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Dispatching strategies of electric vehicles participating in frequency regulation on power grid: A review
Renewable and Sustainable Energy Reviews 68 (2017) 147–152
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Dispatching strategies of electric vehicles participating in frequency regulation on power grid: A review
crossmark
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Chao Peng , Jianxiao Zou, Lian Lian School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Electric vehicles Frequency regulation Dispatching strategies Stability of grid Economy
Recently, with the rapid growth of electric vehicle (EV) and development of Vehicle-to-Grid (V2G) technology, EVs participating in frequency regulation service to support power grid operation has been seen as one of the most promising power grid ancillary services provided by EVs integrated in grid. The dispatching strategy of EVs determines the feasibility and efficiency of EVs participating in frequency regulation, which have been received extensive researches. This paper will review the current dispatching strategies of EVs participating in frequency regulation. At first, the system structure of EVs participating frequency regulation is introduced. The stability and economy of EVs participating frequency in grid are analyzed. Secondly, the existing dispatching strategies are categorized into strategies for stability problem and strategies for economy problem, which are discussed in detailed. Finally, the existing problems and the future researches in EVs participating in frequency regulation on power grid are summarized.
n n SOCmin , SOCmax Minimum/Maximum SOC of EV n in %.
1. Introduction Since the 90's of last century, with the rapid increase of number of vehicles, traditional vehicle exhaust emission has become one of the largest sources of air pollution. Meanwhile, global climate change and fossil energy shortage make EVs instead of the traditional vehicles receiving long-term attention all around the world. In recent years, the developments of motor and battery technology have accelerated this trend. The global number of EVs on road has exceeded 1,000,000 in 2015, according to International Energy Agency (IEA)’s 2016 Global EV Outlook [1]. Vehicles proportion would continue to grow rapidly in the coming decades in many countries. For example, according to data from [2], the number of EVs in 2020, 2030 and 2050 will account for 35%, 51% and 62% in American. Large number of EVs integrated in power grid would bring great challenges for the power grid operation, the large charging load and its changes could bring great impacts on power grid. To solve these problem, Kempton and Letendre proposed the concept of V2G in 1997 [3]. The main idea of V2G is EV battery charge operates in bidirectional mode, i.e., EV batteries could obtain energy from power grid and the energy stored in them could be delivered back to power grid. Thus, the batteries of EVs could be used as energy storage devices or power resources to participate in the grid services. EVs integration could not
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Corresponding author. E-mail address: [email protected] (C. Peng).
http://dx.doi.org/10.1016/j.rser.2016.09.133 Received 12 July 2015; Received in revised form 20 July 2016; Accepted 29 September 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.
only be charging loads in grid, but also can be used as energy storage units and generating units, to provide spinning reserve [4], frequency regulation [5], voltage regulation [6] and other ancillary services. To promote the common development of power grid and EVs, utilizing EVs to provide power grid ancillary services has become an inevitable trend. In general, EVs are averagely parked for 22 h every day [7], capacity of their batteries could not be ignored. The EVs battery's characteristics of large capacity and fast response make EVs very suitable to participate in grid frequency regulation. Therefore, Participating in frequency regulation has become one of the most promising and practical ancillary service provided by EVs. At present, EVs participating in frequency regulation has become a hot research topic. Dispatching strategies determines the feasibility and effectiveness of EVs participating in frequency regulation. Thus, in recent years, many scholars focus on the dispatching strategies to improve the grid frequency stability and make full use of EVs. The two main problems faced by EVs participating frequency regulation are grid frequency stability and economy when EVs participating in frequency regulation. To optimize the operation stability and economy of grid and EVs, many dispatching strategies are proposed. However, there strategies have different optimization objectives and background. Thus, this paper will comprehensively review and categorize the existing dispatching strategies of EVs participating in frequency regulation on power grid. In this paper, the system structure and two ways of EVs participat-
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control are evaluated in [11]. The role of EVs contributes to frequency response in the Great Britain power system is investigated in [12], the simulation results demonstrates that utilizing EVs to stabilize the grid frequency can significantly reduce frequency deviations. In [13], a quasi-Monte Carlo (QMC) based probabilistic small signal stability analysis (PSSSA) method is proposed to assess the power grid frequency stability when EVs participating frequency regulation.
Nomenclature Symbols Rreg (t ) E (t ) Preg (t ) Pr (t ) PregUp (t ),
The meaning of the symbols Power capacity of regulation at time t in kWh Charging energy of EV at time t in kWh Frequency regulation price at time t in $/kWh Unit price of power from grid at time t in $/kWh PregDown (t ) Price of regulation up/down at time t in $/kWh RregUp (t ), RregDown (t ) Power of regulation up/down at time t in kWh SOC n (t ) State of charge (SOC) of EV n at time t in %
3.2. Economy of EV participating in frequency regulation The economy is the important factor to attract EVs participating in frequency regulation. The economy factors mainly are benefits of EV owners, aggregators and power grid operators. Most researches about economy focus on revenue evaluation, economic analysis of EV owners or the power grid operators, the fair allocation of regulation capacity and other economic problems.
ing in frequency regulation are introduced. The stability and economy of EVs participating in frequency regulation are analyzed. The overviews of the existing dispatching strategies of EVs participating in frequency regulation are discussed in detail. Finally, the existing scheduling problem and the future research direction are summarized.
3.2.1. Economy evaluation of EVs participating in frequency regulation Currently, there still not exists a strict and standard analysis tool for the revenue evaluation of EVs participating in frequency regulation. However, it has shown its promising economic value for the grid and EV owners, which would be shown in following:
2. System structure of EVs participating in frequency regulation There are two mode of system structure of EVs participating in frequency regulation, i.e., distributed dispatch system and centralized dispatch system [8]. In distributed dispatch system, the integration points of EVs locate in the public or private areas, which distribute widely. The grid operator controls each individual EV to participate in frequency regulation, as shown in Fig. 1. In centralized dispatch mode, the integration points of EVs locate in a charging station, a parking lot or a community, which are centralized distribution. An aggregator was used to control and manage each individual EV to participate in frequency regulation. The grid operator only manages the aggregators and it does not need to manage each individual EV, as shown in Fig. 2...
1) Reduce peak load through the reasonably charging and discharging behaviors. Thereby, reduce the cost of start of generation units to regulate peak and frequency deviation in power grid. 2) Compensate the grid fluctuation caused by intermittent renewable energy, increase the economy of the grid operation and reduce the cost of investment in the grid. 3) Improve the reliability of power supply. Thereby, reduce the cost of power grid maintenance. 4) Reduce charging cost of EV owners or EV aggregator, who also could obtain additional benefits through participating in frequency regulation. 3.2.2. Economy analysis Now, how to increase EV owners’ or aggregator's benefits became a research hotspot. It has been received more and more attention [14– 19]. By using Swedish and German actual energy market data, the conditions EVs participating in frequency regulation which can generate revenues are analyzed in [15]. The opportunities for EVs to obtain revenues from participating in frequency regulation are evaluated in [16]. According to the simulation, its results demonstrate that under the reasonable dispatching strategy, EVs can not only reduce charging cost, but also can get additional revenue from the grid, which would inspire the EV owners participate in frequency regulation. The economic feasibility of V2G frequency regulation considering the EV battery wear is studied in [17]. An economic evaluation of revenues from primary frequency regulation for an EV aggregator managing EV fleet is discussed in [18]. The economic and environmental benefits obtained from electric delivery trucks participating in frequency regulation are analyzed in [19].
3. Stability and economy problem faced by EVs participating in frequency regulation The problems faced by EV participating in frequency regulation mainly are in two aspects, stability problem and economy problem. The stability problem refers how to maintain the stability of grid frequency and provide frequency regulation services by control or management of the large scale EVs integrated in grid. The economy problem refers to how to increase EV owners or aggregator's benefits and encourage more owners and aggregators to participate in frequency regulation ancillary service. With the rapid increasing of EVs integrated into the grid, more and more researchers pay attention to the stability and economy analysis of EVs participating in frequency regulation. 3.1. Stability of EVs participating in frequency regulation The stability criterion of grid frequency is the power generation must match the load consumption. A mismatch will cause the deviation of the grid frequency from the criterion operating point (50 Hz in China, 60 Hz in USA) [9]. There is a strict criterion of grid frequency fluctuation range. The existing researches on stability of EVs participating in frequency regulation mostly focus on its effects on grid frequency stability and feasibility analysis. The feasibility and practicality of EVs participating in frequency regulation is evaluated in [3]. The impact of EVs integration and the technical potential of frequency response from EVs integrated in grid are analyzed in [9]. A project that EVs participate in frequency regulation service is demonstrated in [10]. The effects of autonomous distributed vehicle-to-grid on power system frequency
Fig. 1. The structure of distributed dispatch system.
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based intelligent dispatching strategy is proposed in [21]. To maintain the load frequency stability and reduce frequency fluctuation of smart grid with electric vehicles and renewable resources, an intelligent load frequency control approach is proposed in [22]. 4.1.1.2. Dispatching strategies based on frequency regulation signal analysis. Regulation signal is based on the deviation between the operation frequency point and the required grid operation frequency point, which is used as guidance for frequency regulation. It would be analyzed by power grid operator or EV aggregator and be used for dispatching charging or discharging power of EVs to provide frequency regulation service. There are some studies on the dispatching strategies based on frequency regulation signal from power grid operator. In [23], a strategy using unidirectional V2G is proposed to control the charging power of EVs to meet the frequency regulation signal requirement. In [24], a dispatching strategy based on EV battery SOC is developed to track frequency regulation signal with energy constraint. Meanwhile, the history statistics of the frequency regulation signal is analyzed, the results demonstrates that it approximately accords with a normal distribution with a mean value of zero and the positive and negative values of the signal are approximately equal in most time. A dispatching strategy which coordinates energy storage devices and EVs to satisfy frequency regulation demand is proposed in [25].
Fig. 2. The structure of centralized dispatch system.
4. Dispatching strategies of EVs participating in frequency regulation The dispatching strategy of EVs participating in frequency regulation is the key to solve the stability and economy problems. The different strategies based on different optimization goal, control the charging and discharging process of EVs to provide frequency regulation service. According to different objectives, the existing dispatching strategies could be divided into two main strategies, dispatching strategies for stability problems and dispatching strategies for economy problems.
4.1.1.3. Dispatching strategies considering on reduction of peak load. The large scale of EVs integrated in grid would bring a large number of loads into grid, which would cause overload. Thus, it is possible to dispatch charging power of EVs to reduce peak load and stabilize the power grid frequency.
4.1. Dispatching strategies for stability problems The dispatching strategies for stability problems stabilize the grid frequency by control or manage the charging or discharging behaviors of EVs integrated in grid. Their objectives are to solve the grid frequency stability problem by utilizing large scale of EVs integrated in grid. According to the different methods of EVs participating in frequency regulation, the dispatching strategies for stability problem can be divided into two classes: dispatching strategies based on grid frequency and dispatching strategies based on regulation capacity.
Some literatures have studied on how to reduce peak load by control the charging power or discharging power of EVs. In [26], a dispatching strategy to reduce the peak load and provide frequency regulation at the same time is proposed. In [27], a dispatching strategy to minimize the load of EVs charging and stabilize grid frequency is developed. To alleviate the power system peak load and frequency fluctuation, a novel dispatching strategy with EVs, combination of inertial emulation for power grid, is proposed in [28].
4.1.1. Dispatching strategies based on grid frequency The dispatching strategies based on grid frequency takes stabilizing the grid frequency as objective. The grid operator or aggregator dispatches the charging and discharging power of EVs according to the grid frequency deviation. When EVs providing frequency regulation service, the frequency fluctuation caused by intermittent renewable energy, the tracking errors of EVs response to the regulation signal and other related factors are need to be considered.
4.1.1.4. Dispatching strategies considering on other factors. Some literatures also consider some related factors on EVs participating in frequency regulation, such as SOC of EV battery, behavior of EV owners and so on. A dispatching strategy based on charging demand by EV owners is proposed in [29]. A suppressing frequency fluctuation strategy using bidirectional V2G is proposed in [30]. A dispatching strategy based on a multi-level architecture for bidirectional V2G is studied in [31]. In [32], a dispatching strategy to ensure the charging demand of EVs and the quality of EVs providing frequency regulation service is studied. In [33], a dispatching strategy based on a novel auction-based optimal distributed V2G coordination method is proposed. A dispatching strategy providing frequency regulation under limit communication condition is studied in [34].
4.1.1.1. Dispatching strategies considering on integration of intermittent renewable energy. Intermittent renewable energy power generation mainly includes photovoltaic, wind and other renewable energy power generation, whose power generation is intermittent and random. If they are connected to the grid, they will lead to the power grid frequency fluctuation. Because of the environmental protection benefits, more and more renewable power generation units will be integrated into grid. The large scale of EVs integration in grid is the ideal tool to stabilize grid frequency fluctuation caused by the renewable energy power generation.
4.1.2. Dispatching strategies based on participating capacity The dispatching strategies based on frequency capacity take optimization of the capacity of EVs participating frequency regulation as objective. The regulation signal represents the capacity of the grid demand, and EVs will compensate the deviation of capacity to maintain the stability of grid frequency. The objective of dispatching strategies based on participating capacity could be given as
How to utilize EVs to compensate the frequency fluctuation caused by renewable energy power generation has attracted more and more scholar's attentions. To maximize the utilization of renewable energy and reduce the impact of EV charging on the power grid, a EVs dispatching strategy of EVs participating in frequency regulation is proposed in [20]. To reduce the frequency fluctuation caused by high penetration of wind power, an intelligent particle swarm optimization
max Rreg 149
(1)
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EVs owners is not only that SOC of EV battery must meet the owners’ travel demand, but also maximizing the benefits of EV owners. Aiming to satisfy above demands, some dispatching strategies considering on demands of EV owners were studied. A dispatching strategy based on dynamic price is proposed in [40]. The core idea of the strategies in [41,42] is to maximize the benefits of EV owners. These strategies were implemented by using two ways, one is achieved by optimizing the battery charging rate, and another is achieved by using dynamic programming to optimize the sequence of charging and participating in frequency regulation.
Rreg is the power capacity of EVs providing to frequency regulation. More EVs could provide the more power capacity which could maintain the power grid frequency stability. In [35], a dispatching strategy based on capacity is developed for EV aggregator to provide regulation service. In [36], a model is proposed to optimize performance of frequency regulation service based on the interaction between EVs and EV aggregator. In [37], a dispatching strategy based on a novel robust frequency regulation capacity scheduling algorithm is proposed. 4.2. Dispatching strategies for economy problems
4.4. Dispatching strategies considering on mobility behavior of EVs
The dispatching strategies based on economy problems control or manage the charging or discharging power of EVs participating in frequency regulation, to optimize the economy benefits of EV owners or EV aggregators. The strategies proposed in most of the literature could be divided into two classes: dispatching strategies based on EV owners’ benefits and dispatching strategies based on aggregator's profits.
The mobility behavior of EVs mainly means that EV should satisfy the travel demand of EV owners as transportation facility at any time. When EVs participate in frequency regulation, EVs may be in discharging state in a certain period. However, EV would leave in random, the SOC of battery may not satisfy the traveling demand of EV owners if EV participate in frequency regulation. Thus, some scholars have studied the dispatching strategies considering on mobility behavior of EVs. A dispatching strategy which considers the EV mobility and takes satisfying the required SOC as objective is proposed in [43]. A dispatching strategy based on EVs dynamic mobility has been studied in [44]. This paper compares the benefits obtained by using average daily number of EVs integrated participating in frequency regulation in German and the benefits by using the dispatching strategy considering different travel habits of EV owners and mobility of EVs. A dispatching strategy considering the traveling behavior in Great Britain power system is proposed in [45]. Because of the probability distribution of EV integrated in grid, there are sometimes when capacity of EVs participating in frequency regulation service is insufficient. Four modes of punishment mechanism of dispatching strategy under regulation capacity shortage situation, is analyzed in [46]. A strategy which considers the benefits of EV owners when EVs could not satisfy the demand of the capacity for frequency regulation is studied in [47].
4.2.1. Dispatching strategies based on the benefits of EV owners The dispatching strategies based on the benefits of EV owners, control and manage the charging and discharging power of EVs to maximize the benefits of EV owners by analyzing the cost of EV owners, benefits under real-time electricity price and price of participating in frequency regulation and coordinating the EV battery SOC and the demand of EV owners. The general calculation model of maximizing benefits of EV owners could be given as,
max Preg (t ) Rreg (t ) − Pr (t ) E (t )
(2)
where Preg (t )and Pr (t ) are the real time price of participating frequency regulation and electricity price respectively, Rreg (t ) and E (t ) are the capacity participating in frequency regulation and charging energy of EV battery. There are some literatures which divide the benefits into benefits obtained by upward frequency regulation and downward frequency regulation as follows [38],
max(PregUp (t ) RregUp (t ) + PregDown (t ) RregDown (t ) − Pr (t ) E (t ))
(3)
4.5. Dispatching strategies considering on the response of EVs to frequency regulation signal
where PregUp (t ) and PregDowm (t ) are the real time price of upward frequency regulation and downward frequency regulation respectively, RregUp (t ) and RregDown (t ) are the power capacity participating in upward frequency regulation and downward frequency regulation. The constraints of above optimization objective always require the charging and discharging power of battery meet the basic constraints of battery devices as follows [39],
−pcimax (t ) ≤ pci (t ) ≤ pcimax (t )
The response of EVs to frequency regulation signal means that as a supplementary device for ancillary services, EV would respond the grid regulation signal to stabilize the grid frequency. The grid operator sends out frequency regulation signal as guidance, which could notify EVs how to participate in frequency regulation. Thus, the performance of response of EVs to frequency regulation signal would affect the performance and benefits of frequency regulation. Thus, some literatures have studied the dispatching strategies considering on the response of EVs to frequency regulation signal, such as the dispatching strategy based on giving a priority for each vehicle [34], the dispatching strategy based on the uncertainty of regulation signal [48], and the dispatching strategy based on Markov random regulation signal and Markov random prices [49], the dispatching strategy based on droopbased control and decentralized iterative algorithm [50], and so on.
(4)
where pcimax (t ) represents the maximum charging power constrain of EVs which is relevant to EVs number integrated in grid at node i at time t and the type of EV. −pcimax (t ) represents the maximum discharging power of EVs. Their value are deduced from the real-time statistics data or simulating charging demand. To avoid the overcharging and over-discharging, the SOC of EV battery should be restricted as follow,
SOCmin ≤ SOC n (t ) ≤ SOCmax
(5)
In addition, other related constraint conditions should be considered, such as mobility behaviors of EV, the demand of the EV owners and the grid frequency stability. The existing optimization objectives and constraints of EV dispatching strategies based on the EV owners’ benefits have been mentioned above. According to the different factors to be considered, the strategies could be divided as follows.
4.6. Dispatching strategies considering on fair allocation between EVs The dispatching strategies considering on fair allocation of EVs control and manage the charging or discharging powers between EVs reasonably according to the capacity of EV battery. These dispatching strategies make the EVs with lower SOC participating in downward frequency regulation in priority, and the EVs with higher SOC participating in upward frequency regulation in priority [51]. Many literatures have studied the dispatching strategies considering on fair allocation between EVs. A linear weight function for fair allocation in
4.3. Dispatching strategies considering on demands of EV owners When EVs participate in frequency regulation, the demand of the 150
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where In and C are the EV aggregator's profits and cost respectively, α is the proportion of earning from EVs participating frequency regulation. Mk is the electric price sold to EVs. At present, many scholars have studied dispatching strategies based on aggregator's profits. They mainly could be divided into two aspects, i.e., the unidirectional V2G and bidirectional V2G. In [56], the dispatching strategies using unidirectional V2G are studied to satisfy user demand and the grid requirements to maximize EV aggregator's profits. In [57], a dispatching strategy which considers not only EVs participating in frequency regulation, but also providing spinning reserve is proposed. In [58], a dispatching strategy based on the mobility behaviors of EVs and EV aggregator's profits is proposed. In [59], a dispatching strategy is proposed to maximize aggregator's profits according to frequency regulation signal. In [60], to make EVs participating in frequency regulation by providing reserve capacity and maximizing aggregator's profits, a dispatching strategy is proposed for bidirectional V2G system. Based on above strategy, an enhanced dispatching strategy is proposed in [61], which considers the cost of battery degradation and controls the charging and discharging rate to optimize aggregator's profits. In [62], a dispatching strategy is proposed to minimize the operating cost of EV aggregator when it participating in frequency regulation.
Fig. 3. A linearly degrading weight function.
upward frequency regulation and downward frequency regulation is developed in [38,42,43], which is shown in Fig. 3.. There are some other dispatching strategies proposed in the literatures to realize fair allocation, such as the dispatching strategy based on water-filling algorithm [51], dispatching strategy based on welfare-maximizing regulation allocation algorithm [52]. 4.7. Dispatching strategies considering other economic factors
5. Conclusion
There are some other economic factors should be considered in EVs participating in frequency regulation, such as the EV battery degradation, the environmental benefits of EVs participating in frequency regulation, and electric price uncertainty and so on. The frequent charging and discharging actions of EVs could affect the battery lifetime. Many literatures consider battery degradation as the cost of EV owners. The cost of EV battery degradation [53] always is given as follow,
Cd =
cb + cl Lc⋅E⋅DoD
With the rapid growth of EV on road and development of V2G technology, EVs have become important auxiliary devices for grid frequency regulation. This paper has described two modes of EVs participating in frequency regulation in brief, and discussed the stability and economic problems in EVs providing frequency regulation service in detail. The existing dispatching strategies of EVs participating in frequency regulation which solves the stability and economy problems are discussed in detail. However, currently, there still exists following problems have not be solved well.
(8) 1) There still existing many various economic factors have not been considered comprehensively in dispatching strategy of EVs participating in frequency regulation, such as the charging or discharging price demand of EV owners, the cost of battery degradation, and so on. 2) Space and time distribution of EVs integrated in grid which would affect the effectiveness of EVs participating in frequency regulation, have not be analyzed well. They could be used to predict the capacity of EVs in advance, to avoid the EV participating capacity insufficiency problem in some times. 3) Most existing dispatching strategy researches have not considered on the electric power loss in EV charging or discharging process. The stability of power grid affected by EV charging or discharging rate has not been analyzed sufficiently. 4) Most existing dispatching strategy researches are studied in theory, further researches should paid more attention on their practicality.
where Cb is the total cost of a brand-new EV battery, cl is the replacement cost of EV battery, Lc is the EV battery cycle life at a certain depth of discharging, E is the total energy capacity of EV battery,DoD is the EV battery depth of discharging. If the battery degradation adds in the objective function as cost in Equ.(8), the effective depth of discharge of battery would be limited, so it would extend the battery lifetime to reduce the battery degradation. Some literatures have studied the other factors. In [54], a dispatching strategy of when EVs participating in frequency regulation considering on environmental incomes is studied. In [42,48], a dispatching strategy considering on the uncertainty of electricity price and regulation capacity price is developed. 4.7.1. The dispatching strategies based on aggregator's profits The dispatching strategies based on aggregator's profits take EV aggregator as an agent between EVs and grid operator. They manage the cost and profits of a large scale of EVs participating in frequency regulation [55]. The objective of them is to maximize the revenue obtained according to the power grid market information and information of EVs. The revenue of EV aggregator comes from two parts: the first part comes from the price difference between buying energy from the grid and that sold to EV owners, another part comes from managing EVs participating in frequency regulation to earn a proportion of fee. Its profits calculation model always given as follows,
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In = α∑t (PregUp (t ) RUp (t ) + PregDown (t ) RDown (t )) + Mk ∑t E (t )
C=
∑ (E (t )⋅Pr (t )) t
(9) (10) 151
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Dispatching strategies of electric vehicles participating in frequency regulation on power grid: A review
crossmark
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Chao Peng , Jianxiao Zou, Lian Lian School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Electric vehicles Frequency regulation Dispatching strategies Stability of grid Economy
Recently, with the rapid growth of electric vehicle (EV) and development of Vehicle-to-Grid (V2G) technology, EVs participating in frequency regulation service to support power grid operation has been seen as one of the most promising power grid ancillary services provided by EVs integrated in grid. The dispatching strategy of EVs determines the feasibility and efficiency of EVs participating in frequency regulation, which have been received extensive researches. This paper will review the current dispatching strategies of EVs participating in frequency regulation. At first, the system structure of EVs participating frequency regulation is introduced. The stability and economy of EVs participating frequency in grid are analyzed. Secondly, the existing dispatching strategies are categorized into strategies for stability problem and strategies for economy problem, which are discussed in detailed. Finally, the existing problems and the future researches in EVs participating in frequency regulation on power grid are summarized.
n n SOCmin , SOCmax Minimum/Maximum SOC of EV n in %.
1. Introduction Since the 90's of last century, with the rapid increase of number of vehicles, traditional vehicle exhaust emission has become one of the largest sources of air pollution. Meanwhile, global climate change and fossil energy shortage make EVs instead of the traditional vehicles receiving long-term attention all around the world. In recent years, the developments of motor and battery technology have accelerated this trend. The global number of EVs on road has exceeded 1,000,000 in 2015, according to International Energy Agency (IEA)’s 2016 Global EV Outlook [1]. Vehicles proportion would continue to grow rapidly in the coming decades in many countries. For example, according to data from [2], the number of EVs in 2020, 2030 and 2050 will account for 35%, 51% and 62% in American. Large number of EVs integrated in power grid would bring great challenges for the power grid operation, the large charging load and its changes could bring great impacts on power grid. To solve these problem, Kempton and Letendre proposed the concept of V2G in 1997 [3]. The main idea of V2G is EV battery charge operates in bidirectional mode, i.e., EV batteries could obtain energy from power grid and the energy stored in them could be delivered back to power grid. Thus, the batteries of EVs could be used as energy storage devices or power resources to participate in the grid services. EVs integration could not
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Corresponding author. E-mail address: [email protected] (C. Peng).
http://dx.doi.org/10.1016/j.rser.2016.09.133 Received 12 July 2015; Received in revised form 20 July 2016; Accepted 29 September 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.
only be charging loads in grid, but also can be used as energy storage units and generating units, to provide spinning reserve [4], frequency regulation [5], voltage regulation [6] and other ancillary services. To promote the common development of power grid and EVs, utilizing EVs to provide power grid ancillary services has become an inevitable trend. In general, EVs are averagely parked for 22 h every day [7], capacity of their batteries could not be ignored. The EVs battery's characteristics of large capacity and fast response make EVs very suitable to participate in grid frequency regulation. Therefore, Participating in frequency regulation has become one of the most promising and practical ancillary service provided by EVs. At present, EVs participating in frequency regulation has become a hot research topic. Dispatching strategies determines the feasibility and effectiveness of EVs participating in frequency regulation. Thus, in recent years, many scholars focus on the dispatching strategies to improve the grid frequency stability and make full use of EVs. The two main problems faced by EVs participating frequency regulation are grid frequency stability and economy when EVs participating in frequency regulation. To optimize the operation stability and economy of grid and EVs, many dispatching strategies are proposed. However, there strategies have different optimization objectives and background. Thus, this paper will comprehensively review and categorize the existing dispatching strategies of EVs participating in frequency regulation on power grid. In this paper, the system structure and two ways of EVs participat-
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control are evaluated in [11]. The role of EVs contributes to frequency response in the Great Britain power system is investigated in [12], the simulation results demonstrates that utilizing EVs to stabilize the grid frequency can significantly reduce frequency deviations. In [13], a quasi-Monte Carlo (QMC) based probabilistic small signal stability analysis (PSSSA) method is proposed to assess the power grid frequency stability when EVs participating frequency regulation.
Nomenclature Symbols Rreg (t ) E (t ) Preg (t ) Pr (t ) PregUp (t ),
The meaning of the symbols Power capacity of regulation at time t in kWh Charging energy of EV at time t in kWh Frequency regulation price at time t in $/kWh Unit price of power from grid at time t in $/kWh PregDown (t ) Price of regulation up/down at time t in $/kWh RregUp (t ), RregDown (t ) Power of regulation up/down at time t in kWh SOC n (t ) State of charge (SOC) of EV n at time t in %
3.2. Economy of EV participating in frequency regulation The economy is the important factor to attract EVs participating in frequency regulation. The economy factors mainly are benefits of EV owners, aggregators and power grid operators. Most researches about economy focus on revenue evaluation, economic analysis of EV owners or the power grid operators, the fair allocation of regulation capacity and other economic problems.
ing in frequency regulation are introduced. The stability and economy of EVs participating in frequency regulation are analyzed. The overviews of the existing dispatching strategies of EVs participating in frequency regulation are discussed in detail. Finally, the existing scheduling problem and the future research direction are summarized.
3.2.1. Economy evaluation of EVs participating in frequency regulation Currently, there still not exists a strict and standard analysis tool for the revenue evaluation of EVs participating in frequency regulation. However, it has shown its promising economic value for the grid and EV owners, which would be shown in following:
2. System structure of EVs participating in frequency regulation There are two mode of system structure of EVs participating in frequency regulation, i.e., distributed dispatch system and centralized dispatch system [8]. In distributed dispatch system, the integration points of EVs locate in the public or private areas, which distribute widely. The grid operator controls each individual EV to participate in frequency regulation, as shown in Fig. 1. In centralized dispatch mode, the integration points of EVs locate in a charging station, a parking lot or a community, which are centralized distribution. An aggregator was used to control and manage each individual EV to participate in frequency regulation. The grid operator only manages the aggregators and it does not need to manage each individual EV, as shown in Fig. 2...
1) Reduce peak load through the reasonably charging and discharging behaviors. Thereby, reduce the cost of start of generation units to regulate peak and frequency deviation in power grid. 2) Compensate the grid fluctuation caused by intermittent renewable energy, increase the economy of the grid operation and reduce the cost of investment in the grid. 3) Improve the reliability of power supply. Thereby, reduce the cost of power grid maintenance. 4) Reduce charging cost of EV owners or EV aggregator, who also could obtain additional benefits through participating in frequency regulation. 3.2.2. Economy analysis Now, how to increase EV owners’ or aggregator's benefits became a research hotspot. It has been received more and more attention [14– 19]. By using Swedish and German actual energy market data, the conditions EVs participating in frequency regulation which can generate revenues are analyzed in [15]. The opportunities for EVs to obtain revenues from participating in frequency regulation are evaluated in [16]. According to the simulation, its results demonstrate that under the reasonable dispatching strategy, EVs can not only reduce charging cost, but also can get additional revenue from the grid, which would inspire the EV owners participate in frequency regulation. The economic feasibility of V2G frequency regulation considering the EV battery wear is studied in [17]. An economic evaluation of revenues from primary frequency regulation for an EV aggregator managing EV fleet is discussed in [18]. The economic and environmental benefits obtained from electric delivery trucks participating in frequency regulation are analyzed in [19].
3. Stability and economy problem faced by EVs participating in frequency regulation The problems faced by EV participating in frequency regulation mainly are in two aspects, stability problem and economy problem. The stability problem refers how to maintain the stability of grid frequency and provide frequency regulation services by control or management of the large scale EVs integrated in grid. The economy problem refers to how to increase EV owners or aggregator's benefits and encourage more owners and aggregators to participate in frequency regulation ancillary service. With the rapid increasing of EVs integrated into the grid, more and more researchers pay attention to the stability and economy analysis of EVs participating in frequency regulation. 3.1. Stability of EVs participating in frequency regulation The stability criterion of grid frequency is the power generation must match the load consumption. A mismatch will cause the deviation of the grid frequency from the criterion operating point (50 Hz in China, 60 Hz in USA) [9]. There is a strict criterion of grid frequency fluctuation range. The existing researches on stability of EVs participating in frequency regulation mostly focus on its effects on grid frequency stability and feasibility analysis. The feasibility and practicality of EVs participating in frequency regulation is evaluated in [3]. The impact of EVs integration and the technical potential of frequency response from EVs integrated in grid are analyzed in [9]. A project that EVs participate in frequency regulation service is demonstrated in [10]. The effects of autonomous distributed vehicle-to-grid on power system frequency
Fig. 1. The structure of distributed dispatch system.
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based intelligent dispatching strategy is proposed in [21]. To maintain the load frequency stability and reduce frequency fluctuation of smart grid with electric vehicles and renewable resources, an intelligent load frequency control approach is proposed in [22]. 4.1.1.2. Dispatching strategies based on frequency regulation signal analysis. Regulation signal is based on the deviation between the operation frequency point and the required grid operation frequency point, which is used as guidance for frequency regulation. It would be analyzed by power grid operator or EV aggregator and be used for dispatching charging or discharging power of EVs to provide frequency regulation service. There are some studies on the dispatching strategies based on frequency regulation signal from power grid operator. In [23], a strategy using unidirectional V2G is proposed to control the charging power of EVs to meet the frequency regulation signal requirement. In [24], a dispatching strategy based on EV battery SOC is developed to track frequency regulation signal with energy constraint. Meanwhile, the history statistics of the frequency regulation signal is analyzed, the results demonstrates that it approximately accords with a normal distribution with a mean value of zero and the positive and negative values of the signal are approximately equal in most time. A dispatching strategy which coordinates energy storage devices and EVs to satisfy frequency regulation demand is proposed in [25].
Fig. 2. The structure of centralized dispatch system.
4. Dispatching strategies of EVs participating in frequency regulation The dispatching strategy of EVs participating in frequency regulation is the key to solve the stability and economy problems. The different strategies based on different optimization goal, control the charging and discharging process of EVs to provide frequency regulation service. According to different objectives, the existing dispatching strategies could be divided into two main strategies, dispatching strategies for stability problems and dispatching strategies for economy problems.
4.1.1.3. Dispatching strategies considering on reduction of peak load. The large scale of EVs integrated in grid would bring a large number of loads into grid, which would cause overload. Thus, it is possible to dispatch charging power of EVs to reduce peak load and stabilize the power grid frequency.
4.1. Dispatching strategies for stability problems The dispatching strategies for stability problems stabilize the grid frequency by control or manage the charging or discharging behaviors of EVs integrated in grid. Their objectives are to solve the grid frequency stability problem by utilizing large scale of EVs integrated in grid. According to the different methods of EVs participating in frequency regulation, the dispatching strategies for stability problem can be divided into two classes: dispatching strategies based on grid frequency and dispatching strategies based on regulation capacity.
Some literatures have studied on how to reduce peak load by control the charging power or discharging power of EVs. In [26], a dispatching strategy to reduce the peak load and provide frequency regulation at the same time is proposed. In [27], a dispatching strategy to minimize the load of EVs charging and stabilize grid frequency is developed. To alleviate the power system peak load and frequency fluctuation, a novel dispatching strategy with EVs, combination of inertial emulation for power grid, is proposed in [28].
4.1.1. Dispatching strategies based on grid frequency The dispatching strategies based on grid frequency takes stabilizing the grid frequency as objective. The grid operator or aggregator dispatches the charging and discharging power of EVs according to the grid frequency deviation. When EVs providing frequency regulation service, the frequency fluctuation caused by intermittent renewable energy, the tracking errors of EVs response to the regulation signal and other related factors are need to be considered.
4.1.1.4. Dispatching strategies considering on other factors. Some literatures also consider some related factors on EVs participating in frequency regulation, such as SOC of EV battery, behavior of EV owners and so on. A dispatching strategy based on charging demand by EV owners is proposed in [29]. A suppressing frequency fluctuation strategy using bidirectional V2G is proposed in [30]. A dispatching strategy based on a multi-level architecture for bidirectional V2G is studied in [31]. In [32], a dispatching strategy to ensure the charging demand of EVs and the quality of EVs providing frequency regulation service is studied. In [33], a dispatching strategy based on a novel auction-based optimal distributed V2G coordination method is proposed. A dispatching strategy providing frequency regulation under limit communication condition is studied in [34].
4.1.1.1. Dispatching strategies considering on integration of intermittent renewable energy. Intermittent renewable energy power generation mainly includes photovoltaic, wind and other renewable energy power generation, whose power generation is intermittent and random. If they are connected to the grid, they will lead to the power grid frequency fluctuation. Because of the environmental protection benefits, more and more renewable power generation units will be integrated into grid. The large scale of EVs integration in grid is the ideal tool to stabilize grid frequency fluctuation caused by the renewable energy power generation.
4.1.2. Dispatching strategies based on participating capacity The dispatching strategies based on frequency capacity take optimization of the capacity of EVs participating frequency regulation as objective. The regulation signal represents the capacity of the grid demand, and EVs will compensate the deviation of capacity to maintain the stability of grid frequency. The objective of dispatching strategies based on participating capacity could be given as
How to utilize EVs to compensate the frequency fluctuation caused by renewable energy power generation has attracted more and more scholar's attentions. To maximize the utilization of renewable energy and reduce the impact of EV charging on the power grid, a EVs dispatching strategy of EVs participating in frequency regulation is proposed in [20]. To reduce the frequency fluctuation caused by high penetration of wind power, an intelligent particle swarm optimization
max Rreg 149
(1)
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EVs owners is not only that SOC of EV battery must meet the owners’ travel demand, but also maximizing the benefits of EV owners. Aiming to satisfy above demands, some dispatching strategies considering on demands of EV owners were studied. A dispatching strategy based on dynamic price is proposed in [40]. The core idea of the strategies in [41,42] is to maximize the benefits of EV owners. These strategies were implemented by using two ways, one is achieved by optimizing the battery charging rate, and another is achieved by using dynamic programming to optimize the sequence of charging and participating in frequency regulation.
Rreg is the power capacity of EVs providing to frequency regulation. More EVs could provide the more power capacity which could maintain the power grid frequency stability. In [35], a dispatching strategy based on capacity is developed for EV aggregator to provide regulation service. In [36], a model is proposed to optimize performance of frequency regulation service based on the interaction between EVs and EV aggregator. In [37], a dispatching strategy based on a novel robust frequency regulation capacity scheduling algorithm is proposed. 4.2. Dispatching strategies for economy problems
4.4. Dispatching strategies considering on mobility behavior of EVs
The dispatching strategies based on economy problems control or manage the charging or discharging power of EVs participating in frequency regulation, to optimize the economy benefits of EV owners or EV aggregators. The strategies proposed in most of the literature could be divided into two classes: dispatching strategies based on EV owners’ benefits and dispatching strategies based on aggregator's profits.
The mobility behavior of EVs mainly means that EV should satisfy the travel demand of EV owners as transportation facility at any time. When EVs participate in frequency regulation, EVs may be in discharging state in a certain period. However, EV would leave in random, the SOC of battery may not satisfy the traveling demand of EV owners if EV participate in frequency regulation. Thus, some scholars have studied the dispatching strategies considering on mobility behavior of EVs. A dispatching strategy which considers the EV mobility and takes satisfying the required SOC as objective is proposed in [43]. A dispatching strategy based on EVs dynamic mobility has been studied in [44]. This paper compares the benefits obtained by using average daily number of EVs integrated participating in frequency regulation in German and the benefits by using the dispatching strategy considering different travel habits of EV owners and mobility of EVs. A dispatching strategy considering the traveling behavior in Great Britain power system is proposed in [45]. Because of the probability distribution of EV integrated in grid, there are sometimes when capacity of EVs participating in frequency regulation service is insufficient. Four modes of punishment mechanism of dispatching strategy under regulation capacity shortage situation, is analyzed in [46]. A strategy which considers the benefits of EV owners when EVs could not satisfy the demand of the capacity for frequency regulation is studied in [47].
4.2.1. Dispatching strategies based on the benefits of EV owners The dispatching strategies based on the benefits of EV owners, control and manage the charging and discharging power of EVs to maximize the benefits of EV owners by analyzing the cost of EV owners, benefits under real-time electricity price and price of participating in frequency regulation and coordinating the EV battery SOC and the demand of EV owners. The general calculation model of maximizing benefits of EV owners could be given as,
max Preg (t ) Rreg (t ) − Pr (t ) E (t )
(2)
where Preg (t )and Pr (t ) are the real time price of participating frequency regulation and electricity price respectively, Rreg (t ) and E (t ) are the capacity participating in frequency regulation and charging energy of EV battery. There are some literatures which divide the benefits into benefits obtained by upward frequency regulation and downward frequency regulation as follows [38],
max(PregUp (t ) RregUp (t ) + PregDown (t ) RregDown (t ) − Pr (t ) E (t ))
(3)
4.5. Dispatching strategies considering on the response of EVs to frequency regulation signal
where PregUp (t ) and PregDowm (t ) are the real time price of upward frequency regulation and downward frequency regulation respectively, RregUp (t ) and RregDown (t ) are the power capacity participating in upward frequency regulation and downward frequency regulation. The constraints of above optimization objective always require the charging and discharging power of battery meet the basic constraints of battery devices as follows [39],
−pcimax (t ) ≤ pci (t ) ≤ pcimax (t )
The response of EVs to frequency regulation signal means that as a supplementary device for ancillary services, EV would respond the grid regulation signal to stabilize the grid frequency. The grid operator sends out frequency regulation signal as guidance, which could notify EVs how to participate in frequency regulation. Thus, the performance of response of EVs to frequency regulation signal would affect the performance and benefits of frequency regulation. Thus, some literatures have studied the dispatching strategies considering on the response of EVs to frequency regulation signal, such as the dispatching strategy based on giving a priority for each vehicle [34], the dispatching strategy based on the uncertainty of regulation signal [48], and the dispatching strategy based on Markov random regulation signal and Markov random prices [49], the dispatching strategy based on droopbased control and decentralized iterative algorithm [50], and so on.
(4)
where pcimax (t ) represents the maximum charging power constrain of EVs which is relevant to EVs number integrated in grid at node i at time t and the type of EV. −pcimax (t ) represents the maximum discharging power of EVs. Their value are deduced from the real-time statistics data or simulating charging demand. To avoid the overcharging and over-discharging, the SOC of EV battery should be restricted as follow,
SOCmin ≤ SOC n (t ) ≤ SOCmax
(5)
In addition, other related constraint conditions should be considered, such as mobility behaviors of EV, the demand of the EV owners and the grid frequency stability. The existing optimization objectives and constraints of EV dispatching strategies based on the EV owners’ benefits have been mentioned above. According to the different factors to be considered, the strategies could be divided as follows.
4.6. Dispatching strategies considering on fair allocation between EVs The dispatching strategies considering on fair allocation of EVs control and manage the charging or discharging powers between EVs reasonably according to the capacity of EV battery. These dispatching strategies make the EVs with lower SOC participating in downward frequency regulation in priority, and the EVs with higher SOC participating in upward frequency regulation in priority [51]. Many literatures have studied the dispatching strategies considering on fair allocation between EVs. A linear weight function for fair allocation in
4.3. Dispatching strategies considering on demands of EV owners When EVs participate in frequency regulation, the demand of the 150
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where In and C are the EV aggregator's profits and cost respectively, α is the proportion of earning from EVs participating frequency regulation. Mk is the electric price sold to EVs. At present, many scholars have studied dispatching strategies based on aggregator's profits. They mainly could be divided into two aspects, i.e., the unidirectional V2G and bidirectional V2G. In [56], the dispatching strategies using unidirectional V2G are studied to satisfy user demand and the grid requirements to maximize EV aggregator's profits. In [57], a dispatching strategy which considers not only EVs participating in frequency regulation, but also providing spinning reserve is proposed. In [58], a dispatching strategy based on the mobility behaviors of EVs and EV aggregator's profits is proposed. In [59], a dispatching strategy is proposed to maximize aggregator's profits according to frequency regulation signal. In [60], to make EVs participating in frequency regulation by providing reserve capacity and maximizing aggregator's profits, a dispatching strategy is proposed for bidirectional V2G system. Based on above strategy, an enhanced dispatching strategy is proposed in [61], which considers the cost of battery degradation and controls the charging and discharging rate to optimize aggregator's profits. In [62], a dispatching strategy is proposed to minimize the operating cost of EV aggregator when it participating in frequency regulation.
Fig. 3. A linearly degrading weight function.
upward frequency regulation and downward frequency regulation is developed in [38,42,43], which is shown in Fig. 3.. There are some other dispatching strategies proposed in the literatures to realize fair allocation, such as the dispatching strategy based on water-filling algorithm [51], dispatching strategy based on welfare-maximizing regulation allocation algorithm [52]. 4.7. Dispatching strategies considering other economic factors
5. Conclusion
There are some other economic factors should be considered in EVs participating in frequency regulation, such as the EV battery degradation, the environmental benefits of EVs participating in frequency regulation, and electric price uncertainty and so on. The frequent charging and discharging actions of EVs could affect the battery lifetime. Many literatures consider battery degradation as the cost of EV owners. The cost of EV battery degradation [53] always is given as follow,
Cd =
cb + cl Lc⋅E⋅DoD
With the rapid growth of EV on road and development of V2G technology, EVs have become important auxiliary devices for grid frequency regulation. This paper has described two modes of EVs participating in frequency regulation in brief, and discussed the stability and economic problems in EVs providing frequency regulation service in detail. The existing dispatching strategies of EVs participating in frequency regulation which solves the stability and economy problems are discussed in detail. However, currently, there still exists following problems have not be solved well.
(8) 1) There still existing many various economic factors have not been considered comprehensively in dispatching strategy of EVs participating in frequency regulation, such as the charging or discharging price demand of EV owners, the cost of battery degradation, and so on. 2) Space and time distribution of EVs integrated in grid which would affect the effectiveness of EVs participating in frequency regulation, have not be analyzed well. They could be used to predict the capacity of EVs in advance, to avoid the EV participating capacity insufficiency problem in some times. 3) Most existing dispatching strategy researches have not considered on the electric power loss in EV charging or discharging process. The stability of power grid affected by EV charging or discharging rate has not been analyzed sufficiently. 4) Most existing dispatching strategy researches are studied in theory, further researches should paid more attention on their practicality.
where Cb is the total cost of a brand-new EV battery, cl is the replacement cost of EV battery, Lc is the EV battery cycle life at a certain depth of discharging, E is the total energy capacity of EV battery,DoD is the EV battery depth of discharging. If the battery degradation adds in the objective function as cost in Equ.(8), the effective depth of discharge of battery would be limited, so it would extend the battery lifetime to reduce the battery degradation. Some literatures have studied the other factors. In [54], a dispatching strategy of when EVs participating in frequency regulation considering on environmental incomes is studied. In [42,48], a dispatching strategy considering on the uncertainty of electricity price and regulation capacity price is developed. 4.7.1. The dispatching strategies based on aggregator's profits The dispatching strategies based on aggregator's profits take EV aggregator as an agent between EVs and grid operator. They manage the cost and profits of a large scale of EVs participating in frequency regulation [55]. The objective of them is to maximize the revenue obtained according to the power grid market information and information of EVs. The revenue of EV aggregator comes from two parts: the first part comes from the price difference between buying energy from the grid and that sold to EV owners, another part comes from managing EVs participating in frequency regulation to earn a proportion of fee. Its profits calculation model always given as follows,
Acknowledgments This work is being supported by the National Natural Science Foundation of China (Grant No. 61201010). References [1] MattP International Energy Agency: electric vehicle battery costs rapidly declining, TESLA cited as leading the pack[Online]. Available: 〈https://evannex.com/blogs/ news/118365957-international-energy-agency-electric-vehicle-battery-costsrapidly-declining-tesla-cited-as-leading-the-pack〉. [2] Duvall M, Knipping E. Environmental assessment of plug-in hybrid electric vehicles Volume1 [July]. Palo Alto, California: Nationwide Greenhouse Gas Emissions; 2007. [3] Kempton W, Letendre S. Electric vehicles as a new power source for electric
In = α∑t (PregUp (t ) RUp (t ) + PregDown (t ) RDown (t )) + Mk ∑t E (t )
C=
∑ (E (t )⋅Pr (t )) t
(9) (10) 151
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