To recap, the central role of the inverter is to convert the direct current delivered by the solar panels to an alternating current that can be used by electrical appliances or fed to the grid.

In the first part of this article, we looked at the vital role of the inverter, and examined some trends – including the drive towards the development of the micro inverter. But it is important to recognise that innovation is also happening in the core inverter technologies.

For example, Eltek Group company Valere claims to be setting the standard for grid-tie string inverters with its THEIA HE-t solar inverters. A 97% maximum efficiency is certainly impressive for a solution incorporating an isolation transformer, made possible by using a high frequency type.

At 21kg, the 4.4kW member of the series weighs substantially less than many competitor products. Because users can earth either the positive or negative terminal on the dc side, the device is suitable for use with modules of any technology, whether monocrystalline, polycrystalline or thin-film. A low start-up voltage coupled with efficient bespoke MPPT ensure high yields, even under fluctuating irradiance conditions. Maximum dc voltage is 600V and the MPPT operates over the range 230–480V dc. IP65 protection ensures that units can withstand wide temperature variations, operating with convection cooling only, as well as high humidity, dust etc.

Installation is facilitated by easy access to all connections in a front cabinet, simple country configuration set-up and, where there are multiple inverters, automatic transfer of settings from one inverter to the others across the THEIA HE-t network.

German inverter giant SMA achieves a peak efficiency of 95% with its Sunny Boy series (1.2 to 3 kW), a figure characteristic of a modern system using a transformer to secure galvanic isolation. An integrated dc disconnect switch simplifies installation, while units embody protection against dc reverse polarity; ac short circuit; earth faults; and over-voltage. Grid parameters are monitored. Units are said to be “suitable for indoor or outdoor installation and convection cooling makes the system virtually maintenance-free”.

The 98% efficiency claimed by SMA for its Sunny Tripower series of 10–17 kW three-phase inverters denotes a transformerless design, acceptable in certain applications. Suited to almost any module configuration, a Tripower inverter can accept inputs of up to 1,000Vdc, has an MPPT range of 320–800V and, as a maintenance aid, monitors string currents and string fuses. A dc overload protector is available as a safety option. Three-phase cable can be connected without special tools in an easily accessible connection area. Bluetooth communications are incorporated.

Grid support

Tripower also illustrates a trend in inverter design whereby advanced systems increasingly incorporate grid support features, helping to make solar PV more acceptable to network operators. For instance, instead of simply disconnecting in the event of a short grid disturbance or outage, inverters are engineered to ‘ride through’ such faults, so contributing to overall system stability.

Another feature now being included in major central inverters is reactive power management. By manipulating their power factor – the ratio of active (core, energy transferring) power to so-called reactive power (less directly useful power that maintains the magnetic fields needed to transmit energy to the consumer) – inverters can deliver reactive power to the grid. This is useful because, for efficient operation, the grid requires a significant proportion of its total power to be reactive. Injection of such power helps provide voltage support and regulation and can eliminate the need for network operators to take grid reinforcement measures in order to provide uninterrupted service during grid disturbances.

SMA's Sunny Tripower inverters are among several from leading manufacturers that provide reactive power control. Small variations in the tightly regulated grid frequency signal the need for more or less reactive power and the inverter responds by varying its voltage-current phase relationship to achieve the appropriate power factor.

Discussing with Renewable Energy Focus the growing grid support role of large inverters, Andreas Schlumberger from Kaco New Energy GmbH of Germany, stressed the importance of reactive power.

“This is a big topic for us,” he declared. “While we don't like to see reactive power that much because it means you have less real (active) power that can be worked, if we want solar installations to take on the role of true renewable power stations, then we must be able to have all the features that a classic power station has. That includes reactive power control.” He added.

“You must also be able to contribute to grid stability. Previously, whenever there was something wrong with the grid, the inverter had to shut down or isolate itself. Now solar plants are getting to such a size that, if we suddenly withdraw from the grid, that might hasten its collapse. So we have to have a fault ride-through mode. This requires a different sort of power transistor [in the inverter's switching circuitry] that can take a certain amount of electrical shock. As well, because you need to stay on the grid even when it's down, your inverter needs a power supply.”

Problems can be exacerbated where the grid system is relatively small, as with some island nations. SunPower Solar Technology, for example, encountered issues with a 1.2 MW solar plant that provides over a quarter of the electrical supply in Hawaii. This utility-scale system was proving disruptive to the grid because of its intermittent power production. SunPower developed power electronics to moderate power, voltage, frequency and reactive power of the plant's output and combined this with a powerful battery system so that disruption no longer took place. Employing similar electronics in large inverters could be useful for a range of utility-scale schemes where network operators have intermittency concerns.

For more modest schemes, reactive power is less of a concern and simpler inverters operate at unity power factor. They may also lack fault ride-through, tripping out in the event of a grid fault. One example, the Macsolar (Sweden) single-phase inverter, is transformerless, boasts a conversion efficiency of up to 97.6% and has a wide MPPT range.

Three models deliver maximum powers of 3.4, 4.5 and 5.6 kW. The Platinum range from Diehl AKO Stiftung & Co KG, while also operating at unity power factor, differ in having isolating transformers. Even so, they achieve efficiencies of around 95%, according to the company. Eight models range from single-phase 2.1kW up to three-phase 4.6kW. Usable with all the leading module technologies including thin film, the devices are said to be particularly environmentally resistant. The company claims that the units can withstand difficult conditions such as the midday heat, rapid temperature changes, high humidity and copious dust.

Inverters at EcoBuild

Among the many other inverter producers featuring their wares at the recent EcoBuild show in the UK, were the following:

Danfoss highlighted advanced digital algorithms in multiple independent MPP trackers used in its Triple Lynx string inverters. Three-phase versions are transformerless but the single phase model has a transformer. The units are lighter than many competitors, making them easier to install. Inverter status can be monitored via the Internet by using a ComLynx Weblogger.

The Fronius IG Plus 3.5 and 4 kW single-phase devices from Fronius International GmbH in Austria achieve peak efficiencies of almost 96% using high-frequency technology. A combination of multiple power stages ensures that maximum power is harvested even on a cloudy day. The stages divide up the work depending on the operating conditions and provide an element of redundancy in case of failure in a single stage. Fuse monitoring is built in and an integrated fused string combiner enables up to six strings to be connected. The IG Plus family also includes three-phase devices and extends from 3.17 to 12.7 kW.

The Radius Home range of inverters from Italy's Gefran SpA spans 2 to 3.6 kW power ratings. Embodying transformer isolation and multiple MPPT channels, units can be operated at full power rating in temperatures of up to +45 deg C. Standard features include integrated protection against islanding, over-temperature, dc and ac over-voltage, over-current, earth faults and software for remote inverter monitoring. Other products from Gefran include Radius Industrial central three-phase inverters of up to 330 kW rating.

Korean company Samil Power has applied expertise gained with variable speed drives and lift controllers to grid-tie PV inverters. Its SolarRiver String inverters span 1.5 to 5 kW; SolarLake models cover the 10–17 kW range and Solar Ocean span 100–500 kW.

Sungrow Power Supply Company from China has over 500 MW of PV inverters installed in China and several overseas countries. Its Sun Access inverters cover the range 3.3 kW (single-phase string inverters) to large four-bay central types of up to 700 kW. Hyundai produces a family of six solar inverters extending from a 3 kW single-phase transformer-less model to three-phase types up to 250 kW having low-frequency transformers. These are natural matches for Hyundai solar modules.

Future developments

For the future, micro-inverter designers will rely on further increased levels of circuit integration and digital processing to deliver products that are increasingly light, compact and smart, while being no more expensive. The distributed micro-inverter solution may well take off in Europe and elsewhere, but reliability will be crucial.

Inverters for utility-scale, grid-connected solar PV will continue to become more intelligent and comprehensive. For example, communication with other sources will enable inverters to monitor the demand and supply sides and tailor their grid inputs according to the amount of solar insolation and the utility supplier's prevailing kilowatt-hour pricing, both of which can vary hour to hour. Continued evolution of power electronics will underpin advanced grid support features, hastening network operator acceptance of large solar plants as an essential element of the future energy mix.

Whether a distributed (micro) or centralised approach is taken, advances in inverter technology will go a long way to making solar plant (and other renewables) acceptable to grid operators. Inverter and power control evolution will go hand in hand with that to the future smart grid. Inverters will stop being over-shadowed by the currently more glamorous solar module side of the business as they are key to solar PV fulfilling its true potential.

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 technology correspondent for Renewable Energy Focus magazine.

Renewable Energy Focus, May/June 2011.