Power Quality Management

Why Power Factor and Power Quality Correction Should be the First Thing You Do!

Very often, installing power factor correction equipment, in a large factory, or even a large corporate office building, yields the biggest single saving on the monthly electricity bill.

What is Power Quality Management?

Power quality management is concerned with the control of the quality or purity of the AC sine wave of the electricity power supply.

Power quality management is typically concerned with the following issues:

• Harmonics and distortion
• Transients
• Power factor
• Voltage sags and dips
• Current and voltage instability

The graph shows a situation in which there are significant distortions on the current wave form caused by various harmonics.

If your factory has any of the following types of plant, you probably have power quality problems:

• Induction heaters or furnaces
• Arc furnaces
• Inverters or variable speed drives
• Large motors that start and stop frequently
• Welding (both spot welding and arc welding) processes
• Any other large inductive loads such as chillers, or refrigerators

Power Factor Correction

Most people only think about power factor correction when they think about power quality.  Probably because power factor is realtively easy to measure (in fact its usually listed on your council bill) and it can be corrected quite cheapily.  The monetary benefits are usually immediate and dramatic if your electricity tariff includes a demand charge.  This means that the council or Eskom monitors your maximum demand each month and then bills you at a set rate on the maximum kVA drawn during the month.  If your power factor is low at the point of maximum demand, it means that the kVA measured by the council is much higher than it needs to be, and you will have to pay a LOT more than you needed to each month as a consequence.  So correcting the power factor back to unity results in immediate and dramatic savings in those cases.

However, cheap power factor correction equipment often fails soon after it is installed.  It is not unusual to find power factor correction equipment permanently disabled in many factories even though it would yield significant savings, simply because the correction equipment keeps blowing up and tripping the production processes.  The reason for this is that cheap power factor correction is usually contactor based.  In other words the reactive elements that correct the power factor are switched in and out as needed by the correction system using ordinary mechanical contactors.  This approach has three significant problems all of which result in rapid wear and tear on the unit:

• Each time the contactors disengage mechanically they arc, which wears out the switch over time, but more importantly introduces large transient spikes into the system which shocks all sensitive electronic devices on the network as well as the corrective capacitors.
• Mechanical contactors are very slow in comparison to the 50Hz AC cycle (20 milliseconds per cycle).  This means it is impossible to connect the reactive elements onto the network at the zero crossing point of the AC wave.  The contactor closes at any arbitrary point in the cycle which could even be at maximum voltage and/or current.  That can cause a very large shock load on the corrective capacitors, dramatically reducing their life.
• Because contactors are very slow, in comparison to the 50Hz wave, those forms of power factor correction equipment cannot be used with the more sophisticated power quality controllers that are available.  The contactor is simply too slow to be of any use.  This is particularly problematic when the load is very dynamic and changes a lot.  In that case the correction equipment tends to step in and out all the time trying to follow the rapidly changing load but never quite succeeding, resulting in rapid wear and tear of the correction equipment.

It is far better to use modern solid state switches which are super fast and dont wear out virtually at all, and which dont introduce switching transients into the system.  If these units are controlled by a super fast controller the above problems are largey solved and the correction equipment lasts a long time while providing superior results even under very dynamic load conditions.

Voltage Dips and Sags

Very often voltage dips are a far larger problem than poor power factor, with much larger monetary consequences.  Voltage dips are sudden, very short dips in the voltage caused by many different facors such as a large motor or other large loads starting up elsewhere in the plant.  Unfortunately they can also be imported from other nearby factories on the same feeder.  Voltage dips can cause drives and other control equipment to shut down or reset unexpectedly in mid process, resulting in plant stoppages and rework.  Most factory managers acknowledge that the cost of such a plant stoppage, together with the labour and over time required to correct the problem and get back on schedule is enormous, often running into millions for a single event.

For this reason it is often crucial to identify a voltage dip problem and install voltage ride-through equipment to reduce the effects of this problem.  Unfortunately voltage dips are usually very short in duration and sporadic in nature and thus they difficult to predict.

Harmonics

Various different types of equipment can cause harmonics on the wave form.  A good example are variable speed drives.  The currents caused by harmonics can cause losses in the lines as well as overloading of transformers.

Transients

These are spikes in the current and voltage caused for example by the starting and stopping of inductive loads.  They wreak havoc with sensitive electronic equipment resulting in damage or failure.

Quite often the costs associated with electronic equipment that fails prematurely due to harmonics and transients is quite high, but usually that high cost goes unnoticed in factories with poor power quality, until proper correction equipment is installed and then people realise that those items are not failing frequently anymore.

Correcting Power Quality Problems

The first step is always proper high resolution measurements.  It is crucial that measurements are taken continuously for at least a full production cycle, so that sufficient data can be gathered to enable a proper analysis of the power quality conditions in your plant at different times in your business cycle.  Voltage dips and transients are usually associated with sporadic events that may happen at odd times.  You need to be measuring at those times to isolate the problem and determine how bad it is before, any corrective actions can be taken.

Unfortunately a lot of the cheaper (and even some of the expensive) power quality meters can only record for a few minutes before their on-board memories fill up.  What you need is a meter that can record for a few days or even a whole month continuously.

It is also preferable that the meter does not average the data over any time periods before storing the information.  Any sort of average over even a short time period is going to lose all information about high speed voltage dips, harmonics and transients.

Once you have a good set of data then your power quality consultant can easily specify the type of correction equipment required.