The risks of feeding VSDs from generators
ABB’s water framework manager, Martin Richardson, explains some of the challenges associated with supplying variable-speed drives (VSDs) from generators – and how to solve them.
In the water industry, we are increasingly seeing customers using generators to provide resilience in case of power supply issues with the public network. However, powering a VSD from a generator poses certain technical challenges that, if not addressed, can cause costly problems down the line, particularly when retrofitting VSDs to applications fed from existing generators.
There are three main issues to consider when installing VSDs onto generators:
- Standard six-pulse VSDs will generally draw more current due to harmonics, and this means that the generator has to provide additional harmonic current as well as fundamental current. Harmonic content can cause issues with the generator’s AVR (automatic voltage regulator). One option is to de-rate the generator by 50% or so to provide the additional current, but this is costly, and only mitigates, rather than eliminates the harmonic content, and does not solve the underlying issue.
- Older generators – and, in particular, the AVR – can react to harmonic content and this can make them unstable to the point where they trip out, even with plenty of de-rate capacity.
- VSDs generate small amounts of earth leakage, which can cause nuisance trips on generators if the correct protection is not selected or set up appropriately.
The first two of the above points can be solved by using an active filter, or an ultra-low harmonic (ULH) drive. ULH drives provide exceptionally low harmonic content compared to conventional six-pulse drives, and require little additional current from the generator, ensuring efficient operation.
Generators provide vital backup power in the event of supply outages, but they are expensive. Spending a little more on a ULH drive or an active filter will far outweigh the cost of a replacement generator, as well as solving the underlying harmonic issues.
Another important factor to consider is the criticality of generators. When a generator is required, it is often because of an emergency, and so it is vital that the generator can be relied upon when needed to ensure system resilience.
ABB offers Lunch ‘n’ Learn training courses that teach you the fundamentals of harmonics and harmonic mitigation, including the specifics of generator operation. To find out more, visit: http://bit.ly/2TDg6Sa
