Managing the Challenges of Three-Phase Power and Energy Measurements

1. Introduction
2. Key Measurements
3. Capabilities and Limitations of Today's Test Tools
4. A Unique Solution from Keysight Technologies
5. Summary
6. Related Information

In today’s world of ever-increasing focus on energy and power efficiency, engineers are under pressure to find every possible way to reduce power consumption in their electronic power conversion systems (such as AC/DC power supplies, DC/DC converters, DC/AC inverters, solar inverters, battery management systems, electric vehicle power control systems, satellite power systems and general aerospace/defense applications) and for devices that draw power from an AC line. To achieve energy-efficient designs, R&D engineers need to make measurements to ensure their designs meet established goals, operate efficiently and behave properly under transient conditions without creating noise or power quality problems.

This paper discusses the types of measurements engineers need to make when they want to reduce power consumption in electronic power conversion systems and the capabilities and limitations of today’s test tools for making these measurements. It also introduces a new test tool that overcomes the limitations of previous tools and makes it faster and easier for engineers to evaluate and reduce power consumption in their electronic power conversion systems.

First and foremost, R&D engineers need to measure DUT power consumption and energy efficiency to determine if their designs are meeting project goals. To measure efficiency, output power must be compared to input power, so simultaneous measurements of input and output power are necessary. As the efficiency of the DUT increases, the accuracy of the power measurement must increase as well or it will be impossible to determine if small power losses are attributable to the power lost in high-efficiency conversion or if they are caused by measurement error. 

Measuring power quality is important as well. For example, engineers need to verify power factor correction. Power factor is the ratio between the real power (watts) flowing into a device and the apparent power (VA) flowing into the device. An energy-consuming device with a low power factor draws more current than a device with a high (near 1) power factor. These higher currents require the power distribution system to have higher capacity than would be required for delivery of just the real power. Higher capacity increases infrastructure costs and wastes energy, so regulations mandate vendors to build power-consuming devices with high (near 1) power factor. Low power factor can also be caused by high harmonic content of current, which creates issues in multiphase power distribution systems where odd harmonics produce excessive current in the neutral wire. R&D engineers strive to design their devices with high power factor and then measure the power factor and harmonic content to verify their designs. 

Many of today’s power conversion systems are meant to supplement the AC power grid. For example, a domestic solar inverter takes DC power from solar panels and converts it to AC power that can be used within a home or be put on the grid. The power coming out of these inverters must be clean and free from noise, have low harmonic content and be well-regulated to avoid polluting the grid. To make sure their output meets these criteria, engineers typically measure frequency, power factor, phase angle (between the sinusoidal AC voltage and current), real or active power (watts), apparent power (VA), reactive power (VAR) and harmonic content (typically to the 40th or 50th order).

In addition to measuring the steady-state characteristics of their DUTs, R&D engineers need to understand the functional performance of their DUTs. Functional testing tends to be dynamic testing, where some test condition is momentarily changed and the behavior of the DUT is observed. One example is inrush current testing. Many devices draw a large amount of current for a short duration when power is first applied. This inrush current is most often due to the charging of capacitors. These inrush currents can be many times larger than the steady-state operating current, so it is important to measure them to ensure proper sizing of fuses, circuit breakers and wiring. Another example is transient testing, where the engineer will produce a momentary disruption in the input power to the DUT and determine if the output of the DUT remains operational and stays within specifications.

The two most commonly used instruments for making these measurements are precision power analyzers and oscilloscopes. The table below compares the basic capabilities of these two classes of instruments.

	Precision power analyzer	Digital oscilloscope
Accuracy	High: 0.1% or better	Medium: Typically 1% to 3%
Resolution	High: 16 bits	Medium: 8 bits to 12 bits
Measurement types	Measures repetitive signals only, suitable for steady-state analysis No single-shot measurements	Measures repetitive signals for steady-state analysis Measures single-shot signals for dynamic analysis
Connection to DUT	Isolated and floating Accepts AC and high voltage Measures current directly	Non-isolated, ground referenced Requires differential probes to measure floating voltages Requires high-voltage probes Requires current probes
Visualization	Limited ability to see waveforms being measured	Primary function of oscilloscope is to visualize the waveforms being measured
End of use	Somewhat cumbersome to set up and operate Used less frequently than other test tools, requiring relearning each time	Familiar and easy to use The "go to" instrument for engineers

As the chart shows, precision power analyzers offer high accuracy and ease of connection to the DUT, making them ideal for steady-state measurements of power consumption, efficiency and power quality. For these measurements, the accuracy of the power analyzer gives R&D engineers the measurement integrity they need. With floating inputs and directly connected current measurement, precision power analyzers make it easy for engineers to connect to their DUTs. While power analyzers offer adequate measurement accuracy, they are somewhat cumbersome to use and lack the ability to characterize power consumption under dynamic conditions.

Only oscilloscopes offer the single-shot measurement capability necessary for dynamic measurements during functional test. Furthermore, by offering a visual picture of what is happening, oscilloscopes allow engineers to gain insight into their DUTs and to identify issues. However, their lower accuracy means that making critical efficiency measurements on high-efficiency converters may not be possible. Because oscilloscopes have ground-referenced, non-isolated front ends, probes are required for floating and current measurements. Probes further reduce measurement accuracy and make oscilloscopes harder to connect to the DUT for power-related measurements.

R&D engineers, therefore, switch between these two instruments depending on the type of measurement they need to make: They use a power analyzer to make accurate measurements and an oscilloscope to visualize repetitive and single-shot events such as turn-on and occurrences of transients. Switching between instruments is time-consuming and makes it difficult to get consistent, reproducible results.

The Keysight IntegraVision power analyzer is the first power analyzer that combines accurate power measurements and touch-driven oscilloscope visualization capability in a single instrument.

The Keysight IntegraVision power analyzer makes it easy for R&D engineers who are designing and testing electronic power conversion systems to access dynamic views of current, voltage and power so they can see, measure and prove the performance of their designs. IntegraVision power analyzers are ideal for R&D engineers who want to quickly and interactively measure AC and DC power consumption, power conversion efficiency, operational response to stimulus and common AC power parameters such as frequency, phase, power factor and harmonics – all with 0.05% basic accuracy, 16-bit resolution and single-shot measurement capability. The power analyzer enables engineers to characterize power consumption under steady-state and highly dynamic conditions with 5-M samples per second digitizing speed and 2.5-MHz bandwidth. The three-phase measurement tools on the new four-channel PA2203A include an easy-to-use wiring configurator, a phasor diagram for phase relationship analysis and  three-phase harmonics analysis.

When engineers are trying to reduce power consumption in electronic power conversion systems, they need to make a variety of high-accuracy and dynamic measurements. Typical test tools for making these measurements are somewhat cumbersome to set up and use. Keysight’s IntegraVision power analyzers overcome these limitations and make it faster and easier for engineers to evaluate and reduce power consumption in their electronic power conversion systems.