with a condensation temperature of-25˚C, suitable for secondary distribution MV GIS. The chemical analysis of gas samples taken from the switchgear after 100 successful interruptions indicate only trace amounts of fluoroketone decomposition products.

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