Smart Grids and Micro-Grids. Umashankar Subramaniam

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As stated in the previous section that different types of storage technologies are available, a comparison of different technology is provided in Table 2.2. The selection of ESS (Energy Storage Systems) depends on the need and application.

2.4 Power Electronics Converter in Microgrid

      Power electronics technology is expanding progressively with the development of various power converters. These converters allow the charging and discharging procedure of ESS, the interconnection of buses, power quality enhancement, etc., [10]. Power converters, including DC-DC converters, AC-DC rectifiers, DC-AC inverters, and AC-AC converters, are discussed briefly.

       2.4.1 DC-DC Converter

      DC-DC converters is a power electronic circuit that converts the DC source’s voltage level to the desired DC output voltage level. DC-DC converters are primarily categorized into three types such as step-down (buck) converter, step-up (boost) converter, and bidirectional DC-DC converter. A step-down converter provides lower voltage output than the input source voltage, whereas a step-up converter raises the DC source voltage to a higher output voltage. The bidirectional DC-DC converter operates in both step-down and step-up modes of conversion with a bidirectional power flow. The DC-DC converter is used for the MPPT operation of photovoltaic (PV) microgrid and the association of the BESS to the DC link of the grid.

Table 2.2 Comparison of physical characteristics for various kinds of energy storage [5, 15].

ESS	Power (MW)	Energy density (Wh/kg)	Power density (W/kg)	Discharge time	Response time	Efficiency (%)	Capital cost ($/kWh)	Life-time (years)	Cycling times
PHS	100 - 5000	0.5 - 1.5	-	1 - 24 hr.	Minutes	70-80	5 - 100	> 50	> 15,000
CAES	5-300	30-60	-	1-24 hr.	1-10 min	41-75	2-50	> 25	> 50,000
FESS	0.25	5-30	400-1500	Sec. - hr.	< 1 sec.	80-90	1000 - 5000	15-20	> 15, 000
SMES	0.1-10	0.5-5	1-10	m. sec - 8 sec.	< 5 sec.	75-80	1000 - 10,000	-	> 50, 000
SC	0-0.3	0.05 - 15	5-10	msec. - lhr.	< 1 sec.	85-98	300 - 2000	4-12	> 50, 000
Pb-Acid	0-20	30-50	75-300	Sec. - hr.	< 10 sec.	75-90	200 - 400	3-15	< 2, 000
Li-Ion	0-0.1	75 - 200	150-300	Min. - hr.	< 10 sec.	65-75	600 - 2,500	5-100	< 10, 000
Ni-Cd	0-40	15-55	150-200	Sec. - hr.	< 10 sec.	60-80	800 - 1500	5-20	1500 - 3000

2.4.2 DC-AC Inverter AC-DC Rectifier

      AC-DC converter (rectifier) converts AC input power into DC output power. The rectifier provides DC power to charge the BESS. Three-phase rectifiers are widely used for the industrial, electrical transportation system and the transmission of energy applications. Moreover, a DC-AC converter (inverter) is a crucial device that connects any DC power source to an AC bus in the power system. Inverters appeared as a promising power converter with numerous medium and high-power applications such as industrial motor drives, variable-frequency drives, EV/HEV, UPS, grid-connected renewable energy sources, HVDC, flexible AC transmission system. The typical inverters and rectifiers model are similar, which allows the converter to operate as either a rectifier or an inverter.

       2.4.3 AC-AC Converter

      AC to AC converter is utilized to convert the AC to another AC with requisite frequency and magnitude. AC to AC converter is mainly classified as cycle converters, Indirect AC to AC converter (DC- Link converter), matrix Converters and hybrid matrix converter. AC to AC converter is used for speed control of drive, the interconnection of AC buses and charging and discharging of ESS.

2.5 Control of Interfaced Converters

      The role of the control and management system in a microgrid is indispensable in addition to the design of converters. The ESS in the microgrid requires a bidirectional power flow for its charging and discharging, which is possible by controlling the required converters. The control of charging and discharging process of ESS for Ac and Dc microgrid is explained in the following section.

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