September 2019

50 n BUILDING SERVICES September 2019 www.drivesncontrols.com How to use VSDs with standby generators M any industries, utilities and public service buildings have a critical requirement to ensure the continuity of their power supplies so that their operations will not compromised if the supply fails. In the case of a hospital, for instance, a loss of power would endanger life, while in a data centre, a power interruption could lead to critical data losses. Take shed-enclosed poultry farming as another example. It is clear that these sheds need a second source of power in the event of the mains supply being lost. Each shed may contain up to 40,000 birds which after about five weeks will have almost achieved their full adult sizes and weights. Each bird gives off a considerable amount of body heat and to keep the birds at the correct temperature as they grow, automatic ventilation systems in the walls and roof are used to increase airflows in the sheds. In the event of a sudden and unplanned power supply failure – caused, for example, by a tree hitting an overhead line in a storm – the near-adult birds in an affected shed would quickly overheat and would start to die in about 15 minutes, so it is essential that power is resumed rapidly. The standard approach in this industry – as in many others – is to use a diesel generator that comes online automatically in the event of a mains power failure. Such a generator must be dimensioned correctly to suit the nature of the load that will be connected. Some “non-linear” loads such variable-speed drives require the generator to have a considerably larger capacity than would be needed for standard loads such as lighting, heating, motors and so on. Distorted voltage The non-linear input current of a typical general-purpose, six-pulse VSD will distort the generator output voltage due to harmonics caused by the drive’s rectifier. This current will differ greatly from a normal linear sinewave shape because the diodes tend to “bite” current out of the supply, leading to a pair of current peaks on both the positive and negative halves of the sinewave. The bigger the motor load on the VSD, the higher the current peaks and the greater the effect on the generator output voltage. In the UK, a typical mains power transformer on the supply network may have a source impedance of 3–5%. When a VSD is connected to such a network, there are normally few issues relating to the performance of the drive because the input voltage will be largely unaffected. However, when the same VSD is connected to a standby generator, the input current will have to be supplied via a much higher source impedance – typically around 20%. Because the impedance of the source is proportional to voltage drop, the generator output voltage may be severely limited at the points in the waveform where the current peaks are at their highest. This can cause the drive to fault trip on under- voltage – or sometimes over-voltage – conditions during ramp-up or steady-speed Non-linear loads such as VSDs can pose problems when they need to be driven by standby generators. Stuart Kemp, a drives application engineer with Dalroad who has 25 years of experience working with VSDs and controls, offers advice on tackling such problems. When ordering a standby generator you need to consider carefully the capacity it needs to support VSD loads

RkJQdWJsaXNoZXIy MjQ0NzM=