THE 8th INTERNATIONAL SYMPOSIUM ON ADVANCED TOPICS IN ELECTRICAL ENGINEERING May 23-25, 2013

Bucharest, Romania

Grid Connection Rules for Electric Cars Integrated as Virtual Power Plant in Smart Grids

Krisztina Leban1, Student Member IEEE, Ewen Ritchie2, Member IEEE, Paul Bach Thøgersen3 , Alin Argeseanu4 … 12Dept. of Energy Technology, Aalborg University, Denmark; 3KK-Electronik a/s, Denmark

3Politehnica University Timisoara, Romania, Faculty of Electrical Engineering and Power Systems kle@et.aau.dk, aer@et.aau.dk, patho@kk-electronic.com,alin_argeseanu@yahoo.com

Abstract This paper reviews the situation of V2G and proposes a solution involving a consolidating fleet manager, and a decision making process for the individual V2G electric car owner.

A grid connection routine for electric vehicles is proposed. The algorithm dealing with decisions to be taken in foreseen circumstances is presented.

Variables and rules employed used in the grid connection control are detailed. Definitions of terms are stated to clarify the functionality of the algorithm.

I. INTRODUCTION

The current Danish Energy Strategy requires that by 2025, 50% of electrical energy in the Danish grid system will be wind power. It may be expected that this will exacerbate the power flow problem as indicated in Fig. 1.

Fig. 1 The impact of ‘more wind energy on the grid. [3].

For improved large scale integration of wind power, which is an intermittent energy source, an energy storage device may be deployed. An example energy storage device could be a battery electric car that may be used to store energy when the demand is low, and deliver energy to the grid, when needed. To provide sufficient storage capacity a large number of parked electric cars, connected to the grid, would be required. In this way it may be possible to support the grid in the presence of many renewable energy sources [1]. If sufficient electric cars were available, the power flow problem and other grid problems could be resolved by integrating them into the grid as virtual power plants (VPP). For example, power regulation could be provided for grid services by allowing bidirectional power flow from the grid to the battery of the cars and vice versa. Generically, this principle is known as V2G (Car to Grid). The idea is attributed to Kempton (University of Delaware, USA, [2]. From the 1970’s, when his work began, to the present day, Kempton and his team of researchers have developed the idea to an applicable state [3], [4], [5].

Fig. 2 presents a schematic drawing of an electric car connected to the grid, V2G. The car battery is charged via the voltage and frequency adjusters (transformer and power converter). The power supply is connected to the battery through a four quadrant traction converter. The same converter controls the motor during drive operation when the grid power supply is disconnected. The power flow is made bidirectional for both charging and extended grid support functions. The on and off board meters quantify the circulating energy. The communication system monitors and transmits data between the grid and the car.

Fig. 2 Electric Car connected to Grid [6].

If a system such as this were to be taken into use, it would need to satisfy the needs of both the grid operator and the car

owners. A problem is that the car is a very small unit compared to the grid. According to [7], power-generation facilities/Adjustable consumption may be classified as:

*Adjustable facilities ≥25 MW *Adjustable facilities ≥10 MW, including adjustable wind power ≥10 MW *Adjustable facilities <10 MW (other than wind power) *Non-adjustable wind power (turbines and farms without the possibility of regulation from a central control room) *Adjustable consumption ≥10 MW *Adjustable consumption <10 MW.

In this classification ‘adjustable’ means ‘controllable by the grid operator from a central control room’. It may be seen that the grid operators like to operate with minimum controllable units of 12 MW, which is much larger than an electric car charging from a Danish domestic power supply of 3*400 V 16A, i.e. about 10 kW. Another problem is that the electric car owner will suffer a reduction in the lifetime of his battery by assisting the grid in this way.

978-1-4673-5980-1/13/$31.00 ©2013 IEEE

II. CAR OWNER REQUIREMENTS

The car owner requires that his car fulfills his transport needs as and when he has to travel. He has invested in what for him is an expensive item and he must have the appropriate return for this. The first priority is that the car is available to drive the required distance at the desired time. That is to say that at the time the driver returns to the car, the state of charge is such that he can complete the next planned journey, and have some reserve, say 100 km worth of charge. Second priority is that the investment has a long lifetime of availability for driving. Implied in the word ‘drive’ are all the components we have learned to accept when we drive an internal combustion engine driven car. The car must keep pace with the rest of the traffic, be silent, keep all passengers comfortably warm or cool, have lights, windscreen wipers, etc. A tertiary priority is to use the battery to help the power system. This means that the car owner will require full compensation for the service he provides to the grid utility owners.

The battery is a very expensive component in an electric car. In recent years the battery of an electric car has proved to have a shorter lifetime than that predicted by the manufacturers. It may be expected that increasing the number of charge/discharge cycles will reduce the battery lifetime. At all times the car owner will need to be certain that his immediate transport needs will be met. For these reasons, any car owner will want to determine if his car is available to support the grid or not, and if he needs immediate charging or can afford to wait. If the car is made available to support the grid, then the car owner will want to review the current remuneration rate for the service provided and decide on this basis if it compensates the expected reduction of battery lifetime caused by providing the service. He needs a simple and quick way of estimating his current overall needs. Fig. 3 illustrates some of the information useful to the car owner in making the decision and gives three buttons enabling him to communicate his immediate wishes. This can form the basis for an algorithm for an automated decision making process to be installed in the car.

Fig. 3 Some of the information required by the car owner and a simple interface enabling the car owner to exert apt influence the decision making process.

III. GRID REQUIREMENTS

For the grid operator, a single electric car is a vanishingly small VPP unit. In Denmark, grid operators wish to control

from the central control room units no smaller than 10 MW. Several methods are in use for regulating power, [13]. An example of these is ‘Regulating Power Ordered via Scheduling’. Another example is ‘Direct Activation of Regulating Power. Both of these require communication between the grid operator and the provider of power, in this case the VPP.

For reactive power greater than 25 kVAr requirements are imposed on wind generators for the control band within which the reactive power must be kept. Fig. 4 shows a typical requirement for exchange of reactive power with the grid.

Fig. 4 Requirements Imposed on a Wind Turbine Generator For Exchange Of Reactive Power With The Grid [14]

The requirement for grid support will vary hour by hour in the course of the day; see Fig. 1. This means that the grid operator will want to vary the price he is willing to pay related to the magnitude of the support he needs. To obtain the required VPP capacity some form of aggregator is proposed, a so-called ‘Fleet Operator’. Each Fleet Operator will have a minimum orderable power rating of 10 MW, obtained by subcontracting to the necessary number of electric cars. The number of electric cars in the fleet must be such that, at any one time, stochastically, sufficient cars are connected to the system and made available for grid support purposes, and willing to offer the service for the remuneration that the grid supplier is offering.

IV. REQUIRED INFRASTRUCTURE

A. Management Strategy When connecting to the grid, a given car will fall under

the management of a certain aggregator company. At this point, in order to be able to function as a part of the aggregator’s smart V2G VPP, the car and the aggregator must be able to communicate effectively.

The more data is sent to the aggregator, the more precisely the situation is known. Logging of events and storing the log for virtually each car would be a main concern. Other issues like hacking and data corruption are some of the consequences of introducing a complex data communication system. Once reliable information is available, interpretation and further handling would yield new business and engineering opportunities

To enable universal simple charging or V2G operation, a standard plug and socket will be necessary. Provision will

need to be made for home charging and for public charging stations. For simple charging, a normal energy consumption meter will be adequate, but for V2G, two way metering will be necessary.

Fig. 5 An example SAE J1772 combo plug [25], provided with connectors for AC, DC charging and data transmission.

B. Plug Standards In order to enable charging at random charging stations, a standard plug and socket will be required. Several types of plugs have been proposed. An example of one of the proposed standard connectors is shown in Fig. 5. This type has charging power connectors for DC and three phase connectors, and also a data connection. It seems that different standard plugs are required, depending on the country of charging.

C. Charging Stations Home Charging Stations

The metering at home charging stations could be arranged in several different ways. As an example, one of them is shown in Fig. 6. As this arrangement allows metering in both directions, and thus V2G grid support, data communication with the aggregator will be necessary.

Fig. 6 Example of a home charging station with direct connection. M1 meter charging supply; M2 meter generation to grid; M3 Meter own domestic consumption.

Public Charging Stations Public charging stations could be provided at public car parking places and also at public ‘gas’ stations. The gas station solution cannot be involved in the V2G scenario, as its business is the high speed charging of cars, enabling them to extend their range on long journeys. The car park solution could provide multiple charging stands, or all stands provided with charging equipment. As there are few electric cars when

they are first introduced, the pragmatic solution will start with a few charging stands on a car park, and gradually increase this as the number of electric cars on the road increases. As this type of car park is public the problem arises that internal combustion engine drivers may choose to park their car in an electric car charging space so that the space is unavailable for charging electric cars. This may be a question that is only soled by introducing legislation.

D. Metering Electricity meters are the universal way of invoicing for consumed electrical energy.

-Metering in cars -Metering outside cars

E. Grid operators F. Local Electricity Distributors G. Standards

As electric cars are a fairly new commercial product, standards for the cars themselves as well as for integration into other systems (like smart grids, intelligent homes) are still being discussed.[25], [26]. To be able to effectively participate in various markets, a standard for the communication elements must be agreed upon. Within the European Union, cars travel ‘freely’ from country to country. • Standards, from the charging station to the grid system • Standards are discussed in [27] • Developing standards for electric cars[25], [26] may be

found in IEEE P2030.1 • power quality[28] • For the plug J1772_201210 (see [25], SAE international

standards). This allows AC and DC charging in USA and other countries using SAE standards.

Compatibility between P1901 communication and 802.x standards is achieved through IEEE P1905. With this, IP-based communications between the car, metering apparatus (distributor), and the charging station may be achieved. P1905 includes wireless communications capabilities. Standards for communication between the car and the charging station through the plug are still under discussion. HomePlug [30] - specifications orientated towards smart grids and implicitly, smart houses. It is based on IEEE 1901. Versions: o HomePlug 1.0 o HomePlug AV o HomePlug AV2

o HomePlug Green PHY o HomePlug Access BPL.

are under consideration

V. DEMONSTRATOR

To demonstrate the possibilities and improve the understanding of V2G a demonstrator was made and tested in the laboratory. The system comprised two Digital Signal Controllers, one each for the decision making system in the car and on the charging unit. A PC was programmed to emulate the aggregator, who manages the interface to the grid controller.

LabviewInterface

DSP ChargingStation

DSPCar

UserButtons

Fig. 7 General arrangement of demonstrator system.

An algorithm implementing the set of grid connection rules was programmed on two Texas Instruments TMS320F28335 Kits. One was for the car and one for the charging station. The aggregator was emulated by a PC. Using a Labview interface, the rules were tested using the boards. Due to continuing discussion of standards and specifications, certain modules were bypassed and simple laboratory solutions were applied instead. An example of this is the communication between the car and the charging station-wires transmitting voltage levels(0-3V). Once standards are known, a communication module will replace the current solution. The setup is shown in Fig. 8

Fig. 8 Photo of the test arrangement showing the two DSC boards.

The user interface was programmed in Labview and is shown in Fig. 9.The objective was to build a simple interface that enables further development as understanding of V2G operation is better understood. The link between the set of grid connection rules, the hardware and the software was implemented. The signals measured were successfully transmitted between the different components of the system.

Control Strategy and Communications -Server analysis and communications structure. Learning Mechanisms - levels [23], [24]. If the car user accepts to share his personal data (e.g. daily schedule), to build up a log, learning algorithms could produce a driving profile. Define the information to be exchanged between the various stakeholders. The information flow between stakeholders should contain a set of data. To ensure compatibility, a default structure of the information is needed. An initial list of fields for the data package is presented below. As the system develops and evolves, additional fields might be added

-Car identification-uniquely identified the car in the world (the assumption that the car could travel/be shipped to any part of the world)

-Billing information-may be supplied on site or linked to the contract settings -Selection of the subset of V2G electric cars, if only a portion is to be activated. -According to grid requirements, random financial optimization, wear on the batteries, fair distribution.

-Contract settings- This is a collection of agreements between the car owner and the managing entity. In special

cases, the contract setting may be a default ‘empty’ one-no agreement-e.g. the car would act as a simple load. The fields in the contract settings may be:

-car owner -car agrees to V2G grid support if the ‘price is right’ -definition of the ‘right price’ -The right of the car user to override V2G grid support (activate simple load button) while under contract -maximum charge value per month -Privacy allowances - use of car data for statistics

Fig. 9 Labview interface of the laboratory system. Decisions are made based on the charging and discharging prices, the battery state of charge and the requirement for grid support.

IT infrastructure - communications server What hardware and software is necessary to use electric cars in VPP mode and intelligent network management? The main hardware is represented by the battery, drive system and platforms for implementing controls. The software will evolve in time as new features and business models are developed. Consequently, the software should be designed in such a way that it would accept new models that may be easily linked to the existing ones. Compatibility between versions is also an important characteristic. For a certain car, features will need to be enabled and disabled respecting the agreement between the agreement between the car owner and the aggregator. This feature alone gives an advantage to the smart car as it allows for flexibility and adaptation to the needs of the stakeholders

VI. SIMULATION MODEL

These rules were first implemented in a Simulink model. After testing for the proper functioning of each decision path,

the code was written in microprocessor language (C code). The developed simulation model comprises the control of the individual car when connected to the supply grid. The flow chart for the simulation model is the same as for the demonstrator.

Fig. 10 Flow chart for charging station DSP.

Fig. 11. Flowchart for the car DSP.

A number of situations have been simulated and dealt with by the controller.

Issues treated: -Monitor the difference between the energy production and consumption. -Connection and correct functioning of car-charging station communication paths: determine if the data bus and power connection enabled/connected and are functioning as desired? -Override capability of the car/user of the grid demands-the car user has primary control over the battery charging/discharging process and the battery state of charge.

Initially the preference profile of the user is recorded. In the contract between the user and aggregator (fleet operator) it should be stipulated whether the car will offer grid support (charge and discharge the battery as the grid needs within driving profile limits). Another factor in the response of the car is the state of charge of the battery: if battery is full, charging is obviously not possible even though the grid is asking for it. The same type of physical limitation applies when the battery is discharged and the grid needs extra energy from the cars. It is important to note that

discharging will not start unless the minimum amount of battery level plus a reserve amount exists. The results of the Matlab Simulink program are presented in Fig. 12 In Table 1, all push-button override possibilities are presented. The logical value ‘1’ is attributed for the enable state and ‘0’ for the disable state. !!!at this point in the design, all logical combinations are presented. Some possibilities might be qualified as ‘reserved’-inactive or disabled in the future to their inapplicability. (Discussion required). The vector values are used to express the upper mentioned possibilities. In the Description column, the red explanations represent issues that are not yet decided on. In Decision CS value (CS=car status-the response the car gives after reviewing the request of the aggregator) In Table 1, Table all push-button override possibilities are presented. The logical value ‘1’ is attributed for the enable state and ‘0’ for the disable state. !!!at this point in the design, all logical combinations are presented. Some possibilities might be qualified as ‘reserved’-inactive or disabled in the future to their inapplicability. The vector values are used to express the upper mentioned possibilities. In the Description column, the red explanations represent issues that are not yet decided on. In Decision CS value (CS=car status-the response the car gives after reviewing the request of the aggregator)

VII. CONCLUSIONS

Some of the issues arising with the possible introduction of grid support features, known as V2G, have been presented and discussed. The main issues are associated with standardization. Additionally, it seems that it would be advantageous to create an aggregated fleet as an intermediary between the grid and the individual electric car. A demonstrator decision making unit has been made and is ready for testing. The decision making process has been emulated in a simulation model using the same algorithm as for the demonstrator. After making his original profile, the car owner has three simple buttons to indicate his choice of service at any charging station.

ACKOWLEDGEMENT

The work presented in this paper is a result of the research project, "Tomorrow’s high-efficiency electric car integrated with the power supply system". The authors acknowledge the contributions from the other project members of WP9 group their inputs. The project is supported by the EU Regional Development Fund, and the Danish Erhvervsstyrelsen (Commerce Agency).

TABLE 1: VEHICLE STATES DUE TO OVERRIDE SETTINGS Simple load

Fast charging

Grid Support

Vector value Description Decision

CS value 0 0 0 0x0 000 all options disabled – car standing by CS=1 0 0 1 0x1 001 grid support enabled Grid support if advantageous for car owner; Standby 0 1 0 0x2 010 fast charging enabled Fast charging to max Standby CS=1 0 1 1 0x3 011 Fast ch. does not support the grid Standby(until resolved) CS=1 1 0 0 0x4 100 simple load enabled Charges to max CS=2; Standby CS=1

1 0 1 0x5 101 simple load and grid support enabled simple load CS=5 until grid support advantageous CS=2 or CS=3 or smart charging?? CS=2 or CS=3

1 1 0 0x6 110 simple load and fast charging enabled standby (until resolved) CS=1 1 1 1 0x7 111 all enabled - choose default mode standby (until resolved) CS=1

Grid -1 – grid needs power

0 – grid is balanced 1-grid needs to store

CT 0 -disconnected 1 – standby 2 - normal charging

3 - discharging 5 - simple load

CS 0 -disconnected 1 – standby 2 - normal charging

3 - discharging 4 - fast charging 5 - simple load

fuel Monitoring of battery level

test In each branch of the code, the variable test takes another value, to indicate the section being executed

Standby mode -1-no standby

0 – passive standby 1-active standby

Fig. 12 Monitored System- Simulation results

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