Fault rate analysis of variable-frequency electric drives in district heating stations under power supply violations

The article considers fail-safety decrease for variable-frequency electric drive (VFD) under power supply violations. It is noted that wide VFD introduction aggravates problem of power generating unit emergency stop in heating station under power supply violations (such as voltage sag or short-time voltage shutdowns (1-3 s)). The reasons for these shutdowns are electric drive acceleration rate decrease under power supply recovery and increase in electric drive work time pause duration. The diagrams describing the processes under VFD power supply violations are shown. The monitoring results of duration and number of voltage sags resulting in FVD failure and occurring in high-voltage switchgear sections 10 kV at a district heat generating plant are submitted. It is detailed that the duration of most deviations is 0.2-0.6 s. Besides, troubles in various inputs in most cases are not synchronized, so there is no full disconnection of a power user. The article gives the data concerning the number of variable-frequency and unregulated electric drive shutdowns of boiler KVGM-100 smoke exhauster at Magnitogorsk peak boiler plant, stated in the period of 2005-2013. The algorithm and details on the developed program of electrical equipment fault rate calculation are provided. As a result of the calculations the average number of shutdowns per year is defined, which is 8 cases per year for VFD and 3 cases for unregulated electric drives. It is noted that these data correspond to the results obtained using other analysis techniques and the results published by other authors. The article marks the research results in creation of VFD with the increased resistance to power supply violations executed by a group of author's. The expediency of designing VFD with two independent power supply inputs is proved.