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Using variable speed drives with standby generators

Author : Stuart Kemp, drives application engineer, Dalroad

19 June 2019

Many industries, utilities and public service buildings have a critical requirement for continuity of power supplied to them to ensure that their operation is not compromised in the case of a supply failure.

In the case of a hospital, a loss of power would endanger life, whereas in a data centre a power interruption could lead to data loss and everything in between.

If we take shed enclosed poultry farming as an example it is clear to see the necessity of having a second source of power in the event of loss of mains. Each shed may contain up to 40,000 birds which after 5 weeks or so have almost achieved full adult size and weight. Each bird gives off a considerable amount of body heat and automatic ventilation systems in the shed walls and roof increase the airflow as they grow to keep the birds at the correct temperature.

In the event of a sudden and unplanned power supply failure due to a tree hitting an overhead line in a storm, for example, the near-adult birds in an affected shed would quickly overheat and begin to die in about fifteen minutes so it is essential that power is resumed very quickly. An appropriately sized diesel generator configured to automatically come online in the event of such a mains power failure is the standard solution in this industry as it is in many others.

Such a generator must be correctly dimensioned to suit the nature of the load that will be connected and some ‘non-linear’ loads such variable speed drives require the generator to have a considerably larger capacity than would be necessary for standard loads such as lighting, heating, motors, etc.

The non-linear input current of a typical general purpose 6 pulse variable speed drive will distort the generator output voltage due to harmonics caused by the drive’s rectifier. This current will differ greatly from a normal linear sine wave shape as 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 sine wave. 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 typically 3-5% and when a VSD is connected to such a network there are normally few issues relating to the performance of the drive as 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 volt 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 operation where the load is increasing or swinging rapidly.

Connecting additional line reactors at the input to the drive only increases source impedance and should be avoided.

Due to the high cost of purchasing a standby generator careful consideration of the capacity required to support the total variable speed drive load should be given prior to ordering it as mistakes can prove very costly. Best practice is to oversize the generator capacity by three or four times in order to reduce its percentage source impedance to a point where the drives would be expected to operate without problems.

Example

A standby generator is required to supply an installation in the event of main power failure.

Connected loads are:

• 2 x 15kW 400V, 3-phase centrifugal pumps operated by two 6-pulse VSD’s

• 22 x 1.1kW 400V, 3-phase fans operated in parallel by four 6-pulse VSD’s

• Total VSD loading = (2 x 23kVA) + (4 x 9kVA) = 82kVA (Actual VSD capacity can be obtained from supplier or manufacturer)

• Minimum generator capacity to support VSD load = 82 x 3 = 246kVA

• Various other loads (lighting, heating, etc) = 4kVA

• Minimum generator capacity to support total site load = 246 + 4 = 250kVA

For some applications it may be necessary to oversize the generator by more than three times and required capacity will increase if other kinds of non-linear loads are connected in addition to the variable speed drives.

If the generator capacity cannot be increased to the size required to support a given VSD load a 1:1 ratio isolation transformer with phase shifted secondary windings connected on the generator output could be considered. Typically, such a transformer would have two secondary windings, one connected in star and one in delta giving a 30° electrical phase shift.

The installed VSD’s would also need to support the connection of a second bridge rectifier which would then connect to the delta winding of the transformer whilst the other rectifier is connected to the star winding, then the newly configured VSD becomes a 12 pulse VSD with significantly reduced peak currents. This option would normally be a last resort as the cost of the transformer and its installation could be far higher than the cost of upsizing the standby generator.

In all cases, the generator manufacturer, the drives supplier along with the suppliers of any other non-linear loads with significant input current should be consulted prior to specifying the generator capacity.


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