BLOG: Power Perspectives

[#1]

Non-linear loads
Non-linear loads create harmonics and voltage distortion, degrading the performance of a generator set. The percentage of voltage distortion is especially critical in data center applications. A generator set's alternator must be properly sized to limit voltage distortion to an acceptable level. Some types of non-linear loads are:

• Uninterruptible Power Supply (UPS)
• Battery Charger
• Elevator
• Variable Frequency Drive Motors – VFD (60 Hz)
• Variable Speed Drive Motors – VSD (50 Hz)

 
Filters & Pulse Rectifiers
To offset the unfavorable effects of non-linear loads, consulting engineers can upsize the alternator or recommend a higher rated (thus more costly) generator set.

However, adding filters and/or increasing the pulse count of pulse rectifiers (e.g. 6 pulse to 12 pulse, 18 pulse or 24 pulse) on a generator set can reduce the harmful effects of non-linear loads. Alternator life can be extended by avoiding the overheating caused by distortion. Power quality can be maintained or improved. Taking those measures can allow for a smaller ekW/kVA rated generator set and reduce the initial purchase cost.

Questions

• As a consulting/specifying engineer or a commissioning engineer, what are the applications or situations in which you recommend the use of filters or higher pulse rectifiers on a generator set?
• What types/size of filters and/or pulse rectifiers do you recommend to balance the cost-benefit trade-off between the purchase price of a filter or pulse rectifier, and the opportunity to downsize the required ekW/kVA rating of the generator set?
• What are some applications in which you have effectively reduced your customer's costs by using filters or pulse rectifiers?
• Are there any problems you have encountered using filters and/or pulse rectifiers?
• What are some rules of thumb you would advocate to a colleague who is faced with a customer site application where harmonics and voltage distortion are a concern?

 
Please post your answers and comments below.

[#2]

At Sturdy I noticed that any thing you do in this direction creates heat which causes problems. Working on aircraft electrical systems at L-3 I find that when the outside temp gets above 90 f, and I'm running tests in the hanger,  the rectifiers start falling offline. The problem is the systems are too small (although well designed and adequate when the aircraft is in flight and at low temps) to work effectively on the ground. Perhaps if a smaller well made system where cooled in some way it would be able to keep up with  the non linear loads.

[#3]

Generally speaking data centre loads, radio transmission stations, telecommunication exchanges, banks,  can have high harmonic load profiles. When we say high harmonics it could be V(THD)  L-L is higher than 10% .

But when we talk about harmonics we have to identify the correct orders of harmonics at the site ie some loads  create  3rd harmonics, and some loads create 5th and 7th harmonics. It could be intermittent or continuous.

I reckon we should apply the filters or rectifires where 5th and 7th harmonics loads are at the site rather than 3rd harmonics. 3rd harmonics can be mitigated by using 2/3 pitch winding and now a days it is more common in most of the generators as a standard feature. Also, in multiple generator applications 3rd harmonics can be mitigated by using neutral switching (ie on 3 phase 4 wire systems ex: 415V 3 phase star TN-C-S , or TT systems), and also, by zig zag earthing transformers.

It is not a good idea to install  filters when you operate generators parallel with the grid. It could be possible to resonate with nearby capacitor banks installed at the neighbour (consumer) connected to the same grid.

First of all it is necessary to take harmonic load profile at the site, and identify the harmonics order and % to the fundamental. Then it is better to do a simulation study using a software. Filters need to be tuned properly , and this will be a trial and error tuning at the site to match the loads, and also, pulse rectifiers may not be suitable for prime and continuous applications, but would be suitable for stand by applications.

Most of the time, economic factors  overpower the technical factors therefore , it is necessary to check  whether it is economically viable even though technically feasible. Also,  we need to consider the maintainability, spares,products support.

If you choosed an oversize generator set for a particular harmonic load application you can guarantee the reliability of the operation as you have one safe source for product support for that generator set. Can we expect the same level of reliability, and operating performance from filters or rectifiers.

In additional to that consider the additional space required to install these filters or  rectifiers, you need to accommodate either in to the switchboard or installed on generator power terminal box. 

In summary I would say if you choosed oversize alternator that would give you a extra kVA required not only to cater for harmonics loads but also for high starting current motors at the site.

In your site if you have intermittent harmonics loads, and the apply of  filters or rectifiers  would be technically feasible and economically viable compared with oversize alternator.

[#4]

Relative to generator sizing and the Cat sizing tool, SpecSizer -

What is a Pulse Rectifier?

The 6, 12, 18 or 24 pulse rectifiers we provide in our Caterpillar generator set sizing tool, SpecSizer are loads.  They are not equipment on the genset.

A standard 3-phase rectifier is a bridge circuit of 6-diodes (2 diodes per phase, one for the positive cycle and one for the negative cycle). On the DC output of the rectifier this configuration produces 6 "ripples" per AC cycle which is why it is called 6-pulse. With a special transformer and two rectifier bridges you can phase shift the input to one of the rectifiers and create 12 "ripples" per AC cycle on the DC output, which is where the term 12-pulse rectifier comes from. Similarly with 3 and 4 bridges you get 18-pulse and 24-pulse rectifiers. The more pulses the less input current distortion to the rectifier and the easier it is for the gen-set to supply.

What is a Filter?

The filter we provide in our Caterpillar generator set sizing tool, SpecSizer is a load.

A filter provides a low impedance path for harmonic currents to flow, reducing the harmonic current seen by the gen-set.

With addition/selection of a rectifier with more pulses, or a filter on a genset, the non-linear load distortion can oft-times be reduced, resulting in the need for a smaller kW/kVA generator set. As always, a cost/benefit analysis must be done by the consultant comparing the cost of the pulse rectifier and/or filter to the cost of a larger kW/kVA genset.