Wind: preventing grid voltage jitters

George Marsh

This article focuses on voltage, the electrical “pressure” that drives current which enables machines and consumer devices to work. Steady voltage is a key grid requirement.

Conventional power generation stations have to ensure that the energy they feed to the electricity grid meets certain criteria for key electrical parameters. In particular, the last thing transmission authorities want if they are to provide consumers with steady supplies is voltage or frequency jitters. It is safe to assume that suppliers of renewable energy will increasingly have to meet similar criteria to conventional power stations before they are allowed to inject power into the grid. In short, they will be obliged by regulators to supply high quality power. Wind energy is the first sector to be challenged by this, particularly as wind turbines and wind farms become ever larger and more significant in power generation terms. Unfortunately, the ability of some wind generators to support grid voltages can vary widely as compared to conventional generators. In addition, if the voltage on the grid goes outside allowable limits at either the turbine or the substation, that turbine or the entire wind farm will trip out (disconnect). A valuable generation resource is thereby suddenly lost and the event itself does nothing for grid stability. It's the familiar conundrum; the highly variable nature of wind versus a grid system that has evolved around a steady state.

Further explanation requires a brief foray into electrical engineering, in particular certain characteristics of alternating current supplies.

Active/reactive power

Ac supplies carry two types of power, active and reactive. Active power, developed when voltage and current are in phase with each other, is directly usable by machines and consumer devices to do work and is therefore ‘useful’ power. Conversely reactive power, developed when voltage and current are out of phase, cannot be converted into useful work per se and is, at least to the consumer, virtually useless. Out-of-phase conditions are associated with different types of electrical device. Wound machines, for instance, are inductive and cause the current to lag behind the voltage, while capacitive devices produce a phase lead. As most motor loads are inductive, capacitors can be used to offset the effect of motors – that is bring the voltage (V) and amperage (A) sine waves back into phase so that the ratio of useful power to total power is nearer to unity or 100%. This ratio is often referred to a ‘power factor’ and can be either leading (capacitive) or lagging (inductive). Operating at leading power factor has the effect of raising voltage and, conversely, operating at lagging power factor lowers the voltage. Thus by mediating the power factor in this way, the performance of the transmission lines can be maximised.

Wind turbines, especially those of the induction type without reactive compensation would draw substantial amounts of reactive power (VARs) from the grid in amounts that fluctuate with the speed at which the wind drives the turbines. This tends to depress system voltage, not infrequently triggering remedial action to maintain voltage fluctuations within the tolerances established by the utility. If this happens at a time when faults or other events occurring on the grid are depressing or elevating the voltage, the wind turbine (or farm) may trip offline, an undesirable consequence that can negatively effect grid stability and causes mechanical and electrical stress.

A traditional way of enhancing reactive power support at wind farms is with mechanically switched capacitor banks. These inject into the system leading phase-based reactive power to counteract the lagging phase VARs drawn by the wind turbine's inductive generator. Restoring the power factor towards unity in this way helps maintain a constant voltage at the point of connection to the grid.

Unfortunately, because capacitors supply discrete amounts of energy, adjustment tends to be in bulk steps rather than in a smooth continuous progression, causing the voltage to rise or fall stair-step fashion. The downside of this approach is that the speed at which the compensation can be applied is generally much slower than with dynamic compensation. Further, once a capacitor has been switched out of circuit, a period of time – typically five minutes – must pass before this bank can be used again. Moreover, mechanical capacitor bank switches have a finite number of duty cycles and service lives. Nevertheless, capacitors are the lowest initial cost option and will remain central to voltage regulation solutions in many cases.

However, subtler control methods are available, thanks to the growing affordability and capability of power electronics. Electronics are already instrumental in converting the output from direct current generators to alternating current at the frequency required for grid matching (normally 50 or 60Hz). This is accomplished by an inverter, more properly regarded as a converter, which switches direct current alternately positive and negative to create a square wave and then removes various harmonics of the base frequency to convert the square wave into a grid-compatible sine wave. Electronics operating on a similar basis enable converters to regulate voltage by feeding VARs into the system as required. Thus reactive power does have a useful role in regulating voltage.

Power electronics can manage VARs more efficiently, more dynamically and more smoothly than capacitors alone. One system currently enjoying considerable success in the wind energy sector is the D-VAR® system (D for dynamic) developed by American Superconductor Corporation. AMSC claims to have over 70% market penetration at present. According to Chuck Stankiewicz, Executive Vice President, AMSC Power Systems, some 33 wind farms worldwide are so far equipped with the system, covering nearly 3GW of wind power.

The basic building block from which all AMSC D-VAR systems are created is the company's Power Module™ or inverter (converter). Configured in a modular enclosure, this is able to produce up to four megaVARs continuously. Operating numbers of these units in parallel leads to large integrated systems able to supply the power ratings and sizes required for particular installations. These modular arrays can be expanded as required, providing capacities of four to over 100 MVARs. The modular approach confers high redundancy with no single-point failures and easy fault rectification if a problem does occur. Each module has its own integral control circuitry, and is integrated with an overall master system controller. Large systems can be bulky since a basic 4MVAR installation would occupy an 8ft by 8ft footprint.

Reliability of the AMSC system is said to be ‘high and still on an upward trend’, with reported availability on a global basis as high as 99.9%. System condition is tracked remotely via satellite, ethernet or local phone lines so that technicians at AMSC headquarters will receive an alarm should a system develop a fault or trip out. They can use received data to identify the cause of a trip, an over-voltage event for instance, and reset the affected unit if the fault is considered transient or not serious. Typically, D-VAR installations for offshore wind farms would be located at sub-stations ashore.

Harnessing the rapid response of contemporary high-power electronics underpins the dynamic nature of the system along with its ability continuously to track grid voltage changes and react immediately to them. As Chuck Stankiewicz told Renewable Energy Focus:

‘Capacitors alone do not react fast enough to provide the dynamic correction that grids often need, but using them in conjunction with D-VAR provides a much more capable solution. D-VAR has become a de facto standard, sometimes alone but more often used in conjunction with capacitors. Although the power electronics solution costs more than capacitors alone, by the time you factor in the various efficiencies, the lower maintenance due to reduced gearbox stress and the fact that turbines stay on line across a wider range of conditions, savings that accrue over time make the system highly cost effective.’

The advent of reliable insulated gate bipolar transistors (IGBTs) capable of handling high power levels – 100 amps at 1200Vdc for example – has made it possible to improve on previous-generation silicon-controlled rectifier (SCR)-based devices, though the company still produces products based on this technology for large transmission system applications. The IGBTs (or SCRs) carry out the first-stage chopping of the WT's generator output to create the initial square wave. IGBTs enable this to be done at a higher pulse repetition frequency than with SCRs, such frequencies being desirable because they result in lower system losses. This helps explain why the company can claim an efficiency for its basic power module of 98.5%, though another 1% loss can be incurred in an associated step-up transformer.

A pulse width modulation system is used to convert the high-frequency square wave to a sine wave of the required grid frequency. IGBTs, which are sourced from a number of suppliers, require active cooling, so forced air cooling is incorporated within the power modules. Operation is possible in ambient temperatures ranging from -40 deg C to + 50 deg C.

AMSC has engineered the D-VAR system to accommodate considerable voltage variations, both high and low. Systems can be designed to allow the wind farm to ‘ride through’ low voltage events that would cause many older wind generation systems to trip out. Indeed, operators in a number of countries, including for example Spain which has numerous wind farms, are considering upgrading their installations by retrofitting systems that provide dynamic reactive compensation. AMSC says that D-VAR is eminently suitable for this application and is in touch with operators who could benefit. The system's dynamic characteristics also enable it to tolerate and provide mitigation for the wind farm for electrical grid high voltages, in some cases providing correction for a level of up to 115 to 120% of nominal for two to three seconds in the D-VAR's overload mode.

The D-VAR sensing and control scheme continuously monitors the voltage at the wind farm collector bus. When the voltage rises or falls to a level outside a preset band, the system responds instantaneously to stabilise the voltage. D-VARs can also act as ‘shock absorbers’, protecting the wind farm from voltage disturbances that may occur on the transmission grid. This helps prevent nuisance tripping, so maximising overall power output, to the benefit of operating revenue.

Chuck Stankiewicz concedes that several other power electronic-based VAR management solutions are available from players like ABB, Siemens and others. He attributes AMSC's competitive success to sound design and the fact that the primary focus of AMSC Power Systems is just that, power systems, rather than transmission and network schemes or traction control. He is highly confident about the prospects for D-VAR, expressing the view that:

‘With most countries in the world now committed to cutting their carbon dioxide emissions and wind energy being an important means for achieving this, wind farms will before long be everywhere. As farms and turbines grow bigger and as wind-derived power accounts for a larger proportion of total generated power, electricity grids will be more at risk and will stiffen up their requirements relating to voltage, frequency, power factor and so on. We can only benefit from this, given the effectiveness of our solution. D-VAR will also be useful, in due time, to operators of other variable renewables such as wave and tidal power.

Looking further ahead, more innovation will be needed. As renewable energy sources start to produce maybe 20 or 25% of all generated power in some locales, there will be major strategic grid issues. The concept of a Smart Grid, able to react quickly to changes, is much discussed and in the United States some very significant players, such as IBM, Honeywell etc, are investing strongly in this. In the meantime, we’ll be working to make D-VAR with higher power densities and therefore smaller; more efficient, even more maintainable and more affordable.’

Other reactive power regulating devices are available. Static VAR compensators based on thyristor/silicon controlled rectifier switching are produced by AMSC and others, though today the advantages of IGBTs tend to be more attractive for wind energy application. Unified power quality conditioners (UPQC), which combine statcom (D-VAR) techniques with series capacitors, can be effective in regulating parameters on line, but affordability may be an issue for wind farm operation. Similar considerations apply to unified power flow controllers (UPFC) though these effective devices, based on active filtering, are useful in certain applications requiring particularly high power quality.

Other systems known to power quality specialists include the distributed static synchronous compensator. AMSC has its Super VAR, a dynamic synchronous condenser system that can be used as a grid shock absorber. Perhaps the most forward looking potential solution in sight, however, is to obviate the need for all such devices by developing synchronous ac generators that deliver power at the right steady voltage and frequency levels. Alternatively, asynchronous generators can be used with mechanical or fluidic slip systems such as WinDrive.

In an interesting related development, AMSC is likely to be asked by the US Department of Commerce to put forward concepts for an 8-10MW direct drive HTS generator suitable for offshore wind application. ‘HTS’ signifies high-temperature superconductor, a technology that applies accessible superconductivity to greatly increase the efficiency of wire-based machines, including generators. This would exploit technology incorporated by AMSC in a powerful electric motor it developed for the US Navy and, significantly, enable the American Superconductor Corporation to utilise its founding discipline in the wind energy field.

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