The poor ratio between DC link and AC output voltage is one of the biggest disadvantages most utility-scale inverter systems entail. PV plants are based on a maximum DC input voltage of 1000V, directly connected to the DC input terminals of the inverter. To maintain an operating voltage window which is sufficient for MPPT, in the past this prevented the usage of a direct transformer-less inversion into line voltages above 320 to 360 Vac. At that point, Power-One came in and developed a novel circuit topology conceived and designed to specifically address the demanding needs of utility-scale applications. Power-One’s new AURORA ULTRA central inverters with an output power of up to 1.4 MW are the first commercially available products using the company’s proprietary inverter concept.

The aim of the patent-pending technology was to remove the intrinsic limitations of direct conversion architectures, thus allowing a voltage conversion at industrial standard 690 Vac. At the same time, the conversion efficiency should be maintained or further improved over the widest possible input range. Additional design targets included a modular construction for all main building blocks to ensure easy maintenance and spare parts standardization and a compact outdoor construction, which help to meet the requirements of large solar parks even better.

The New Topology

The innovative multi-level circuit topology for this three-phase PV inverter has been identified and optimized after accurate simulations. It includes an integrated DC/DC system as well as a multi-level inverter and offers the best efficiency performance of up to 98.7% over an input voltage range of 500-1000 Vdc when inversion is made onto an output voltage nominally set at 690 Vac─a remarkably higher voltage with respect to conventional architectures. The DC input voltage can even fall below this limit without significantly affecting the efficiency, while standard single stage direct conversion systems may suffer from production losses. Moreover, the efficiency increases towards the middle of the input operating voltage range. System design can be optimized for maximum energy harvesting at the maximum possible string length and minimum DC current, so that DC cable and distribution costs have been reduced as well.

An important property of this topology is the zero common mode noise. Unlike conventional systems, the AURORA ULTRA does not generate high frequency switching voltage components between output terminals and ground. This is achieved by the output capacitive filter network connected to the mid-point of the DC link and prevents high frequency currents circulation in the ground loop. In ungrounded PV array applications, this common mode noise-free operation prevents that large leakage currents are developed through the PV array parasitic capacitance in the ground loop, thereby allowing the connection of multiple inverters in parallel at the common AC output. This reduces the system installation costs and complexity without producing detrimental effects on the inverter and PV array.

As a consequence of the new inverter concept, accurate evaluation of AC voltage level is no longer needed. Energy losses by suboptimal array configurations can also be minimized as may occur during peak hours and hot climates or as a cause of the module ageing effects. Furthermore, the extended DC input range extends the duty cycle of the PV system and allows maximum energy harvesting under low irradiance conditions in the morning or at sunset, as well as during partial shading. Another advantage of the new topology is the optimized utilization of standard IGBT modules. External clamping or freewheeling diodes are no longer needed, as the body diodes integrated in the IGBT module are used for freewheeling and clamping purposes thus eliminating the need to add expensive external fast recovery diodes.

Preventing Module Degradation

Depending on the module type used for an installation, irreversible performance degradation effects have been observed especially on amorphous silicon panels (a-Si) caused by a negative potential relative to the ground. The common method to prevent such long-term degradation effects is to refer the PV generator to the ground potential in order to eliminate large DC voltage offsets. While the so-called TCO (Transparent Conductive Oxide) corrosion effect has been extensively analyzed and shall be prevented by negative grounding, a more recent study shows that wafer-based crystalline silicon technology is also not completely immune to Potential Induced Degradation effects (PID)1). In addition, other module technologies display reversible phenomena induced by surface polarization effects, which require a positive-grounded array in order to prevent the performance degradation effect. To allow either positive or negative grounding systems usually requires an inverter with galvanic isolation, adding cost and weight to the equipment.

This does not cause particular problems in single-inverter installations. However, in a larger plant a common grounding strategy is usually employed so that all PV array sub-fields have the same grounded pole. In this configuration, a separate galvanic isolation is required in order to prevent recirculation currents among different inverters. This does not only severely affect the system flexibility, availability and costs, but also makes the use of multiple and independent transformerless inverters connected to the MV grid via a common simple winding transformer impossible. To ensure the necessary insulation, the system requires dedicated transformers for each inverter or a multiple winding special transformer─adding further costs, complexity and losses to the overall system. In addition, the energy losses due to downtimes induced by ground faults increase as large PV arrays are usually bundled together in one inverter.

While PV module manufacturers, development institutions and scientists are working to identify and remove these effects, power electronics can offer cost effective alternative solutions to prevent these detrimental effects. Power-One’s new ULTRA central inverters using the new circuit topology offer a patented solution for this problem by adding a dedicated control scheme at the grounded terminal without affecting the costs or the system safety. Thus positive or negative grounded systems reach the same performance as traditional ungrounded crystalline-based PV power plants. Furthermore, PV generators can be designed according to the desired schemes with independent PV arrays connected to each inverter to form separate MPPTs. Different operating voltages may occur due to different string sizing, different irradiance or thermal conditions can also be controlled. This eliminates the need to combine the arrays of all inverters connected to the same transformer winding in parallel to form a single MPPT. In addition, the control scheme ensures that a failure in one or more grounding branches does not affect the whole plant leakage regulation: hot plugging and redundancy are therefore possible. An arbitrary leakage current limit can be set to match the specific needs of the plant and prevents any detrimental effect on the PV installation.

Modular Liquid Cooling

Even the most efficient and high performance inverter design may fail completely if it does not meet the construction characteristics expected in this professional market segment. An environmental-proof construction using an IP65 enclosure which allows mounting in a free-field installation without additional infrastructures is just one of those requirements. An elaborate cooling concept is even more important.

To respond to these requirements, Power-One decided on using a passive liquid cooling construction. Offering much higher heat-extraction capabilities with respect to forced air-cooled heat sinks, the liquid-cooled cold plates are ideally suited for large inverter systems. The closed-circuit system cools down the power stage section with a pressured mixture of water and glycol which is pumped into the pre-formed cooling coil by the air/water exchanger. To ensure a proper pressure gradient the chillers are mounted on the rooftop of the inverter system. An additional secondary air/water exchanger is used to cool the rest of the magnetic and electromechanical components that cannot be effectively attached to the cold plate. This method allows to segregate the active parts of the unit in a watertight and pollution-free chamber, further improving the system long-term immunity against aggressive agents in the harshest environments like deserts or in high humidity and seafront areas. This also extends the maintenance cycle and related costs.

To ensure the optimum trade-off between weight, dimensions and the lowest specific costs at maximum efficiencies, 350 kW single cold-plate IGBT power modules were selected. The overall weight of the whole power converter assembly has been limited to a maximum of 50 kg to ensure easy maintenance and installation by two operators. Quick coupling interconnections of the power conversion module assembly with the upstream and downstream electrical currents as well as to the primary hydraulic circuit ensure a maintenance-friendly construction. Non-spill fluid connectors enable a safe connection of the hydraulic circuit to the cold plate, preventing leaks to the adjacent high voltage DC and AC connections during insertion or disassembly.

To reduce the deployment and installation costs even further the inverter system has been assembled in a compact lightweight single cabinet. Thanks to its modular structure which is made of front accessible and extractible subassemblies, installation and maintenance procedures are rather easy. The inverter system can also be monitored via Ethernet communication and two independent RS-485 communication interfaces for inverter and intelligent string combiner monitoring. The new AURORA ULTRA central inverters are compliant to BDEW (German Federal Association for Energy and Water) and FERC 661 (Federal Energy Regulatory Commission).

Outlook

To develop savings opportunities well beyond the mere component-level cost reduction is and will remain the mission for inverter technology manufacturers in the future. In addition, the improvement of diagnostic and fault-detection capabilities as well as the compatibility with energy storage requirements will also become more and more important. And in achieving these goals, the use of power electronics will continue to play a significant role.

This new inverter concept for utility-grade applications shows that many inverter systems still offer a huge savings potential. Besides optimizing the system-level BOS, lifecycle costs can be significantly reduced at the same time, for example by simplifying the installation, operation and maintenance procedures.

REFERENCES

1) J. Berghold, O. Frank, H. Hoehne, S. Pingel, B. Richardson, M. Minkler: Potential Induced Degradation of Solar Cells and Panels; September 2010.