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PHIL KREVELD: Power factor… Quality matters

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Power factor correction can lead to additional business for electrical contractors. Phil Kreveld covers the considerations.

Maximum demand in Australia has been growing much more strongly than aggregate terawatt/hours, particularly in the northern states.

Yet even in the south, kVA demand has become an important consideration in distribution companies’ capital expenditure. As a result, consumers in areas previously not subject to kVA tariffs are facing substantial increases in their energy bills.

Electrical contractors willing to take an interest in this field can make an economic case for installing power factor correction gear, and thereby gain increased business.

This article deals with technical aspects and some matters relating to installation and service.

 

Power factor

A power factor of 0.9 lagging – not too shabby, you might think – implies a phase angle difference between voltage and current of almost 26°.

In kVA terms there’s an 11% increment compared with kilowatts, and the possibility of a sharp increase in the power bill. To correct the power factor to 1.00 requires almost half the kilowatt value in kVArs.

In short, that’s where the cost-benefit calculations come in: weighing up the cost of correction against potential savings in electricity.

Correcting the power factor to 0.95 requires about one-seventh of the kilowatts in kVArs, reducing kVA to 5% of kilowatts thus minimising the chance of a bigger power bill.

The power factor in discussion is the displacement power factor, equal to the cosine of the phase angle difference between voltage and the fundamental 50Hz current – ie: cos φ where φ is the phase angle.

It’s not the total power factor, because that takes into account current harmonics as well. There is some doubt on the sensitivity of metering in respect of harmonics in a particular installation. However, it’s a safe assumption that the kVA (when based on Kilowatts divided by the displacement power factor) gives the demand on which the tariffs would be based.

 

Correction equipment

With the exception of electronic loads such as IT power supplies, most loads draw lagging current.

Correcting kVArs is almost invariably handled by capacitors, their leading current offsetting the load current lagging reactive component.

Commercially available equipment mostly employs capacitor-based correction. However static var compensation equipment is also available. The latter type uses solid-state switching to generate anti-phase current.

A note of caution: the different technologies have advantages and disadvantages. Electrical Connection does not offer editorial advice. We highlight matters so that readers can investigate technical features and, where necessary, seek independent advice.

Based on the technical literature, it seems that some static var compensation equipment (in particular STATCOM, see below), offers important advantages. These include stepless adjustment of reactive compensation, and leading as well as lagging power factor correction.

 

Harmonics

Office buildings, malls, supermarkets, etc, have high harmonics generating loads.

There’s a common assumption that the displacement power factor is close to unity, but this is not always borne out by measurements.

Thyristor-based converters in variable-speed drives for fans, chillers, compressors, etc, have lower power factors than insulated gate bipolar transistor (IGBT) and gate turn-off (GTO) thyristor switched converters. The latter two having a power factor very close to 1.00.

Phase-fired thyristor equipment – used in heating, gluing, heat treatment, etc – has a poor displacement power factor. Phase firing at 30° (1.7 milliseconds from the zero-crossover) produces a displacement power factor of 0.78. The harmonics content for the current is close to 50%.

Harmonics further reduce the total power factor but have no bearing on displacement power factor.

 

Harmonic filtering

It’s often assumed that harmonic filtering improves power factor, yet this is true only for total power factor.

Many active harmonic filters provide limited displacement power factor correction. However, their total kVAr capacity invariably has to be apportioned between harmonics mitigation and displacement power factor correction, thus limiting one or the other capacity range.

In some installations, capacitor-based correction equipment is combined with active filtering. In such cases, care must be taken to place the capacitor bank at the incomer, and not on the bus feeding the harmonics-generating load.

 

The STATCOM

The equipment type referred to here is properly called a static synchronous compensator.

It is often referred to as a STATCOM, and a simplified schematic is shown in Figure 1.

50Figure 1: Schematic for a STATCOM

The circuit looks a lot like those employed in active harmonic filters, but in this case the control scheme is different in important ways.

Although details of the compensation will vary between commercially available equipment, in each case the fundamental current is extracted by some means – fast Fourier transform (FFT), notch filter, etc.

Pulse width modulation, adjustable per phase, basically allows adjustment of power factor per phase and also the balancing of phases.

For many installations, phase balancing is a very important feature requiring a dynamic response, in that phase loading fluctuates.

There is little or no detail in suppliers’ technical literature on how phase balancing is achieved. One method is possible by way of control protocols converting unbalanced currents into symmetrical components. See Figure 2 for a brief description of symmetrical components.

It is advisable to examine phase balancing features closely and seek practical verification if that feature is crucial for an installation.

51Figure 2: In general, unbalanced currents can be resolved into a-b-c (positive sequence), a-c-b (negative sequence) and a0-b0-c0 zero sequence components.

The negative sequence controller injects capacitive or inductive negative sequence currents of opposite phase to that of the load negative sequence currents.

As a result, the network sees symmetrical load current and phase voltages without exchanging power with the network. Note: the zero component flows only in a four-wire distribution system or otherwise in delta windings of transformers

 

Other compensation

The use of thyristors and GTOs as switching elements for capacitors – and sometimes parallel connected inductors – is well established in MV and HV transmission.

The STATCOM had its start in MV distribution, and more recently it has come into use in LV networks. The term static var compensator is generally reserved for equipment switching passive components.

 

Capacitor-based correction

Metallised polypropylene (MPP) capacitors have greatly improved capacitor bank reliability compared with earlier types using polychlorinated biphenyl (PCB) dielectric material.

MPP capacitors are durable and generally offer a self-healing feature. When they short out, the conductor area surrounding the shorted area vaporises, thus removing the shorted circuit. The capacitor continues operating, with slightly lower capacitance.

The self-healing feature is convenient, but if capacitors continue operating beyond their design constraints they start to have multiple shorts and can lose capacitance more rapidly.

Capacitor banks are generally connected in delta circuits, using power factor as measured on one phase as the basis for power factor correction.

Specialised contactors are sometimes employed with early-make contacts in series and a current-limiting resistor to help ‘form’ the capacitor, followed by the later main contact closure.

Some contactors employ the use of small, air-core inductors to limit inrush current. However, this technology increases the response time of power factor correction equipment. Zero voltage cross-over thyristor (triac) control diminishes inrush current and improves response time.

 

Preventive maintenance

Temperature degradation, in terms of component value and reduced lifetime, is a  serious problem.

It should be dealt with by regular preventive maintenance testing, including capacitance measurement.

52Figure 3: Capacitor lifetime versus temperature.

Twice yearly check-ups, including verification of capacitance values, are recommended. But this is rarely done in practice, even though it is desirable for continued effective operation of the equipment.

Problems occur with frequently exercised capacitor banks, particularly in regard to inrush current. This can be several orders of magnitude larger than under steady-state conditions, thereby causing contacts to weld.

 

Selecting correction equipment

Significant harmonics, load unbalance and power factor variations must be taken into account.

It is a very good idea to carry out a thorough power quality survey before specifying power factor correction equipment.

Specifications for new installations may describe in detail the equipment to be quoted on, but they lack information on power quality at the site. This adds to potential future problems such as voltage regulation, resonance and imbalance.

For existing installations a power quality survey should be conducted over a reasonably long period. This will allow as many possible variations in electrical parameters as possible to be captured and contrasted against incoming power line conditions. Problems in power quality at that stage cannot easily be compensated.

Although not a direct power factor issue, phase balancing is very important. Many types of load – in particular induction motors – respond badly to phase imbalance.

About Phil Kreveld

Phil Kreveld

Phil Kreveld is an electrical engineer with broad experience in electrical and electronic instrumentation, including relay testing power and power quality analysis.

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