Terminology

Electrical power systems suffer various types of disturbances that reduce the quality of supplied electricity. These disturbances increase cost by reducing efficiency and equipment life to a varying degree, dependent on the system in question.

 

In an AC systems these disturbances are phase shift differences between current and voltage waveforms for example caused by induction motors or equipment with capacitive loads. Harmonic disturbance induced by frequency converters, PC computers or solid-state DC converters can also increase phase shift and loss of efficiency.

 

Surge disturbance generated by sudden change in load, such as inductive motors starting up. Generally three types of loads in electric systems are referred to; resistive, inductive and capacitive (inductive and capacitive loads are called reactive loads). To better understand the effect this has on a power system, the load types, their behavior and related terminology needs to be explained.

 

Resistive loads

A pure resistive load, like a tungsten light bulb, does not cause disturbance in electric systems. An AC current voltage waveform that has only a pure resistive load and no disturbances could look like the waveform on Figure 1. Here current and voltage waveforms are following one another perfectly or what is called voltage and current are in phase.

 


 

 

 

 

 

 

           Figure 1 . Current / voltage waveforms in phase (PF=1)

 

Inductive loads

Inductive loads include motors, transformers, and solenoids. In a purely inductive AC circuit, current lags behind voltage by 90°. Current and voltage are said to be "out of phase." (Se Figure 2). Inductive circuits, however, have some amount of resistance. Depending on the amount of resistance and inductance, AC current will lag less than 90° depending on the amount of resistance.

 


 






           Figure 2 . Current / voltage waveforms in a pure inductive load.

 

Capacitive loads

Capacitive loads; these include power factor correction capacitors and filtering capacitors. In a purely capacitive circuit, current leads voltage by 90°. (see Figure 3). Capacitive AC circuits, however, like inductive loads have some amount of resistance. The amount of resistance and capacitance determines how mush AC current will lead voltage.

 


 

 

 

 

 

 

 

    Figure 3 . Current / voltage waveforms

 

Although resistive loads like a tungsten light bulb does not cause disturbance in electric systems, resistive loads such as switch mode power supplies, frequency converters and PC-computers cause harmonic disturbance (Se Figure 4).

 

These loads distort both the current and the voltage waveform. Just as resistance is opposition to current flow in a resistive circuit, reactance (induction and capacitance) is opposition to current flow in a reactive circuit. It should be noted, however, that where frequency has no effect on resistance, it does affect reactance.

 

An increase in applied frequency will cause a corresponding increase in inductive reactance (increase in phase difference) and a decrease in capacitive reactance. Therefore harmonics and surges can have significant effect on induction machines power consumption.

 


 







     Figure 4 . Harmonic current in current / voltage waveforms.

 

The power factor

Regardless of load type, power is measured via the delivery rate of energy, in electrical circuits, it is expressed as the mathematical product of Volts and Amps (Power=Volts x Amps). However, in AC power systems, a complication is introduced; current may flow into and back out of the load without delivering energy. This current, called reactive or harmonic current, it goes out of phase and can increase in frequency. This phenomena gives rise to an "apparent" power (ApP) which is larger than the "actual" power (AcP) consumed. This difference between the apparent power and the actual power is, in most cases, pure loss, or power spent without work. The ratio between actual power and apparent power is called power factor (PF) or cos(?). The apparent power (ApP) is expressed as the Volt-Amp or VA rating for the user. Therefore, the actual power (AcP) in any AC system is the VA rating multiplied by the power factor. Inductive and capacitive loads have power factor rating less the unity or less then 1. These loads are called, as stated before, reactive loads. Figure 5 displays a voltage current curve that is in phase or PF=1.

 


 

 

 

 

 

 

      Figure 5 . PF in unity, optimal power quality

 

This could be a curve for a pure resistive load like a tungsten light bulb. Here the apparent power (ApP)=(AcP), all the power generated is consumed by the load. Figure 6 on the other hand could be a waveform for a pure inductive load.

 

 

 

 

 

 

 

 

      Figure 6 . Pf=0, power half the power consumed does not work.

 

Here the phase difference is 90° and therefore PF=0, the actual power (AcP) begin used for work equals the reactive power (RaP) doing no work. System efficiency is zero. This is not a very likely scenario, but Figure 7, on the hand, presents a more likely scenario.

 

 

 

 

 


 

     

 

      Figure 7 . PF at 71% of unity.

 

 

This could be a reactive circuit that has equal resistance to reactance, her the phase difference is 45° and therefore PF=0.71

 

An electrical system with a low system quality, e.g. large amount of face shift and plagued by harmonics and other phenomena, requires more power than is actually consumed to produce work. Power consumption is directly related to cost, not only when it comes to electric bills, but also when it comes to cable and generator size (in ships f.x.). A system that has a power factor as low as 0.65 requires 35 - 40% larger generator and cables to deliver the power actually used for work. This also implies to systems that are connected to large power grids via transformers ore directly coupled. The transformer must be 35-40% larger and the electric bill is higher than need be.

 

Furthermore, if a large consumer, connected to a national power grid, is producing harmonics and large phase shifts back in to the national grid, the power company must correct for these errors and as a result, impose penalties to compensate for cost. A secondary affect from poor electric quality is equipment life and performance, large phase shifts and harmonic disturbances cause heat in most electric equipment, induction motors can experience straining magnetic forces that increase power consumption even further and shorten life, radar equipment and sonar can display noise patterns that hide week signals and diminish signal quality.

 

In short, all electric equipment is designed to operate at optimum on power that is noise free and has no phase difference; most can operate on poor quality power but at a cost, less then optimal performance and higher power consumption.

ELCorrect Ecological Electric - Skutahraun 2 - 220 Hafnarfjordur - Iceland - ee@rafehf.is

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