A key technology required for the exploitation of renewable resources is the humble inverter. Widely regarded as a ‘black box’ component, highly efficient inverters are crucial to enable the widespread introduction of grid-integrated price parity renewables. David Appleyard reports.Simple enough in conception, inverters, so-named because they perform the opposite function of a rectifier, convert direct current (DC) into alternating current (AC). Used in an endlessly varied range of applications from electronic power supplies to bulk power transmission, in the renewable sector the inverter converts DC from sources such as PV modules to AC either for local use or at voltages more suitable for export to the transmission grid.
In addition, in the case of variable-speed generation devices – such as wind turbines, marine tidal and similar – an inverter is essential for the device to connect to the grid and supply code compliant power. This includes almost all modern wind turbines – which are variable speed – the one exception being turbines that are equipped with a hydrodynamic torque converter that regulates generator speed.
Aside from grid connection, there are numerous advantages associated with the use of an inverter in the wind sector. This includes the ability to help balance the grid, supplying reactive power for instance, and the possibility of reducing the output from a machine during the evening to minimize noise, while still maintaining a grid-code-compliant supply.
There are three main types of inverter system used in the renewables sector, grid-tie (grid-connected), stand-alone and hybrids. In grid-tie systems the solar panels or wind turbines feed the inverter, which in turn supplies the grid. Grid-connected inverters are usually optimized for one type of generator, such as PV, and generally operate at a higher DC voltage than stand-alone inverters. The power output may either be sold to the local utility company or, in the case of commercial and domestic systems, offset a portion of the power used on site.
Stand-alone systems do not have the ability to supply power to the grid and will usually include battery systems that are charged by the renewable technology before the inverter is used to supply mains-quality AC power. Hybrid grid-tie inverters also use batteries, allowing both stand-alone and mains supply operation, although this does result in some efficiency loss when in grid supply mode: with some older inverters, losses can be up to 50% of the available power. To protect utility transmission line workers, inverters are also required to cut grid supply in the event of a grid failure. Switching between mains supply and stand-alone, or island mode, can be achieved by either passive or active islanding detection.
Simple inverters operate by running a DC input into at least two power switches, rapidly turning these switches off and on, and thus feeding opposite sides of a transformer. The transformer converts this DC input to alternate sides into an AC output, which may be a simple square wave, a modified sine wave, or a true sine wave depending on the complexity of the inverter and its intended application.
In the case of high-voltage transmission used in national transmission grids, a good-quality sine wave voltage supply is required in order to work efficiently. Other applications may be happy with a simple square wave, although simple wave inverters, while cheaper, are less efficient in most applications. However, as costs have fallen the most basic square wave systems, which are only suitable for running some power tools, incandescent lights or heating elements and similar, are becoming increasingly rare.
More complex circuitry allows outputs of a modified sine wave, which is in effect a stepped square wave. Suitable for most domestic appliances, such outputs may not work with micro-electronic systems or more sensitive devices and motors will use about 20% more power when supplied with a modified sine wave, rather than with a true sine wave. Nonetheless, modified square wave inverters are a good choice for many applications since their high surge capacity lets them start motors, while a high efficiency lets them run small appliances economically.
For any grid-connected generation system, one of the most important technical issues is the power quality, with power factor and harmonic consideration significant influences. The most complex inverter systems deliver a true sine wave output that can be of better quality than that supplied from the mains network.
Transistors and various other types of solid state devices have long been incorporated into inverter circuit designs, but since early transistors were not capable of handling the voltages found in most inverter applications it was the development of the thyristor or silicon-controlled rectifier (SCR), in the mid-1950s, that allowed solid state inverter circuits to be developed. Now available in higher voltage and current ratings, semiconductors which can be switched using control signals have become the preferred design for inverter circuits. Today, with the exception of some wind applications, the vast majority of such devices used in the renewable energy sector are microelectronic in design.
A wealth of different inverter design architecture and numerous strategies exist. The waveform can be filtered using capacitors and inductors with low-pass filters allowing the fundamental frequency of the waveform through, while limiting the passage of harmonics. Feedback rectifiers or antiparallel diodes may be used when the switch is off to deal with inductive load currents, since most loads contain inductance.
In renewable applications, inverters are typically designed to provide power at a fixed frequency, in which case a resonant filter can be used to block many of the undesirable harmonics.
The quality of an inverter can be expressed by using Fourier analysis of its output waveform to calculate the total harmonic distortion (THD) of the ideal pure sine wave, though to a large extent the quality of output is characterized by the intended application. The most important inverter parameters are rated DC and AC power, maximum power point (MPP) voltage range, maximum DC/AC current and voltage and rated DC/AC current and voltage. Other parameters are power consumption in standby mode, power in sleeping (night time) mode, power factor, distortion, noise level and such like.
Meanwhile, inverter efficiency is a ratio of AC power out and DC power in as shown:
∏ = AC Power / DC Power
However, inevitably inverters have flaws, modern electronic inverters are very efficient over a wide range of outputs, but some power is required simply to keep the inverter running and they are less efficient when running small loads. Efficiencies are typically rated at 90%–95%, although actual field efficiencies may be less, with some systems consuming power at night for example. Efficiency ratings are usually stated with reference to a resistive load, such as a heating element, but with some applications the efficiency is more accurately broken into two parts – the efficiency of the inverter, and the efficiency of the waveform.
Inverters are also much less efficient when used at the low end of their maximum power, so sizing the inverter for its intended application is a key factor in determining system efficiency. Undersizing the inverter will cause overloading and shutdown or power limitation, while oversizing will see standing losses increase, reducing overall efficiency and increasing the purchase costs. Achieving high efficiency in the face of a varying power output presents design challenges. However, energy management can reduce peak demand, allowing the inverter to be sized at close to the average, rather than theoretical peak, demand. Input voltage to the inverter depends on inverter power – for small domestic systems of say 100 watts an input voltage of 12 volts would be appropriate. For larger systems, inverters can be connected in parallel if higher powers are needed, and for the biggest systems three-phase inverters are available. Along with charge regulating electronics, three-phase inverters are also often used for high power applications, such as utility-scale transmission and HVDC.
Solar PV inverters
One of the world’s fastest growing energy technologies is grid-connected solar photovoltaic, particularly in Japan, southern Europe and the USA. Manufacturers of photovoltaic inverters include SMA Technologie AG, SatCon Technology Corp, Studer Innotec, Xantrex, Fronius, Sputnik and Mitsubishi, among others. Typical grid-connected PV installations feature so-called ‘central inverters’ which are frequently connected as master–slave systems. Under these conditions subsequent inverters are only switched on when solar radiation is above a certain threshold or if the main inverter fails.
String inverters are connected to strings of modules, and are used in applications across a wide range of power outputs. They allow more reliable operation than a single central inverter. Furthermore, maintenance for string inverters may be cheaper – even for large systems – since untrained personnel can exchange them in case of failure, whereas for central inverters considerable, and costly, expertise is required for servicing.
Among the more popular grid-tie inverters in Europe is the SMA ‘Sunny Boy’ system, which is designed to be used with a series wired string of 6–24 modules, depending upon inverter type.
Meanwhile, module inverters are mostly used in small systems; while they may also be suitable for larger systems, cheaper central or string inverters are more frequently used.
Inverters are the most sophisticated electronic devices installed in photovoltaic systems – there are various types in use. In line-commutated inverters, thyristors, are used as switching elements. Line-commutated inverters are not suitable for use in stand-alone systems because AC voltage is required to turn off the thyristors, although self-commutated inverters can operate without AC grid voltage.
In most cases, grid-connected inverters use a current control scheme, which has the advantage of a higher power factor and better transient current suppression. Grid-tie inverters also automatically shut down in the event of a high or low AC grid voltage or frequency, or in the event of grid or inverter failure.
The move to transformerless systems
Transformer-based inverters usually have a much higher maximum surge rating than electronic-based systems. However, transformerless inverters have been increasingly used in PV systems as they are considerably more efficient and can be produced at a much more competitive price.
Developed nearly 30 years ago, the earliest systems used metal oxide semiconductor field effect transistors (MOSFETs) as switching transistors to produce a stepped output voltage. In 1982, the first pulsed transformerless inverter using MOSFETs was developed at the Swiss Federal Institute of Technology in Zurich with an efficiency of 95%.
Pulse-width modulated, self-commutated transformerless inverters remain the most common solution in use today for PV systems, as grid-commutated thyristor devices have increasingly been squeezed out of the market. This is because grid-commutated inverters tend to have a smaller voltage range and need higher reactive power to operate. Indeed, transformerless inverters continue to advance around the world and have achieved a market of about 70%, although the US market is still dominated by transformer-based inverters since standards required DC grounding (earthing), which is not possible with transformerless inverters. The new releases of UL1741 and the NEC2008 no longer require DC grounding so clear the way for transformerless inverters. However, there is still a 600V DC voltage limitation, which particularly constrains three phase transformerless inverters. The devices typically achieve peak efficiencies of up to 98% and European efficiencies of 97.7%.
Development of SiC (silicon carbide) MOSFETs is expected to achieve a significant reduction of switching and conduction losses – of more than 25% – resulting in a peak efficiency of 98.5% and a European efficiency of more than 98% for an entire inverter. Next-generation power switches based on SiC are expected to become commercially available over the next few years. Due to the high switching-speed of SiC semiconductors, in future switching frequencies will be increased further, thus significantly reducing the size and weight of the inductive components of inverters, and consequently costs.
Indeed, in recent weeks the Fraunhofer Institute for Solar Energy Systems (ISE) has set what it says is a new record for inverter efficiency, at 98.5%, using SiC transistors
In a test using prototype silicon carbide-based MOSFETs, manufactured by CREE, Inc., Fraunhofer researchers report they reduced the power dissipation by 30%–50% when compared with traditional silicon-based transistors. ‘Silicon carbide components switch faster and have a smaller forward bias power loss than traditional silicon-based transistors,’ explains Dr Bruno Burger, head of the Power Electronics Group at Fraunhofer ISE. The Fraunhofer team achieved the result with a single-phase inverter and a nominal power rating of 5 kW.
Certainly, other organizations are also exploring SiC technologies. For instance, SemiSouth Laboratories, Inc. recently announced that, in trials, its enhancement-mode SiC junction field effect transistor (JFET) had significantly improved the efficiency of an off-the-shelf inverter commonly used in residential and commercial solar power energy systems.
Replacing the existing transistors with SemiSouth’s version allowed the inverter to reduce losses by as much as 50% in the grid-connected, low-frequency isolated inverter designed with conventional silicon Insulated Gate Bipolar Transistors (IGBTs).
The new enhancement-mode JFET can be used as a direct replacement for silicon MOSFETs and IGBTs in virtually any off-the-shelf converter or inverter design, the company says. Vess Johnson, SemiSouth’s president and CEO, comments, ‘The fact that the JFETs can be used as a drop-in replacement means that the barrier to entry has been greatly reduced and that designers working with these devices will be able to see immediate performance and efficiency improvements and will be able to drive new and better products to market much faster.’
Along with efficiency improvements, cutting the size and weight, and improving the operational flexibility, another development has seen a leap in the availability of inverter communications systems. A number of companies are now offering inverters that can communicate wirelessly with the internet to enable remote monitoring and diagnostics.
Xantrex, for instance, recently launched its Gateway wireless monitoring system for small-scale solar power installations. The communication component keeps the owner or operator, at any location, informed about the system’s operation and energy production by connecting the single-phase grid-tie inverter to the internet. It logs the system’s performance data directly from the inverters and transmits that information to Yahoo™ Widget-based software. The system can monitor a network of up to 20 such inverters. Fronius is among the other companies offering similar wireless transceiver systems.
One issue that remains unresolved to date is that of standards. Differences between, for instance, Europe’s IEC, the c-Tick standard in Australia, GOST in Russia, and the UL and c-UL safety standards in the United States and Canada mean that manufacturers wishing to supply international markets must frequently, in effect, produce two or three designs to achieve an all-but-identical result.
A key objective for the industry is to prepare and publish international standards for all electrical and electronic technologies, including inverters, so that components or systems manufactured in one country can be sold and used in all others.
The renewable energy boom across Europe and elsewhere has opened up an unprecedented market for solar energy-based inverters. Inverters for solar energy systems account for some 99.4% of the renewable energy market, according to recent research from analysis firm Frost & Sullivan, and in Europe at least, revenues are expected to increase at a compound annual growth rate (CAGR) of 24.9% out to 2011.
Chandni Raj, research analyst for the firm, observes: ‘PV inverters offer many advantages; first of all ease in implementation in the urban environment for high consumption generation of electricity even in dim sunlight. They also have limited impact on surroundings. Th ese are two key factors that are encouraging the market. Moreover, they offer better control over power consumption and lower electricity bills.’
Germany takes the lion’s share of sales in Europe, say Frost & Sullivan, maintaining a clear lead as a major producer and consumer of PV inverters. Raj comments: ‘The German trump card is not an excess of sunshine over other regions. It is the far-sighted vision and support of the German government … feed-in tariffs (FITs) and incentives worked like a magic wand, accelerating renewable energy growth.’
Germany is followed by Spain as another PV inverter hotspot. ‘Spain has made amazing strides in the renewable energy-based inverters industry in a short span of time owing to the generous government subsidies,’ adds Raj, saying that in Europe, Italy, the UK, Austria, Switzerland, Denmark and the Netherlands have growing markets, while Greece and Portugal are evolving to be highly-promising. ‘Some countries are growing fast. Some others are showing interesting signs of expansion. The European market as a whole now sees soaring sales and spiralling growth,’ says Raj.
With significant market growth also seen in the USA and Asia, a large number of players and new entrants are seeing the market for PV inverters become more competitive. As a result, prices are expected to fall and products with innovative features and greater efficiency will flood the marketplace.
This anticipated activity is being reflected in the number of new investments in production capacity which have been announced over the past few months.
For instance, Sputnik Engineering AG is increasing its annual production capacity for string and central inverters with its new facility, which went into initial operation in March in the Biel suburb of Port, Switzerland. The company aims to see its capacity reach 400 MW annually by the end of the year.
The new Sputnik plant will assemble central inverters with outputs from 50 kW upwards. Sputnik managing director Christoph von Bergen identified Germany, Spain and Italy, along with France and Greece as areas showing significant market growth. ‘While the sales to Spain grew by 250%, they increased in Italy by 200% and in France by 180%. At the end of 2007 Sputnik closed a sale on the delivery of 2 MW to Greece,’ he said.
SMA Technologie AG, meanwhile, has begun construction on what it says is the world’s largest solar inverter factory in Kassel, Germany, near its headquarters in Niestetal.
The company is expecting continuous growth in the coming years and to meet this increasing demand is developing a new 15,000 m2 production facility that is designed to be completely CO2-neutral and has a virtually independent power supply with a MW-scale BIPV system. ‘The demand for SMA’s solar inverters is continuously increasing, confirming the growth trend. Expanding our production capacities is an important step to further improve our international competitiveness’, says chief executive Günther Cramer.
It seems that with exciting technological developments set to offer greater efficiencies and higher power ratings, coupled with the industry-wide ramp-up that will inevitably result in cost reductions, the role of the inverter in delivering the price parity goal for the renewables industry is assured.
David Appleyard is Associate Editor of Renewable Energy World magazine.
With thanks to Dr Bruno Burger of the Fraunhofer ISE