Practical PFC for improved energy efficiency
02 October 2006
Mike Thornton explains how PFC (Power Factor Correction) equipment can help to reduce electricity bills as well as meeting the latest requirements for environmentally friendly buildings
Power factor (PF) is essentially a measurement of how effectively electrical power is being used. The higher the PF – and in an ideal case it would be unity - the more effectively electrical power is being used, and vice versa. A distribution system’s operating power is composed of two parts: active (working) power that performs the useful work and reactive (non-working) magnetising power. The active power performs the useful work, while the function of the reactive power is to develop the magnetic fields required by inductive devices such as transformers, computers, fluorescent lighting, welding equipment, furnaces and ac induction motors.
In general, power factor decreases (Ø increases) with increased inductive loads. Therefore, when more inductive reactive power is needed, more apparent power is also needed. The geometric relationship between apparent and active power is traditionally expressed by the right angle triangle relationship of PF = kW/kVA = cos Ø.
Why improve low power factor?
Low power factor means poor electrical efficiency. If low power factor is not corrected, the utility must provide the non-working reactive power in addition to the working active power. This results in the use of larger generators, transformers, bus bars, cables, and other distribution system devices that otherwise would not be necessary. Since the utility’s capital expenditures and operating costs are higher, they will naturally want to pass these higher expenses down the line to industrial users in the form of power factor penalties. In the majority of tariffs currently in operation, a reactive power charge is most often made for reactive power units (kVArh) in excess of one third of the active power units (kWh) – this equates to an average target operating power factor 0.95 lag.
A low power factor could also restrict expansion by limiting the capability to add further loads without major reinforcement of the site power infrastructure. And it might have implications for security of supply due to peak currents, which could potentially cause fuse failure and tripping.
Reducing C02 generation
An additional driver for improving power factor is the growing need to reduce system power losses and consequent harmful generation of CO2. In fact, specific mention of power factor is made in the Building Regulations 2000, Document L2A ‘conservation of fuel and power in new buildings other than dwellings’ which sets out the factors determining TERs (Target CO2 Emission Rates) and BERs (Building CO2 Emission Rates). As part of the ‘Enhanced management and control features’ the use of power factor correction to enable the whole building to achieve a power factor of at least 0.95 lag, allows for an adjustment in the BER.
For example, if the CO2 emissions due to electrical consumption were 70 kgCO2/m2/year without power factor correction (PFC), then the provision of PFC to achieve a PF of 0.95 would enable the BER to be reduced by 70 x 0.025 = 1.75 kgCO2/m2/year
Producing reactive power
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. So, we have an item of electrical equipment that can be used to offset a proportion of the reactive power drawn from the supply – reducing the reactive power supplied by the electricity company and improving the power factor.
Power capacitors are static devices, with no moving parts, so maintenance is minimal. They are also electrically very efficient so their use on a network makes no significant increase in the active power requirement from the utility.
Capacitor -based PFC equipment is available in a variety of formats, either for installation within panels or freestanding, depending on the application. Its benefits are most clearly demonstrated by some practical applications.
Blackpool Pleasure beach
When the dramatic ‘Bling’ ride opened at Blackpool Pleasure Beach in 2004, the owners of the park called in ABB to help them deliver the electricity required without having to undertake a total upgrade to the power network. The white-knuckle experience places a big demand on the three-phase power network and was expected to draw around 1,400A per phase. But with the installation of one bank of PFC capacitors totalling 300kVAr the supply current has reduced to 1,200A per phase and the power needed to operate the ride has decreased by around 25%.
With total energy savings running at up to £2,000 per month during the peak season, the ABB equipment is expected to pay for itself in less than three years. This was not the first time that ABB’s PFC technology had helped the park add a ride without upgrading the whole power supply. It was a similar story when the Valhalla ride, claimed to be the world’s biggest most spectacular dark ride, opened in 2000.
Following the installation of the ABB automatic power factor correction equipment, comprising two banks of capacitors totaling 900 kVAr, the supply current was reduced to 1500A per phase. The power required to operate the ride was reduced from 1.5MVA to a little over 1MVA – a reduction of approximately 25%, realising cost savings similar to those achieved on the Bling ride .
Mike Thornton is with ABB
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