The worldwide demand for photovoltaic equipment continues to expand and mature; new market entrants bring sophisticated technology developed in parallel markets to bear on the needs for greater efficiency, reliability and improved returns on investment.

Critically, inverter system availability, which is easily overlooked, has the most important effect on the overall plant yield. What are the chances of an inverter fault occurring, how long to recover from the fault and what are the consequences on plant profitability should the worst happen? Take a moment to place a value on one day of total failure and relate that to a percentage of inverter cost. Not unreasonably you could multiply this by the life expectancy of the inverter in years and typically 20% of inverter capital cost is returned. Traditionally those who consider such scenarios tend to opt for smaller inverters in larger quantities thus the loss of any single inverter is seen as less catastrophic. This is balanced by an increased likelihood of failure due to the number of inverters deployed and by increased cost per kVAp of the inverters. In reality this means that there is actually no increase in availability but a large increase in cost from the additional inverters, shelters, medium voltage switchgear, transformers MV cabling to each inverter, access roadways and additional SCADA connections.

Emerson Control Techniques’ unique ‘Master Master’ transformerless grid-tie solar inverter system, known as SPV, is constructed using parallel connected 175 kVAp inverter modules to produce any desired power rating up to 1,590 kVAp. The inverter designs are based on mature products that are well-proven in thousands of demanding industrial applications worldwide. It uses a unique control methodology that enables the inverter to automatically match its capacity to the available power from the PV array. In doing so, the SPV significantly reduces inverter switching losses which in turn enhance plant efficiency particularly under low load conditions. Inverter availability is improved through minimizing the run time of individual sections and through the adoption of a ‘Rotary Master’ where each inverter module is exercised equally as the turn-on sequence rotates daily.

The ‘Master Master’ approach has relatively few points of common failure, allowing the plant designer to select a smaller number of higher power inverters to benefit from best cost per kVAp, with corresponding reductions in balance of system equipment. Take for example the SPV2700 (nominally 1,590kVAp) which internally has 9 parallel connected inverter modules. Should an inverter module fail in the SPV2700, it is automatically isolated from both the AC and DC power sources and the system continues to operate with peak capacity reduced by 11% or 175 kVAp. Depending on the prevailing weather conditions and the installed ratio of DC kW to AC kVA the loss of a small amount of inverter capacity may not impact at all on revenue generation.

“It’s easy to see why reliability and quality count in this market,” states Andy Green, Emerson Control Techniques Industry Development Manager for PV Inverters. “In all key markets in the U.S.A. and Europe, Control Techniques’ utility-scale PV inverter systems are aggressively establishing themselves as the new benchmark with Euro and CEC weighted efficiencies exceeding 97.6% (98% peak) as well as having inherently reliable inverters that are fault tolerant.

The SPV system has a very flat load/efficiency curve and is able to maximize energy yield in low to medium light conditions. Inverters remain near to peak efficiency down to 5% of load whereas many inverters start to see efficiency roll off at 20% of load. Inverter turn-on and turn-off is only 900 W regardless of kVA rating, a figure that is guaranteed through the use of power measurement prior to start up rather than purely assess voltage which is dependent upon temperature.

Some 400 MWp of SPV inverters are now operating worldwide and this is expected to increase by over 200% in the next 12 months.

The PV market is still young and developing. In this situation, the credibility and track record of suppliers is crucial. Will they support their products? Will they still be around in 10 or 20 years? Investors need to have confidence in the long-term performance, reliability and support of the chosen inverter system, the most critical part of any PV system. The SPV system is backed by the US$21 billion Emerson group with its power generation experience, unsurpassed R&D and international manufacturing resources and supported by a comprehensive network of engineering centers in 31 countries, providing ‘local’ advice and back-up.

Regarding R&D, the pursuit of technology leadership is one of four business imperatives at Emerson that serve as the foundation for steady growth and drive the organization. The success of this focus is clearly indicated by the fact that overall in Emerson new products represent 37% of its total sales.

Further information: Emerson Control Techniques (http://www.emersonct.com/)