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In many regions of the world, the topography of the landscape and the high cost of power line extensions prevent a connection to the electric network. In these cases, diesel generators are commonly used for a wide range of applications, particularly in remote areas as stand-alone systems. However, the rising fuel price and the associated fuel transportation costs, combined with the desire to reduce emissions of carbon dioxide, are making this option less desirable.

Powering a micro-grid with a hybrid power system (HPS) that uses photovoltaic modules, as shown in Figure 1, presents a promising alternative to diesel-only generators due to the latter’s cost and environmental concerns. A typical HPS combines PV energy and/or other renewable energy sources with a diesel generator. In addition, battery storage systems are used to mitigate the problems associated with the intermittent nature of the PV energy and increase system reliability and performance.

In order to attend to the growing demand for stand-alone power systems in remote areas, new hybrid power converters for medium- and high-power solutions are being developed. These converters are ideal for replacing a diesel generator-based system with an HPS. Multiple converters are commonly used to achieve the integration of different sources and batteries in an HPS. Such systems are classified according to their configurations.

In a DC-coupled configuration, the different sources are connected to a common DC distribution bus that is shared through independent converters. The system can supply power to the AC loads or interface with a utility grid through a bidirectional inverter. A backup diesel generator can be also connected.

In an AC-coupled system, the different energy sources are integrated through independent power electronic converters to a common AC bus. A proper power-sharing control scheme is required to achieve a desired load distribution among the different converters.

However, due to needs for higher power density, compact structures, better dynamic performance and real-time energy management of such systems, these configurations will have worse characteristics than those of an integrated power converter system.

 

Multiport converter topologies

In recent years, the progress in power electronics has facilitated the development of integrated power converters that are capable of interfacing with and controlling energy sources concurrently. Emerging multiport topologies are based on a unified converter topology with multiple inputs that are capable of interfacing with different sources, storages and loads.

Instead of using separate power electronic converters for each energy source, multiport converters have the advantages of requiring fewer components and having a lower cost, more compact size and better dynamic performance.

Many topologies have been proposed to create a multiport interface. Among them, one simple topology is to interface several converter stages to a common DC bus based on buck, boost and buck-boost structures with interleaved modulation that reduces the total current ripple.

Multiport converters are also constructed from half-bridge or full-bridge topologies with magnetic coupling via high-frequency transformers and soft switching modulation. They can meet isolation requirements and also have bidirectional capabilities. They offer high power density and more flexible output voltage level.

Other topologies with zero-voltage switching or zero-current switching can be implemented for all main switches to allow higher efficiency at higher switching frequency, which will lead to a more compact design.

In all these topologies, the use of new semiconductor devices - such as silicon carbide and gallium nitride semiconductors - is rapidly advancing. Devices made out of these materials can be operated at much higher frequencies, leading to compact multiport converters, which can enhance the overall performance of an HPS.

 

Multiport architecture

A typical architecture of a multiport converter is shown in Figure 2 and consists of the following subsystems:

• PV input ports: Each port can manage a different PV array with an advanced algorithm for maximum power point tracking (MPPT) to maximize the PV array output.
• Other renewable energy source (RES) input ports: These ports can manage other renewables - such as wind generators and fuel cells - but they can also connect more PV sources.
• Battery storage system ports: These bidirectional ports are responsible for managing the charging and discharging of the battery system. They can also support other storage technologies, such as supercapacitors and flywheels.
• Consumption ports: These ports operate as inverters, generating a pure sine wave three-phase voltage, adapting the output power to that demanded by the installation loads. If more than one port is working, a proper power sharing control scheme is required to achieve a desired load distribution among the different ports. These ports also can be connected to the distribution grid. In this case, they will manage the energy flow between the HPS and the distribution grid. When the backup generator or the distribution grid has to recharge the batteries, these modules operate as a rectifier.
• Backup diesel generator port: This port can be connected to a backup diesel generator to avoid any blackout in the HPS in case the renewable generation and storage systems cannot cover the load.

 

Control strategy and modes

Multiport converters need an intelligent power management control in order to boost energy harvesting; protect the batteries; and increase overall system functionality, reliability and efficiency. Typically, they use a centralized control structure that determines and assigns active and reactive output power references of each port while keeping its AC output voltage and frequency at the desired level.

The measurement signals of all ports are sent to a centralized controller that acts as an energy supervisor and makes decisions on control actions based on all measured signals and according to predetermined constraints and objectives. The centralized controller will prioritize and manage energy utilization among the various energy sources of the HPS by sending the control signals to the corresponding ports.

The multiport architecture shown in Figure 2 always assures energy supply and - with a simple control strategy - gives priority to the MPPT methods for PV sources. It operates in the following modes:

• Mode 1: When the energy available in PV ports is greater than the demand, the surplus energy is stored in the batteries. The batteries will preserve extra solar power for backup use.
• Mode 2: Once the battery system is fully charged, the battery port is maintained in an idle state to prevent overcharging, and the power generated in the PV ports is adjusted to meet the demand. The MPPT is disabled, and only part of the solar power is used. 

• Mode 3: When the connected load requires more energy than is being generated by PV sources, the system taps the required energy from the battery system.
• Mode 4: Once the battery’s state of charge is lower than the limit set in the control unit, the backup generator is engaged to assure the energy demand of the loads. The generator is usually brought online to assist only during periods of high loads or low renewable power availability. In this mode, the inverter output port can work as a rectifier in order to charge the batteries, or as an inverter in order to feed the loads if PV power is higher than the maximum recharge power of the batteries.

If the grid-connected configuration is employed instead of a diesel generator, there will be another scenario in which power flows from the grid to the battery. For example, it is usually useful to feed grid energy into the battery system when the utility price is low at night. When operating Mode 2 while grid-connected, the MPPT is never disabled, and the extra solar power is fed into the grid.

Multiport converters have the advantages of high power density, easy control, minimum investment requirements and reduced operating costs over schemes with multiple converters. The whole system is more compact and has lower overall mass.

Due to centralized control, no communication devices between separate converters are needed. As a result, communication delay and error can be avoided. The whole system is more reliable and has better dynamic performance.

Compared to a diesel generator-based system, the initial high costs of an HPS are offset by lower operation, maintenance and fuel costs. The HPS operator can look forward to reliable access to electricity by combining PV energy with multiport technology.

Remote areas lacking resources to keep up with diesel fuel prices and without hope of being connected to the public grid in the medium term can turn to HPS as the most suitable, environmentally friendly and cost-competitive solution for their electrification. S

 

Javier Villegas Núñez is the power electronics systems manager at GPtech in Bollullos de la Mitación, Spain. He can be contacted at 34 954181521, ext. 2120, or at javier.villegas@greenpower.es.