Analyses of vacuum circuit breaker switching transients in medium voltage networks with respect to LC filters of solar converters T. Kuczek, M. Florkowski, W. Piasecki ABB Corporate Research Center 13A Starowislna St., Krakow, Poland [email protected], [email protected], [email protected]Abstract: This paper presents possible transient states related to vacuum circuit breaker operation in PV power plant. Case study of transformer de-energization under no-load conditions was analyzed. The variation with respect to up-to-date studies is related to determination of solar converters LC filters influence on multiple arc re-ignitions occurrence during vacuum circuit breaker operation. Research covers both laboratory measurement as well as PSCAD simulations. key words: vacuum circuit breaker, transients, switching, LC filters, solar converters, simulations verification I. INTRODUCTION The motivation for this study was driven by significant development and expansion of photovoltaic (PV) power plants in modern electrical power systems. Photovoltaic market has grown by a factor of hundreds of percent [1] during last 13 years. It covers both rise of small household installations as well as large photovoltaic plants with the peak power capability of 500 kW p and more [2]. Photovoltaic cells generate power that is dependent on solar irradiation and ambient temperature [3, 4]. Voltage and current at DC are converted by means of power electronic inverter to the AC side and then transferred to the medium voltage level by LV/MV transformer [6]. Such circuit may be subjected to de- energization operation that is usually performed by vacuum circuit breakers (VCB). At certain network conditions such operation may result in high peak of the transient recovery voltage, which can lead to significant overvoltages generation at the transformer terminals. Such effects were analyzed for example in [7] and [8], but for other applications like no-load transformer or arc furnace transformer, not for PV related. This paper is focused on determination of influence of solar converter’s LC filters on transformer de-energization during no-load conditions. Research covered both laboratory experiments as well as PSCAD numerical simulations. II. PV PLANT LAYOUT OF CONCERN The layout of concern in this study is presented in Figure 1. PV panels generate DC voltage and current, which are dependent on solar irradiance and ambient temperature. Manufacturers of PV panels provide nonlinear characteristics of generation with respect to above mentioned factors [3], [4]. The DC power has to be converted into AC by means of DC/DC and DC/AC power electronic converters. For DC/DC boost converters are most commonly utilized since DC voltage has to be appropriately adjusted in order to transfer the power by the inverter from PV panels into the external grid. The LC filters are required in order to limit the ripple and the THD in voltage and current [5]. They are designed according to desired output current, voltage, switching frequency and allowable limits of peak-to-peak ripple and THD. For example, 5% value can be found in the literature [6]. PV LV VCB external grid DC DC DC AC MV MV cable LC Figure 1. Equivalent circuit of grid connected PV plant As mentioned earlier, LV/MV transformer may be subjected to switching off operations by means of vacuum circuit breaker. This phenomena is well described in literature [7, 8, 9]. However, this paper focuses on influence of converter’s LC filters on switching conditions during vacuum circuit breaker operation. This is due to the fact that those filters may change the total impedance as seen from operated vacuum circuit breaker’s terminals, thus influencing the natural frequency of the circuit being switched. Based on above clarification, several laboratory tests of switching off operations were conducted in network prepared on AGH University of Science and Technology in Kraków, Poland. Measurement results allowed to develop PSCAD model that was utilized for experimental results verification as well as further analyses. III. VACUUM CIRCUIT BREAKER OPERATION Vacuum circuit breakers use vacuum as a quenching medium for electrical arc suppression, which appears across breaker’s contacts during any switching operations. Thanks to this method, the dielectric withstand between circuit breaker contacts is approximately 10 times larger than in air at atmospherical pressure. As a result, it is possible to decrease the gap between contacts inside the vacuum chamber of the circuit breaker. Several conditions have to be fulfilled in order
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Analyses of vacuum circuit breaker switching
transients in medium voltage networks with respect to
LC filters of solar converters
T. Kuczek, M. Florkowski, W. Piasecki ABB Corporate Research Center
Abstract: This paper presents possible transient states related
to vacuum circuit breaker operation in PV power plant. Case study of transformer de-energization under no-load conditions was analyzed. The variation with respect to up-to-date studies is related to determination of solar converters LC filters influence on multiple arc re-ignitions occurrence during vacuum circuit breaker operation. Research covers both laboratory measurement as well as PSCAD simulations.