1      Scope

This Part of IEC 61968 specifies the format and rules for exchanging modeling information based upon the CIM and related to Distribution Network Data.

The intention of this Part of IEC 61968 is to allow the exchange of instance data in bulk. Thus, the imported network model data should be sufficient to allow performing network connectivity analysis, including network tracing, outage analysis, load flow calculations etc. This part could be used for synchronizing geographical information system databases with remote control system databases.

This Part is closely linked to 61970-452 Energy Management System Application Program Interface (EMS-API) CIM Network Applications Model Exchange Specification. Thus this document has been written in order to reduce its maintenance. It describes only differences with 61970-452. Nevertheless, as 61970-452 is a future international standard, this part still has duplicate information with 61970-452, in order to be more understandable.

It uses the CIM RDF[1] Schema presented in IEC 61970-501 as the meta-model framework for constructing XML[2] documents containing power system modeling information. The syntax of these documents is called CIM XML format. Model exchange by file transfer serves many useful purposes, specially when some applications need to have the complete network model defined. Though the format can be used for general CIM-based information exchange, in this Part of IEC 61968 specific profiles (or subsets) of the CIM are identified in order to address particular exchange requirements.

2      General

Given the CIM RDF Schema described in IEC 61970-501, a DMS power system model can be converted for export as an XML document, see Figure 1. This document is referred to as a CIM XML document. All of the tags (resource descriptions) used in the CIM XML document are supplied by the CIM RDF schema. The resulting CIM XML model exchange document can be parsed and the information imported into a foreign system. This part of IEC 61968 is aligned to CIM Model version 11, CPSM 3.0 profile.

Figure 1 – XML-Based DMS Network Data Configuration

Similar to using any programming language, implementers have many choices when creating a CIM XML document. The RDF syntax itself can be used in several ways to achieve the same basic result. The way one approaches the CIM RDF Schema can yield various forms when producing a CIM XML document. The following Clauses discuss the style guidelines for producing a CIM XML document. Such guideline rules are important to communicate and follow when producing these documents because they simplify and facilitate the software written to unambiguously interpret the model information.

Some comparisons have been made between CIM RDF and CIM XSD. Annexes A, B, C and D are extracted from articles and documents discussing CIM RDF and CIM XSD. A Distribution Management System can use only a CIM XSD message types architecture, but CIM RDF has three advantages :

·      A UML model is a graph model and RDF helps to describe the graph model. XSD describes a hierarchical model which suits the message type approach.

·      RDF is more readable and understandable by people working in the electrotechnical field.

·      It is a basic requirement to build ontologies.

If required, tools would ensure the compatibility between CIM-RDF and for instance IEC 61968-4, IEC 61968-3 message types concerning distribution network model representation.

3      Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

IEC 61968-1, Application integration at electric utilities - System interfaces for distribution management - Part 1: Interface architecture and general requirements

IEC 61968-3, Application integration at electric utilities - System interfaces for distribution management - Part 3: Interface for network operations

IEC 61968-4, Application integration at electric utilities - System interfaces for distribution management - Part 4: Interfaces for records and asset management 

IEC 61970-301, Energy management system application program interface (EMS-API) - Part 301: Common Information Model (CIM) base

IEC 61970-501, Energy management system application program interface (EMS-API) - Part 501: Common Information Model Resource Description Framework (CIM RDF) schema

4      Future standards documents related to this part

The following documents are taken into account even if they have not been published as FDIS yet.

Extensions to CIM for Distribution : IEC 61968-11 System Interfaces for Distribution Management- Interface Reference Model

This document is used during interoperability tests : IEC 61970-452, EMS-API – Part 452: CIM Model Exchange Specification

IEC 61970-552-4, EMS-API – Part 552-4 : CIM XML Model Exchange Format

5      CIM RDF describing Distribution Networks

In this part of the IEC 61968 standard, the object is to describe a CIM RDF model for the Distribution networks. It has the same  objective as  the NERC Common Power System Model (CPSM)  Profile that has been agreed to at the Transmission level (reference: http://www.w3.org/TR/2004/REC-rdf-primer-20040210 subclause 6.5, and IEC 61970-452). At the Distribution level, several kinds of application exist such as Network Operation, Asset Management, Customer Information, Network Planning, Work Management, etc. Efforts on standardization of these applications are conducted at the IEC through the Technical Committee 57. For more information, refer to http://www.cimuser.org web site.

Electric utilities use power system models for a number of different purposes. For example, power system simulations are developed for planning and security analysis. An operational power system model may consist of thousands of classes of information. In addition to using these models in-house, applications inside an individual utility need to exchange system modelling information, both for planning and operational purposes (e.g. coordinating transmission and distribution networks and ensuring reliable operations). However, individual utilities use different software packages for these purposes. As a result, the system models are stored in different formats, making exchange among these models difficult. The exchange of model data is difficult and requires specific interface development for data exchange between each pair of applications. Consequently, the individual utilities recognize the need to agree on common definitions of the power system entities and relationships to facilitate the future data exchange requirements.

The CIM defines most of objects inside an electric utility as classes and attributes, as well as the relationships among them. The CIM uses these object classes, their attributes and relationships to support the integration of independently developed applications among vendor specific DMS applications. CIM represents a canonical data model to support data exchange between each part of a DMS system such as asset management, distribution planning, etc.

Based on the NERC CPSM Profile for the transmission network, this part of IEC 61968 proposes a CIM-RDF profile for modelling Distribution networks. This part of IEC 61968 defines a CDPSM profile (Common Distribution Power System Model). IEC 61968-13 will mention the differences between this part of IEC 61968 and CPSM profile when they occur.

The data is intended for initial configuration of distribution network applications includes the applications such as distribution load flow calculation, dynamic network coloring, stability studies due to the impact of Distributed Energy Resources on Distribution Networks, Distribution remote control system data management, exchange of data between TSO (Tranmission System Operator) and DSO (Distribution System Operator), etc.

Consequently the proposal is mainly based on IEC 61970-301, without, at the present time, the Asset classes found in IEC 61968-11. In the future, assetType attribute of Asset class will be used instead of PsrType if CIM IEC 61968-11 is normalized and incorporated officially in the CIM. In this part of IEC 61968, class Location is defined in the IEC 61968-11 packages.

This part of IEC 61968 is valid for three-phase balanced and unbalanced distribution networks. It is described as a single phase network and may have single- or two-phase components such as single-phase laterals and transformers[3]. However, some users may find it convenient to restrict the proposed profile to include only the subset of three-phase balanced networks and exclude support for single phase components. In the sections which follow the term “partial-phase devices” is used to describe components having less than three phases.

6      Issues related to partial-phase devices modeling

6.1       General

The IEC 61970-301 standard already has support for partial phase conducting devices through the phase-code attribute which may be a combination of any or all of the letters A, B, C, and N. In general, one can think of a partial phase conducting device as being the same as a full 3-phase device with some of the phases missing.

6.2       Impedances of unbalanced and partial phase devices

IEC 61970-301 specifies impedance of conducting devices in terms of the real and reactive positive and zero sequence impedance. Unfortunately, this is only valid for perfectly symmetric three-phase networks where all 3 phases have the same value of self-impedance and the same mutual impedance value.

The impedance of unbalanced 3-phase conducting devices such as AC line segments shall be specified as a three by three complex matrix where the diagonal terms specify the self impedance of each phase and the off-diagonal terms specify the mutual impedance between each phase pair. These values can be computed using Carson’s equations based on the geometric mean radius, the linear resistance and the geometric arrangement of the three phases on the pole. IEC 61970-301 provides all the parameters necessary in the Conductor and WireArrangement classes. For 2-phase devices, the impedance matrix is two by two and for single-phase devices, it is a complex scalar specifying the self impedance of the single phase conductor.

6.3       Switches

IEC 61970-301 allows only two states for a switch device, i.e. open and closed. Thus for a 3-phase switch it suggests that all three phases of the switch always operate together and it does not support the situation where, for example, phase A of the switch is open while phases B and C are closed. Of course, a single-phase switch may be open or closed.

6.4       Partial phase continuity in radial networks

Many distribution networks are operated radially meaning that there is only one path for power to be supplied to any conducting device. For all phases of a device in a radial network to be properly energized, all devices upstream shall have the same phases present. (For example, it is not possible to energize all the phases  of a three phase device via a partial phase upstream device.)

However, this requirement is not enforced in this part of IEC 61968. Rather, it is up to the importing DMS to check if this requirement is satisfied throughout the network.

7      CIM Classes Used and Corresponding RDF

7.1       General

There is a large variety of voltage combinations in a substation. Also, in general substations may  contain one, two or more voltage levels. The applications needing such “substation type” information will deduce the substation type from the voltage levels it contains

In general, substations may  contain one, two or more voltage levels, the substation type will be deduced by analyzing the voltage levels a substation contains. The class PSRType can be used to distinguish these different substations. Class Location can be used to define the absolute position of a Substation.

Annex E gives a complete example of a Distribution Network Data represented through CIM-RDF. It has to be pointed-out that this complete example has been successfully tested during CIM interoperability tests conducted by EPRI in 2004, 2005, and 2006.

From the standpoint of a data producer (exporter), the document describes a minimum subset of CIM classes and class data which must be present in an XML formatted data file to comply with CDPSM Minimum Data Requirements. From the standpoint of a data recipient (importer), the document describes a subset of the CIM that an importer could reasonably expect to receive in an XML data file designed to be compliant with the CDPSM Minimum Data Requirements (See IEC 61970-501).

7.2       BaseVoltage and VoltageLevel

For every operating voltage found in the network we create a BaseVoltage. An ACLineSegment is associated to a BaseVoltage. A TransformerWinding is associated to a BaseVoltage. PowerTransformer should be contained in a Substation.

Every Substation is associated with one or more VoltageLevel-s, each of which is in turn associated with the corresponding BaseVoltage.

All the objects of the network, except ACLineSegment, PowerTransformer and Transformer Winding should be contained within a VoltageLevel.

7.3       Containment hierarchy roots

The CPSM 2.0 profile of base CIM defines HostControlArea to be at the root of containment hierarchy. In contrast, this specification defines HV/MV Substation as the root of the containment hierarchy.

7.4       HV/MV Substation

The containment hierarchy begins by HV/MV Substation.

7.5       MV/MV Substation

7.6       MV/LV Substation

If HV/LV Substation and LV/LV Substation have to be modeled, they will follow the same principles as above.

In IEC 61968-13 all conducting equipment shall be a member of either a substation or of a feeder. Normally all substation equipment is housed in a physical enclosure such as a building or a fenced area. A feeder is generally outside a physical enclosure and consists of a collection, or connected set, of AC line segments, switches, transformers (which may or may not be considered as a substation), etc. See further discussion of the feeder container object under “Line” later in this document.

In addition IEC 61968-13 shall support generalized equipment containers to group a set of connected conducting devices – for example the CompositeSwitch device of IEC 61970-301.

7.7       Junction

In the CIM, devices are connected to each other by connecting a terminal of a device to a common ConnectivityNode. A connectivity node may have any number of terminals connected to it.

In a Distribution network, most ConnectivityNodes are contained in substations. However in some cases (e.g. a tapped distribution line), ConnectivityNodes may be located on lines which are outside of substations. IEC 61970-301 defines the Junction class to indicate such connectivity nodes. In this case, the ConnectivityNode and the Junction shall be located in a virtual Substation.

However, a typical distribution network generally has many connectivity nodes outside of substations along a feeder. Since these connectivity nodes serve no purpose other than to connect two or more devices, it is generally not necessary to also specify them as a Junction.

7.8       Switch[4]

Switches are contained either by VoltageLevel or by Bay. If Switches are contained by VoltageLevel, Bay is not required. The abstract Switch is used only when we do not know the detailed class.

IEC 61968-13 supports the following kinds of Switch devices:

Breaker (exist in CPSM) able to interrupt fault currents greater than normal load currents.

LoadBreakSwitch (exist in CPSM) able to interrupt normal load currents only.

Disconnector (exist in CPSM) no current interrupt capability.

Fuse (doesn’t exist in CPSM) able to interrupt fault currents.

Jumper (doesn’t exist in CPSM).

GroundDisconnector (doesn’t exist in CPSM).

7.9       Bay

IEC 61970-301 supports the Bay object as a collection or container of a set of switch devices and connectivity nodes inside a substation. Generally, a substation will contain several, usually identical, bays containing connectivity nodes for incoming or outgoing lines (feeders)  Outgoing and incoming feeders are distinguished by the class PSRType. (PSRType and Location are not mandatory).

This data is not mandatory. If Switches are contained by VoltageLevel, Bay is not required.

7.10     BusbarSection

Figure 2 describles the connectivity of a busarsection which has only one Terminal.

Figure 2 – Connectivity of BusbarSection

7.11     PowerTransformer

IEC 61968-13 supports transformer objects and their tap changers exactly as defined in IEC 61970-301.

While an Autotransformer in reality does not have two distinct windings, it is acceptable in IEC 61968-13 to model it as having two windings similar to conventional transformers in order to define the voltage ratio. However, in distribution systems, line voltage regulators are sometimes used to compensate for line voltage drop. Line voltage regulators frequently are Autotransformers which have a nominal 1:1 voltage ratio, but generally operate at slightly off-nominal taps to provide a voltage boost. There is a special problem defining the leakage impedance of such devices since at nominal tap position, the leakage impedance is essentially zero. Therefore for autotransformers with a nominal 1:1 voltage ratio, the leakage impedance shall be defined with the tap at maximum tap position.

There are dozens of distribution transformer winding configurations which cannot be simply transformed into Y-Y equivalents as is commonly done for balanced transmission modeling. Therefore, more information is needed than is provided below in order to accurately model many transformer types. However, comprehensive transformer modeling would push the size and detail of the profile beyond practical usability. Depending on the need, Kersting IEEE models could be used as a guide to an appropriate level of transformer detail to extend this profile.

The associations for PowerTransformer containment are:

Substation -> PowerTransformer -> TransformerWinding

The TransformerWinding -> BaseVoltage link should be used. The model needs only BaseVoltage instances that correspond to TransformerWinding’s voltage levels.

7.12     MV/MV Transformer

A MV/MV transformer or auto-transformer is a PowerTransformer as described above.

7.13     Line

In IEC 61970-301 a line is a conductor connecting nodes usually in two different substations. However, IEC 61970-301 also allows for the modeling of a tapped line connecting more than two substations, but it imposes the requirement that the tap junction be contained in a “dummy” or collapsed (or fictitious) substation.

In distribution networks, it is more common to use the term “feeder” instead of the term ”line”. A feeder can be considered as a tapped line having, in general, several ACLineSegments,  junctions or  taps. In addition, a feeder may also contain switch devices,  MV/LV distribution transformers, capacitors, line voltage regulators. Because of this potentially large number of junctions, it is considered impractical to insist that all such devices and junctions be contained in a substation. Instead, it is sufficient to indicate them as members of a feeder only. However, it is also acceptable to model feeder devices, such as an MV/LV transformer and related switches, to be contained in a distribution substation which in turn is a member of a feeder. A Substation cannot be a member of a feeder or a line. If there is a Substation, it is necessary to split the feeder or the line into two different feeders or lines.

To be consistent with CPSM, any ConnectivityNode and any equipment except ACLineSegment, PowerTransformer and TransformerWinding should be in a VoltageLevel itself in a Substation. PowerTransformer and TransformerWinding should be in a Substation.

Each  Line  has a list of GmlPosition. The list of GmlPositions in the file reflects a precise order using sequenceNumber attribute, if the line has to be drawn.

7.14     ACLineSegment

A distinction between Asset characteristics of the Line Segment  and the operational (PowerSystemResource) characteristics of the Line Segment shall be made. For instance, the maximum ampacity can be modeled by the amprating attribute of WireType class (Current carrying capacity, expressed in amperes, of a wire or cable under stated thermal conditions). To reflect the operational value of this attribute, then a Measurement can be used with Limit and LimitSet classes.

7.15     WireArrangement

The WireArrangement needs an enumeration of phase in order to make the Carson’s Equations calculations for impedances. Currently, one WireArrangement is needed per phase and neutral of an ACLineSegment. Eventually, this information should be moved into the Assets  package,  including each phase’s x,y position where ground level is assumed to be at y = 0 for reference.

For a balanced 3 or 4 wire case, an ACLineSegment instance  is described as follows :

For an unbalanced case where impedances shall be derived with Carson’s Equations, the X,Y wire arrangement data shall be supplied as well as wire type impedance per unit length.

An EquivalentSource represents a High Voltage Source which can generally be considered as an “infinite bus” capable of supplying whatever load is connected to it.

7.16     Compensator

Compensator represents either a capacitor or a reactor. They are distinguished from each other by the sign in the value of the attribute mVArPerSection. If mVArPerSection is positive, it is a capacitor. If it is negative, then it is a reactor. A Compensator can have either one or two Terminals, which means it can be either a shunt device or a series device.

7.17     StaticVarCompensator

A StaticVarCompensator has only one Terminal, even if it represents a coil. SVC is a always shunt device.

A StaticVarCompensator represents either a capacitor or a reactor. They are distinguished from each other by the capacitiveRating (for capacitor) and inductiveRating (for reactor).

An example of a capacitor:

An example of a reactor:

7.18     EquivalentLoad

According to IEC 61970-301, an EnergyConsumer is a generic user of energy - a  point of consumption on the power system model. According to IEC 61968-13, a MV  customer is a CustomerLoad, a LV customer is an EquivalentLoad. An EquivalentLoad has the attribute customerCount  having its value greater than one to indicate the number of customers attached. The voltage level is specified by a voltage level that contains this EquivalentLoad.

The abstract EnergyConsumer is used only when we do not know the detailed class (as for Switch, it is not recommended).

7.19     Using CustomerLoad, GeneratingUnit and SynchronousMachine to model Distributed Energy Resource

For a Distributed Energy Resource (DER), we generate CustomerLoad, SynchronousMachine and GeneratingUnit to model it. When it consumes energy, we take data from CustomerLoad. When it produces energy, we take data from SynchronousMachine and GeneratingUnit.

A DER can have two different contracts: Energy Consumption and Energy Generation. A DER can be a Voltage regulator. For a DER, we need to define its rated active Power and rated reactive Power. When P is positive, it is consuming energy. When P is negative, it acts as producing energy.

7.20     GeneratingUnit

In most distribution networks, embedded generation is not intended to supply all load and can only operate while there is also a transmission source of supply. Thus embedded generators should be modeled as generators and not as an equivalent source. The output of an embedded generator may be specified by a curve and it may be specified as a P,Q schedule or a P,V schedule[5].

In CPSM, these curves do not exist. There are the GrossToNetMWCurve which defines net power and gross power of the group and the MVArCapabilityCurve that defines Qmin and Qmax.

Note that in the case of a P,Q generator it is also acceptable to model it simply as a negative load, Connectivity Nodes and Terminals.

Connectivity Nodes and Terminal classes of the CIM topological model are used to describe the connectivity model. GeneratingUnit.initalMW is used to represent normal Active power (P).

7.21     SynchronousMachine

SynchronousMachine.baseMVAr is used to represent reactive power (Q).

7.22     HostControlArea

This class is not mandatory in the IEC 61968-13 (CDPSM) profile. We list it here since it is used in CPSM profile hierarchy.

7.23     SubControlArea

This class is not mandatory in the IEC 61968-13 (CDPSM) profile. We list it here since it is used in CPSM profile hierarchy.

8      Adequation between IEC 61968-3 (CDPSM) and IEC 61968-4

In IEC 61968-4 (Records and Asset Management), NetworkDataSet Message Type is defined. In order to prove that the standard is consistent and that the semantic is shared whatever XML support used (RDF, XSD), the following table highlights differences between CDPSM profile defined in IEC 61968-13, and Cim elements used in NetworkDataSet Message Type. A comment is given when necessary. It has to be mentioned that IEC 61968-13 relies on CPSM profile, as a consequence IEC 61970-301 is used, thus PowerSytemResource is principally the base class which is used. On the other end, IEC 61968-4, and NetworkDataSet message type, relied on all CIM classes, and extensions made in CIM by wg14 , thus Asset class is also used as a base class.

The message type NetworkDataSet.xsd is based on CIM version 10 revision 7.

The message structure used by 61968-3 and 61968-4 standard parts are described in 61968-1 part.

NOTE In order to be concise, if the same set of elements is found in the NetworkDataSet message type, a global name to is used to refer to it. For instance : TerminalSubSet.

The Message is composed of two blocks described in Table 1 and Table 2 :

MessageHeader
MessagePayload

Table 1 - Header of NetworkDataSet  Message Type

 Table 2 – Message Payload of NetworkDataSet Message Type

Based on Table 2 , it can be said that IEC 61968-13 is, at the present time, a subset of IEC 61968-4, as it does not include any asset related class.

9      Adequation between CDPSM and CPSM

Annex G compares CDPSM and CPSM through some CIM-XML-RDF files examples.