REDUCING HARMONICS WITH FILTERS OR ACTIVE FRONT-END DRIVES?

 

 

Harmonics are waveforms in multiples of the standard network frequency, i.e. 250 Hz waveform on a 50 Hz network is the 5th harmonic. The harmonic waveform represents energy that cannot be used by devices on the network and may cause equipment to behave erratically.Motors can overheat or get noisy; meters give false readings; lights flicker; and cable insulation break down.

Even if none of these symptoms are experienced, users still must comply with the G5/4 regulations to ensure that other users are not affected by any harmonic waveforms generated.

 

ASSESSING THE TOTAL HARMONIC LOAD

Harmonics are caused by nonlinear loads on the network, such as fluorescent lights, computers and AC drives.With AC drives, extra care is needed, as a site can have many of these installed.When adding further AC drives, the total harmonic load must be considered.

Most drives have six-pulse diode rectifiers, which give about 40 percent harmonic distortion. Twelve-pulse rectifiers were introduced some years ago, which reduce total harmonic distortion to about 12 percent. However, in recent years, active front-end drives have taken over. These have a harmonic distortion of only 2 to 4 percent.

 

Fig 2. Comparisons of current distortion

HARMONICS TREATMENT INSIDE THE DRIVE

With an active front-end drive, harmonics are dealt with inside the drive and never enter the network. The traditional solution is to filter out harmonics after they have been created.

ABB’s active front-end drives use their drive control platform DTC (direct torque control) "back-tofront”. DTC is normally used to control the output to the motor terminals to achieve the required motor speed and torque. In the active front-end drive, it is used to rearrange the incoming waveform, ensuring that the output from the drive becomes a smooth waveform, free from harmonics.

The active front-end drive is a safe and easy option, guaranteed not to add damaging harmonics to the network. Unlike passive filters, it also coexists happily with generators on the network. Its disadvantage is its cost, which is about 10 to 20 percent higher than a traditional arrangement with a drive and a filter.

 

PASSIVE FILTER NEEDS SITE SURVEY

For a passive filter solution the characteristics of the network must be taken into consideration. The total harmonic voltage distortion of the non-influenced mains voltage has to be less than two percent and the ratio of shortcircuit power to installed load must be at least 66. Under these conditions, the total harmonic distortion of the mains current of the AC drive is reduced to between 5 to10 percent and the passive filter solution will be sufficiently effective. However, establishing whether these conditions exist requires a site survey, which may well counteract any cost benefit achieved.

Problems may also occur in networks that have an imbalance between the phases. Under these conditions, the passive filter is less effective. The problems will be particularly severe at low load, around 20 percent, where the distortion is already high. In this range, distortion may increase from, say, 8 to 12 percent. At full load, the increase will be less pronounced, perhaps from 5 to 6 percent, but still significant.

 

INCREASED LOSSES

Another drawback of the passive filter is the higher losses, particularly at reduced load. Comparisons between passive filters and low harmonic drives are frequently made at full load. However, a true comparison should be made at reduced load – the reason the drive is installed is to enable operation at lower speeds. The comparison should therefore be made around 75 percent load, which is closer to the actual operating point for most pump motors.

The increase in losses tends to be greatest in low power installations. For instance, a 22 kW six-pulse drive with a 43 A filter can have losses of 873 W, while a low harmonic drive of the same size has losses of 490 W, which means the losses of the filter solution are a full 78 percent higher. The passive filter can also increase the voltage in the drive DC link, which will put additional stress on the capacitors in the drive. To allow for this a 10 percent larger drive should be selected if using a passive filter. In addition, some passive filters are not effective across the whole harmonic spectrum but only at the lower end. Also, power factor can be very poor at low load; 0.5 at 20 percent, for instance, is not unusual.

EASY INSTALLATION WITH ACTIVE FILTER

The active front-end drive is easier to install, as you only have to consider the mains voltage, the pump absorbed power and the fault level of the network. For a passive filter, you have to consider all of these plus the existing power factor correction. The filter also needs to be disconnected at low load, which means that additional circuitry needs to be designed into the panel.

A generator supply complicates the issue further.With an active front-end drive you only have to consider the above elements. For a passive filter, you have to consider the above plus: generator fault level, disconnection of passive filter when on generator, second harmonic prediction when on generator, as well as the load factor per scheme. Passive filters cannot be operated when on generator supply, so the panel builder has to install circuitry to disconnect the filters when the generators start up. However, disconnection of the filter increases the level of harmonics on the network, which can cause the generator’s voltage regulator to trip.

INDIVIDUALLY DESIGNED

A passive filter needs to be designed for each site reflecting the conditions of the site at a specific time. If conditions change, the filter may have to be changed. Passive filters should only be considered when there is a small network with a small number of pumps, no generator supply and consistent load conditions. Under these circumstances, the passive filter can provide a cost benefit. Otherwise, it is safest to go with a low harmonic drive.

FOLLOW THE HARMONICS RECOMMENDATION – IT’S THE LAW

Few users seem aware that the harmonics regulations document G5/4 is actually the law. The full title of the document is "Energy Networks Association Engineering Recommendation G5/4-1 – Planning levels for harmonic voltage distortion and the connection of non-linear equipment to transmission systems and distribution networks in the United Kingdom.”

Taking the word "recommendation” literally, many seem to think the regulations can safely be ignored. Nothing could be further from the truth. The document came about as a guideline during the time when the electricity industry was still in government hands.When the industry was privatised, it became a legal requirement for the electricity companies to enforce G5/4 and part of their license to operate. Although the industry may have changed, the document, with its title, still stands.

Users who fail to follow G5/4 can in the worst case be disconnected from the network, although this is unlikely to happen.What does occasionally happen, however, is that new installations are refused connection because they do not follow G5/4.

The document is written on the assumption that harmonics are unavoidable and that users will have to share the resulting pollution. Equipment up to 16 A, i.e. devices compliant with BS EN 61000-3-2, is effectively ignored, as it would be impractical to try to control the number of TVs and washing machines connected. However, for higher voltage and power, the document calls for an evaluation of the prospective pollution.With higher rating follows more stringent evaluation.

The application of G5/4 is likely to become stricter in the future. Installations of small generators of renewable energy are expected to become more numerous and these produce additional levels of harmonics. This means that harmonics will increase in parts of the network that were previously relatively unaffected and that the total harmonic load on the network will increase. As a result, it will be necessary to treat also existing unauthorised polluters more strictly.