The combined forces of electrical utility deregulation and energy conservation have generated a demand for a whole new class of line-voltage power meters. New electrical loads of all types now require sophisticated labels describing the energy consumption and the cost of operation.
Evaluation engineers are being challenged to provide accurate, detailed descriptions of the power-consumption profile of new line-powered electrical devices. Due to the continued emphasis on efficiency, electrical designers are becoming increasingly dependent on semiconductors to provide precise control of line current, thereby obtaining the lowest possible power consumption. As a result, the traditional tools and methods for measuring power no longer provide accurate or complete results.
Electrical loads which have semiconductors in the line circuits include everything from large motor drives to fluorescent lighting ballasts. To properly evaluate one of these devices, you need a line-voltage power meter.
Power meters must measure and record power in kilowatts, the true power factor (PF), the displacement power factor (DPF), true-rms current, total harmonic distortion (THD), and the harmonic current spectrum up to at least the 31st harmonic. Harmonic measurements are particularly important if the device being evaluated will be sold in European countries where local governments place strict limits on harmonic currents generated by loads connected to the power line.
Traditional power meters used analog multipliers or Hall-effect devices to accurately measure power consumption, but the analog circuits were unable to measure harmonic distortion. Modern power meters typically use a sampling circuit in conjunction with a microprocessor to provide both the power consumption and the harmonic information. Good accuracy for power measurements can be achieved with a rate of about 100 samples per line cycle. A bandwidth of at least 2 kHz is required to measure harmonics up to the 31st.
Power meters should include the capability to accurately measure all the parameters mentioned plus a few added features to make the measuring job safer and easier. Some power meters have enhanced memory capabilities and a record mode that takes a comprehensive snapshot of data.
A serial interface makes it possible to download information to a PC. Application software allows data to be viewed on screen in graphics and tables. Data and graphics can be easily transferred to a spreadsheet or word processor for custom reports or saved to a disk file for documentation and future use.
Safety Considerations
The safety rating of power meters is important because the measuring task involves making direct connection to power line voltage. The continuous voltage rating of the test instrument should be higher than the worst-case measurement requirement. For example, a 600-V rating would be appropriate for evaluating a load connected to a 480-V source. In addition to the voltage rating, look for an IEC 1010 Overvoltage Category, which defines the transient overvoltage that the power meter can withstand without damage.
IEC-1010, the new international safety standard, establishes Overvoltage Installation Categories I through IV. The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient will be attenuated or dampened as it travels through the impedance (AC resistance) of the system.
A higher overvoltage-category number refers to an electrical environment with higher energy available and the potential for higher voltage transients. For example, a CAT-II rated instrument should only be used at the end of a long branch circuit. A CAT-III rated instrument is qualified for use on a load connected directly to a service panel.
Look for a power meter bearing the symbol and listing number of an independent testing agency such as UL or CSA. This evidence of testing and acceptance by a listing agency often is required by your insurance company.
Current Clamps
When measuring line current, a moveable-jaw current clamp is preferred. This method avoids breaking the circuit, which can involve dangerous bare conductors. Current clamps invariably have upper and lower limits to their accurate measuring range. When choosing a current clamp, look carefully at the upper end of the specified range and verify that the clamps handle the highest current you want to measure. Usually, you can measure below the clamp’s lower limit by simply wrapping extra conductor turns through the jaws and dividing the measured value by the number of turns. Make sure the jaws close properly before making the measurements.
An Application
To help explain the terminology and use of a modern power meter, let’s look at a typical application. Suppose you are asked to evaluate a major lighting retrofit. The job involves replacing 5,000 fluorescent ballasts in a large commercial building. The existing ballasts are reaching the end of life, and a decision has been made to replace all of them under one contract.
This is a good opportunity to reduce the building’s power consumption and possibly improve the power factor of the lighting load. You immediately call the sales people from the three largest ballast suppliers and ask for specifications and samples.
What you need is a side-by-side performance test using a good power meter. The test results for each ballast must include the power in watts, a DPF, a true PF, and the THD of the line current. The DPF is the traditional power factor where the load inductance shifts or displaces the current from the voltage.
The true PF takes into account the affects of harmonic current. Since harmonic current cannot do useful work, the true PF always will be lower than the DPF whenever the line current contains harmonics (Figure 1).
The THD indicates the percentage of harmonic current with respect to the fundamental (THDF) or the total rms current (THDR). A good power meter will show both values so a direct comparison can be made to whatever terminology is used in the ballast specification.
Figure 2 shows a diagram of the ballast performance comparison test. This setup minimizes the variations due to line voltage and line impedance.
In the test setup shown, a good power meter can easily compare the electrical performance of different ballast designs. In reality, however, the electrical performance will not be the only factor influencing the selection process.
Since the line voltage will affect power consumption and line impedance will influence harmonics, the ballasts are simultaneously switched on to the same power source using the same set of lamps. The current clamp encloses 10 turns of wire to simulate a string of 10 ballasts. The power meter measures watts, PF, and THD.
Conclusions
Selecting a power meter is not difficult if you take the time to carefully analyze your needs. Start with the range of voltage and current you want to measure. Remember that you can extend a current clamp’s lower end of range by wrapping extra turns around the jaws of the clamp.
Look carefully at the overvoltage ratings and the IEC 1010 category number. Choose the higher ratings for higher transient overvoltage-withstand capability. If you need to keep extensive documentation records, look for a power meter with enhanced memory, PC compatibility, and supportive software. These features may cost more, but will save a lot of time in the future.
About the Author
Cliff Asbill is the senior product specialist for the Industrial Group at Fluke. He has been with Fluke since 1974, starting as a field service technician. Later, Mr. Asbill
moved to positions as a service engineer specializing in system digital multimeters and switches and then application support of power harmonics analyzers. Fluke, P.O. Box 9090, Everett, WA 98206, (425) 356-6357, e-mail: [email protected].
Copyright 1998 Nelson Publishing Inc.
May 1998