Saving Energy Through Load Balancing and Scheduling

You can find significant energy-saving opportunities through load balancing and load scheduling when using power logging test tools. Whether conducting an energy audit or troubleshooting to find energy losses, a typical power logging session should record several factors, including voltage, current, power trends, transients, and event logs.

Although analysis of each parameter is important in the overall scheme of reducing energy costs, the focus of this article will be on reducing your energy bill by balancing loads across a 3-phase distribution system and scheduling the operation of certain loads to reduce energy demand. Therefore, we'll concentrate on taking voltage and current unbalance readings to determine load balancing issues and on recording power and related events to identify load scheduling issues.

Load balancing

System designers and electricians usually try to balance loads across a 3-phase distribution system during installation. Loads are calculated in accordance with NEC Art. 220 based on their volt-amperes (VA) or kilovolt-amperes (kVA), rather than watts (W) or kilowatts (kW). This provides an accurate analysis of the ampere values that will flow in the circuit. Even though noninductive load ratings are expressed in watts or in kilowatts, these wattage ratings can be considered the equivalent of the same rating in volt-amperes or kilovolt-amperes. Understanding how loads are calculated and the associated units of measurement allows for proper power quality instrument setup, results interpretation, and corrective action decisions.

Electricians install equipment and usually divide kilovolt-amperes between phases such that each phase will carry an equal amount of load. This concept, however, holds true in theory only. In the practical world, the 3-phase system is rarely “perfectly” balanced. Load unbalance (imbalance) manifests as a voltage and current unbalance. Therefore, you must monitor and record both voltage and current to determine the extent of the load imbalance in a system.

You can use one of two methods to determine an unbalance situation. The first, which many refer to as the IEEE or NEMA method, uses a digital multimeter (DMM) to take spot voltage readings. To use this method, you measure the three phase-to-phase voltage readings (AB, AC, BC), sum their values, and then divide by three to obtain the average voltage. Any phase voltage reading that deviates by more than 1% from the average value warrants corrective action.

While this process should be one of your first steps when troubleshooting an identified unbalance problem, it will not provide for an accurate analysis over time as loads cycle on and off. This requires trending or power logging.

Some makes of power quality analyzers and power loggers use a mathematical tool called the “method of symmetrical components” to analyze unbalance. This method not only simplifies the voltage imbalance concept by providing a graphical representation of unbalance in vector format, but it also provides accurate and detailed information to be used for analysis. Basically, these instruments split each phase voltage and current into three separate components: the positive, negative, and zero sequences.

The positive sequence component represents the normal voltage or current in a balanced 3-phase system. The negative sequence voltage or current is created by an unbalance in the system and results in overheating in inductive loads such as motors and transformers. This component is also responsible for reducing motor torque and can affect speed. The zero sequence component represents the unbalanced current that flows in the neutral of the 3-phase, 4-wire system. This results in energy losses in the form of heat in conductors and transformers.

The EN50160 power quality standard, “Voltage Disturbances: Voltage Characteristics in Public Distribution Systems,” (from the Leonardo Power Quality Initiative) sets the maximum unbalance at 2% at the point of common coupling (PCC), which is the ratio of the negative sequence component (V_neg) to the positive sequence component, or the zero sequence component (V_zero) to the positive sequence component. Neither value should exceed 2% (Figure on page 18). If these limits are exceeded, you should isolate and correct the source of the problem, or energy losses could become substantial. After a power logging session, download the data to a PC and analyze.

You must also consider the options available when setting a recorder instrument to log voltage unbalance parameters, including the following values:

• Frequency

• Unbalance V_neg %

• Unbalance A_neg %

• Voltage (positive, negative, and zero sequence)

• Amperes (positive, negative, and zero sequence)

The more data available to analyze, the more likely you are to come up with energy-saving ideas. However, do not be overwhelmed with data. You should select only the specific set of data you wish to analyze at one time. Armed with knowledge of what single-phase equipment was operating at what time — and with an up-to-date one-line diagram of the distribution system — you can now isolate loads and equalize them across all three phases to correct the unbalance problem.

Load scheduling

In addition to reducing energy costs through load balancing, you can also create immediate energy savings through load scheduling, which is the part of energy load management that minimizes demand.

Electric utilities charge large commercial and industrial customers a “peak demand penalty” in addition to the total usage of electricity over the billing period. “Maximum demand” is the maximum amount of electricity used by a customer at any point in time. The electric utility must be capable of supplying this load, must size its distribution equipment accordingly, and will therefore charge the customer to be able to meet this need.

This maximum electrical energy usage, or demand, is averaged over a 15-min. period (typical), and determines the rate schedule at which a customer will be charged. Peak demand is usually caused by a spike in power consumption, most often when multiple loads come on simultaneously. These additional penalties can be high and add significantly to the cost of electrical energy. It only makes sense to minimize the amount of peak power being used, if at all possible. Power logging provides this opportunity.

If you are attempting to more effectively schedule load operation, conduct power logging recording sessions to measure energy usage over time and identify large loads that operate concurrently. Use one-line diagrams to determine the energy demand of various loads and compare them to operating needs. Do not look for just one particular load as the culprit. Quite often, you must work with operations personnel and adjust a process by staggering cycling times, or complete certain processes during off-peak hours to reduce demand. You must also work together with operations management to review the electrical energy bill and compare it to collected power logging data to aid in making the best plant operations and energy cost savings decisions.

You should always begin monitoring at the service entrance to determine total energy usage. Set the averaging time in the power logger to 15 min. if that's the time used by your utility. (Consult with your utility to determine the actual averaging time, as 30 min. may also be used.) In addition, monitor power for an entire billing cycle of 30 days. This will provide the most comprehensive information. Consider repeating this on a seasonal basis, as power consumption requirements change.

Import the data to your PC upon completion of the logging session and search for periods where energy usage exceeds desired limits. Identify the equipment creating the peak demand charges by observing excessive uses of active power (kW). Finally, adjust plant operations and processes as necessary, to minimize demand charges.

You can play a large role in reducing plant energy costs by setting up a power quality instrument to record needed voltage and power values. By using software to analyze the recorded data, you can reap the benefits of significant energy savings through proper load balancing and load scheduling.

Barnett is training director for American Trainco, Engelwood, Colo., and technical author for Fluke Corp., Everett, Wash. He can be reached at [email protected].