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Practical power factor correction for LV power distribution designers

04 October 2011

Power factor correction is becoming an increasingly important aspect of low voltage, three-phase power distribution system design as the electricity suppliers impose heavy surcharges for poor power factor control on their already cash-strapped customers. The trick is to maintain power factor above a figure of 0.95 lag, but how achievable is this? David Knapper offers a solution to the problem.

There is a growing emphasis on power factor (PF) among the bigger consumers of three-phase electric power who face excess reactive penalty charges from their suppliers when the average power factor at their facilities is lower than 0.95 lag. These penalty charges have a significant impact on electricity bills, so consultants and specifiers are now requiring power factor correction (PFC) equipment to be integrated within low voltage (LV) power distribution panels.

PF is essentially a measure of how effectively a distribution network uses its electrical power. The higher the power factor – and in an ideal case it would be unity – the more efficiently it is being used, and vice-versa. The electrical power consumed by a distribution system has two components: active (working) power that performs the useful work and reactive (non-working) magnetising power required by inductive devices such as transformers, computers, fluorescent lighting, welding equipment, furnaces and ac induction motors.

The PF represents the ratio between active and reactive power on the network and the more inductive loads there are then the more likely it is that the power factor will be ‘poor’ – that is, substantially less than 0.95 lag.

Why improve power factor?
A low power factor means poor electrical efficiency. If it goes uncorrected then the utility must provide the non-working reactive power in addition to the working active power. This requires larger generators, transformers, busbars, cables, and other distribution system devices that otherwise would not be necessary. Since the utility’s capital expenditures and operating costs are higher, they naturally want to pass these higher costs down the line to their customers as power factor penalties.

In addition to the immediate effect on electricity bills, a low power factor can also have a significant long term impact on a business by limiting its capability to add further loads – such as a new production line for a manufacturer or new press for a printing company - without major reinforcement of the site power infrastructure.

It could also have implications for security of supply and availability of production plant due to the presence of high peak currents which could potentially blow fuses and trip circuit breakers. Furthermore, excess heat generation in cables, switchgear, transformers and other network equipment can result in premature failure and thus poor capital equipment returns.

How is power factor corrected?

In electrical terms, capacitance is considered as a ‘reactive power’ component but in fact its characteristic in an electric circuit is to neutralise or compensate for the inductive reactive power. As a result, PFC capacitors are used effectively to offset a proportion of the reactive power drawn from the supply, thus reducing the reactive power supplied by the utility and improving the system power factor/efficiency. This will allow excess reactive power penalty charges to be reduced immediately or even eliminated completely.

Moreover, by improving the overall site operating PF, it may be possible to reduce the Authorised Supply Capacity, to be agreed with the Regional Electricity Company, giving a further potential financial saving.

Capacitors are electrically very efficient, so their use on a network makes no significant increase in the active power requirement from the utility.

De-tuned or not de-tuned?
The presence of harmonics on the network where a LV distribution panel is to be installed could cause specific problems for PFC capacitors, resulting in general reliability issues such as nuisance tripping or even premature failure.

The simple solution is to add a reactor to create a ‘de-tuned’ installation. In general, this approach is always recommended for any applications where the network harmonic content is known to be greater than 15 to 20%. To avoid future problems it is often good practice to specify de-tuning reactors as a matter of course.

David Knapper is with ABB

PFC: the benefits in summary
Installing PFC equipment in LV distribution systems offers a number of important benefits:
 Reducing and possibly eliminating expensive utility penalties for a poor power factor
 Improved energy efficiency – reduced system currents and kW losses
 Security of supply - reduction in peak currents prevents fuse failure and loss of supply
 Release of additional capacity available within existing network equipment so that the overall system load can be increased without the need to invest in additional infrastructure
 Environmentally friendly - reduced kWh losses mean that less power needs to be generated, so less CO2 is produced, which contributes to carbon reduction commitments
 Increased service life of infrastructure – since the amount of heat generated within cables, switchgear, transformers and other equipment is reduced

Effective reactive power compensation
ABB can supply a range of automatic reactive power compensation equipment to meet the specific PFC needs of power distribution system designers. The latest addition to its range is the new Vector shelf (illustrated), which features the company’s CLMD03 dry capacitor technology.

Vector capacitor shelves are able to deliver up to 100kvar in a single shelf, and come complete with stage fuses and contactors to switch the individual capacitor steps. This means they are suitable for use with conventional, varying loads. CLMD technology also includes a patented sequential protection system, ensuring reliable and fail-safe operation.

CLMD03 provides all the advantages of dry capacitor technology in a compact case. The materials of construction and ventilation technique ensure efficient heat dissipation, which is why the system is able to offer up to 50kvar in a single unit.

For more information about ABB’s Vector series PFC equipment, including ratings, dimensions and other specifications email, call 0151 357 8400 or visit the website. 

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