Auto-Mapping and Configuration Method of IEC 61850 Information

energies

Article Auto-Mapping and Conﬁguration Method of IEC 61850 Information Model Based on OPC UA

In-Jae Shin 1, Byung-Kwen Song 2,* and Doo-Seop Eom 1

1 Department of Electrical Engineering, Korea University, Seoul 02841, Korea; [email protected] (I.-J.S.); [email protected] (D.-S.E.) 2 Department of Electrical Engineering, Seokyeong University, Seoul 02173, Korea * Correspondence: [email protected]

Academic Editor: Gianfranco Chicco Received: 24 June 2016; Accepted: 20 October 2016; Published: 1 November 2016

Abstract: The open-platform communication (OPC) unified architecture (UA) (IEC62541) is introduced as a key technology for realizing a variety of smart grid (SG) use cases enabling relevant automation and control tasks. The OPC UA can expand interoperability between power systems. The top-level SG management platform needs independent middleware to transparently manage the power information technology (IT) systems, including the IEC 61850. To expand interoperability between the power system for a large number of stakeholders and various standards, this paper focuses on the IEC 61850 for the digital substation. In this paper, we propose the interconnection method to integrate communication with OPC UA and convert OPC UA AddressSpace using system configuration description language (SCL) of IEC 61850. We implemented the mapping process for the verification of the interconnection method. The interconnection method in this paper can expand interoperability between power systems for OPC UA integration for various data structures in the smart grid.

Keywords: open-platform communication (OPC) uniﬁed architecture (UA); IEC 61850; system conﬁguration description language (SCL); abstract communication service interface (ACSI); smart grid

1. Introduction In the energy field, smart grids are a much-discussed topic. Many views on smart grids exist, which has led to many definitions of what is understood as a smart grid [1,2]. Power information technology (IT) protocols have a complicated network configuration because of the presence of a variety of protocols. The open-platform communication (OPC) unified architecture (UA) (IEC62451) middleware has a simple network configuration because it can be integrated into a single protocol. OPC UA is the communication protocol for the smart grid (SG) application platform. It was standardized by the IEC TC57 group in December 2008. The abstract approach of the OPC UA enables extensions of the application area, so the focus is on general data exchange within any domain, and it can be used for integrated automation concerns [3–5]. The IEC 61850 is the communication protocol and the substation’s service model. It has been used in digital substation systems and smart distributed systems. The top-level SG management platform needs independent middleware to transparently manage power IT systems. Interconnection modules between power IT and OPC UA in the top-level SG management system have to be developed to manage the supervisory control and data acquisition (SCADA) protocol [6–8]. In this contribution, we focus on communication technology mappings of the IEC 61850 and introduce the OPC UA as an alternative way of communication based on the IEC 61850 model. We designed mapping between OPC UA AddressSpace and IEC 61850 model information using UA’s UaModeler. We implemented System Configuration description Language (SCL) for the OPC UA

Energies 2016, 9, 901; doi:10.3390/en9110901 www.mdpi.com/journal/energies Energies 2016, 9, 901 2 of 16 Energies 2016, 9, 901 2 of 16 UaModeler. We implemented System Configuration description Language (SCL) for the OPC UA AddressSpace module. We converted all the SCL filesfiles to the OPC UA AddressSpace. We We verified verified IEC 61850 model mapping onto OPC OPC UA UA using using KEMA’s KEMA’s UniCA UniCA IED IED (level (level A A certificate) certificate) Server Server Simulator. Simulator. OPC UA UA was was developed developed using using UA’s UA’s UaModeler UaModeler and and a C++-based a C++-based system system development development kit. The kit. TheIEC IEC61850 61850 client client module module was was dev developedeloped using using SISCO’s SISCO’s MMS-EASE MMS-EASE Lite Lite library, library, and and the the IEC IEC 61850 server was using KEMA’s UniCAUniCA IEDIED (intelligent(intelligent electronicelectronic device)device) simulator.simulator. The paper is organizedorganized asas follows.follows. InIn SectionsSection 2,2.1 we and discuss 2.2, we Power discuss IT PowerProtocols—the IT Protocols—the OPC UA OPC(IEC62451) UA (IEC62451) and IEC 61850 and standards. IEC 61850 In standards. Section 3, we In Sectionsoutline the 2.3 IEC and 61850 2.4, model we outline mapping the IEConto 61850 OPC modelUA. Section mapping 4 outlines onto OPC the UA.experimental Section3 outlines results an thed experimentalperformance resultsevaluation. and performanceSection 5 concludes evaluation. the Sectionpaper and4 concludes shows some the paperfuture and perspectives. shows some future perspectives.

2. IEC IEC 61850 61850 Model Model Mapping Mapping onto onto OPC OPC UA UA

2.1. OPC UA (IEC62451) Standard OPC UA (IEC62451) was was standardized by the IEC TC57 group in DecemberDecember 2012.2012. The classic OPC was separated separated into into OPC OPC Data Data Access Access (DA), (DA), OP OPCC Historical Historical Data Data Access Access (HDA), (HDA), OPC OPC Alarms Alarms & &Conditions Conditions (AC), (AC), and and OPC OPC Extensible Extensible Markup Markup La Languagenguage (XML). (XML). The The classic classic OPC OPC features were integrated into OPC OPC UA. UA. Additionally, Additionally, service-orie service-orientednted architecture architecture (SOA) (SOA) was was added added to to OPC OPC UA UA [3]. [3]. The information information model model of of integrated integrated OPC OPC UA UA is shown is shown in Figure in Figure 1 [4].1[ The4]. classic The classicOPC can OPC be canbuilt beon built top on of top the of theOPC OPC UA-based UA-based inform informationation model. model. Vendor-specific Vendor-specific models models and domain/technology-specificdomain/technology-specific models models can be built built on top top of of the the OPC OPC UA-based UA-based information model and the classic OPC, exposing their specificspecific informationinformation via OPC UA. OPC UA AddressSpace, provided by OPC UA, is defineddefined along with general information models based on object type. All information models using OPC UA definedefine the following: (i) the entr entryy points into the address space used by clients to navigate navigate through through the the instances instances and and types types of ofOPC OPC UA UA servers; servers; (ii) (ii)the thebase base types types that thatbuild build the root the rootfor the for different the different types types of hierarchies; of hierarchies; (iii) the (iii) built-in the built-in but extensible but extensible types types (i.e., object (i.e., object types types and data and datatypes); types); and and(iv) (iv)the theserver server object object that that provides provides capability capability and and diagnostic diagnostic information. information. OPC OPC UA information models, as well as complex models, cancan provide simple information. The base modeling concepts of OPC UA are nodes and references betweenbetween nodes. Each node contains attributes, such as ID and name.

Figure 1. Information model of integrated OLE (object(object linking and embedding) for process control OPC uniﬁedunified architecture (UA).

OPC UA software layers are shown in Figure 2. The complete software stack can be implemented OPC UA software layers are shown in Figure2. The complete software stack can be implemented with C, .Net, or JAVA. OPC UA is not limited to these programming languages and development with C, .Net, or JAVA. OPC UA is not limited to these programming languages and development platforms, however, only these environments are currently used for implementing the OPC platforms, however, only these environments are currently used for implementing the OPC foundation foundation UA stack deliverables [9]. UA stack deliverables [9].

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Figure 2. OPC UA address space. Figure 2. OPC UA address space. Figure 3 shows the OPC UA AddressSpace. The AddressSpace model specifies the building FigureFigure3 shows 3 shows the OPC the OPC UA AddressSpace. UA AddressSpace. The The AddressSpace AddressSpace model model specifies specifies the the building building blocks blocks to expose instance and type information and thus the OPC UA meta-model used to describe to exposeblocks instance to expose and instance type information and type information and thus theand OPCthus UAthe OPC meta-model UA meta-model used to used describe to describe and expose and expose information models and to build the OPC UA server AddressSpace. The OPC UA informationand expose models information and to buildmodels the and OPC to UAbuild server the OPC AddressSpace. UA server AddressSpace. The OPC UA The AddressSpace OPC UA is AddressSpace is modeled as a set of nodes that are accessible by OPC UA clients using OPC UA modeledAddressSpace as a set of is nodes modeled that as are a set accessible of nodes by that OPC are UA accessible clients by using OPC OPC UA UAclients services. using OPC The nodesUA in services.services. The Thenodes nodes in the in the AddressSpace AddressSpace are are used used to representrepresent real real objects, objects, their their definitions, definitions, and theirand their the AddressSpace are used to represent real objects, their definitions, and their references to each other. referencesreferences to each to each other. other. The The view view is is a aspecific specific subset ofof the the AddressSpace AddressSpace that that is of is interest of interest to the to the The view is a specific subset of the AddressSpace that is of interest to the OPC UA clients [10]. OPC OPCUA clientsUA clients [10]. [10].

Figure 3. OPC UA AddressSpace. Figure 3. OPC UA AddressSpace. 2.2. IEC 61850 Standard Figure 3. OPC UA AddressSpace. 2.2. IEC 61850 Standard 2.2. IEC 61850IEC 61850 Standard is a communication standard for electrical substation automation systems. It is the IECstandard 61850 protocol is a communication used in digital substation standard systems for electrical and smart substation distributed automation systems. The systems. goal of the It is the standardIEC 6185061850 protocol series is a used communicationis to in provide digital in substationteroperability standard systems for between electric and theal smart IEDssubstation distributedfrom different automation systems. suppliers systems. The or, goal more It ofis the IECstandard 61850precisely, protocol series between is used to functions provide in digita to interoperabilityl besubstation performed systems by between systems and for the smart power IEDs distributed ut fromility automation different systems. suppliers while The residing goal or, of more the precisely,IEC 61850in equipment between series isfrom functions to providedifferent to beinsuppliers.teroperability performed IEC 61850- by between systems7 describes forthe power IEDsthe relationships fr utilityom different automation between suppliers other while parts or, residing more inprecisely, equipmentof the between IEC from 61850 functions different series [11]. suppliers.to be performed IEC 61850-7 by systems describes for power the relationships utility automation between while other residing parts of The models of the Abstract Communication Service Interface (ACSI) provide [12]: thein equipment IEC 61850 seriesfrom different [11]. suppliers. IEC 61850-7 describes the relationships between other parts of theThe• IEC modelsThe 61850 definition series of the of[11]. Abstract the basic Communication model of the utility Service information Interface models (ACSI) contained provide in [ 12IEC]: 61850-7-3 The models(common of data the Abstractclasses for Communication utility automation Service applications), Interface IEC (ACSI) 61850-7-4 provide (compatible [12]: logical • The definitionnode classes of and the compatible basic model data of classes the utility for utility information automation models applications), contained and in IEC IEC 61850-6 61850-7-3 • (commonThe (thedefinition substation data of classes the conf basiciguration for utilitymodel language). automationof the utility applications), information models IEC 61850-7-4 contained (compatible in IEC 61850-7-3 logical • node(common The classes definition data and classes compatibleof information for utility data exchange automation classes service for utility applications),models. automation IEC applications), 61850-7-4 (compatible and IEC 61850-6 logical node classes and compatible data classes for utility automation applications), and IEC 61850-6 (the substationThe following configuration overall classes language). are defined: Server, Logical Device, Logical Node, and Data Object. (the substation configuration language). • The definition of information exchange service models. • The definition of information exchange service models.

The following overall classes are defined: deﬁned: Server, Logical Device, Logical Node, and and Data Data Object. Object.

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Energies 2016, 9, 901 4 of 16 The IEC 61850 information model is shown in Figure4. The information model is an abstract representationThe IEC of 61850 the devices information inside model the is substation. shown in Figu There logical4. The information devices (LDs) model comprise is an abstract a virtual representationrepresentation of physicalof the devices devices, inside such the as substati relayson. or The bays logical in the devices substation. (LDs) Ancomprise LD includes a virtual many logicalrepresentation nodes (LNs). of LNs physical are virtual devices, representations such as relays or of bays components in the substation. of devices, An such LD includes as switches, many circuit breakers,logical and nodes voltage (LNs). meters. LNs are Data virtual objects representation (DOs) ares representationsof components of of devices, the common such as information switches, of nodes.circuit All devicesbreakers, follow and avoltage common me namingters. Data scheme; objects even (DOs) if the are manufacturer representations is different, of the common it is possible information of nodes. All devices follow a common naming scheme; even if the manufacturer is to access the same device. DOs are typed by common data classes (CDC) from IEC 61850-7-3 or IEC different, it is possible to access the same device. DOs are typed by common data classes (CDC) from 61850-7-2 [12,13]. IEC 61850-7-3 or IEC 61850-7-2 [12,13].

FigureFigure 4. IEC4. IEC 61850 61850 information information model (Source: (Source: IEC IEC 61850-7-2). 61850-7-2).

Functional constraints (FC) play a crucial role in the definition of the information models and in Functional constraints (FC) play a crucial role in the definition of the information models and the services used to access the various parts of the information model. The FC is not shown in the in the services used to access the various parts of the information model. The FC is not shown object reference. The FC information may be mapped into the object reference in a Specific in theCommunication object reference. Service The Mapping; FC information IEC 61850-8-1 may maps be mapped the FC between into the the object logical reference node name in a and Specific Communicationthe data name Service (Relay1:MMXU1$CF$PhV). Mapping; IEC 61850-8-1 FCs mapsare defined the FC as between follows: theMX—measurements; logical node name ST— and the data namestatus; (Relay1:MMXU1$CF$PhV).CF—configuration; RP—unbuffered FCs are report defined control as follows: blocks; LG—log MX—measurements; control blocks; ST—status; BR— CF—configuration;buffered report RP—unbufferedcontrol blocks; GO—Generic report control Orient blocks;ed Object LG—log System control Event blocks;(GOOSE) BR—buffered control blocks; report controlGS—Generic blocks; GO—Generic Substation Status Oriented Event Object(GSSE) Systemcontrol Eventblocks; (GOOSE)SV—substituted control values; blocks; SE—setting GS—Generic Substationgroup Statusediting Event[14]. (GSSE) control blocks; SV—substituted values; SE—setting group editing [14]. The languageThe language for for the the configuration configuration of of electricalelectrical substation IEDs IEDs is iscalled called System System Configuration Configuration description Language (SCL) [15]. It is used to describe IED configurations and communication description Language (SCL) [15]. It is used to describe IED configurations and communication systems systems according to IEC 61850-5 and IEC 61850-7. It allows the formal description of the relations according to IEC 61850-5 and IEC 61850-7. It allows the formal description of the relations between the between the utility automation system and the process (substation, switchyard). utility automationTable 1 shows system all the and groups the process in the (substation,logical node switchyard).defined in IEC 61850-7-4 [16]. This group is Tabledefined1 showsas approximately all the groups 90 logical in nodes, the logical such as node the common defined applications in IEC 61850-7-4 and supply [ 16 of]. substation This group is definedequipment. as approximately One major 90focus logical is the nodes, definition such of as information the common models applications for applications and supply relating of to substation the equipment.Protection One and major Protection-related. focus is the definition The two groups of information constitute modelsalmost half for of applications the logical node. relating This to the Protectionshows and the high Protection-related. importance of the The Protection two groups for th constitutee safety and almost stable half operation of the logicalof the power node. system. This shows the high importance of the ProtectionTable 1. IEC for 61850 the safety logical andnode stable(LN) groups operation [16]. of the power system.

LogicalTable Node 1. IEC Groups 61850 logical node Number (LN) groups of Logical [16]. Nodes System logical nodes 3 LogicalProtection Node functions Groups Number of 28Logical Nodes Protection related functions 10 SystemSupervisory logical control nodes 5 3 ProtectionGeneric references functions 283 ProtectionInterfacing related and archiving functions 104 SupervisoryAutomatic control control 4 5 MeteringGeneric and references measurement 8 3 Interfacing and archiving 4 Automatic control 4 Metering and measurement 8 Energies 2016, 9, 901 5 of 16

Table 1. Cont.

Logical Node Groups Number of Logical Nodes Energies 2016, 9, 901 5 of 16 Sensors and monitoring 4 SensorsSwitchgear and monitoring 4 2 InstrumentSwitchgear transformer 2 2 InstrumentPower transformer transformer 2 4 Further powerPower systemtransformer equipment 154 TotalFurther number power ofsystem logical equipment nodes 15 92 Total number of logical nodes 92 2.3. Design of OPC UA AddressSpace for IEC 61850 2.3. Design of OPC UA AddressSpace for IEC 61850 Figure5 shows the whole design flow of OPC UA AddressSpace for IEC 61850. First, the engineer Figure 5 shows the whole design flow of OPC UA AddressSpace for IEC 61850. First, the has to define IEC 61850 logical node type using UA’s UaModeler—Section 2.3. Second, the user needs engineer has to define IEC 61850 logical node type using UA’s UaModeler—Section 2.3. Second, the to generateuser needs IEC to 61850 generate logical IEC node61850 instancelogical node using instance SCL using to OPC SCL UA to OPC AddressSpace UA AddressSpace Tool—Section Tool— 2.4. IEC 61850Section logical 2.4. IEC node 61850 type logical source node file type and source IEC 61850file and logical IEC 61850 node logical instance node source instance file source represent file the IEC 61850represent information the IEC 61850 in OPC information UA. in OPC UA.

Figure 5. The whole design flow. Figure 5. The whole design ﬂow. OPC UA provides extensible information models. The information models of OPC UA are OPCmapped UA after provides generating extensible the abstract information object on AddressSpace. models. The Therefore, information the IEC models 61850 ofinformation OPC UA are mappedmodel after has generating to be modeled the onto abstract OPC UA object AddressSpace on AddressSpace.. The following Therefore, describes the how IEC the 61850 IEC16850 information is modelmapped has to to be the modeled OPC UA onto AddressSpace. OPC UA AddressSpace. The following describes how the IEC16850 is mapped toThe the mapping OPC UA of AddressSpace.the IEC 61850 information model onto OPC UA AddressSpace is shown in Figure 6 [1]. The OPC UA AddressSpace changes the model from the IEC 61850 information to the The mapping of the IEC 61850 information model onto OPC UA AddressSpace is shown in IEC62541 information. Then, it has and holds as nodes on AddressSpace. Figure6[ 1]. The OPC UA AddressSpace changes the model from the IEC 61850 information to the To structure the objects, three standard UA reference types are used [2]: IEC62541 information. Then, it has and holds as nodes on AddressSpace. To– structureHasComponent the objects, describes three part standard of the UArelationsh referenceip between types are LN used and [its2]: attributes as well as between CDC and its attributes. Furthermore, it is used for the grouping by FC. - HasComponent– Organizes is describesused to group part the of theCDC relationship attributes by between FC. LN and its attributes as well as between

CDC– HasTypeDefinition and its attributes. connects Furthermore, the LN itattributes is used with for the the groupingaccording byCDC. FC. - Organizes is used to group the CDC attributes by FC. - HasTypeDeﬁnition connects the LN attributes with the according CDC.

Mapping example: The model of the XCBR, one of the logical nodes (LNs) of the IEC 61850, using UA’s Modeler tool can be mapped by reference to Figure5. The XCBR LN of the IEC 61850 is the circuit breaker (CB) of the substations or distributed devices [12]. It is mapped to object type because of the LN class. Additionally, it has many common data classes (CDCs) of the attribute type [11]. CDCs are modeled to object type. CDCs reference the LN data deﬁned in the IEC 61850. The Single Point Status (SPS), one of the CDCs, provides status information. The quality of the measured value (q) and the Figure 6. Mapping of the IEC 61850 information model onto OPC UA AddressSpace.

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Sensors and monitoring 4 Switchgear 2 Instrument transformer 2 Power transformer 4 Further power system equipment 15 Total number of logical nodes 92

2.3. Design of OPC UA AddressSpace for IEC 61850 Figure 5 shows the whole design flow of OPC UA AddressSpace for IEC 61850. First, the engineer has to define IEC 61850 logical node type using UA’s UaModeler—Section 2.3. Second, the user needs to generate IEC 61850 logical node instance using SCL to OPC UA AddressSpace Tool— Section 2.4. IEC 61850 logical node type source file and IEC 61850 logical node instance source file represent the IEC 61850 information in OPC UA.

Figure 5. The whole design flow.

OPC UA provides extensible information models. The information models of OPC UA are mapped after generating the abstract object on AddressSpace. Therefore, the IEC 61850 information model has to be modeled onto OPC UA AddressSpace. The following describes how the IEC16850 is mapped to the OPC UA AddressSpace. Energies 2016The, 9, 901mapping of the IEC 61850 information model onto OPC UA AddressSpace is shown in6 of 16 Figure 6 [1]. The OPC UA AddressSpace changes the model from the IEC 61850 information to the IEC62541 information. Then, it has and holds as nodes on AddressSpace. timestamp of the measured value (t) are modeled to the variable [11]. The predefined data type is To structure the objects, three standard UA reference types are used [2]: referred to in Modeler. The XCBR LN has LN data: Loc (local control behavior), EEHealth (external equipment– HasComponent health), EEName describes (external part of equipment the relationsh nameip between plate),and LN soand forth, its attributes as the HasComponent. as well as between CDC and its attributes. Furthermore, it is used for the grouping by FC. The HasComponent ReferenceType is used, exposing the configuration of a device as a component of – Organizes is used to group the CDC attributes by FC. the device. The attribute types of LN refer to the CDC, such as SPS to HasTypeDefinition [1,10]. – HasTypeDefinition connects the LN attributes with the according CDC.

FigureFigure 6. Mapping 6. Mapping of theof the IEC IEC 61850 61850 information information modelmodel onto onto OPC OPC UA UA AddressSpace. AddressSpace.

Data-type mapping is the most basic information in the rules of mapping. IEC 61850 BasicTypes are consistent with OPC UA BaseDataTypes. Table2 shows the data-type mapping table for mapping the IEC 61850 and OPC UA.

Table 2. Data-type mapping.

IEC 61850 BasicTypes OPC UA BaseDataType BOOLEAN Boolean INT8 Number-Integer-Sbyte INT16 Number-Integer-Int16 INT24 Number-Integer-Int32 INT32 Number-Integer-Int32 INT128 Number-Integer-Int64 INT8U Number-Integer-Uinteger-Byte INT16U Number-Integer-Uinteger-UInt16 INT24U Number-Integer-Uinteger-UInt32 INT32U Number-Integer-Uinteger-UInt32 FLOAT32 Number-Float FLOAT64 Number-Double ENUMERATED Enumeration CODED ENUM Enumeration OCTET STRING ByteString VISIBLE STRING String UNICODE STRING String

Figure7 shows ACSI server class mapping. The server class represents the external visible behavior of a device. It is composed of 1 to N logical device classes. The attribute ServiceAccessPoint identifies a server within the scope of a subnetwork. The attribute LogicalDevice identifies a logical device that is contained in a general server. The attribute File identifies a file system included in a general server. The attribute TPAppAssociation identifies a client with which a server maintains a two-party application association. The attribute MCAppAssociation identifies a subscriber with which a server maintains a multicast application association. ServiceAccessPoint is mapped to the variable node class type. The remaining attributes are mapped to the object node class type [6,10,12]. Energies 2016, 9, 901 7 of 16

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Figure 7. Abstract Communication Service Interface (ACSI) server class mapping. FigureFigure 7. Abstract 7. Abstract Communication Communication ServiceService Interface (ACSI) (ACSI) server server class class mapping. mapping. ACSI logical device mapping is shown in Figure 8. The logical device classes comprise three or ACSIACSI logical logical device device mapping mapping is is shown shown inin FigureFigure 88.. The logical logical device device classes classes comprise comprise three three or or more logical node classes. The attribute LDName unambiguously identifies an instance of a logical moremore logical logical node node classes. classes. The The attribute attribute LDName LDName unambiguouslyunambiguously identifies identiﬁes an aninstance instance of a oflogical a logical device withindevice the within scope the scopeof a subnetwork. of a subnetwork. The The attribut attributee LDRef LDRef identifies identifies the reference the reference path of patha logical of a logical device within the scope of a subnetwork. The attribute LDRef identiﬁes the reference path of a logical device within the scope of a subnetwork. The attribute LogicalNode is a list of all logical nodes that devicedevice within within the thescope scope of ofa subnetwork. a subnetwork. The The attribut attributee LogicalNodeLogicalNode is is a lista list of allof all logical logical nodes nodes that that are containedare contained in a general in a general logical logical device device class. class. LDName LDName and and LDRef LDRef are mappedare mapped to the variableto the variable Node Node are containedclass type. in LogicalNode a general logical is mapped device to the class. object LDName node class and type. LDRef are mapped to the variable Node classclass type. type. LogicalNode LogicalNode is mapped is mapped to to the the object object node node classclass type. type.

Figure 8. ACSI logical device mapping.

The ACSI logical node class is a composition of LNName, LNRef, DataObjects, DATA-SET, BRCB (buffered report control block), and URCB (unbuffered report control block) [9]. The attribute

FigureFigure 8. 8. ACSIACSI logical logical devicemapping. mapping.

The ACSI logical node class is a composition of LNName, LNRef, DataObjects, DATA-SET, BRCB (buffered report control block), and URCB (unbuffered report control block) [9]. The attribute

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The ACSI logical node class is a composition of LNName, LNRef, DataObjects, DATA-SET, BRCB (buffered report control block), and URCB (unbuffered report control block) [9]. The attribute LNName unambiguously identifies a logical node within the scope of a logical device. The attribute LNRef is the unique path name of a logical node. The attribute DataObject is a data object contained in a logical node. The attribute DataSet is a list of all data sets that are included in a logical node. The attribute BRCB is a list of all buffered report control blocks that are comprised in a logical node. The attribute URCB is a list of all unbuffered report control blocks that are contained in a logical node. LNName and LNRef are mapped to the variable node class type. The remaining attributes are mapped to the object node class type. The ACSI data object class is a key element in IEC 61850. It is a composition of DataObjectName, DataObjectRef, and DataObjectType. The attribute DataObjectName unambiguously identifies a data object within the scope of a logical node. The attribute DataObjectRef is the unique path name of a data object. The DataObjectType is the CDC type. Table3 shows an example of the common data class (CDC—Data Object). The CDCs are defined in IEC 61850-7-3 [13]. CDCs are a configured common data attribute (base data type or data structure) and they are defined for use in IEC 61850-7-4 [16]. CDCs define the relation between their attributes and the functional constraint as well as the possible trigger options. CDC specifications are classified as status, measurand, controls, status settings, analog settings, and description. DPS (double point status) belongs to status.

Table 3. Example of common data class: DPS (double point status) mapping.

PDS Type Name Type Deﬁnition Data Type Node Class Type Data Name Data Attribute Status stVal Enum_stVal Enumeration Variable q Quality_type BaseDataType Variable t TimeStamp_type BaseDataType Variable Substitution subEna DataAttribute_Type Boolean Variable subVal Dnum_stVal Enumeration Variable subQ Quality_type BaseDataType Variable subID DataAttribute_Type String Variable Conﬁguration, description and extension d DataAttribute_Type String Variable dU DataAttribute_Type String Variable cdcNs DataAttribute_Type String Variable cdcName DataAttribute_Type String Variable dataNs DataAttribute_Type String Variable

Table4 shows an example of mapping using the IEC 61850 information model deﬁned in IEC 61850-7-4. The logical node is to be composed of several CDCs. The logical node is composed of three categories of data, and status information can be information that controls the logical node. The categories are deﬁned in several classes of data. The logical node is mapped to the OPC UA node model based on the previously mapped CDC. The UaModeler tool is used to implement the OPC UA AddressSpace for IEC 61850 ACSI mapping. The UaModeler tool produces the source code for types of mapped IEC 61850 models (e.g., XCBR type). It can easily implement information modeling, and it is represented graphically for the OPC UA AddressSpace. The generated source code is C++, ANSI C, .NET [17]. Energies 2016, 9, 901 9 of 16

Table 4. Example of logical node: XCBR mapping. Energies 2016, 9, 901 9 of 16 XCBR Type Name Type Definition Modeling Rule Data Type Node Class Table 4. Example of logical node: XCBR mapping. LLName ObjectName_Type Mandatory BaseDataType Variable Data XCBR Type Data from Override Instance Name Type DeﬁnitionMandatory Modeling Rule Data TypeObject Node Class Common_LN_Type Declaration LLNameLoc SPS_Type ObjectName_Type Mandatory Mandatory BaseDataType Object Variable Data Data fromEEHealth Common_LN_Type OverrideINS_Type Instance DeclarationOptional Mandatory Object Object EENameLoc DPL_Type SPS_Type Optional Mandatory Object Object EEHealthOpCnt INS_Type INS_Type Mandatory Optional Object Object EENamePos DPC_Type DPL_Type Mandatory Optional Object Object OpCnt INS_Type Mandatory Object BlkOpnPos SPC_Type DPC_Type Mandatory Mandatory Object Object BlkOpnBlkCls SPC_Type SPC_Type Mandatory Mandatory Object Object ChamotEnaBlkCls SPC_Type SPC_Type Optional Mandatory Object Object ChamotEna SPC_Type Optional Object SumSwARs BCR_Type Optional Object SumSwARs BCR_Type Optional Object CBOpCap INS_Type INS_Type Mandatory Mandatory Object Object POWCap INS_Type INS_Type Optional Optional Object Object MaxOpCap INS_Type INS_Type Optional Optional Object Object

2.4. SCL to OPC UA AddressSpace Module Implementation The IEC 61850 server uses SCL files files and prefer preferencesences for the server. The OPC UA AddressSpace may configureconfigure the IEC 61850 model information using SCL files,files, because the IEC 61850 server configuresconfigures the information model using SCLSCL files.files. The The SCL SCL to OPC UA AddressSpace module converts the IEC 61850 model information in the SCL file file into OPC UA AddressSpace information. The mapping typestypes inin SectionSection 2.3 2.3 help help the the conversion. conversion. The The OPC OPC UA UA AddressSpace AddressSpace creation creation process process is performedis performed in in two two steps. steps. Figure 99 shows step 1. 1. Step Step 1 1creates creates the the TDF TDF (type (type definition definition file) file) and and NDF NDF (node (node definition definition file) withfile) withthe Data the DataMap Tool. Map Tool.The Data The Map Data Tool Map maps Tool the maps IEC the 61850 IEC model 61850 information—logical model information—logical device, logicaldevice, node, logical functional node, functional constraint, constraint, data object, dataobject, and da andta attribute. data attribute. TDF is TDF type is definition type definition file for file IEC for 61850IEC 61850 node node in SCL in SCLfile. NDF file. NDFis node is nodedefinition definition file for file IEC for 61850 IEC 61850 node nodein SCL in file. SCL TDF file. and TDF NDF and make NDF SCLmake information SCL information simple; simple; they help they to help create to create logical logical node nodeinstances instances for a predefined for a predefined logical logical node nodetype intype OPC in OPCUA. With UA. WithTDF TDFand NDF, and NDF, the mapping the mapping process process time timeis reduced is reduced to less to than less thanone minute. one minute. The TheTDF TDFand andNDF NDF are shown are shown in Figure in Figure 10. 10.

Figure 9. StepStep 1: 1: Creation Creation of of TDF TDF (type (type definiti definitionon file) file) and NDF (node definition definition file).file).

Energies 2016, 9, 901 10 of 16

(A) (A) ( (BB))

Figure 10. Examples of TDF and NDF. (A) Example of a created TDF file. Parentheses indicate the data Figure 10. Examplestype—(TL) of TDF is the typeand of NDF. logical node, (A) (FC) ExampleExample is the functional ofof aa createdcreatedconstraint, and TDFTDF (TA) ﬁle.file. is a type Parentheses of data indicate the data type—(TL) isattribute. the type Brackets of indicate logical the node,name of the(FC) data is and th the thee datafunctional for each data constraint, type and braces and indicate (TA) the is a type of data name of the type. (B) Example of a created NDF file. Parentheses indicate the data type—(LD) is an attribute. Bracketsinstance indicateindicate of a logical thethe device, name name (LN) of of is the anthe instance data data andof an a logicald the the node, data data (FC) for foris eachthe each functional data data constraint, type type and (DO) and braces braces indicate indicate the thename name of the of the type; type;is (theB )data Example(B object,) Example and (DA) of a ofis created the a datacreated attribute. NDF NDF Br ﬁle.ackets file. Parentheses indicate Parentheses the name of indicate the indicate data and the the the data fordata type—(LD) type—(LD) is an is each data type and braces indicate the name of the type. aninstance instance of aof logical a logical device, device, (LN) (LN) is an is instance an instance of a of logical a logical node, node, (FC) (FC) is the is functional the functional constraint, constraint, (DO) (DO)is the is data the object, data object, and (DA) and is (DA) the datais the attribute. data attrib Bracketsute. Brackets indicate indicate the name the of name the data of the and data the dataand the for dataeach for data each type data and type braces and indicate braces indicate the name the of name the type. of the type.

Figure 1111 showsshows step step 2. 2. The The SCL SCL to OPCto OPC UA UA AddressSpace AddressSpace module module generates generates the OPC the UA OPC source UA sourcecode for code building for building the OPC the UA OPC server UA usingserver theusing TDF the and TDF the and NDF. the The NDF. generated The generated OPC UA OPC source UA sourcecode is conﬁguredcode is configured as the OPC as UAthe AddressSpaceOPC UA AddressSpace of the OPC of UA the server OPC for UA the server IED represented for the IED in representedthe IEC 61850 in model.the IEC The 61850 code model for the. The types code of for mapped the types IEC of 61850 mapped model IEC was 61850 created model in was Section created 2.3. Thus,in Section generated 2.3. Thus, code generated is valid for code IEDs is valid or any for IEC IEDs 61850 or any systems IEC 61850 due itsystems is able todue receive it is able as inputto receive and asSCL input ﬁle. and SCL file.

Figure 11. Step 2: Generation of OPC UA source code.

3. Experimental Experimental Results Results and and Performance Performance Evaluation Evaluation The OPC OPC UA UA server server contains contains the the OPC OPC UA UA server server module module and and IEC IEC 61850 61850 client client input/output input/output (I/O) (I/O)module. module. The OPC The UA OPC server UA servermodule module configures conﬁgures the OPC the UA OPC AddressSpace UA AddressSpace for the IEC for the61850 IEC model. 61850 Themodel. OPC The UA OPC client UA can client access can the access IEC 61850 the IEC information 61850 information model by model searching by searching for a node for in a the node OPC in UAthe OPCAddressSpace. UA AddressSpace. If the OPC If theUA OPC server UA synchronizes server synchronizes with the with datathe of the data actual of the IEC actual 61850 IEC server, 61850 the OPC UA client may be used to obtain the data in the IEC 61850 server without using the IEC 61850 protocol. Figure 12 shows a network diagram between IEC 61850 and OPC UA. The OPC UA client requests the service for the OPC UA server. The OPC UA server module is transmitted to the IEC 61850 client I/O modules via the interprocess communication (IPC) service required. The IEC 61850

Energies 2016, 9, 901 11 of 16 server, the OPC UA client may be used to obtain the data in the IEC 61850 server without using the IEC 61850 protocol. EnergiesFigure 2016, 912, 901 shows a network diagram between IEC 61850 and OPC UA. The OPC UA client requests11 of 16 the service for the OPC UA server. The OPC UA server module is transmitted to the IEC 61850 client I/Oclient modules I/O modules via the deliver interprocess the required communication service (IPC)to the service IEC required.61850 server. The IECTable 61850 5 shows client I/Otest modulesenvironments. deliver the required service to the IEC 61850 server. Table5 shows test environments.

Figure 12. Network diagram between IEC 61850 and OPC UA.

Table 5. Test environments [[17–20].17–20].

Component OPC UA Client OPC UA Server IEC 61850 Client IEC 61850 Server Component OPC UA Client OPC UA Server IEC 61850 Client IEC 61850 Server OS Ubuntu 14.04 Ubuntu 14.04 Ubuntu 14.04 Windows OS Ubuntu 14.04 Ubuntu 14.04 Ubuntu 14.04 Windows KEMA’s UniCA Tool UA’s UaExpert KEMA’s UniCA Tool UA’s UaExpert IED Simulator IED Simulator UA’s OPC UA SDK Sisco’s MMS-EASE SDKSDK UA’s OPC UA SDK 1.3.1 Sisco’s MMS-EASE Lite 5.02 1.3.1 Lite 5.02 LanguageLanguage C++ C++ C C

We havehave performedperformed an IEC 61850 and OPC UA interlockinterlock test using SCL filesfiles with the IED information of the four manufacturers—Table 66;; thesethese areare thethe SCLSCL filesfiles thatthat composecompose thethe realreal IED.IED. All four SCL filesfiles werewere successfullysuccessfully convertedconverted toto thethe OPCOPC UAUA AddressSpace.AddressSpace.

Table 6. The number of nodes for each data type and rate ofof conversion.conversion.

Metric IED A IED B IED C IED D Metric IED A IED B IED C IED D Logical Device(Num) 2 4 1 4 LogicalLogical Device(Num) Node(Num) 2 80 4 217 81 1 116 4 LogicalData Node(Num) Object(Num) 80 979 217 2532 840 81 809 116 Data Object(Num) 979 2532 840 809 Data Attribute(Num) 2795 6276 2095 2108 Data Attribute(Num) 2795 6276 2095 2108 Conversion (%) 100% 100% 100% 100% Conversion (%) 100% 100% 100% 100%

Figure 13 shows the results of IEC 61850–OPC UA interconnection. Figure 13b is the IEC 61850 modelFigure information 13 shows using the KEMA’s results of UniCA IEC 61850–OPC IED simulator UA interconnection.[19]. Figure 13a shows Figure that 13b UA’s is the Expert IEC 61850 tool model(OPC UA information client) accesses using KEMA’s the IEC UniCA61850 model IED simulator information [19]. by Figure searching 13a shows for nodes that UA’s in the Expert OPC toolUA (OPCAddressSpace UA client) [5]. accesses The results the IEC of 61850the IEC model 61850–OPC information UA interconnection by searching for include nodes inlogical the OPC device, UA AddressSpacelogical node, functional [5]. The results constraint, of the data IEC 61850–OPCobject, and UAdata interconnection attribute. GEDeviceF650 include logical is a Logical device, Device logical node,for the functional generator constraint,role. GGIOs data (generic object, process and data input/output) attribute. GEDeviceF650 are logical nodes. is a This Logical node Device is used for only the generatorto model process role. GGIOs devices (generic that are process not predefined input/output) by LN aregroups logical in a nodes. generic This way. node CF (configuration), is used only to modelDC (description), process devices EX (extended that are not definition), predefined MX by (measurands), LN groups in and a generic ST (status) way. CFare (configuration), the functional DCconstraints. (description), AnIn is EX a data (extended object with definition), the CDC MX MV (measurands), (measured values) and ST type. (status) The data are theattributes functional (db, constraints.RangeC, units) AnIn belong is a to data MV object CF. with the CDC MV (measured values) type. The data attributes (db, RangeC, units) belong to MV CF.

Energies 2016, 9, 901 12 of 16 Energies 2016, 9, 901 12 of 16 Energies 2016, 9, 901 12 of 16

(a) (b) (a) (b) FigureFigure 13.13. ResultsResults of of IEC IEC 61850–OPC 61850–OPC UA UA interconnection. interconnection. (a)( aUA’s) UA’s Expert Expert tool tool (OPC (OPC UA UA client); client); (b) Figure 13. Results of IEC 61850–OPC UA interconnection. (a) UA’s Expert tool (OPC UA client); (b) (KEMA’sb) KEMA’s UniCA UniCA IED IED (intelligent (intelligent electronic electronic device) device) Simulator Simulator (IEC (IEC 61850 61850 server). server). KEMA’s UniCA IED (intelligent electronic device) Simulator (IEC 61850 server). We have performed simulations in an environment as shown in Figure 12. We simulated We have performed simulations in an environment as shown in Figure 12. We simulated throughputWe have for performed the IEC 61850 simulations and OPC inUA an network enviro nmentarea, and as theshown results in areFigure shown 12. in We Figure simulated 14. We throughput for the IEC 61850 and OPC UA network area, and the results are shown in Figure 14. throughputused GetDataValues for the IEC service. 61850 andThe OPCthroughput UA network of IE Carea, 61850 and is the 65,000 results byte/s, are shown and the in throughputFigure 14. We of We used GetDataValues service. The throughput of IEC 61850 is 65,000 byte/s, and the throughput usedOPC GetDataValuesUA is 20,000 byte/s. service. The The OPC throughput UA network’s of IE Cth 61850roughput is 65,000 is lower byte/s, than and that the of throughputthe IEC 61850 of of OPC UA is 20,000 byte/s. The OPC UA network’s throughput is lower than that of the IEC 61850 OPCnetwork, UA isand 20,000 this result byte/s. means The OPCthat theUA number network’s of messages throughput of IEC is lower 61850 than is greater that ofthan the that IEC of 61850 OPC network, and this result means that the number of messages of IEC 61850 is greater than that of OPC network,UA. The OPCand this UA result can efficiently means that transm the numberit a greater of messages number ofof items.IEC 61850 is greater than that of OPC UA. The OPC UA can efﬁcientlyefficiently transmittransmit a greater numbernumber ofof items.items.

Throughput Throughput 80000 80000 60000 60000 40000 40000 20000

Data Size(byte) 20000 0 Data Size(byte)

0 1 9 17 25 33 41 49 57 65 73 81 89 97 1 9 105 113 121 129 137 145 153 161 169 177 185 193 201 209 217 225 233 17 25 33 41 49 57 65 73 81 89 97 105 Time(s)113 121 129 137 145 153 161 169 177 185 193 201 209 217 225 233 Time(s) OPC UA IEC61850 OPC UA IEC61850

Figure 14. OPC UA and IEC 61850 throughput. Figure 14. OPC UA and IEC 61850 throughput. Figures 15 and 16 show the round-trip time (RTT) of IEC 61850 (IEC 61850 client module— KEMAFigures IED Simulator) 15 and 16 andshow OPC the UA round-trip (UA Expert—OPC time (RTT) UA of serverIEC 61850 module). (IEC We 61850 used client GetDataValues module— KEMAservice. IED The Simulator) RTT of IEC and 61850 OPC is UA less (UA than Expert—OPC 8 ms. On the UA other server hand, module). the RTT We of usedOPC GetDataValuesUA is less than service.20 ms, thus The theRTT RTT of IEC of OPC61850 UA is lessis higher than 8than ms. IECOn the61850. other (1) hand, The OPC the RTT UA ofserver OPC module UA is less conveys than 20 ms, thus the RTT of OPC UA is higher than IEC 61850. (1) The OPC UA server module conveys

Energies 2016, 9, 901 13 of 16

Figures 15 and 16 show the round-trip time (RTT) of IEC 61850 (IEC 61850 client module—KEMA Energies 2016, 9, 901 13 of 16 IED Simulator) and OPC UA (UA Expert—OPC UA server module). We used GetDataValues service. ThetheEnergies information RTT 2016 of, 9 IEC, 901 received 61850 is lessfrom than the 8IEC ms. 61850 On the client other module hand, to the the RTT OPC of UA OPC Client; UA is (2) less The than OPC 2013 ofUA ms, 16 thusserver the module RTT ofperforms OPC UA a proces is highers for thanmapping IEC 61850.the IEC (1)61850 The data OPC model UA serverand OPC module UA AddressSpace conveys the the information received from the IEC 61850 client module to the OPC UA Client; (2) The OPC UA informationmodel. All information received from about the mapping IEC 61850 data client was module exchanged to the correctly. OPC UA Client; (2) The OPC UA server moduleserver module performs performs a process a proces for mappings for mapping the IEC the 61850 IEC data 61850 model data andmodel OPC and UA OPC AddressSpace UA AddressSpace model. Allmodel. information All information about mapping about mapping data was data exchanged was exchanged correctly. correctly.

Figure 15. Round-trip time (IEC 61850: IEC 61850 client module—KEMA IED Simulator). Figure 15. Round-trip time (IEC 61850: IEC 61850 client module—KEMA IED Simulator).

Figure 16. Round-trip time (OPC UA: UA Expert—OPC UA server module). Figures 17 Figureand 18 16. are Round-trip Wireshark time IO (OPC graphs UA: UAof IECExpert—OPC 61850 GOOSE UA server message module). (IEC 61850 client module—KEMA IED Simulator) and OPC UA (UA Expert—OPC UA server Module), respectively. Figures 17 and 18 are Wireshark IO graphs of IEC 61850 GOOSE message (IEC 61850 client We used GOOSE control block service. The IO graph of GOOSE is 120–160 packets/s. On the other module—KEMA IED Simulator) and OPC UA (UA Expert—OPC UA server Module), respectively. hand, the IO graph of OPC UA is 43–46 packets/s, thus the IO graph of GOOSE is higher than OPC We used GOOSE control block service. The IO graph of GOOSE is 120–160 packets/s. On the other hand, the IO graph of OPC UA is 43–46 packets/s, thus the IO graph of GOOSE is higher than OPC

Energies 2016, 9, 901 14 of 16

Figures 17 and 18 are Wireshark IO graphs of IEC 61850 GOOSE message (IEC 61850 client module—KEMA IED Simulator) and OPC UA (UA Expert—OPC UA server Module), respectively. Energies 2016, 9, 901 14 of 16 WeEnergies used 2016 GOOSE, 9, 901 control block service. The IO graph of GOOSE is 120–160 packets/s. On the14 other of 16 hand,UA. (1) the GOOSE IO graph messages of OPC UA support is 43–46 real-time packets/s, communications thus the IO graph for ofcritical GOOSE messages; is higher (2) than All OPC devices UA. UA.(1)sending GOOSE (1) GOOSE GOOSE messages messages messages support support shall real-time continue real-time communications to communicationssend the message for critical for with critical messages; a long messages; (2)cycle All time, devices(2) All even sendingdevices if no sendingGOOSEstatus/value messagesGOOSE change messages shall has continue occurred. shall tocontinue send the to message send the with message a long with cycle a time, long evencycle if time, no status/value even if no status/valuechange has occurred. change has occurred.

Figure 17. Wireshark IO (input/output) graphs: packet (IEC 61850 Client module—KEMA IED FigureFigureSimulator). 17. 17.Wireshark Wireshark IO (input/output) IO (input/output) graphs: graphs: packet packet (IEC 61850 (IEC Client61850module—KEMA Client module—KEMA IED Simulator). IED Simulator).

Figure 18.18. Wireshark IOIO graphs:graphs: OPCOPC UAUA packetpacket (UA(UA Expert—OPCExpert—OPC UAUA serverserver module).module). Figure 18. Wireshark IO graphs: OPC UA packet (UA Expert—OPC UA server module). 4.4. Conclusions Conclusions 4. Conclusions WithWith thethe development development of technology,of technology, the smartthe smart grid hasgrid been has certified been certified as the underlying as the underlying structure With the development of technology, the smart grid has been certified as the underlying ofstructure the core of strategy the core for strategy energy saving.for energy The saving. power networkThe power requires network a unified requires platform a unified through platform the structure of the core strategy for energy saving. The power network requires a unified platform interworkingthrough the interworking of various power of various protocols. power OPC protoc UAols. adopted OPC aUA unified adopted platform a unified because platform it provides because a through the interworking of various power protocols. OPC UA adopted a unified platform because modelit provides for meta-information a model for meta-information and an expandable and an ex typepandable with an type object-oriented with an object-oriented model. The model. main aimThe it provides a model for meta-information and an expandable type with an object-oriented model. The ofmain this aim paper of hasthis beenpaper to has demonstrate been to demonstrate how to convert how the to OPCconvert UA the information OPC UA modelinformation through model the mainthrough aim the of analysisthis paper of thehas IECbeen 61850 to demonstrate standards and how implement to convert the the SCL OPC to UAOPC information UA AddressSpace model throughmodule theusing analysis IEC 61850 of the SCL IEC files. 61850 The standards SCL to OPC and UAimplement AddressSpace the SCL module to OPC can UA convert AddressSpace all SCL modulefiles. This using paper IEC has 61850 shown SCL a process files. The in whichSCL to the OPC SCL UA file AddressSpace (IEC61805 model) module is converted can convert to the all OPCSCL files.UA address This paper of thehas OPCshown UA a processserver. Wein which were theable SCL to viewfile (IEC61805 and change model) the valueis converted of IEC to61850 the OPC data UAinformation address ofin the OPC UAUA clientserver. using We werethe technique able to view proposed and change in this thepaper. value All ofinformation IEC 61850 about data informationmapping data in wasthe OPCexchanged UA client correctly. using Thethe techniqueapplication proposed of the OPC in this UA paper. offers All great information potential aboutin the mapping data was exchanged correctly. The application of the OPC UA offers great potential in the

Energies 2016, 9, 901 15 of 16 analysis of the IEC 61850 standards and implement the SCL to OPC UA AddressSpace module using IEC 61850 SCL files. The SCL to OPC UA AddressSpace module can convert all SCL files. This paper has shown a process in which the SCL file (IEC61805 model) is converted to the OPC UA address of the OPC UA server. We were able to view and change the value of IEC 61850 data information in the OPC UA client using the technique proposed in this paper. All information about mapping data was exchanged correctly. The application of the OPC UA offers great potential in the smart grid field. This paper contributes to the identification of information and communication technologies (ICTs) capable of satisfying the communication requirements for future power systems.

Acknowledgments: This research was supported by the Energy Technology Development Project through the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy (MOTIE) (No.: 20131020400660 and No.: 20131020402080) and Seokyeong University (2016). Author Contributions: In-Jae Shin and Byung-Kwen Song designed the study and the simulation. In-Jae Shin performed the simulation and data analysis. In-Jae Shin wrote the paper, with assistance and editing from Byung-Kwen Song, Doo-Seop Eom. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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