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A conventional utility-scale PV plant contains a large number of PV modules connected in series and parallel to form strings and sub-arrays, which are combined to feed the inverters. The inverters are then connected to the medium-voltage (MV) electric grid through a low-frequency (LF) transformer.
Transformerless and high-frequency transformer topologies are limited to converters with power of up to a few kilowatts. Hence, an LF transformer in higher power three-phase systems provides a galvanic isolation and perform the connection to the MV grid.
PV architectures for large PV plants can be further classified into two types. The first is centralized architecture. The central inverter performs a unique maximum power-point tracking (MPPT) algorithm for all the strings and interfaces to the MV grid.
The second option is multi-string architecture: The strings are connected to DC/DC converters that perform maximum power-point tracking and convert the PV string output voltage into a controlled bus. The DC bus feeds a central inverter that interfaces to the MV grid.
Two-stage converters allow the PV panels to operate over a wider voltage range than is possible with a centralized architecture and reduce losses due to panel mismatch and partial shading. On the other hand, the DC/DC converter increases the costs and decreases the conversion efficiency.
Although both architectures described above use central inverters, utility-scale centralized architectures currently represent the state-of-the-art for megawatt-scale PV plants due to their low cost-per-watt, easy maintenance and high conversion efficiency.
Currently, single-stage PV inverters are available with ratings ranging from a few kilowatts up to 1 MW. For high-power (more than 100 kW) PV systems, most commercial inverters use a two-level, single-stage, three-phase full bridge structure.
This topology consists of six switches - typically insulated-gate bipolar transistor (IGBTs) - with anti-parallel diodes. The switching frequency for utility-scale PV inverters is within a range of a few kHz and usually uses space-vector modulation instead of sinusoidal pulse-width modulation (PWM) to fully utilize DC link voltage.
New inverter requirements
Next-generation utility-scale PV inverters must evolve to meet the technological advances of PV cells, semiconductor parts, magnetic components and smart grid integration. The products are expected to meet the following requirements, based on future trends:
- Higher power ratings. The trend in the industry is toward higher power ratings for inverters, because the inverter cost per watt decreases as inverter power increases. Therefore, inverters with power ratings up to a few megawatts may be offered to the commercial market.
- Higher voltage ratings. Another trend, for both the DC and AC side of the inverter, is higher system voltages, in order to reduce wire costs and power losses. In practical terms, customers will wind up using smaller cross-section cables, fewer generator connection boxes and less cabling at the DC end. This setup enables a further reduction in balance-of-system costs.
On the DC side, currently, most large PV systems have been operating with a DC voltage limit of 1,000 V. In order to increase cost savings and efficiency, and accommodate technological advances in PV cells that allow for operation at 1,500 V, components rated at 1,500 V DC are beginning to be developed.
Therefore, topologies for medium-voltage grid integration of megawatt-scale PV inverters are moving toward multilevel structures.
New system architectures with medium-voltage DC busses up to 1,500 V have become good candidates for these PV commercial applications. Therefore, central inverters with a DC input voltage of up to 1,500 V DC will be required.
Within electrical standards where the listed voltage limit is still 1,000 V, the limiting factor will be how long it takes for these standards to be updated - and for stakeholders to define the testing process to answer questions on safety and reliability.
However, with PV arrays in a bipolar configuration with midpoint grounded, it would be possible to double overall DC voltage without violating isolation voltage limits. In this case, the PV inverter could work at higher DC and AC voltages to reduce costs and improve its efficiency, but it would not reduce the number of cables in the DC side.
On the AC side, the output voltage of the inverter depends on DC voltage range. One useful reason to increase PV array voltage is to allow higher AC voltage, reducing costs and boosting efficiency.
- Improved power conversion. In general, approximately 99% power conversion efficiency is required for the next generation of large-scale PV inverters. A higher voltage rating will allow the inverter’s power to increase while maintaining the same current and will also help to boost the efficiency.
- Better reliability, modularity and scalability. PV inverters must improve their reliability in order to match the lifetime of PV modules. Field test data have demonstrated a PV panel lifetime of more than 20 years.
Statistically, central inverters are more reliable than distributed architecture because of their fewer components and easy maintenance. However, distributed structure provides easy scalability and redundancy, and reduces the potential effects on the system in case of individual failure.
For next-generation PV inverters, multiple units will be connected in parallel in order to reach a higher power rating with modularity and scalability. Modular and redundant topologies will be adopted in the design for reliability of the inverter.
- Smart grid functionality. PV inverter controls must be able to enhance grid reliability and power quality, and support grid voltage and frequency stability. With the emergence of smart grid technologies and the increase in PV penetration, future PV inverters for large solar plants will need to incorporate several grid-control functionalities.
First, they must meet low-voltage and high-voltage ride-through and frequency ride-through requirements. Power-level limiting functions will allow the grid operator to control solar power output.
Additionally, AC voltage control at the point of interconnection must be performed by reactive power injection or consumption. Reactive power provision can be provided by PV inverters that have an extended VA rating.
PV inverters can also provide real power and primary frequency response to frequency deviations from scheduled frequency, as well as be able to control the rate of increase of power in any case, and the rate of decrease of power when a power level limit is defined.
Additional benefits
The aforementioned trends and inverter requirements will drive the research and development of next-generation PV inverters. Traditional two-level inverter topologies will not be able to fulfill the power and voltage rating requirements and efficiency requirements.
Therefore, topologies for medium-voltage grid integration of megawatt-scale PV inverters are moving toward multilevel structures. These inverters can often provide higher voltages and power ratings. Multilevel topologies allow for the usage of IGBTs and diodes with breakdown voltages that are lower than the actual DC-link voltage, improving switch-performance characteristics.
For example, a DC-link voltage of 1,500 V can be handled with 1,200 V IGBTs in a three-level topology. However, 3,300 V IGBTs are necessary in a two-level topology. 1,700 V IGBTs are not enough to manage this DC-link voltage because of voltage overshoot when the current is rapidly switched.
As 1,700 V and 3,300 V components are slower than 1,200 V components, in a three-level converter, the dynamic losses are significantly reduced, and the efficiency can be increased. Although the number of switches in the active current path in a multilevel topology is increased - causing higher conduction power losses - the reduced losses of lower blocking voltage devices compensate for the additional conduction power losses.
Multilevel inverters can also lower total harmonic distortion and offer lower common-mode voltages. Voltage and current harmonics are reduced due to the increase in the number of levels’ voltage to modulate compared with two-level topology.
Multilevel inverters offer improved output waveforms, as well as lower electromagnetic radiation and switching stress. This will reduce the effort for filtering and isolation in the filter inductor.
Finally, multilevel inverters can operate with a lower switching frequency than other types, due to an inherent increase in the output voltage waveform quality. The superior output voltage waveforms of the multilevel topologies enable operation at a lower switching frequency, thus improving the efficiency of the system, and provide better power quality, complying with the more demanding grid codes found at higher power levels.
Multilevel inverters are not without their drawbacks, including higher complexity and more components to be handled, which makes the circuit more sensitive to parasitic effects. In order to avoid such disadvantages, careful power module design is critical. S
Product: Utility/Large-Scale Inverters
Multilevel Topologies: Can New Inverters Improve Solar Farm Output?
By Javier Villegas Núñez
With 1,000 VDC utility-scale solar projects more common, new inverter technologies could offer increased advantages.