ELECTRICAL AND electronic engineers face a particular challenge where, as in most utility-scale cases, grid connection is required. Especially given the growing penetration of wind power into the energy infrastructure, prompting tougher regulations for grid connection.

Fortunately, engineers seem equal to the challenge, even if not all the necessary solutions are yet in place. A big help for them has been the growing capability of solid-state electronics. Large semiconductor devices such as insulated gate bipolar transistors (IGBTs) and integrated gate commutated thyristors (IGCTs) can today manage unprecedented power levels at the high switching rates required for PWM, the predominant waveform manipulation technique. Connecting multiple IGBTs in parallel provides the basis of a modular approach that enables power converter suppliers to cater for up to several megawatts.

Wind turbines designed to operate at one or two fixed speeds can have their outputs fed straight to the grid, or at least via a voltage step-up and fault-isolating transformer. Rather than generator speed varying with the wind, the energy harvested does. The original utility-scale turbines pioneered by the Danes had squirrel cage induction generators of this type which could be directly connected. A big advantage is the relative simplicity and low maintenance of these machines. No power converter or complex controller is needed, at least for turbine ratings of up to about 2 MW.

However, most turbines today operate at variable speed so that energy can be extracted over more of the wind strength range. These require power products that either convert the ‘wild’ AC from generators operating at fluctuating speeds to a stable AC output synchronised in frequency and phase to the grid, or exercise control over the generators such that they run at constant speed. Precise arrangements depend on the type of generator used.

Today, some 70% of all onshore wind turbines utilise a doubly fed induction generator (DFIG), in which windings in both the stationary and rotating parts (stator and rotor) participate in the power transfer process. Useful in applications where drive shaft speed varies, as in wind turbines, this type of generator can run at double the speed of a singly fed type and is noted for high power density, torque and efficiency, along with competitive cost. Such generators are normally driven via a gearbox since they need to rotate much faster than the wind turbine rotor – typically at 1500 rpm. (Actual speed depends on the number of electrical poles in the generator.)

Induction generators may or may not require brushes or commutators, depending on whether or not there is a separate winding on the rotor to provide excitation, but they do require grid connection to work. This is because, to keep rotor speed constant, reactive power is either drawn from or fed into the grid. (Reactive power is that form of electrical power which, unlike active power, cannot directly be converted into useful work but creates the magnetic force that electrical machines require for their operation.) Any tendency of the generator's rotor to turn faster, impelled by the wind turbines' drive train, is balanced by reactive power drawn from the grid. This increases magnetisation and magnetic drag within the machine so slowing the rotor. This also raises the amount of induced current available for feeding to the grid.

A big plus for the DFIG is that only the power in the rotor is non-synchronous and has to be converted. Power in the stator is in synch with the grid and can be fed directly to it. Having to manage just a third of the total rated power (typically) means that the converter can be smaller, lighter, less complex and less costly than the full-power converters that competing generator types require, while also needing less energy for its own operation. DFIG power converters are often configured as twin converter units, connected back-to-back via a DC link. The role of the machine side unit is to control the generator rotor current, and hence its torque, and control the power factor. The grid-side converter controls the voltage on the DC link.

In addition to mediating the flow of reactive power between grid and generator, the power converter conditions the generator's output to meet grid requirements for frequency, phase, minimal harmonic distortion (i.e. a near perfect sine wave) etc. It also uses phased switching to secure ‘soft’ (lacking in electrical spikes or transients) connection and disconnection to the grid. Increasingly, it is required to provide electrical support for the grid similar to that expected of conventional utility-scale power plants. For example, converters are required that will enable turbines to ‘ride through’ short-duration network-side faults, such as voltage dips, without tripping off line, since uncontrolled disconnects of large multi-megawatt turbines can be seriously de-stabilising.

Another support measure demanded by network operators is the feeding of reactive power into the grid during grid outages, when such power is required in order to retain magnetisation across the network. Thus, converters are expected continuously to control power factor – the ratio of active power to total power generated – in response to voltage cues from the grid. A power factor of unity is the normal working ideal since this maximises the active power component, but departures either side of this may occur in line with network changes on the one hand and wind strength changes on the other.

A converter may also provide monitoring functions, delivering statistics of active and reactive power, conversion efficiency and other metrics.

Yet another type of wind turbine generator, becoming popular offshore because it enhances overall reliability thanks to not needing a gearbox, is the synchronous generator with separate excitation. One disadvantage is that this non-inductive system requires a direct in-line, full power converter. Another is the high number of poles needed (100 to 300), resulting in large machine and nacelle diameters.

Permanent magnet generators are also becoming increasingly viable, thanks to the advanced magnetic materials that have become available in recent years.

Commercial power solutions

Unsurprisingly, wind turbine power management solutions tend to come from suppliers who have developed electrical power management solutions for other industrial sectors. The Power Systems Division of the Asea Brown Boveri (ABB) Group, for example, has based its wind energy power converters on industrial drive modules. According to the Swiss-headquartered company, new stricter grid codes will require turbines to maintain their power level during grid events such as voltage and frequency drops. Other requirements will be an ability to limit generated power to a level below the maximum when required by network operators, an ability to keep operating during reclosing operations after short circuit problems in the grid, and capability to provide reactive power during a network fault. To stay abreast of grid requirements and help drive the technology forward, ABB invests strongly in R&D and is active in grid code and power quality working groups. It operates a laboratory in Finland dedicated to investigating the low voltage ride-through capability of various wind turbine systems.

ABB offers medium voltage converters with power ratings up to 14 MVA, suitable for grid connection of even the largest contemporary turbines. Converters for doubly-fed machines feature reactive power control, high conversion efficiency and low total harmonic distortion (THD). Systems are based on modular power electronic building blocks (PEBBs) which offer high power density, while providing fault ride-through capability and other grid support features. Available in different configurations within a standard cabinet design, systems can be totally enclosed and protected to IP 54 level against precipitation, dust, sand, salt and other hazards. Air or liquid cooling can be chosen according to the expected ambient conditions.

Given the need for wind power converters to be able to operate continuously for long periods unattended, reliability and durability are crucial. Components are selected and tested with these qualities in mind. For example, line filters use dry film capacitors (rather than leak-prone electrolytics) for long life. Use of encoders, another known source of unreliability, has been avoided. Warm-up resistors are used to keep circuitry warm and dry. Modules are designed so that they are readily maintained on site, even in the nacelle. Converters have built-in self-monitoring facilities that provide data which engineers can view remotely, in real time if necessary, for status monitoring and diagnostic purposes.

Offshore solution

A particular focus is the offshore environment where, increasingly, directly-driven permanent magnet synchronous generators are used because, given that these lack the gearboxes that account for a high proportion of mechanical faults onshore, they offer improved reliability over long periods of unattended operation. A disadvantage is that they require full-scale in-line power converters.

One way to counter the need to pass ever larger electrical currents through power converters and turbine interconnections as turbine power increases, raising their bulk and complexity, is to increase the working voltage. (W=VA where W is power in watts, V is volts and A is amps). This also avoids the alternative need to place bulky voltage transformers in the nacelle.

In line with this raised voltage trend, ABB has engineered a series of ‘medium-voltage’ converters that are particularly suited to powerful offshore wind turbines. These avoid the need to connect several low voltage converters in parallel to handle the same amount of current, with consequences for nacelle dimensions and weight.

Dual inverter modules connected back-to-back via a DC link are based on IGCTs, a superior form of semiconductor switch well proven in industrial applications. IGCTs are noted for their low losses at high switching frequencies, resulting in high conversion efficiency and reduced size of harmonic filters. Systems are currently available for turbine powers of up to 5 MW (NB the converter rating needs to be substantially higher than the wind turbine rating to allow for the turbine overload capability). They are said to provide sufficient reactive power compensation, fault ride-through and other connection attributes to meet emergent grid codes.

According to ABB expert Markus Eichler, the company's PCS 6000 three-phase converter, including all necessary water cooling and other auxiliaries as well as the power module core, is sized to fit neatly inside turbine towers. As well as the IGCTs that carry out the rapid-switched PWM-based frequency conversion process, circuitry includes a harmonic suppression filter on the grid side and an edge filter on the generator side. The grid filter is an inductor/capacitor combination allied with a damping circuit for the lowest-order harmonic. The filter on the generator side limits the rate of voltage rise at the generator terminals.

The DC link between the dual inverter units is protected by a voltage limiter that helps ensure smooth ride-through of short-term grid faults. This also minimises torque oscillations at the turbine during grid disturbances. ABB says its technology results in low ripple currents, which in turn brings low torque ripple on the generator side and therefore less stress on the drive train. Dynamic voltage control enables the system to react rapidly to both balanced and unbalanced grid faults (the latter are faults that affect the three electrical phases differently), enabling compliance with strict grid codes.

A four-quadrant topology that permits bi-directional current flow enables the generator system to be ‘motored’ for maintenance and testing purposes. This is useful, for example, in enabling engineers to motor the main turbine rotor to a precise alignment for maintenance.

Final connection to the grid is via a transformer, which provides necessary galvanic isolation in case of a fault. Special measures are embodied in the PCS 6000 to ensure that, as part of the grid connection procedure, voltage is ramped up slowly allowing the transformer to synchronise without an excessive inrush of current. This soft start capability helps prevent voltage dips during connection and, where necessary, enables large transformers to connect successfully to weak grids. PCS 6000 can also absorb and inject reactive power, avoiding the need for the separate VAR compensators that can be required with other generator/converter combinations. A voltage limiter unit (brake chopper) ensures that active power can be dissipated during periods when the generator is delivering 100% reactive power to the grid.

Control of the power converter is exercised by a standard programmable logic controller (PLC) via a sub-controller at the converter itself. The PLC responds to start/stop signals received over a digital data link from the master turbine control centre. It outputs to various relays and switches and passes dynamic control signals to the pulse width modulator. Generator currents serve as control feedback. An internal algorithm ensures that the converter operates the machine at maximum torque per ampere over the entire power range via a phase-locked control loop. An additional role of the control system is to record all important parameters and the status of the converter along with a timestamp during a fault. ABB personnel can analyse the situation in relation to recorded transient events, from remote locations.

While solutions from ABB are representative of advanced grid-friendly practice today, a number of other players are in the business too. Siemens Energy, like ABB, is active in medium voltage power conversion for wind energy and develops new conversion philosophies and topologies at its Power Converter Competence Centre in Keele, Germany. Its NetConverter full power conversion system is said to efficiently decouple generator and turbine dynamics from the grid, while offering flexible turbine response to voltage and frequency control, as well as fault ride-through and output adjustment.

Converteam markets its ProWind frequency converters in powers of 1.5–3.2 MW. Advanced fault ride-through technology along with power factor control enables them to stabilise the grid during even strong fluctuations, so complying, says Converteam, with the latest grid codes. Recently the company supplied its 1000th ProWind converter to REpower, prompting Dr Georg Moehlenkamp, Vice President Systems and Drives, to boast that the energy generated by the 1000 associated turbines operating at full power roughly equates to that from two large nuclear reactors. Last year, REpower's MM turbine equipped with Converteam's ProWind converter systems was the first DFIG-equipped wind turbine to gain Germanischer Lloyd's System Service Ordinance (SDL Windy) certification.

In the USA, Power-One has developed a 2.5 MW converter intended for both onshore and offshore use with permanent magnet generators. Based on IGBT modules, the system operates at 5500 VDC and offers 98.5% conversion efficiency at full load. Modules can, says the company, be combined to provide capability for wind turbines of up to 10 MW. Total harmonic distortion of less than 3% ensures that the power supplied to the grid is of the required quality.

GE Wind Energy, which fields power conversion solutions for its variable-speed turbines, argues that its WindVAR reactive compensation technology improves prospects for wind power in areas where weak rural distribution grids have previously discouraged investment in new schemes. This is because WindVAR and similar electronics can actually strengthen a weak grid. WindVAR is an optional feature within GE's power management suite but, with more than 2000 turbines around the globe now equipped with it, it seems clear that many clients prefer it to more conventional alternatives such as capacitor banks. Advanced electronics also secure the low-voltage ride-through capability that network operators require and help provide active damping of the WT system.

American Superconductor Corporation (AMSC) offers, through its AMSC-Windtec subsidiary, similar benefits with its PM3000W converter module, catering for turbine powers of up to 6 MW. AMSC also holds out the prospect that dramatically higher efficiencies could be secured in future electrical machines through the use of superconductor technology.

Scientific Electronics Pt Ltd (SETEC) has developed an IGBT inverter/converter system suitable for direct drive and doubly fed induction generators. It offers fault ride-through, reactive power compensation, cancellation of harmonics up to the 25th order and single or three-phase power handling at levels up to 3.2 MVA.

For harmonic and reactive current cancellation, SETEC applies an active grid control technique based on measuring the harmonic currents and compensating them by injection of opposite-phase current. The latter is created by PWM of the output of a two or three-phase IGBT inverter and a DC capacitor bus. SETEC claims that, in contrast to traditional capacitor circuits as a source of reactive current, compensation by active control is continuous without transients and switching distortions. Inductive reactive power is also available.

Overload protection, load balancing and automatic re-start functions are included, along with self tuning of phase and current for synchronisation. The system automatically adjusts to the network impedance and lends itself to parallel operation of modules for higher power capability. It readily interfaces with PCs for system monitoring purposes.

Direct drive future?

Present trends suggest that direct drive turbines with permanent magnet generators are the way of the future, particularly offshore but also onshore. Siemens, Enercon and Goldwind are among several manufacturers to have launched direct drive turbines recently while Vestas has favoured the ‘half-way house’ of a geared permanent magnet solution for its latest 3 MW model and its planned medium-speed, geared V164 7 MW offshore wind turbine. Former NREL Chief Engineer Sandy Butterfield, now CEO of Boulder Wind Power, believes that by 2015 most utility-scale turbines will be direct drive because of their superior energy capture, gearbox-free reliability and availability.

Further evolution in power electronics will, it seems, make it possible to meet the challenge of full-power conversion for these ever more powerful turbines, while further enhancing efficiency. Moreover, the capability of power products to step up to ever higher voltages will make them a natural partner to the high voltage direct current (HVDC) transmission lines that will take future offshore wind farm power to the shore. Converters will also contribute to smarter operation as traditional Supervisory Control and Data Acquisition (SCADA)-based control schemes give way to more sophisticated computer based solutions providing improved oversight of all that happens on remote wind farms.

About:

Engineering roles in high-vacuum physics, electronics, fl ight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.

Renewable Energy Focus, May/June 2011.