The Role of Electrical Vehicles for Power Quality and Security during Outages in the Distribution Grid

Victor Astapov, Sambeet Mishra, Ivo Palu Sergei Trashchenkov Department of Electrical Engineering and Mechatronics Department of Engineering Tallinn University of Technology Pskov State University Tallinn, Estonia Pskov, Russia [email protected] [email protected]

Abstract—The power grid is facing challenges to ensure the Through the V2G implementation, energy system reduces power quality and security while new and greenhouse gases emissions and costs. Smart units are increasing in the power grid. The share of electrical applications of charging stations as a part of energy management vehicles (EVs) are increasing in recent period to reduce the system can also supply energy on the smaller scale, such as emissions and efficient use of resources. The EVs can play a Vehicle-to-House (V2H) and Vehicle-to-Building (V2B) [9]. sustainable role in grid stability through power infeed to the grid and act as a storage unit. However, the role of EVs in outage Flexibility of EVs can be used on a different time scales for scenarios requires more investigations to ensure the quality and the different purposes. There are some studies about V2G for the security prospective. This paper sets out to outline the key factors control [10] shows that V2G can reduce capacity of and their inter-dependencies during a for a energy storages in the systems with high penetration of distribution grid with EVs. For this investigation, various grid renewables. EVs can support the distribution grid in such simulations are conducted, and the results are presented. circumstances. The article [11] claims that EV fleet capacity, as a primary reserve should not exceed the reserve capacity from Keywords— DigSILENT; power quality; simulations; conventional rotating units. selfhealing; V2G; V2H; EV; drop Another important issue about V2G is spatial and temporal distribution of EVs [12], [13]. Amount of energy enabled for I. INTRODUCTION discharging on a specific charging station can be intermittent as The gradual transition of the transportation sector from fossil well as an output of sources. Data acquisition fuels to renewable electricity [1] and hydrogen [2] is taking and providing stimulation measures to use V2G when and where place in many countries. For instance, Norway provides it is needed for grid are tasks for V2G aggregators and Internet incentives, such as free parking, access to public bus lanes, road of Things developers and digitalization integrators [14]. toll waivers, a free network of EV charging stations and tax Motivations for EV owners to participate in V2G programs is benefits to stimulate the private adoption of electric vehicles also a topic for studies. Discharging the battery during the times (EV) [3]. Norway is known as a country where the sales of when the EV is on the charging station leads to additional vehicles with internal combustion engines will be terminated in degradation of it and reduce life durability. Publications [15], 2025. Such a systematic policy bears fruit: according to the [16] are dedicated for calculating the cost of discharging and official statistics, 60316 EVs were sold in 2019 [4]. It is more choosing optimal amount of energy to inject to the grid for the than 40 % of the market. Other EV leaders in Europe are maximum benefit for both grid and EV owner. Germany, the UK, and France [5]. The global leader with more than 1 million sales per year is China. The Government of China EVs as an emergency power source for self-supporting of the stimulates EV development and restricts investments in internal are described in [17], [18]. Suppling vital loads by combustion engines manufacturing [5]. energy from EV batteries helps to decrease the cost of outages. In this paper, we explore usage of EV as a support against The main driver of EV adoption policy is the Global Climate voltage drop during abnormal operation mode of double-fed Change agenda. Transportation sector share of global line. When double-fed line misses the primary connection to the greenhouse gas emissions is about 14-16 % [6]. Together with grid and is switched to the secondary one the voltage drop can the energy sector share (electricity and heat generation, overstep the boundaries of power quality requirements. The transmission and distribution give about 30 % of global European standard EN 50160 stipulates that the voltage in emissions [7]) it is almost a half of all man-made emissions. EV distribution grids is to be kept between 90% and 110% of its are considered as a one of the main sources of flexibility for nominal value during 95% of the supplied time [19]. EV energy systems with high penetration of renewables [8]. EVs batteries as a voltage source are investigated. In this study, we help to balance energy system due to Vehicle-to-Grid (V2G) introduced the main parameters of grid and EV that impact on technology [1] without launching additional fossil fuel power the voltage. plants during peak hours. Flexible charging infrastructure allows bidirectional energy exchange between the grid and the battery.

978-1-7281-9510-0/20/$31.00 ©2020 IEEE II. MODEL DESCRIPTION For the wider observation and further analysis, quasi- For the study, the existed is taken from dynamic simulations are performed for the period of one week DigSILENT Power Factory [20] library. It includes 0.4 kV and in December, which includes the maximum of load. From Fig. 3 10 kV grid, which supplied by external grid 30 kV. In fact, this and Fig.4, we can see that in normal regime, the voltage in all is the representation of the real electrical grid located not far feeder satisfies requirements and lines are not overloaded. Here from Gottingen (Germany). is line LN_0914 is the first section of feeder which is closest to substation. For the analysis, the feeder FD230 0.4 kV between substations 13-1 and 16-1 is taken. On the Fig.1, it is marked with grey color and red arrows. In normal regime, the breaker at Sub13_1 (lower) is open. Feeder supplied by Sub16_1 (upper). The number of customers on this feeder is 80, with peak power demand of 420 kW.

Fig. 3. FD 230 minimum and maximum in normal regime.

Fig. 4. Line LN_0914 loading in normal regime.

III. CASE STUDY Since lines are made with cables, and there is no sectioning with breakers, here we consider at the moment only faults with substation. In the particular case, it is Sub16_1. Whatever it is (, 10 kV line or 10 kV fault), there is no

supply any more for feeder FD230. There should some Fig. 1. Model for simulations. Distributed grid and feeder FD230. commutations to exclude fault from the loop and supply customers. In fact, now customers supplied from substation With the DigSILENT software, it is possible to see the 13_1. Simulations are made for the same period. However, since voltage magnitude at the specific time in all nodes of the feeder. the different distribution of loads among the feeder, now we can Fig.2 is the result of simulations, which made for maximum load observe voltage drops, which exceed limits as it presented on at 7 PM 21st of December. Fig. 5. Here it should be noticed, that depending on power flow, DigSILENT considers the same sections as different feeders. I.e. Feeder FD230 now is FD159 because it supplied from another substation Sub13_1. LN_0272 it is a first line if look from Sb13_1.

Fig. 5. FD159 minimum and maximum voltages in case of reconfiguration after the fault.

Fig. 6 shows that first sections of feeder are overloaded also. Fig. 2. Voltage at the nodes in normal regime. Together with the voltage problem, this is the issue to solve, because DSOs do not allow operating grid in such a regime, as lines could be damaged.

Fig. 6. Line LN_0272 loading in case of reconfiguration after the fault.

The most remoted from substation 13_1 customer at ND_1728 has biggest voltage drops. Here is voltage drop is around 19% as it presented on Fig.7.

Fig. 8. Voltage at the nodes in case of reconfiguration after the fault and centralized infeed of 80 kW.

However, as closer charging station to the supplying substation, more power it necessary. Table I represents results of simulations.

TABLE I. RESULTS OF SIMULATIONS Node ND_1728 ND_2077 ND_2079 Location from 420 meters 280 meters 140 meters substation 13_1 (end of line) (approx. 2/3) (approx. 1/3) Necessary amount of 80 100 170 power, P, kW EV share to the 19.0 24.8 40.5 maximum load, Ps/Ppeak, % Minimum voltage at ND_409; ND_1727; ND_1728; the any node of feeder, 0.8997 0.8987 0.8979 node; Umin, p.u. Fig. 7. Voltage at the nodes in case of reconfiguration after the fault. In case of distributed support from individual customers we One of the option to solve particular voltage problems are include into the model additional EV and spread them along the grid reinforcement and installation of OLTC (on load tap feeder. We assumed, that every 8-10th customer has own EV changes) which both increase costs [21]. The grid with charger and able to provide power support for the grid. We reinforcement is not considered here, because authors assume, made simulation for 8 and 10 EVs. Here we need to remember that EV’s can appear one by one and we show later that DSO that each model of EV has own individual capacity of battery can solve problem getting benefits from EV’s installation. and charging/discharging current. The parameters are taken Despite the costs, the installation of OLTC can reduce voltage from [23] and presented in Table II. drop, but will not manage with line overloading. TABLE II. PARAMETERS OF EVS As mentioned above there is a possibility to improve voltage Vehicle and Battery capacity, Max. charge/discharge power profile with infeed at some nodes of power from EV batteries. model C, kWh P, kW There are several options to control inverters such as const V, BMW i3 18.8 7.4 const Q, Q(P) and Q(V) [22] to provide a support to a grid, but Ford Focus 23 6.6 the most common and simple is const Q=0 which is applied in Nissan Leaf 30 6.6 the simulations. Moreover, there are two approaches to provide Tesla Model S 70 10 support: centralized and decentralized. In the first case, it could Firstly, the simulations made for 10 EVs, which can provide be charging station and the second case is individual charger of 10 kW infeed each. After that for 10 EVs with 6.6 kW discharge customers, when EV is plugged to the grid at customer side. In power. The positive thing in distributed support is that, power case of charging station, it is obviously, that to solve voltage flow is smoother and it helps to avoid in the feeding issue, it is better if the charging station located close to the node. point. From another side, we can see that in this case it should In this case, the feeder needs infeed around 80 kW from the be more EV connected to the grid compare to the case with batteries. The results of simulations are presented on Fig.8. charging station close to the end of feeder. The results are presented in Table III. TABLE III. CONFIGURATIONS OF EVS  Logistics. Where is and how much nodes for infeed. It Local infeed determined by logistic (streets and buildings location) 8 EVs x 10 kW 10 EVs x 10 kW 10 EVs x 6.6 kW pattern and type of charging station (centralized or local for a Amount of power, 80 100 66 house). P, kW EV share to the 19 23.8 15.7  Time of fault. When support is needed. maximum load, P/Ppeak, %  Power demands. What power is necessary to keep Minimum voltage at ND_1728; ND_1728; ND_1728; voltage in the limits. It depends on number and location the any node of 0.896 0.908 0.876 of stations, their type. feeder, node; Umin, p.u.  Time of power demands. For how long grid needs Results presented at Tables I and III obtained for the support. It depends from time of restoration and time of maximum load. It means that depending on time of an accident fault, which in turn determinates by and the amount of EVs support might be lower and correspondingly availability of vehicle (people on the way home, office, the number of EV plugged to the grid at the moment. For etc.). example we can see, that in case of 80 kW support needed it is  Support power. The amount of power can be provided only during few hours in considered week, when it is not enough depends on number of vehicles and their nominal output to keep voltage in range with such amount of power. With infeed power. of 66 kW it can be happened 19 hours in January.  Duration of power support. For how long batteries can provide depends on capacity of battery. It depends energy can be provided, i.e. present SOC of EVs batteries and minimum allowed SOC during discharge. As shown through simulations in case of equal availability of EV and their capacity, logistics plays the very important role. If the charging station located near substation, which shut down, the power for support is significantly less, than in other cases. In case of radial low voltage grid, it is more efficient to locate charging stations near substation with normally closed-circuit Fig. 9. FD159 minimum and maximum voltages in case of reconfiguration breaker. after the fault and decentralized infeed of 80 kW. Distributed infeed has advantages and challenges. It creates Coming back to the time of outages, we can identify that it less over voltages, but at the same time it makes things more is also a very important issue, because if it coincides with complicated in terms of control and operation. Generally, demand peaks, when the grid needs more support and more EV charging at home needs to increase the main fuse at the customer plugged to the grid. However, these peaks in turn, can concur connection point and this way increases customers’ costs. This with time of EV charging, which increases the total available approach is worse in sense of voltage profile and more capacity. For example, in the considered model, during working demanding to the number and brand of cars. Therefore, there is days peaks are after 7 PM, which means, that most probably a requirement for regularization of the EVs charging point and customers at home and their EVs are available for support. At procedure. The key factors and their interaction is possible to the same time, batteries SOC could be low after the day usage present as on Figure 10. and close to agreed minimum SOC with aim to constrain discharge the energy can be provided. The time of restoration also plays important role. As we can see from Fig. 9 in worth situation grid needs infeed during 3 hours. During that time grid needs infeed about 120 kWh.

IV. CONCLUSION During some types of faults in the low voltage, grid there is a possibility to supply consumers via reconfiguration in case of radial grid. However, such kind of solution might bring the problem in the voltage level. This situation is possible to avoid with the support of EV and their batteries, i.e. with the power infeed from batteries and this way changing a power flow. The possibility of support low voltage grid from EV has many input parameters and values need to be taken into account. Based on the simulations, the main aspects can be categorized as follows: Fig. 10. Key factors of EVs support and their interactions V. FUTURE WORK Industrial and Information Systems, Mangalore, 2010, pp. 481-485, doi: 10.1109/ICIINFS.2010.5578655. It is indicative from the simulations that the EVs can play an [11] A. Zecchino, A. M. Prostejovsky, C. Ziras, and M. 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