QUESTION
The power factor is low at our facility. Where is the best place to install power factor correction capacitors?
ANSWER
The best place to install capacitors is at the equipment creating the low power factor condition. Typically this means at any induction motor greater than 20 HP. This can be achieved by directly connecting the capacitor to the starter so that it comes on with the load and is disconnected when the load is turned off.
This strategy applies VAR's where they are necessary and releases system capacity. The size of the necessary capacitor should be confirmed by the manufacturer. Overload relays may require adjustment. Avoid connecting capacitors to solid state loads.
However, when there are numerous motors, an abusive environment, or where electronic equipment is involved, an alternate strategy may be more effective. Usually this entails a large, controlled bank which is connected to the motor control center or distribution switchgear. This strategy is favoured by capacitor bank manufacturers because they can treat your problem with one centralized piece of equipment without a detailed analysis of your plant loads.
It also treats the power factor issue where it counts; at the utility billing point. With this method, only the VAR's required to eliminate penalties are applied. Because of the associated varying capacitance, it is necessary to perform a harmonics analysis before and after installation to ensure that damaging harmonic resonances will not be created. Avoid non-controlled, fixed banks as they may create a leading power factor situation in times of low load. This situation could be harmful to sensitive electronic equipment.
The ultimate choice may be either of the above or a combination of both ,depending on the number and type of low power factor loads in your facility.
QUESTION
I'm continually seeing blown electronic control boards on one machine in our plant. The manufacturer is saying it is not his problem. What is causing this and how can I fix it?
ANSWER
The most likely cause of this type of failure is a repeating voltage transient which is being impressed on your equipment by a source internal or external to your plant.
External sources could be lightning activity, utility switching activity, or switching transients caused by other facilities on the same system.
Internal problems could be caused by malfunctioning electronic equipment, power factor capacitor switching, or other electronic equipment generating high levels of noise or harmonics. The other possible cause to consider is that the equipment containing these boards has been mis-applied for the function it is trying to perform.
If possible, high speed transient capture monitoring equipment should be connected to verify and quantify the problem. Tracking down the problem may be difficult without scheduling production outages to isolate the source.
There are products available which are marketed to solve these types of problems. Some of these are legitimate and will solve your power quailty issues. Note however, they will only band-aid the problem and tend to be expensive solutions.
The best way to correct or avoid these problems is to fix the problem at its source. Ensure that sensitive equipment is properly protected with an isolation transformer and/or some form of surge protection. Ensure the equipment is grounded as per manufacturer's recommendations and the ground is taken all the way back to the main substation ground using a copper conducting path. Conduit grounding is not good enough.
The third way to avoid these problems is to carefully evaluate whether the electronic equipment has the capability to handle your application. If not, some changes are required. Make sure that you purchase quality equipment. When it comes to lost production, there are no medals given for buying cheap equipment.
There is a reason for the failures. The best solution is to find the source of the problem and eliminate it.
QUESTION
We have a new installation coming up which will involve a lot of electronically sensitive equipment. What steps do I need to take to ensure that my electrical power system will handle it?
ANSWER
The key to preventing potential power quality problems is doing your homework at the design and specification stage. Taking the time and resources to do this correctly will result in a safe, reliable, and trouble free installation.
Retain the services of an electrical engineer or a reputable contractor who is experienced in power quality issues. Pay them to develop a set of specifications and installation details which will form the basis of your power system design for this project and future installations. Note that the Canadian Electrical Saftey Code is a minimum standard which focuses on electrical safety issues. It is not sufficient as a guideline to establish the requirements for power quality.
Prepare drawings and specifications to describe the power and grounding system you need. Request bids from reputable, experienced contractors. Ensure that the installation is completed according to the specifications and ensure any field changes are recorded on the drawings.
The Institute for Electrical and Electronics Engineers (IEEE) is an international organization which has numerous standards and guidelines relating to power system design, power quality, and grounding. Contact them for further information (www.ieee.org).
David is President of Wintek Engineering. You can forward your questions or comments to him at wintek@wintek-eng.com. Some of these questions will be addressed in future issues of Electricity Today. ET