converter
2.1 Introduction
Intermittency and fluctuation of renewable energy sources coupled with increased penetration pose a challenging problem for microgrid (MG) from the perspective of maintaining grid stability and delivering reliable power. Distributed energy storage (DES) is the best option to mitigate the uncertain power generation challenge posed by renewable energy while maintaining grid reliability with reduced microgrid operational cost. The excess renewable generation is stored by DES and is used when it is useful from either a technical perspective (e.g., voltage and frequency regulation) or an economic perspective (e.g., energy arbitrage). From the perspective of improving power quality and frequency regulation, energy storage systems (ESS) having high power density and fast response is required. For load levelling as well as peak shaving requirements in the microgrid, ESS (Energy Storage Systems) having high energy density and long discharge time is required. This chapter intends to explore the need and application of ESSs to deliver the electric power to costumers that meet the reliability and quality standard. Characteristics of different ESS technologies and their relative pros and cons are discussed, which is the key to select ESSs for a particular application in a microgrid. An overview of interfacing circuit for integrating ESS to microgrids such as DC-DC converter and DC-AC converter is also discussed. The development of simulation model of a popular bidirectional DC-DC converter which is used for controlling the charging and discharging of the ESS to microgrid is discussed as an example of application of ESS in microgrid. To control real and reactive power flow to the grid, modelling and control of voltage source inverter (VSI) based grid tried converter that interfaces ESS and grid is provided. Discussion on simulation results of the developed model in MATLAB-Simulink is provided to show the ability of VSI system for charging the battery or providing power for microgrid as per the requirement mandated by higher level controller of the microgrid.
2.2 Need of ESS (Energy Storage Systems)
The power management of renewable energy source based microgrid is complicated by virtue of the intermittencies and unpredictability issues of renewable distributed generators. The flexibility in a microgrid can be established with Energy Storage Systems (ESS) having the ability to operate as a load or generator, which can eventually balance the fluctuation in the microgrid and significantly improves the microgrid stability. Depending on the requirement of the microgrid, ESS is required to fulfil the objectives of maintaining grid reliability with reduced microgrid operation costs. Some of the function that may be required to be performed by ESS is listed as follows:
1. Load-levelling and Peak-shifting: The renewable systems produce energy only during the availability of the natural source regardless of the demand during peak hour. Thus, this difference in energy may lead to power overflow and losses. This energy gap can be moved by using ESS (Energy Storage Systems) to meet the peak demand and level the load curve. The other way for levelling the load curve is to instruct the consumers on energy efficiency, demand response programs, cost of the microgrid operation and energy utilization and management using ESS. This approach favours to utilities in generation expenses, line loss reduction, and volt support [1].
2. Power Quality Support: The ESS can support the microgrid or main grid for power quality improvements such as low-voltage ride through (LVRT) and voltage regulation. The capability of generators to stay operative under voltage sag conditions without getting disconnected from the grid is known as low-voltage ride-through (LVRT). The implementation of ESS can substantially improve the low-voltage ride-through of the generator. Moreover, The ESS can also be used to improve the voltage regulation because of its rapid voltage support.
3. Smart Grid: Smart grids are turning into a noticeable option for efficient energy transmission and distribution from multiple power supplies. ESS in a smart grid ensures the reliability of energy supplied to the consumers. It also includes upcoming prosumers such as smart houses and electric vehicles as major contributors of grids to avoid network over-loads by regulating and controlling the inconsistent power [2].
4. Electric Vehicles: Plugin and hybrid Electric vehicles are becoming prevalent and attracting recent research. Recent research in battery management system (BMS), drives and battery are promoting electric vehicles (EV) at a fast pace and with drop in cost of ESS. The ESS of the EV (Electric Vehicles) can also feedback the power to the grid under appropriate planning and scheduling. More than one type of batteries can also be coupled together in the form of hybrid ESS to deliver the required energy [3].
5. Smart Homes: ESS is an integral part of energy sustainable buildings and smart homes. An associated smart energy management system can efficiently manage energy storage and power utilization. The accurate data of consumption and subsequent emissions can also be provided. Smart homes efficiently use the energy with lesser emission.
6. Uninterruptible Power Supply (UPS): UPS is the significant application of ESSs in microgrids, particularly for the islanded microgrids. Renewable energy sources, such as solar photovoltaic and wind turbines, may unexpectedly stop producing power because of clouds, nightfall or lack of wind. However, turning on the backup generators may take few seconds [4]. To avoid the chances of outages in the microgrid during this period the ESS in the microgrid rides through the power shortage.
2.3 Available ESS (Energy Storage Systems) Technologies
The energy storage system has been used in power systems for a very long time. The energy storage technology also enables high intervention and incorporation of variable renewable energy sources for grid application. It has a critical role in improving the working abilities of the grid. ESS can be categorised as mechanical, electro-chemical, electrical and thermal systems based on its characteristics, as shown in Table 2.1.
Table 2.1 Classification of ESS [5].
| Mechanical | • Pumped Hydro Storage (PHS)• Compressed Air Energy Storage (CAES)• Flywheel Energy Storage Systems (FESS |
| Electro-Chemical | • Hydrogen Storage• Battery Energy Storage Systems (BESS) |
| Electrical | • Super-conducting Magnet Energy Storage (SMES)• Electric Double Layer Capacitors or Super-capacitors (SCES) |
| Thermal | • Molten Salt Storage• Adiabatic CAES |
2.3.1 Type of ESS (Energy Storage Systems)
This subsection discussed different types of ESS (Energy Storage Systems) technology available that can be utilized in the microgrid. The categorised ESS technology along with its corresponding advantages and disadvantages is enumerated [6, 7].
1. Mechanical storage: Various technologies that falls under this category are:a. Pumped Hydro Storage (PHS): Pumped Hydro Storage has been extensively implemented for a very long period and is considered as a developed technology of energy storage system for power grid applications. PHS projects store water in the reservoir/pond placed at higher altitudes in times of energy availability with aim to save the energy and then electrical energy can be produced by transforming the penitential energy to electrical energy during the release of stored water through turbines. Significant benefits of pumped hydro system are:Huge power and energy ratingProlonged lifespanHigh efficiencyLess discharge lossesThe geographical dependence, large site area and long gestation periods are the major obstructions in this type of energy
