Energy cost reduction through load balancing and shedding

Overcoming power factor degradations, higher energy costs and greater carbon dioxide emissions when controllers fire in phase angle.

03/17/2009


Power factor is an important aspect that drives up energy costs and therefore must be considered by operations and control engineers. This issue is particularly important when SCR (silicon controlled rectifier) power controllers are firing in phase angle.

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Most utility companies apply a surcharge when the power factor goes below 0.9 (or 90%). In one year’s time, a power factor below this rate can translate to thousands or even tens of thousands of dollars, depending on the size of the installation.
To better understand how this can affect your energy costs , consider this scenario: The demand charge represents the cost per kW multiplied by the greatest 15-minute demand reached in KW during the month for which a power bill is rendered. However, the demand is subject to power factor adjustment. Utility companies reserve the right to measure such power factor at any time. Should measurements indicate that the average power factor is less than 90%; the adjusted demand will be the demand as recorded by the demand meter multiplied by 90% and divided by the percent power factor. For example with the greatest demand at 500kW and a power factor at 0.7, the demand charge would be increased by 28%. At $9.72/kW, that’s a $17,000 penalty a year with this relatively low demand example.
Adding in the issue of phase angle firing , the power factor decreases rapidly with output power. At 50% power, the power factor is only 0.7. At 25% power, the same power factor decreases even more at 0.5—resulting in even more penalties. In addition, phase angle firing creates all sorts of disturbances on the grid, such as harmonics, radio frequency interference, line losses, energy waste (KVAR) and transformer overheating. A manufacturer will eventually be forced to increase the capacity of its equipment to compensate for these disturbances, for example by installing active or passive systems such as costly capacitors.
Eurotherm has recently released a white paper describing these power issues in detail and how its 2 emissions.
Two key components are included in the PLM function: load balancing (or load sharing) and load shedding.Load balancing is a strategy of equally distributing power of different loads to obtain an overall power consumption as stable and balanced as possible thus eliminating peaks of power. This is in contrast to a random firing system. For example, each zone controlled by an EPower SCR controller, is defined by an output power, cycle time, and a maximum power (max capacity), which can be pictured as a rectangle. Rather than letting these rectangles pile up randomly, the EPower controller is said to uniformly distribute them, thereby ensuring that at any given moment the overall power is as stable and balanced as possible.
It is important to understand that the predictive load management function does not change the output power but rather balances and shifts the power evenly hereby eliminating any sort of disturbance. The result is optimum load management through intelligent load balancing and load sharing, a strategy that will eliminate peaks and flicker and even out overall power usage.
To access the Eurotherm white paper, click here .
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– Edited by David Greenfield , editorial director
Control Engineering News Desk
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