Abstract

The advances in renewable power generation technologies and modernization of power systems allow the growing proliferation of renewable energy resources (RERs) in the form of microgrid (MG). The integration of microgrids in the distribution system changes the passive distribution system into an active distribution system. The uncertainty in power generation from RERs is causing the major operational challenge to meet supply-load demand balance. Therefore, the electrical energy storage system (ESS) has become an important component of the microgrid system. The energy storage operating time limits have a great impact on the operating cost as well as on the life cycle of the storage. In this research work, the dual energy storage system (DESS) including battery storage (BS) and pump hydro storage (PHS) has been investigated to understand the impact of the minimum operating time limit on the optimal operating cost. Three operating cases have been studied to obtain the optimal operating cost of microgrid system operating with single energy storage and with DESS for different operating time constraints. The optimal operating time increases the effective life cycle of the storage and optimizes the operating cost very effectively. General algebraic mathematical systems (GAMS) modeling for mixed integer nonlinear optimization problem has been developed and simulated using the discrete and continuous optimizer (DICOPT) solver. The results show that the optimal minimum operating time constraint has a great impact on the optimal operating cost of the system.

Keywords: Microgrid, renewable energy resources, pump hydro storage, battery storage, dual-energy storage

7.1 Introduction

The global energy demand has been increased drastically in the last few decades due to modernization and technological advancements in the existing power systems to improve reliability and economic growth. The major share of the power comes from fossil fuels to fulfill the power requirement of the system among all available resources either renewable or conventional. The proper and planned minimum utilization of these fossil fuels leads to a reduction in environmental and global warming concerns [1]. At the same time, dependency on fossil fuels has been reduced by integrating the green renewable energy resources (RERs) and small-scale distributed generations (DGs) like solar photovoltaic (PV), wind turbine (WD), small hydro generators, diesel generators, biogas, geothermal, fuel cell, etc. The variability in power demand–generation balance caused by intermittent resources can effectively countered by the energy storage system (ESS) as an essential part of modern power system [2]. Otherwise, the intermittent nature of the renewable resources brings the system power balance into question and leads to unreliable operation [3]. Increase penetration of these intermittent renewable resources leads to frequent undesired power fluctuations which need storage systems that are capable of managing the system power balance. The battery storage (BS) and pump hydro storage (PHS) can complement each other for different power fluctuations in the system. The dual energy storage system (DESS) is better than the individual storage system as it possesses the advantages of both storage systems. The system stability and realibity under question due to frequent variations in generation and demand can be managed by controlling both demand and generation.

The deregulation and technological innovations in modern power system allow the participation of different scaled distributed generators working in a group known as a microgrid (MG) with capabilities to operate either in islanding (IM) or grid-connecting mode (GCM) [4, 5]. The microgrid operating with the proper storage provision can manage the variability of the available intermittent resources very effectively [6]. The microgrid includes small-scale schedulable and non-schedulable power generators, electrical loads (fixed and adjustable or a combination of both), energy storage systems (single or multiple), and a controller. In GCM, the microgrid manages the variability through power exchanges with storage (if available) and the utility grid. Energy storage of the MG can effectively counter the variability of RERs by scheduling its charging or discharging modes whenever power stress occurs.

Pump hydro and battery storage systems are highly utilized ESSs among many available in modern power systems to manage the system variability and to obtain the optimal operating cost. Due to inherent properties the battery and pump hydro based ESSs are most suitable for respectively small power and large power variations [7]. Therefore, storage provides an effective operation of MG system having large integration of the intermittent resources. The battery storage system requires additional power electronics devices for power conversion as compared to the pump hydro storage system to make it compatible with the utility grid. Therefore, the operating cost of the battery storage is higher than the pump hydro storage [8]. It is crucial to select an effective and suitable storage system for the microgrid operation as both storage systems have many advantages and disadvantages along with different operating costs. The performance of the MG system during power stress periods can be improved effectively by using a combined battery and pump hydro based energy storage systems in the form of DESS [9]. The DESS manages the small frequent and large slow power variations effectively through the battery and pump hydro based storage units, respectively. Therefore, a joint operation of the pump hydro and battery storage in a microgrid system can manage the power fluctuations either due to power generation or due to load demand effectively by complementing each other [10].

In past, significant studies have been carried out to address the impact of increasing integration of highly intermittent RERs in the existing power system along with controlling system variability [11, 12]. The large optimization problems of the power system have been solved with reliable solutions by implementing deterministic methods. A few deterministic optimizing scheduling of microgrid have been conducted and analyzed for grid-connected and islanding modes of operation in electricity markets [13–16]. The intermittency of the power generation from various RERs adversely affects the system reliability, power quality, and optimal operation in large. This sometimes leads to a locally optimal solution value rather than the global optimal solution for the microgrid operation. In [17–20] the microgrid issues have been studied by implementing stochastic and heuristic approaches. The latest studies addressing the energy management of microgrid systems in literature have been tabulated in Table 7.1 based on sources, type of storage, approach used, uncertainty in generation and application, etc., for optimal solutions.

lightning attachment procedure optimization (LAPO) and artificial bee colony (ABC), teaching-learning-based optimization (TLBO).

In literature, microgrid either in grid-connected mode [1, 5, 24, 26, 34] or islanded mode [25, 28, 30] of operation have been investigated considering different RERs and storage systems. The main objective of the present research is to optimize the overall operational cost of the microgrid system. It has been found through reviewing the existing literature that appreciable work has been done in considering energy management for improving system efficiency, reliability, and effective participation in electricity markets. The cost-effectiveness of individual storage systems has been investigated for managing the system power variability caused by load demand and intermittent resources. However, the impact of the minimum on and off time of the DESS has not been investigated for the GCMG system.

Present study, focuses on the use of dual energy storage system to manage the system variability associated with the intermittent solar and wind plants and compare it with results of MG operating with either battery storage or pump hydro storage as single storage. Three cases have been simulated as mixed integer nonlinear programming (MINLP) problem incorporating different technical and operational constraints on the system components. The MG system has a limited power exchangeable capacity with the grid depending mainly on the transmission line capacity. The impact of minimum on and off time limits of the individual and dual storage system on the system’s operational cost. Due to the nature of the technical and operational constraints of the system, this optimization problem becomes a non-linear problem.

The promising features of the present proposed work are summarized as follows:

1. Optimal operating cost solution has been proposed for GCMG system incorporating the battery storage and pump hydro storage as a dual energy storage.
2. Mathematical equation modelling of shiftable and controllable loads considering minimum time of operation, and total energy consumed to manage the supply-demand effectively.
3. The impact of minimum on and off time of the storage system of single and dual ESS on the overall operational cost has also been evaluated for the MG system.

The simulated results obtained and presented in the present research work evaluate and analyze the impact of the minimum on and off-time scheduling on the operational cost of the MG system for managing the system power supply-demand balance.

The rest of the chapter is arranged into four sections. Section 7.2 elaborates the mathematical equations used for the modeling of diesel generator, PV, WD, BS, PHS, and adjustable loads based on operational and technical constraints of the MG system operating with limited power exchange capabilities. The problem formulation for analyzing the impact of the minimum operating time constraint of the storage system on the optimal cost of operation under investigation has been explained in Section 7.3. Section 7.4 explores the system data, simulation results, and explainations of the MG system under study and section 7.5 finally concludes the study.

7.2 System Mathematical Modelling

The grid-connected microgrid system under this study consists of diesel generators, wind turbine power plants, solar plant, adjustable loads, fixed load, and the DESS (battery and the pump hydro). This section expresses the various components and the processes adopted in mathematical equation form operation of grid-connected MG system in an electricity market operating with a DESS including battery storage and pump hydro storage for optimal operation. The ESS not only manages the variability of the intermittent resources but also gives confidence to the market player to participate in the electricity market. The storage cannot be operative all the time as they need the minimum charging time to be able for discharging. Therefore, the minimum charging and discharging time requirement of the energy storage system becomes an important constraint to be explored. To demonstrate the impact of minimum charging/ motoring and discharging/generating modes of the DESS on the optimal operation of the GCMG system has been developed and analyzed using a MINLP subject to various technical and operational constraints. The power demand and generation are both dynamic in nature and therefore power exchange between the grid and the microgrid has been considered through a constraint. The system operator manages the power exchange limit and plans the scheduling of resources one day-ahead of actual scheduling knowing the predicted patterns of generation, load demand, and energy prices. The DESS along with the power exchange option has been applied to minimize the cost of GCMG operation. Various components of the MG system have been described in mathematical equations as below.

7.2.1 Modelling of Wind Turbine Power Generator

The wind carries energy and can be converted into a suitable form of energy using wind turbines. A wind turbine converts the mechanical energy associated with the wind into electrical energy. The total power generated from the wind turbine at t^th the time interval can be mathematically represented as (7.1).

(7.1)

Where, P_{gen wind(t)} is the power generated by the wind turbine power generator at t^th time interval, ρ_wind(t) is the air density of the wind at t^th time interval, V_wind(t) is the velocity of the wind at t^th time interval, A_rotor is the swept area of the wind turbine blades, η_wind is the efficiency of the wind turbine and C_wind is power coefficient of the wind turbine [35, 36].

7.2.2 Modelling of Solar Power Plant

The generated power of the PV array is highly intermittent and varies with the environmental conditions. The power generation at t^th the time interval can be mathematically expressed as (7.2).

(7.2)

Where, P_{gen solar(t)} is the power generated by the solar plant at t^th time interval, A_Csolar is the area of the solar array and I_Rsolar(t) is the solar irradiation at t^th time interval η_solar is the efficiency of the solar unit/plant [35, 36].

7.2.3 Modelling of Conventional Diesel Power Generator

The diesel generator in a microgrid system produces power during the time intervals when the other distributed generators are not able to meet the system demand due to intermittency in the renewable power generation from renewable resources like solar and wind along with the load demand. The fuel cost of the generator is governed by the fuel cost coefficients associated with the respective generator and can be expressed as a quadratic function of the power generated (7.3).

(7.3)

Where, P_gen(i,t) is the power generated from the i^th diesel generator at t^th time interval, and a_i, b_i, and c_i, are the fuel coefficients of the i^th diesel generator, respectively.

7.2.4 Modelling of Combined Heat and Power (CHP) and Boiler Plant

The mathematical modelling of the cost of power generation from the combined heat and power plant has been considered similar to the diesel generator and can be expressed as the quadratic function of the power generated and the fuel coefficients as in (7.4). The boiler plant is modelled as a linear function of the power generated as (7.5)

(7.4)

(7.5)

Where, C_chp(j,t) and C_br(k,t) are the fuel cost of the combined heat and power plant and the boiler plant respectively for the j^th CHP and the k^th boiler at t^th time interval. P_chp(j,t) and P_br(k,t) are the power generation from the CHP plant and boiler plant, respectively.

7.2.5 Modelling of Dual Energy Storage System

ESS become an integral part of the microgrid system due to the presence of solar and wind renewable intermittent power resources. The principal function of the energy storage system is to manage the power balance by operating in charging or discharging modes whenever put under stress either due to fluctuation in generation or demand or a combination of generation and demand. The battery storage system can manage the frequent small power variations of the system, whereas the pump hydro storage plant is suitable for managing slow variations of large magnitude. The dual energy storage system has the advantages of both the storage systems and the power variability of any nature can easily be managed. Both the energy storage systems have a similar operation.

7.2.5.1 Battery Bank Storage System

The battery bank storage system consists of the number of batteries. The state of charge of the battery bank storage system at t^th the time interval will be evaluated based on the energy stored or energy delivered at end of the charging or discharging intervals and expressed by SOC as in (7.6). The operating cost of the battery bank storage system is assumed to be fixed by the charging and discharging power and expressed as (7.7).

(7.6)

(7.7)

Where, SOC_bs(s,t) is the state of charge of the s^th battery bank at t^th time interval, η_{bs dch} and η_{bs ch} are the efficiency of battery bank during discharging and charging mode of operation respectively, m_bs(s,t) and n_bs(s,t) are the binary variable indicating discharging and charging modes, respectively. Δt is the minimum time interval of charging and discharging mode.

7.2.5.2 Pump Hydro Storage System

The pump hydro storage plant consists of a reservoir to store the water from other resources like rivers, lakes, and other man-made or natural reservoirs, etc. The state of the reservoir at t^th time interval will be evaluated based on the energy stored or delivered at end of the pumping or generating intervals and expressed by SOC_ph as in (7.8). The operating cost of the pump hydro storage system is assumed to be fixed by the motoring and generating power and expressed as (7.9).

(7.8)

(7.9)

Where, SOC_ph(p,t) is the state of the reservoir of the p^th pump hydro plant at t^th time interval, η_{ph gen} and η_{ph mot} are the efficiency of pump hydro storage during generating and motoring mode of operation respectively, m_ph(p,t) and n_ph(p,t) are the binary variable indicating generating andmotoring modes, respectively. Δt is the minimum time interval of charging and discharging mode.

7.2.6 Modelling of Power Transfer Capability

The presence of unpredictable renewable energy resources can effectively countered with power exchange option for the grid connected microgrid system to manage its power balance when it operates without energy storage system or with storage having limited capacity. It also helps in optimizing the operating cost of the system by trading the excessive power with the grid. The selling and purchasing power costs can be the same or different depending upon the service requirements. In this research, the selling power cost is 20 percent higher than the purchasing power cost considered a penalty factor to support the grid. The net power exchange cost is modeled as (7.10) [2, 37].

(7.10)

7.3 Objective Function and Problem Formulations

Objective function (OF)

The coordination of all the resources, loads and dual-energy storage systems for the optimal operational cost of the system is the main objective of the problem subject to technical and operational constraints. The objective function is represented by (7.11) [2, 37, 38].

(7.11)

Constraints associated with power balance (7.12), power exchange between the grid and the microgrid (7.13), dispatchable DGs (7.14)-(7.18), battery storage (7.19)-(7.24), pump hydro storage (7.25)-(7.30), adjustable loads (7.31)-(7.33).

7.3.1 Operational and Technical Constraints

A practical system must fulfill certain operational constraints for a realiable and secure operation. All parameters must remain within the limitations set by the system operator in the form of constraints [2, 9, 27, 37–39].

(7.12)

(7.13)

(7.14)

(7.15)

(7.16)

(7.17)

(7.18)

(7.19)

(7.20)

(7.21)

(7.22)

(7.23)

(7.24)

(7.25)

(7.26)

(7.27)

(7.28)

(7.29)

(7.30)

(7.31)

(7.32)

(7.33)

The objective function considered in (7.11) minimizes the daily operational cost of microgrid, which includes the cost of power generation from the diesel generators, cost of power exchange, cost of charging-discharging power of battery storage, cost of motoring and generating of the pump hydro storage, cost of power generation from combined heat and power plant and the cost of power generation from the boiler unit. The power balance equation (7.12) of the system always ensures that the total sum of the power generation from the system resources, storage, and power exchange must match the system power demand. Depending upon the system requirements the power exchange between the microgrid and the grid can be positive (power purchased), negative (power sold), or zero. The power exchange is limited by the transmission line capacity connecting the microgrid with the grid (7.13). The constraint in (7.14) defines the power generation limits (maximum and minimum) of the dispatchable units at any time interval, and technical constraints (ramp rate up/down and minimum operation time on/off) are defined in (7.15)-(7.16) and (7.17)-(7.18), respectively, where I_g is the binary variable representing the commitment state of the dispatchable unit (I_g =1 when unit is committed and I_g = 0 when unit is not committed). In (7.19) and (7.20) the charging and discharging power limits are defined on the battery storage unit based on the mode of operation. m_bs and n_bs are the binary variables indicating the mode of operation (m_bs = 1 means charging mode 0 otherwise and if n_bs = 1 means discharging mode, 0 otherwise). (7.21) ensures the condition that charging and discharging of the battery storage will not occur simultaneously. The storage level of the battery bank is defined by (7.6) and (7.22) ensures that the storage level is within the minimum and the maximum capacity of the storage bank. The operating time limits on the charging and discharging modes are defined by (7.23) and (7.24) respectively. Similarly, (7.25) and (7.26) define the motoring and generating mode power limits on the pump hydro storage unit. The m_ph and n_ph are the binary variables indicating the mode of operation of the pump hydro unit (m_ph = 1 means motoring mode, 0 otherwise, and if n_ph = 1 means generating mode, 0 otherwise). (7.27) ensures the condition that motoring and generating modes of the pump hydro unit will not occur simultaneously. The storage level of the pump hydro storage is defined by (7.8) and (7.28) ensures that the storage level is within the minimum and the maximum capacity of the pump hydro storage. The operating time limits on the motoring and generating modes are defined by (7.29) and (7.30) respectively. In addition to these constraints (7.31) represents the power limits (minimum and maximum) on the adjustable loads of the system, the operating time limit is defined in (7.32), and (7.33) describes the energy consumption limits on the loads over an operating cycle of 24 hours.

7.4 Simulation Results and Discussion

The system under investigation has three dispatchable diesel generators, one combined heat, and power plant, one boiler unit, two renewable plants solar and wind one each, one dual energy storage system comprising one battery and pump hydro, five adjustable loads, one fixed load and with limited power exchange capability. Table 7.2 and Table 7.3 represent the cost coefficients and other characteristics of the schedulable, and non-schedulable storage units, respectively. Table 7.4 shows the characteristics of adjustable loads of the microgrid system. Figure 7.1 and Figure 7.2 show the dynamic electrical and thermal load demands of the system and the energy price in the electricity market, respectively. Figure 7.3 shows the intermittent renewable power generation from solar and wind power plants along with the net aggregated renewable power generation of the microgrid. The three different operating cases have been modeled, simulated, and analyzed to obtain the impact of the minimum operating time on the net operating cost of the grid-connected microgrid system under study as given below:

• Case 1. Microgrid operating only with the battery storage system.
• Case 2. Microgrid operating only with the pump hydro storage system.
• Case 3. Microgrid operating with dual-energy storage (battery and pump hydro storage) system.

The 24-hour optimal scheduling has been obtained for a grid-connected microgrid system for three operating cases. The intermittency of the renewable solar and wind turbine has been considered in optimization modeling using the variations in power generation over the scheduling horizon. The maximum power exchange possible between the grid and the microgrid has been assumed to be 15 MW. The balance between the power supply and demand in the present system has been managed through limited power exchange possible between the grid and the microgrid along with controlling the minimum operating time of the storage system either battery, pump hydro storage or dual energy storage. The minimum operating time for charging/motoring and discharging/generating mode of respective storage system has been considered to be 2 Hrs and 3 Hrs, respectively. The impact of the minimum operating time on the economical operation of microgrid has been obtained and analyzed.

Figure 7.4 shows the variation in power exchange takes place between the grid and microgrid while operating in three operating case scenarios. It has been found that in case 3 microgrid operating with dual storage system requires less amount of power from the grid as compared to the microgrid system operating with one storage either battery (case 1) and pump hydro (case 2).

Microgrid systems can operate either with battery and pump hydro storage or with a dual storage system to manage the system variability. The operating conditions for pump hydro and the battery storage systems have been considered the same for the comparison purpose. The dual storage system counters the variability more effectively as compared to the individual storage system. The charging/motoring and discharging/generating schedules of both storage systems are depending upon the system power requirements and the state of charge at a particular instant of operation. In this study, the minimum operating time for charging/motoring and discharging/generating modes of battery/pump hydro storage system respectively have been assumed to be 2 hours and 3 hours. The state of charge has been considered to be same for the initial and final interval of scheduling. It is assumed to be 2MWh for the individual storage system and 1 MWh for each storage of dual energy storage system in current study. The operating schedule and the state of charge of the storage systems under all three operating cases have been shown in Figure 7.5 for cases 1 to 3. The variations in the state of charge of the storage systems indicate that the storage energy has been utilized to manage the power balance in the microgrid system through different modes of storage operations. Results show that the state of charge for case 1 and case 2 are identical except at the 21^st and 22^nd hours of an operation, mainly due to variations in the operating modes to minimize the operating cost. It has been also found that the net state of charge for dual storage systems is less as compared to microgrid system operation with one energy storage system either battery or pump hydro storage.

The variations in the power generation of the diesel generators have been shown in Figure 7.6(a)–Figure 7.6(c) for three cases of operation. The power generation from all three diesel generators varies according to the price variations and state of storage. The power generation during the initial hours of scheduling is less, due to the higher state of charge available to meet the system demand. Figure 7.7(a)–Figure 7.7(b) represents the charging-discharging and motoring-generating modes of operation during the operational cases of microgrid when operated with battery storage and pump hydro storage individually, respectively. Figure 7.7(c)–Figure 7.7(d) show the variations of the charging-discharging and motoring-generating modes of operation separately during the microgrid operation with DESS. The pump hydro storage provides more charging power and therefore maintains a higher level of charge.

The microgrid system under study has five adjustable loads with operating time shifting and power controlling option during operating time depending upon the power availability and market energy prices. Figures 7.8(a)–(c) shows the scheduling of the adjustable load and it was found that the shiftable loads (load 1 – load 4) and controllable loads operate as per the operational and technical constraints to provide little flexibility to the system for managing optimal dispatch. Different scheduling has been noticed for different operating cases. The results obtained show that the loads operates within the time slots mentioned for the operations and consumed energy as per specifications given in Table 7.3. These variations in operating times of the various loads are due to the market prices and the power availability from other economical resources.

The operating cost of the GCMG system under study includes the cost of power generation from all energy resources, cost of charging/motoring and discharging/generating modes of operation of DESS, cost of power exchange between the grid and the microgrid. In this present study, it is assumed that the microgrid purchase power from the grid at 80 percent and sell power to grid at 120 percent of the energy price rate at a particular time interval. The overall operational cost of the microgrid changes with the operating conditions. The operating cost of microgrid system associated with different operating cases have been tabulated in Table 7.5.

The minimum operating cost of $7,263.1 has been observed in case 1 when the minimum battery charging and discharging time fixed to 3 and 2 hours, respectively. This is due to the minimum power exchange cost for maintaining the power balance. Similarly, in case 2 the minimum operating cost of $7,195.8 has been observed for the minimum operating time limit of 3 and 2 hours, respectively. The operating cost in case of case 2 is less than in case 1 due to the less cost of operation of pump hydro and the power exchange as compared to the battery storage system. But during a dual energy storage operation (case 3), the cost has been found that system cost increases if the microgrid operates at the same operating time conditions on the storage. It has been found the optimal cost of operation of $7,472.1 when storage operates with a minimum time of operation limits of 1 hour of charging/motoring and 2 hours of discharging/generating mode. It has been concluded that the minimum time of operation of the storage system has a great impact on the optimal operation of the microgrid system and therefore the minimum time of operation limit must be considered carefully.

7.5 Conclusion

This chapter presents a grid-connected microgrid system having a dual energy storage system for optimal scheduling while operating with a minimum operating time constraint on the storage system. Day-ahead electricity market scheduling has been obtained for the microgrid system involving solar, wind, diesel, boiler, combined heat, and power plants and fixed and adjustable loads. Thermal load demand is considered to be fulfilled by the combined heat and power units and the boiler system. The microgrid manages the load demand very economically and effectively when the optimal operating minimum time of operation on the storage systems has been applied. The results obtained from the simulations confirm the impact of the minimum operating time limit set by the system operator on the storage systems on the operating cost the system. The dual storage systems are a little costlier than the individual storage system of same capacity but helps in reducing effectively net power demand variations of the system. At the same time it reduces the net carbon emission by reducing the power generation from the diesel generators or generators using fossile fuels for generations. Therefore, it is recommended that the microgrid system with the flexible dual system with flexible operating time limits is more suitable for managing the system variability as compared to the system operating with battery storage and pump hydro storage systems individually.

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