Evaluation of OCPP and IEC 61850 for smart charging electric vehicles

Interoperability of charging infrastructures is a key success factor for E-Mobility. Standards like ISO/IEC 15118 and IEC 61851-1 are developed to ensure base level interoperability of front-end communication and signaling processes for smart charging between electric vehicles and charge spots. With the Open Charge Point Protocol (OCPP) a forum of European industry members also moves towards a common back-end protocol for charge spots intending to reduce and secure overall investment costs. However, in the current form OCPP lacks means for enabling grid services based on smart charging. In this paper the authors provide a review of today's state of the art in ISO/IEC standardization of the V2G Interface and furthermore detail how OCPP could leverage existing standardization efforts for grid automation from IEC 61850 in order to overcome its shortcomings.


Introduction
With the introduction of EVs certain challenges arise for the power grid since it was deployed at times, when such additional loads were not yet considered.Assuming mass market adoption of EVs this inevitably comes along with high chances for concurrent charging processes and therefore overload of low voltage grid segments.In order to address these issues and ensure interoperability between charging infrastructures of various vendors, standardization bodies like ISO and IEC are busy defining corresponding standards for E-Mobility.As of today, core efforts were spent in front-end issues of E-Mobility, like plugs, in-& outlets, electrical and safety requirements, as well as the front-end communication interface in order to ensure common access to EV charging infrastructures.Back-end and grid integration approaches were developed individually as part of research and pilot projects leading to proprietary solutions reflecting the situation on today's market.However, in terms of operational aspects as well as for securing the industry's investments in charging infrastructures it may be beneficial to standardize back-end integration aspects of charging infrastructures in the long term.According to the European Commission's climate and energy targets set in 2007 as well as following directives like 2009/28/EC [1] on the use of energy from renewable sources, EVs are considered to provide their share reducing CO2 emissions in the transport sector.Being shiftable loads as well as regularly available energy storages in the low-voltage grid, they are furthermore supposed to provide a certain benefit to grid stability in conjunction with increased penetration of renewable energy sources.In that sense the integration of EVs and charging infrastructures need to be extended beyond the current scope of purely operational aspects being covered by today's solutions.In order to integrate seamlessly in tomorrow's grid infrastructures a look into the IEC standardization roadmap [2] is necessary.They define IEC 61850 as core standard for grid automation and consider it as the core driver of Smart Grid development in the mid-and long-term.Hence this work analyzes the alignment of the Open Charge Point Protocol (OCPP) -today's EVS27 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 1 World Electric Vehicle Journal Vol. 6 -ISSN 2032-6653 -© 2013 WEVA Page most commonly used and openly available backend communication protocol for charging infrastructures -and IEC 61850 in order to overcome today's lack of grid integration for charging infrastructures of EVs.In sections 3 and 4 both approaches for back-end integration are evaluated and differentiated from each other.In section 5 the authors summarize the lessons learned and provide an outlook on smart charging using OCPP and IEC 61850.But before going into such detail, some background on operational aspects of charging infrastructures and how plugged-in EVs can be characterized as grid assets is needed.Hence, the following section provides an overview on current state of the art considering the E-Mobility V2G Interface.

Standardization Landscape of the E-Mobility V2G Interface
Standardization for the V2G Interface in E-Mobility already has a long history which lately experienced major increase of interest and development.The global interest in reducing CO2 emissions in the transport sector and its resulting economical interests impacting two of the worlds largest industrial domains (utilities and automotive) led to extensive industrial support in research and development.The resulting standardization landscape of E-Mobility can be classified in the following four domains: 1. Plugs (in-& outlets) 2. Charging Topologies 3. Communication 4. Security

Safety
The remainder of this section will walk through each domain highlighting corresponding ISO/IEC standardization work.An overview of the landscape is shown in Figure 1.
Electric Vehicle

Plugs (in-& outlets)
The IEC 62196 standards series is defined in TC23/SC23 and currently comprises three parts.

Charging Topologies
Different charging topologies need to be considered for conductive AC-and DC-based dedicated charging equipments.

Communication
In order to define the V2G  The envisioned high level communication protocol will allow for advanced interaction between the grid and connected EVs for authentication & authorization, accounting, load leveling management and further added-value services.The primary scope of the standard is limited to the communication between the EVCC and SECC as illustrated in Figure 3. is particularly interesting for the E-Mobility domain.However, the scope of IEC 61850 is much broader and originates from the substation automation domain.The standards comprises more than ten parts where those of relevance for this work will be more closely discussed in section 4.

Security
Due to the extended amount of Machine-to-Machine (M2M) communication in the context of E-Mobility as introduced in 2.3, there is substantial need for information and communication security in the context of smart charging EVs.
In contrast to conventional road vehicles, EVs regularly connect to public and heterogeneous infrastructures and exchange partly confidential information.For the OEM domain this is a fairly new use case scenario with special requirements in terms of information security as well as privacy.Hence, information security plays a key role in the definition of ISO/IEC 15118-2.Next to securing the communication channel between the EVCC and the SECC through the establishment of a TLS channel, some endto-end security requirements are addressed between the EVCC and secondary actors, like charge spot operators, service providers, etc. (see Figure 3).Most dependencies on such external secondary actors are limited to authentication/authorization for service consumption, tariffing/billing issues or grid operation purposes like negotiation of dynamic grid limits.Message elements of ISO/IEC 15118-2 with relevance regarding these aspects are handled through endto-end security by XML security, particularly XML Signature [5] and XML Encryption [6]

Back-End Communication Relevance
The review of the E-Mobility standardization landscape from previous subsections allows us to assess the relevance of each domain regarding front-and back-end communication.The OCPP specification currently defines the following list of largely self explanatory message sets which are shown in Table 2.All these messages are defined in the WSDL description and describe atomic operations between the invoking client and its corresponding service.Other than WS-based message transactions, OCPP also defines FTP file transfers (see Figure 5) as part of the firmware update process (FTP download triggered by Update Firmware) and for diagnostic purposes (FTP upload triggered by Get Diagnostics).Based upon the experience the authors gathered during the implementation process of OCPP 1.5 clients and services we provide our lessons learned from following three perspectives: Functional scope/limitations of OCPP, issues and drawbacks from an operational / technical point of view, and maturity of the OCPP 1.5 specification.

Functional Scope and Limitations
Starting with the functional scope of OCPP 1.5 we found in section 3.1 that in its current form OCPP is highly tailored towards the charge spot operator's business.It fulfills all main requirements regarding the charge process, such as authentication and authorization, transaction handling, metering, reservation of a charge spot.It also supports maintenance tasks like status notifications, configuration and firmware handling, diagnostics, and internal support functions like resets, clearing of caches etc.It provides means for vendor-specific and therefore proprietary extensions for an operator's internal use.However, grid-related services, e.g. demand management by utilizing the functionality of ISO/IEC 15118 are currently missing.Smart charging of EVs in the sense of a grid service is therefore not yet considered in OCPP 1.5.The specification furthermore does not consider Role Based Access Control (RBAC) which might become essential in the future.

Technical Issues and Drawbacks
The use of SOAP-over-HTTP has many advantages in terms of the software development process for charge points due rich tool support, as well as out of the box syntactic interface conformance based upon the provided WSDL definition.However, the use of SOAP-over-HTTP also leads to huge message overheads which may be an issue for some operators and could be optimized without loosing out on convenience of Web Services.Particularly in case of wireless back-end communication links like cellular radio access and in case of large scale charging infrastructures, this issue might lead to rather expensive maintenance costs for operators.By utilizing presentation layer formats like Efficient XML Interchange (EXI) these issues could be alleviated.As the authors already showed in [8] using EXI provides a compression rate of up to 5-10% for the message payload size of V2G messages in ISO/IEC 15118.Similar improvement would be feasible in case of using EXI for OCPP.Further improvement could be yield by opting for ICMP ping-like heartbeat messages instead of using a SOAP-based Request-Response MEP.

Specification Maturity
Based upon our own experience with implementing OCPP we rate the maturity of the specification as average.Requirements should be clearly identified by a numbering scheme in order to allow for requirement traceability in the development as well as in the testing phase.Conformance and interoperability issues are quite common for various OCPP implementations since the specification is quite vague e.g. in terms of for handling security.With the help of example stubs and skeletons provided by the OCPP forum as well as third party conformance testing tools like EURISCO's OCPP TestBench (see Figure 6), concrete conformance and testing tools are available today, alleviating the lack of a concrete conformance test specification.

State of E-Mobility in IEC 61850
In today's published series of IEC 61850 standards, EVs are not yet considered in IEC 61850-7-420 [11] as Distributed Energy Resources (DERs), even though EVs could act as energy storage for volatile energy generators such as wind power plants or PV plants.The authors already proposed an object model for electric mobility in [12] which was furthermore published in more detail as IEC Technical Report 61850-90-8 in the meantime [13].As shown in these previous works the proposed object model satisfies the requirements of ISO/IEC 15118 as well as IEC 61851-1 in terms of monitoring and This section briefly summarizes the underlying approach.The first step in the modeling process was the analysis of information being provisioned according to the E-Mobility standardization landscape described in detail in section 2. As part of these standards a multitude of control and monitoring information is exchanged between technical components being involved in the charging process in order to ensure an automated and safe charging process.However, for managing EVs as DERs in the grid only a subset of this information is necessary and must be identified.In order to ensure a scalable system design it is explicitly required to reduce level of detail in terms of information provisioning for upstream stakeholders, e.g.infrastructure operators.The second step in the modeling process addressed the operators view on an interconnected EV.In accordance with the first step, necessary input and output parameters for monitoring and control purposes must be identified and modeled accordingly.The modeling approach taken for this work adheres to IEC 61850-7-420 guidelines.(green) describe dynamic configuration information of an entity which may change over time and may be influenced from a secondary actor, like the EVSE operator.Measurements (red) are observed at the respective entity and also represent dynamic information.The actual information being provided by the respective entities is summarized in the common information model (orange) in Figure 9 and is mapped to the following three newly defined LNs of the E-Mobility Object Model: 1. DESE: This LN represents an EVSE which may house several outlets and contains information related to monitoring and controlling of the EVSE.

DEOL:
This LN represents an individual EVSE outlet and contains information related to monitoring and controlling of the outlet.

DEEV:
This LN represents a connected EV and contains information on an EV connected to an EVSE.If the connection / plug status indicates that no EV is connected the DEEV LN is unregistered.
The DSCH LNs in Figure 9 are re-used from IEC 61850-7-420 and cover the two way charge schedule negotiation handshake of ISO/IEC 15118 (see also [14]).In addition and as shown in Figure 10, a charging infrastructure operator may include further LNs known from the already existing portfolio of LNs in IEC 61850-7-2, -7-4, -7-420 if necessary in order to represent the charging infrastructure's setup.Note, Table 3 only refers to newly defined LNs as part of [13] that are also shown in Figure 9.Other services and respective functions may be added by the means of already existing LNs according to IEC 61850-7-2, -7-4, -7-420, e.g.metering capability through the Logical Node MMTR.

Evaluation of IEC 61850 for E-Mobility
In line with the evaluation of OCPP in section 3.

Technical Issues and Drawbacks
The issue with IEC 61850 today is that coming from the substation domain it is not well integrated in heterogeneous environments due to its currently very restrictive and inflexible binding technology which is based on the Manufacturing Message Specification (MMS) dating back to the 1990s.New binding mechanisms based on Web Service technologies like SOAP or REST would allow much better integration with assets from other domains as well as common IT back-end systems and would furthermore simplify development for spot operators due to a much larger tool-chain support.These aspects including a set of solution concepts are also already detailed by the authors in [15] and supported by others in [16] and [17].

Specification and Standard Maturity
The proposed E-Mobility Object Model for IEC 61850 was published as technical report in TC57.It is currently revised to align with ongoing standardization processes of surrounding standards in the E-Mobility landscape, e.g.ISO/IEC 15118.In general, IEC 61850 has a very long history and builds upon a mature set of specifications.However, the complexity of the standard, its long learning curves, and the complex standardization processes certainly are disadvantages.New Web Servicebased bindings might help to simplify application of IEC 61850 and gain larger audiences in the industry.Especially in the domain of DERs it may help for quicker adoption rates, like it has already been adopted for wind power generation in IEC 61400-25-4.

Lessons Learned
The previous analysis showed, that the technical approaches taken in OCPP 1.5 and IEC 61850 with the proposed E-Mobility Object Model are quite different.Also from a functional point of view there are only marginal overlaps between both candidates.OCPP 1.5 clearly addresses the business domain of charge spot operators which are not necessarily directly interested in smart charging of EVs.IEC 61850 on the other side is all about grid automation and therefore closer to grid stakeholders like plant and distribution system operators.With the trend towards an increasing amount of distributed energy supply, the need for integrating EVs as DERs with a certain storage capacity and controllable demand curves will become more crucial in the long-term.From a technical perspective our evaluation on OCPP and IEC 61850 for smart charging of EVs results in the following three feasible scenarios: • Leveraging the IEC 61850 E-Mobility Object Model as Meta-Model for enabling Smart Charging in OCPP.
• Integrated approach for OCPP and IEC 61850 based on new IEC 61850-8-2 Web Service Binding.
• Outlook on future releases.

IEC 61850 E-Mobility Meta-Model
The first option describes the situation where one or several implementations exist using the IEC 61850 E-Mobility Object Model as Meta-Model and abstract service interface but without the Specific Communication Service Mapping (SCSM) according to IEC 61850.This approach would ensure the use of a harmonized and commonly agreed information model across various back-end communication protocols.However interoperability would fail on the interface level due to the lack of vertical integration of different SCSMs.  2 and 3.

Outlook
The market may also develop in an entirely different direction based upon a different/new approach not considered in this work.At the time of this writing OCPP 2.0 is also being prepared for release.According to the OCPP roadmap, many of the issues highlighted in this paper are being addressed as part of the new release.Most importantly, OCPP 2.0 will integrate support for ISO/IEC 15118, which marks the most important point of criticism in this work.However, since OCPP is solely tailored towards the management and operation of EV charging infrastructures, it will not provide seamless integration with other types of DERs in the future as opposed to IEC 61850.The same remains true for all other types of dedicated charging infrastructure back-end protocols.A plant operator who is envisioning to consolidate demand and supply from various heterogeneous sources, e.g.VPP, would need to support a multitude of protocol stacks coming along with an additional processing burden for the plant controller as well as potential interoperability issues and versioning conflicts.This sought of software fragmentation would furthermore complicate the software development and maintenance processes for the controller.In such scenarios, the option in 5.2 with support of IEC 61850 would be beneficial, since all tasks could be handled according to the same paradigms set by IEC 61850 across all types of DERs being used throughout a heterogeneous plant setup.Hence, operators who in the

Figure 1 :
Figure 1: Digest of main ISO/IEC standards for the scope of the E-Mobility V2G Interface based on[3]

Figure 6 :
Figure 6: Screenshot of EURISCO OCPP Test Bench Software with SOAP Messaging Log

Figure 10 :
Figure 10: Example AC-Charging Deployment Scenario of the E-Mobility Object Model

NOT in scope of ISO/IEC 15118: Backend Communication Interfaces to Secondary Actors Electric Vehicle Primary Actors IN scope of ISO/IEC 15118:
World Electric Vehicle Journal Vol. 6 -ISSN 2032-6653 -© 2013 WEVA Page Today, the IEC standards landscape does not cover a specific back-end protocol for EV charging infrastructures.One potential candidate may be IEC 61850 which focuses on grid automation for various types of grid assets.Its latest advancements in the direction of automation in the context of Distributed Energy Resources(DER) such an alternative carrier will also bring along new use cases even for conductive charging scenarios.A complementary set of use cases suitable for wireless carriers are therefore defined in ISO/IEC 15118-6.An additional alignment specification targeting document ISO/IEC 15118-7 will introduce all differences and new message sets compared to the core message specification (ISO/IEC 15118-2) in powerline communication based scenarios.However, all these parts are in very early stages of the standardization process.IEC 61851-24 defines the digital communication between a DC charging equipment and an EV for the control of the charging process.The standard is still work in progress and currently details various different technical approaches in its annex.tion in any way.The same applies for the proposed work in IEC 61980-2 for inductive charging which is supposed to be referencing the upcoming parts 7 and 8 of the ISO/IEC 15118 series.However, at this time all standardization efforts regarding wireless communication for either inductive or conductive charging of EVs are in very early stages.EVS27 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 3

Table 2 :
Supported Operations in OCPP 1.5 EVS27 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 5 World Electric Vehicle Journal Vol. 6 -ISSN 2032-6653 -© 2013 WEVA Page EVS27 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium for several outlets, the DSCH schedules either refer to a single outlet and its corresponding EV or an aggregate of all managed outlets and the corresponding set of EVs.The integrated sequence including the ISO/IEC 15118 charge parameter negotiation process as well as handling of the DSCH Local Load Limit Profile and the DSCH Local Load Reservation Profile is detailed in Figure 11.

Table 3 :
Overview of Supported Operations of IEC 61850-90-8 E-Mobility Object Model World Electric Vehicle Journal Vol. 6 -ISSN 2032-6653 -© 2013 WEVA Page EVS27 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 9 [13]escribed previously, the need for a Web Service binding for IEC 61850 becomes more and more important with new sorts of DER types being added to the IEC 61850 portfolio.Since OCPP is already based on a SOAP-based message framework and binding mechanism, it would be attractive for charge spot operators to use the same message framework and communication stack and integrate both OCPP and IEC 61850.The advantage of such a solution would be, that EV charging infrastructures could still be integrated as portfolio enhancement into other IEC 61850-based pools of DERs, like combined heat & power, PV etc. without breaking either the IEC 61850's or OCCP's paradigms.In order to combine the existing functionality of OCPP with automated grid services through smart charging of EVs according to[13], additional service(s) based on the IEC 61850 E-Mobility Object Model must be deployed on the charge spot, next to the OCPP Charge Point Service.Figure12illustrates the overall approach.The WSDL of the new IEC 61850 services corresponds to the IEC 61850 ACSI and maps the E-Mobility Object Model which is defined in XML Schema to the supported set of operations and messages (WS-based SCSM).The applicable set of operations resulting from a combined setup with OCPP 1.5 and the IEC 61850 E-Mobility Object Model would be a joint version of Tables World Electric Vehicle Journal Vol. 6 -ISSN 2032-6653 -© 2013 WEVA Page EVS27 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 10