Engineers, electricians and technicians can find significant energy saving opportunities through load balancing when using power logging test tools. This article focuses on how to reduce energy bills by balancing loads across a three-phase distribution system. Correct load balancing is a quick way to reduce electrical energy usage.
System designers and electricians usually balance loads across a three-phase distribution system during installation. Loads are calculated in accordance with Article 220 of the National Electrical Code based on their volt-amperes (VA) or kilovolt-amperes (kVA), rather than watts or kilowatts (kW). This provides an accurate analysis of the ampere values that will flow in the circuit. Even though non-inductive load ratings are expressed in watts or kilowatts, these wattage ratings can be considered the equivalent of the same rating in volt-amperes or kilovolt-amperes. Understanding this concept of 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 divide kVA between phases so that each phase will carry an equal amount of load. This concept, however, holds true in theory only. In the practical world, the three-phase system is rarely perfectly balanced. Load unbalance (imbalance) manifests as a voltage and current unbalance. Technicians, therefore, must monitor and record voltage and current to determine the extent of the load imbalance in a system.
|Figure 1. Unbalance. In this example, the Vneg of 9.2 percent indicates expected overheating of three-phase motors and transformers, and reductions of motor torque and speed. The Vzero of 8.2 percent indicates excessive current flow in the neutral because of the unbalance with expected overheating of conductors and transformers.|
Technicians use one of two methods to determine an unbalance situation. The first uses a digital multimeter (DMM) to take spot voltage readings. To use this method, measure the three phase-to-phase voltage readings (AB, AC, BC). Sum their values and divide by three to obtain the average voltage. Any phase voltage reading that deviates by more than 1 percent from the average value warrants corrective action. While this process should be one of the first steps when troubleshooting an identified unbalance problem, it will not provide for an accurate analysis over time because loads cycle on and off. This requires trending or power logging.
Leading power quality analyzers and power loggers use a mathematical tool called the method of symmetrical components to analyze unbalance. This simplifies the voltage imbalance concept and provides accurate and detailed information to be used for analysis. The analyzers and recorders automatically perform the calculations, and technicians only need to read the results.
|Figure 2. Unbalance Trend. This is the trend function of the Fluke 435 meter screen in Figure 1. By selecting Cursor ON and then moving the cursor along the signatures, the amount of unbalance can be identified at different times. These trends should then be compared to operation of single-phase equipment during these times to determine the source of the unbalance.|
The instruments split each phase voltage and current into three separate components: the positive sequence, the negative sequence and the zero sequence. The positive sequence component represents the normal voltage or current in a balanced three-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 three-phase, four-wire system. This results in energy losses in the form of heat in conductors and transformers.
The EN50160 power quality standard sets the maximum unbalance at 2 percent at the point of common coupling. The analyzer or power logger displays and records the percentage. This percentage is the ratio of the negative sequence component to the positive sequence component, or the zero sequence component to the positive sequence component.
|Data can be easily exported to a spreadsheet program for further analysis.|
Technicians don’t have to memorize the theory-but only learn to operate the equipment and read the results. When checking for unbalance, select Unbalance on the power quality analyzer menu. Then, read the ratios on the meter screen. Neither the Vneg nor the Vzero values should exceed 2 percent. See Figure 1, Unbalance. If these limits are exceeded, isolate and correct the source of the problem or energy losses could become substantial.
Because the meter screen on the power quality analyzer updates constantly, the analyzer meter function is best suited for checking for unbalance at a particular point in time. Select Trend when in the unbalance mode to observe negative sequence and zero sequence voltages over time. After recording has stopped, use the cursor to determine at what time the unbalance occurred and its severity. See Figure 2, Unbalance Trend.
|Figure 3. Power Log Frequency/Unbalance Tab. By selecting the various checkboxes at the top of the Power Log software Frequency/Unbalance Tab, the various sequence components can be compared. In this comparison, the negative sequence voltage component and its effect on amperage are plotted.|
After a power logging session, download the data to a PC and analyze using the Power Log software. This software analyzes voltage unbalance recordings from several types of recording instruments, including the Fluke 345 and 435.
Technicians must also consider the available options when setting a recorder instrument to log voltage unbalance situations. The Power Log software allows for plotting of the following values if they are present in the recorded data:
- Unbalance Vneg percent;
- Unbalance Aneg percent;
- Voltage: positive, negative and zero sequence; and
- Amperes: positive, negative and zero sequence.
Once the data is downloaded to the PC, select the Frequency/Unbalance Tab in the Power Log software. View data graphically by selecting the minimum, maximum or average value of each desired set of information. See Figure 3, Power Log Frequency/Unbalance Tab. For further analysis of the data, export the data to a spreadsheet by selecting Export data under the File tab. You can then compare the exact dates, times and voltage unbalance values to other plant data. Engineers and technicians, 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, can now isolate loads and equalize them across all three phases to correct the unbalance problem.
Engineers, electricians and technicians play a large role in reducing plant energy costs. If a power quality instrument is set up to record needed voltage and power values, and software is used to analyze the recorded data, utilities can reap the benefits of significant energy savings through proper load balancing.