Hybrid Energy Storages Power Converter Topologies

A HESS can be connected to an MG through different topologies. Different topologies can be employed to combine HPS and HES [252]. A comprehensive review of HESS topologies is presented in [180]. Power converter topologies can be classified as active, semi-active, and passive [178].

In passive topology, two storages with same voltage are simply connected together, that is, efficient, simple, and cost-effective topology [88]. Since the terminal voltage of the storages is not regulated, the power distribution between HPS and the HES units is mainly determined by internal resistances and their voltage-current characteristics. As a result, the available energy from the HPS is very limited, and it acts as a low pass filter for the HESS.

In the semi-active topology, a power converter is inserted at the terminals of one of storages, while the other storage is directly connected to the DC bus [253]. Although the use of a converter requires extra installation space and increases the cost, this class of topology offers more controllability and dispatch capability. There are different semi-active HESS topologies that are reviewed in [132, 253]. The use of the extra converters in this topology makes a better working range of the HESS [132].

Active HESS topologies consist of two or more energy storages in which each energy storage unit is connected to the system by the separate power converter. The active HESS topology can be further broken down as series active topology, parallel active topology, isolated active topology, and multilevel active topology, depending on how high energy storage and high power storage are connected to the two or more converters and eventually to the microgrid. Although the complexity, losses, and cost of the system increase, this class of topology has certain advantages. The advantage of this configuration is that all powers of storages can be controlled actively. Sophisticated control schemes can be implemented in a fully active HESS which is the most commonly used configuration [254]. The parallel active hybrid topology utilizes two converters for power control of the storage 1 and the storage 2. While in the traditional parallel topologies, the energy is exchanged via the common DC bus and the two cascaded converters that negatively affects the overall efficiency of the system. To solve the mentioned problem, a reconfigurable topology is proposed in [255]. Compared to conventional active topologies, the reconfigurable topology has advantages including reduction in the DC-bus capacitor size, increased efficiency during the energy exchange modes, and the capability of reconfiguration. In some researches, multilevel converters are used as hybrid storage power converters [256]. Using a multilevel structure, the system reliability and power quality can also be enhanced. Meanwhile, connecting multiple energy storage in one converter reduces costs and coordination control complexity. However, the number of power electronics switches and capacitors in the multilevel converters are high and their control is more complicated. Isolated multi-port converters for HESS have been used in [257]. Each structure has its own advantages and disadvantages, but in recent years, the use of the active structure has been considered due to its high capabilities. For HESS connection to MG, the various class of power converter can be used [258]. To select a power converter, various parameters such as efficiency, cost, reliability, and flexibility should be considered [178].

Comparison of Different Topologies

The HESS topology directly affects energy management strategy. In the passive topology, there is no direct control of the storage power. In the semi-active topology, the output power of one of the storages is uncontrollable and the voltage of the other should be same as the DC bus. The active structure controls the output or input power of both storages by a rational control strategy, in cost of lower efficiency. Appropriate topology should be selected regarding different factors such as costs, efficiency, controllability, complexity, and flexibility. Table 4.5 compares the

TABLE 4.5

Comparison of Different Hybrid Storages Topologies [178]

HESS topologies from different operational aspects. The passive topology is simple and cost-effective but noncontrollable. In terms of controllability and flexibility and taking into account different constraints, such as SoC, the active topology exhibits the best performance, but its cost and complexity is high. The semi-active topology introduces limited controllability with lower cost [178].