Quality in numbers

Although a marginal factor over the past decade, the amount of harmonic current disturbance experienced by electrical installations is increasing continuously.

This rise can be tracked to the vast growth in computer use, telecommunications and power electronics, all of which represent non-linear loads that cause harmonic disturbance.

This phenomenon concerns most of today's electrical distribution systems, whether in the commercial, industrial or residential sectors, and has a negative effect on most installations, including factory assembly lines, data processing equipment and colocation sites.

The eroding quality of electrical power should be of great concern to all end-users. Take for example, the Internet service provider (ISP) sector – downtime caused by low quality power could potentially cost millions of pounds. A simple power spike could easily cause an ISP's servers to fail and the knock-on effect would be catastrophic. If businesses are offline for any period of time the potential loss of earnings could reach millions of pounds.

Rectifing the problem
The presence of harmonics in the upstream circuits is due to the fact that ups use a rectifier to draw power from the input ac distribution system. The rectifier charges the battery at a constant voltage and supplies dc power to the inverter. Without the necessary conditioning equipment, harmonic disturbance reinjected in the mains will ultimately affect other sensitive equipment sharing the same network.

Traditionally, ups vendors used either six-pulse or 12-pulse rectifiers to covert ac to dc. The 12-pulse rectifier is achieved through the combination of two six-pulse bridge rectifiers and a phase-shifting transformer.

However, due to a significantly higher component count (an extra rectifier and a phase-shifting transformer) the 12-pulse rectifier suffers reduced reliability and lower efficiency when compared to its six-pulse counterpart.

The mean time between failure (mtbf) is of critical concern when selecting a ups system; the higher the mtbf the better, as this means that the unit will function efficiently for longer periods between repair. Unfortunately, the 12-pulse rectifier demonstrates a relatively low mtbf when compared to the six-pulse.

More or less?
Reliability is just one concern when considering the best ways to reduce harmonic distortion. To allay such problems and fears, ups vendors must implement more efficient systems to reduce total harmonic distortion.

It must be remembered that it is the rectifier that causes a great deal of harmonic distortion. Generally the total harmonic distortion current (thdi) of a six-pulse rectifier is around 35%, while the 12-pulse commonly experiences thdi of around 12%. Both values fall well short of current standards for the maximum thdi – IEEE 519-2 (USA) stipulates that the thdi level must not exceed 5·5%. Therefore, to limit thdi levels in either scenario a harmonic filter must be used.

The typical dominant harmonics for a six-pulse rectifier are fifth and seventh (six plus one), while for the 12-pulse they are 11th and 13th (12 plus one). However, with the 12-pulse rectifier the skin effect phenomenon must also be addressed ie d = 1/2p=r x 105/f, where d is the depth of current penetration in mm, f is the frequency in Hz and r is the resistivity of conductor. This reduces the penetration depth of current flow at higher harmonic levels.

Figure 1 shows the components involved in a typical ups with total harmonic management (thm) in dealing with harmonic distortion. It also highlights how the thm system eliminates the corresponding harmonic currents depending on the spectrum of the rectifier current.

In essence, one must weigh up the benefits of using a six-pulse rectifier with a single additional filter plus increased reliability to reach the desired 5·5% thdi, or a 12-pulse rectifier with the same additional filter. However, as the higher component count of the 12-pulse rectifier equates to a drastically lower mtbf value, to achieve the same 5·5% thdi, realistically a six-pulse rectifier must be chosen due to its far superior mtbf.

Harmonic filters
Traditionally, 12-pulse rectifiers were implemented due to poor availability of high quality harmonic filters. Today however the choice is far greater, allowing the six-pulse rectifier to come to the fore.

There are four main types of harmonic filter:

• series connected choke – this is the most economical and used option throughout variable speed drive (vsd) applications. The choke is connected in series with the vsd, thus reducing the thdi from 67% (a raw six-pulse rectifier) to around 35%. However, this technology is very basic and less effective at lighter loads, therefore not employed by the majority of ups vendors;
  
• phase-shifting transformers – this solution uses multiple six- or 12-pulse rectifiers, with phase-shifting transformers situated between rectifier units. For example, if two 12-pulse rectifiers are phase-shifted, the thdi is reduced to around 7% (table 1). For two six-pulse rectifiers, this figure equates to around 10%. However, other design criteria need to be analysed when evaluating such a solution:
  – skin effect, as the phase-shifting will create higher harmonic resultant currents (11th & 13th and 23rd & 25th), depending on the bridges being phase-shifted;
  – inherent inrush current required by the phase-shift transformer;
  – additional physical space required for the transformer and external logistics eg extra heat generated;
  
• passive lc filters – this is the basic harmonic filter, specifically tuned for certain harmonics using a combination of chokes and capacitors. Typically, for six-pulse rectifiers, lc filters can reduce thdi from 35% to 5·5%, while 12-pulse is reduced from 12% to nearer 6%.
  Passive lc filters are a popular and common solution used throughout the ups industry, but it cannot be ignored that the filtering achieved is directly related to the percentage load that is presented to the ups ie the higher the load, the lower the thdi. Similarly, this has an impact on the input power factor as seen on the mains side;
  
• active harmonic filters (conditioners) – this is perhaps the optimum solution as it reduces the inherent weaknesses of most of its predecessors. In effect, as the active harmonic conditioner is a current injection device, it is not dependent on the magnitude of the ups load current but purely cleans harmonic pollution between the second and 25th harmonics (figure 2). With the correct selection, thdi levels can be maintained at around 4%, irrespective of load and other criteria. This exceeds the requirements of the industry standard EN 61 000-3-4 (table 2).

Other advantages of the active harmonic conditioner is that it can be installed as a retrofit, and has no risk of resonance. Use of an active harmonic filter does not have any detrimental effect on the mtbf value of the system since it is a parallel device.

Despite the higher initial capital outlay to install such a device, the return on investment is achieved through its ease of installation, better performance and reduced footprint – some vendors now manufacture ups with integral active harmonic conditioners, reducing footprint size even further.

Such a device more than exceeds today's pollution standards and pre-empts future regulations, while offering fast-switching and rapid reaction within two cycles, thus delivering a total harmonic management (thm) solution.

With its lower component count and higher mtbf, six-pulse rectifiers offer end-users far greater efficiency and effectiveness. In relation to thm, combining six-pulse rectifying technology with active harmonic conditioning produces reduced thdi levels and equally important, yet more difficult to quantify, enhancements in the end-users' peace of mind.

As for the future, with the advent of more efficient six-pulse rectifiers and integral active harmonic conditioners reducing physical footprint and capital outlay, official pollution standards can be met with relative ease, while providing low thdi.