Wind turbines can push high-frequency harmonics into the grid

Power grid quality control: new challenges for engineers

The recently introduced IEC 62586 standard has defined a range of test techniques for power quality, but what is the impact of this standard and of what should HV engineers be aware

Concerns over power quality in all parts of the electricity grid, down to the level of individual consumers, have led to an increased demand for instrumentation that can detect and record problems with the supply. Digitally controlled equipment and concerns over renewables have made measurements of harmonics and other problems much more important.

Megger product manager Andrew Sagl points to several issues that are possible with renewables, as well as other electrical equipment: “The state of the grid changes as new technologies are added.” He cites unbalanced voltages caused by single-phase generators connected to a three-phase grid, reverse power flows and rapid voltage changes as issues that need to be addressed, particularly with renewables.

National Grid decided to upgrade its instrumentation to look for problems that are exacerbated by the inclusion of large-scale wind farms and other renewables. Renewables can create power quality issues that the company’s existing equipment could not detect, particularly with the high-?frequency harmonics produced by wind turbines. Even harmonics above the hundredth need to be monitored to ensure that they do not pass a limit.

Furthermore National Grid installed custom test equipment based around CompactRIO modules made by National Instruments which capture and store power-quality analytics to track trends and watch for problems.

Peter Haigh, senior power systems engineer, says the equipment lets National Grid produce a ‘heat map’ of total harmonic distortion. “As we deploy more we can increase resolution of displays of these trends. The additional value that we get is that we can take trends over time that enables us to plan the system more efficiently.”

The issues of power quality with renewables are leading to new designs of power converters for large-scale systems, such as work by Clemson University in South Carolina, USA.
J Curtiss Fox, director of the Duke Energy Electric Grid Research, Innovation and Development (eGRID) centre, has built a 15MW-capable hardware-in-the-loop (HIL) simulator originally designed to test methods to improve the ability of grids to ride-through faults. This has now been expanded to test many other power-quality and reliability issues.

“A big challenge is cleaning up 15MW of unconditioned power and transmitting that into the grid. What we have done is look at the inverter and embedded controller in the turbine have a complex interaction with the grid,” Fox says.

Based on a combination of PXI boards and field-programmable gate arrays to provide faster responses to events than is possible with software control, the HIL simulator captures real-time data from electrical equipment and can be used to prototype control loops that will ultimately be embedded in the controllers for the power converters used in generators such as wind turbines. The NI controllers drive a TECO/Westinghouse power amplifier that has the ability to generate arbitrary high-power waveforms.

Ensuring consistency

Downstream from the core grid, power quality has become a key issue for both utilities and their customers. Despite the widespread adoption of the IEC 61000 standard intended to control their production, a growing range of electrical devices are pushing harmonics onto the network.

“Very high frequency harmonics, from 2 to 150kHz, are created from high-frequency switching devices, such as CFL lighting, LED lighting, inverters as well as commercial electronics,” Sagl says. “Very high frequency harmonics will have very low voltage values. However, due to their high frequency they will push substantial current through capacitive devices. This will lead to EMI filters failing.”

The IEC acknowledges that the IEC 61000 standard that deals with power-quality measurements is at its heart a basic EMC publication and so lacks specific tests to deal with the situations often encountered in power networks. In a paper for the 2014 China International Conference on Electricity Distribution, Reinhard Kuntner and Naibu Ji of test specialist Omicron pointed out that instruments from various manufacturers can produce different results for power-?quality measurements despite conforming to IEC 61000.

This lack of commonality led to the development of IEC 62586, which is a product-level standard intended to provide a consistent set of tests for measurements such as power frequency, supply magnitude, flicker, dips and swells, voltage harmonics and inter­harmonics, flagging and clock uncertainty.

Omicron has developed systems that allow instrument makers as well as installers and calibration specialists to test their equipment’s ability to comply with IEC 62586. Some of the tests are complex, demanding that power-quality parameters be swapped during tests.

Devices are beginning to appear that are designed to IEC 62586, such as Schneider Electric’s PowerLogic PM8000. The company envisages uses in checking that the quality of electricity meets contractual obligations, particularly in hospitals, data centres and manufacturing operations.

“If a power-quality event does occur, the patented disturbance direction detection feature helps identify the location by determining if it occurred upstream or downstream of the meter, so correcting the problem is much faster,” claims John Straughn, offer manager at Schneider.

As support for IEC 62586 becomes more widespread and companies become increasingly concerned about their power quality, this type of instrumentation is likely to become more commonplace.

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