Smartening up the grid: challenges for testing technologies

To have a stable and reliable grid, not only are existing standards for power consumption and harmonic influences becoming stricter for products but also new standards are starting to appear for all energy sources connected to the grid.

Maintaining a balanced grid poses several challenges to power engineers from both grid operators and grid participants. The power outputs from different sources have different levels of distortions from those with switched-mode supplies (solar, wave and wind), and these are usually greater than those from constant-mode supply (e.g. coal, gas and nuclear).

With a multitude of renewable and non-renewable power stations feeding the grid, engineers in charge of ensuring a balanced grid need robust testing and accurate measurements to reduce the impact of noise, distortions and harmonics from multiple sources.

Similarly, power generation stations and large consumers also need to evaluate the effects of their power outputs and usage levels on the grid and on other users.

Therefore, power engineers have a growing need to correctly and accurately characterise the behaviour of all power system components in the smart grid taking into account questions such as:

• What is the level of AC/DC voltage and current to be evaluated?
• At what frequency range and/or bandwidth do measurements need to be made?
• What wiring will be used? Single-phase, three-phase or a combination of both
• What is the shape of the signal: sinusoidal, PWM or a more complex waveform?
• Is the power repetitive? Stable? Intermittent? Continuous or fluctuating?
• Are cycle-by-cycle or sub-cycle power transients required to be measured?
• How distorted are the waveforms? Are the signals noisy?
• What is the power factor?

The testing technologies used for these evaluations need to be robust enough to ride through disturbances such as soft switching and soft starting that occur during and after power outages or responses to peak demands.

Choosing the power measurement solution that is right for you.

Greater certainty required
Engineers are now faced with growing network complexity and therefore the number of measurements and tests they must carry out in their role has greatly increased. As a result, they must be able to accurately measure beyond just voltage or current uncertainty and take power uncertainty, as a whole, into consideration.

Once engineers know what accuracy they need to achieve they can decide on the appropriate measurement technology to use. The technology needs to match the application needs of operating bandwidth, voltage, current, accuracy and number of inputs.

In addition, depending on waveform complexity, types of computations, and electrical mechanical measurement needed, one or more of the following requirements below may be needed:

• Fast and automatic updates of measurement range or update rate to measure input signals fluctuating in amplitude or frequency
• Specifications not only at a power factor of unity but also at power factors applicable to the needs of the application and accounting for uncertainty contributions from internal phase shifts
• Harmonic and flicker analysis capabilities based on IEC standards
• Measurement ranges with high crest factors to capture distorted signals or large, unexpected peaks
• Computation of electrical parameters in star, delta and other wiring configurations
• Functionality and sampling rates for analysing PWM and other complex waveforms
• Measurement of physical parameters such as torque, mechanical power, slip, rotation speed, temperature, pressure, strain etc.
• Time-domain measurements for analysing cycle-by-cycle or sub-cycle power transients

Are you achieving the levels of accuracy you need?

Which test instrument?
There are a variety of instruments in the market that can potentially serve a user’s power measurement needs. Depending on circumstances, the user may need the waveform analysis of an oscilloscope, the high accuracy of a power analyser or a hybrid combination of the two with flexible data acquisition added into the mix.

But the underlying power measurement principle behind each of these instruments is essentially the same - sampling the voltage and current waveforms simultaneously, multiplying them together after acquisition, integrating the resultant instantaneous power readings over a whole number of fundamental waveform cycles and then dividing by the time.

Depending on the resolution of the A/D converter and the sampling rate however, there are two broad categories that power measurement instruments fall under.

Streaming or averaging type instruments: These include the traditional power meters and power analysers, and use high resolution at the analogue/digital conversion stage and instantaneously compute and integrate the voltage, current and power values in order to achieve continuous measurements and high accuracies.

Digital storage type instruments: These include digital storage oscilloscopes, ScopeCorders and Power Scopes, and acquire data at high sampling rate, store the data in an acquisition memory and then process it for output.

During processing of the sampled data however, there is ‘dead time’ when the instrument is not reading the input waveform, thus missing the data points for continuous measurements.

Application considerations
There is a broad spectrum of applications in which today’s grid operators will require one or more of these test instruments I’ve described above:

• Testing the electrical and mechanical efficiency of inverters in renewable energy systems
• Measuring the power consumption of home and office equipment in standby and operation modes
• Evaluating the effects of load switching and short circuits in power distribution systems
• Measuring no-load losses in transformers
• Checking inrush current of Lighting ballasts and harmonic analyses of LED switching circuits
• Testing compliance to harmonics and flicker standards for different equipment classes.

Applications like inverter motor drives and wind energy need both DC and AC signal analysis along with a combination of electrical and physical phenomena. Several industries now adhere to standards for power consumption in startup, standby and operation modes. Modern electronic circuits in lighting, home appliances and office equipment often feature high speed switching techniques to reduce power consumption and bring about opportunities for component miniaturization. But this also introduces opportunities for harmonic and interharmonic interference at higher frequencies which, when poorly measured, can lead to poor performance. In applications such as no-load testing for transformers, high accuracies are expected at power factors as low as 0.002.

Compliance testing, certification and validation

Industries today have to meet a number of governmental and regulatory standards to ensure product efficiency, safety, comfort and productivity for homes and businesses. Compliance to standards for standby power consumption (EN 50564 and IEC 62301) or harmonics and flicker (IEC/EN 61000-3-2 and IEC 61000-4-7) for different classes of electrical and electronic equipment affect both market validation (fit for use) and product differentiation for competitive advantage. A power analyser that can guarantee its accuracy over specified operating conditions is the ideal solution for this stage.

Power measurements in these applications should not only be guaranteed for accuracy, but also be repeatable and stable over time for the specified ranges. One can gain quantifiable confidence in a measurement system only through regular accredited calibration of an instrument’s performance against a standard of known accuracy.

Infographic: Getting started with electrical power measurements

You can learn more about Yokogawa solutions for power measurements across the development cycle by visiting our website.