This section summarizes the use of a reduced-order CIM [1] to support feeder modeling for the North American circuits and use cases considered in GridAPPS-D. The full CIM includes over 1100 tables in SQL, each one corresponding to a UML class, enumeration or datatype. RC1 used approximately 100 such entities, mapped onto 100+ tables in SQL. Subsequent versions of GridAPPS-D use a triple-store database, which is better suited for CIM.
The CIM subset described here is based on CIM100_501-20v1_302CDV20v26_457-20v22, which includes new modeling for DER compliant to IEEE 1547-2018, and other changes proposed from the GridAPPS-D project.
Common Information Model Diagrams¶
Figure 1 through Figure 20 present the UML class diagrams generated from Enterprise Architect [2]. These diagrams provide an essential roadmap for understanding:
How to ingest CIM XML from various sources into the database
How to generate native GridLAB-D input files from the database
For those unfamiliar with UML class diagrams:
Lines with an arrowhead indicate class inheritance. For example, in Figure 1, ACLineSegment inherits from Conductor, ConductingEquipment, Equipment and then PowerSystemResource. ACLineSegment inherits all attributes and associations from its ancestors (e.g., length), in addition to its own attributes and ancestors.
Lines with a diamond indicate composition. For example, in Figure 1, Substations make up a SubGeographicalRegion, which then make up a GeographicRegion.
Lines without a terminating symbol are associations. For example, in Figure 1, ACLineSegment has (through inheritance) a BaseVoltage, Location and one or more Terminals.
Italicized names at the top of each class indicate the ancestor (aka superclass), in cases where the ancestor does not appear on the diagram. For example, in Figure 1, PowerSystemResource inherits from IdentifiedObject.
Please see GridAPPSD_RC4.eap [3] in the repository [4] on GitHub for the latest updates. The EnterpriseArchitect file includes a description of each class, attribute and association. It can also generate HTML documentation of the CIM, with more detail than provided here.
The diagrammed UML associations have a role and cardinality at each end, source and target. In practice, only one end of each association is profiled and implemented in SQL. In some cases, the figure captions indicate which end, but see the CIM profile for specific definitions, as described in the object diagram section. Usually, the end with 0..1 multiplicity is implemented instead of the end with 0..* or 1..* multiplicity.
Network Models in CIMHub¶
Nearly every CIM class inherits from IdentifiedObject, from which we use two attributes:
mRID is the “master identifier” that must be unique and persistent among all instances. It’s often used as the RDF resource identifier, and is often a GUID.
Name is a human-readable identifier that need not be unique.
Figure 1 shows how equipment is organized into a Feeder. Each piece of Equipment has Terminals that connect at ConnectivityNodes, which represent the “buses”. Please note that in GridAPPS-D, we do not use TopologicalNode at all. In some CIM use cases, primary for transmission, the TopologicalNode coalesces ConnectivityNodes that are connected by buswork and closed switches into a single connection point. Instead, substation buswork models need to explicitly include the low-impedance branches and switches between ConnectivityNodes. We assume that modern distribution power flow solvers have no need of TopologicalNode.
Figure 1: Placement of ACLineSegment into a Feeder. In GridAPPS-D, the Feeder is the EquipmentContainer for all power system components and the ConnectivityNodeContainer for all nodes. It’s energized from a Substation, which is part of a SubGeographicalRegion and GeographicalRegion for proper context with other CIM models. For visualization, ACLineSegment can be drawn from a sequence of PositionPoints associated via Location. The Terminals are free-standing; two of them will “reverse-associate” to the ACLineSegment as ConductingEquipment, and each terminal also has one ConnectivityNode. The Terminal:phases attribute is not used; instead, phases will be defined in the ConductingEquipment instances. The associated BaseVoltage:nominalVoltage attribute is important for many of the classes that don’t have their own rated voltage attributes, for example, EnergyConsumer.
Figure 2: There are four different ways to specify ACLineSegment impedances. In all cases, Conductor:length is required. The first way is to specify the individual ACLineSegment attributes, which are sequence impedances and admittances, leaving PerLengthImpedance null. The second way is to specify the same attributes on an associated PerLengthSequenceImpedance, in which case the ACLineSegment attributes should be null. The third way is to associate a PerLengthPhaseImpedance, leaving the ACLineSegment attributes null. Only conductorCount from 1 to 3 is supported, and there will be 1, 3 or 6 reverse-associated PhaseImpedanceData instances that define the lower triangle of the Z and Y matrices per unit length. The row and column attributes must agree with ACLineSegmentPhase:sequenceNumber. The fourth way to specify impedance is by wire/cable and spacing data, by association to WireSpacingInfo and WireInfo. If there are ACLineSegmentPhase instances reverse-associated to the ACLineSegment, then per-phase modeling applies. There are several use cases for ACLineSegmentPhase: 1) single-phase or two-phase primary, 2) low-voltage secondary using phases s1 and s2, 3) associated WireInfo data where the WireSpacingInfo association exists, 4) assign specific phases to the matrix rows and columns in PerLengthPhaseImpedance. It is the application’s responsibility to propagate phasing through terminals to other components, and to identify any miswiring. (Note: this profile does not use WireAssemblyInfo, nor the fromPhase and toPhase attributes of PhaseImpedanceData.)
Figure 3: The EnergySource is balanced three-phase, balanced two-phase or single-phase Thevenin source. GridAPPS-D use a three-phase EnergySource to represent the transmission system. See Figures 13-18 for DER modeling. The EnergyConsumer is a ZIP load, possibly unbalanced, with an associated LoadResponse instance defining the ZIP coefficients. For three-phase delta loads, the phaseConnection is D and the three reverse-associated EnergyConsumerPhase instances will have phase=A for the AB load, phase=B for the BC load and phase=C for the AC load. A three-phase wye load may have either Y or Yn for the phaseConnection. Single-phase and two-phase loads, including secondary loads, should have phaseConnection=I (for individual).
Figure 4: There are eight different kinds of Switch supported in the CIM, and all of them have zero impedance. They would all behave the same in power flow analysis, and all would require many more attributes than are defined in CIM to support protection analysis. The use cases for SwitchPhase include 1) single-phase, two-phase and secondary switches, 2) one or two conductors open in a three-phase switch or 3) transpositions, in which case phaseSide1 and phaseSide2 would be different. RatedCurrent may different among the phases, e.g., individual fuses on the same pole may have different ratings.
Figure 5: On the left, LinearShuntCompensator and LinearShuntCompensatorPhase define capacitor banks, in a way very similar to EnergyConsumer in Figure 3. The kVAR ratings must be converted to susceptance based on the nominal voltage, nomU. Note that aVRDelay is really a capacitor control parameter, to be used in conjunction with RegulatingControl on the right-hand side. The RegulatingControl associates to the controlled capacitor bank via RegulatingCondEq, and to the monitored location via Terminal. There is no support for a PT or CT ratio, so targetDeadband and targetValue have to be in primary volts, amps, vars, etc. Capacitor banks may _respond_ to any of the RegulatingControlModeKind choices, but it’s not expected that capacitor switching will successfully regulate to the targetValue.
Figure 6: PowerTransformers may be modeled with or without tanks, and in both cases vectorGroup should be specified according to IEC transformer standards (e.g., Dy1 for many substation transformers). The case without tanks is most suitable for balanced three-phase transformers that won’t reference catalog data; any other case should use tank-level modeling. In the tankless case, each winding will have a PowerTransformerEnd that associates to both a Terminal and a BaseVoltage, and the parent PowerTransformer. The impedance and admittance parameters are defined by reverse-associated TransformerMeshImpedance between each pair of windings, and a reverse-associated TransformerCoreAdmittance for one winding. The units for these are ohms and siemens based on the winding voltage, rather than per-unit. WindingConnection is similar to PhaseShuntConnectionKind, adding Z and Zn for zig-zag connections and A for autotranformers. If the transformer is unbalanced in any way, then TransformerTankEnd is used instead of PowerTransformerEnd, and then one or more TransformerTanks may be used in the parent PowerTransformer. Some of the use cases are 1) center-tapped secondary, 2) open-delta and 3) EHV transformer banks. Tank-level modeling is also required if using catalog data, as described with Figure 9. (TransformerStarImpedance and several PowerTransformer attributes are not used. Star impedance attributes on PowerTransformerEnd and magnetic saturation attributes on TransformerEnd are not used.)
Figure 7: A RatioTapChanger can represent a transformer tap changer on the associated TransformerEnd. The RatioTapChanger has some parameters defined in a direct-associated TapChangerControl, which inherits from RegulatingControl some of the same attributes used in capacitor controls (Figure 5). Therefore, a line voltage regulator in CIM includes a PowerTransformer, a RatioTapChanger, and a TapChangerControl. The CT and PT parameters of a voltage regulator can only be described via the AssetInfo mechanism, described with Figure 8. The RegulationControl.mode must be voltage. (Note: RegulationSchedule, RatioTapChangerTable and PhaseTapChanger are not used.)
Figure 9: Many distribution software packages use the concept of catalog data, aka library data, especially for lines and transformers. The catalog mechanism for transformers will associate a TransformerTank (Figure 6) with TransformerTankInfo (here). Many TransformerTanks can share the same TransformerTankInfo data, which saves space and provides consistency.It’s important that TransformerEndInfo:endNumber (here) properly match the TransformerEnd:endNumber (Figure 6). The shunt admittances are defined by NoLoadTest on a winding / end, usually just one such test. The impedances are defined by a set of ShortCircuitTests; one winding / end will be energized, and one or more of the others will be grounded in these tests. (OpenCircuitTest is not used, nor are the current, power and voltage attributes of ShortCircuitTest).
Figure 10: The catalog / library mechanism for ACLineSegment will have a WireSpacingInfo associated as in Figure 9. This will indicate whether the line is overhead or underground. phaseWireCount and phaseWireSpacing define optional bundling, so these will be 1 and 0 for distribution. The number of phase and neutral conductors is actually defined by the number of reverse-associated WirePosition instances. For example, a three-phase line with neutral would have four of them, sequenceNumber from 1 to 4. Each WirePosition’s phase is determined by the ACLineSegmentPhase with matching sequenceNumber, i.e., the phases need not be numbered in any particular order. On the left-hand side, concrete classes OverheadWireInfo, TapeShieldCableInfo and ConcentricNeutralCableInfo may be associated to ACLineSegmentPhase. It’s the application’s responsibility to calculate impedances from this data. In particular, soil resistivity and dielectric constants are not included in the CIM. Typical dielectric constant values might be defined for each WireInsulationKind.
Figure 11: The CIM state variables package was designed to report power flow solution values on the distribution system. It could also report state estimator solutions as a special case of power flow solutions. Voltages are measured on ConnectivityNodes (i.e., not TopologicalNodes), power flows are measured at Terminals into the ConductingEquipment, step positions are measured on TapChangers, status is measured on ConductingEquipment, and on/off state is measured on ShuntCompensators for Switches. The “injections” have been included here, but there may not be a use case for them in distribution. On the other hand, solution values for current are very common in distribution system applications. These should be represented as SvPowerFlow values at the solved SvVoltage.
Figure 12: Measurements are defined in the Meas package. They differ from the state variables package, in that the values are measured here and not calculated or estimated. Each Measurement is associated to a PowerSystemResource, and in GridAPPS-D for now, also a Terminal that belongs to the same PowerSystemResource. (Non-electrical measurements, for example weather, would not have the Terminal). The measurementType is a string code from IEC 61850, with PNV, VA, A and POS currently supported. The Measurement has a name, mRID, and phases. In GridAPPS-D, each phase is measured individually so multi-phase codes like ABC should not be used. Pos measurements will be Discrete, for such things as tap position, switch position, or capacitor bank position. The others will be Analog, with magnitude and optional angle in degrees. Each MeasurementValue will have a timeStamp and mRID inherited from IdentifiedObject, so the values can be traced. (Note: IOPoint is a placeholder class with no attributes, inherting from IdentifiedObject. Further, it’s acceptable to supply an empty or short non-unique name for each MeasurementValue.)
Figure 13: Power Electronics attributes are the minimum needed to support a time series power flow solution. For simple short-circuit calculations, maxIFault is provided as the inverter fault contribution in per-unit of rated current. When PowerElectronicsConnectionPhase is not present, the inverter is assumed to be balanced three-phase. The type of associated PowerElectronicsUnit determines whether the inverter is for solar or storage (wind is not currently used in GridAPPS-D). If the inverter employs a SmartInverterMode of voltVar, voltWatt or loadFollowing (storage only), then a Terminal should be associated through RegulatingControl, especially for loadFollowing. If the inverter will regulate its own Terminal, then the explicit Terminal association may not be needed. However, there are more attributes needed in CIM to define smart inverter functions. This might be done in harmonization with IEC 61850, which does define smart inverter function parameters. The existing CIM RegulatingControl attributes are probably not applicable, so they have been hidden in Figure 13.
Figure 14: Rotating Machines are three-phase balanced, either synchronous or asynchronous. The SynchronousMachine ikk attribute and most of the AsynchronousMachine attributes are provided to support short-circuit calculations according to IEC 60909. The GeneratingUnit class is needed to define minimum and maximum power limits. In the full CIM, GeneratingUnit is an abstract class with descendants HydroUnit, ThermalUnit and NuclearUnit, but in GridAPPS-D we don’t currently distinguish between those types. If the SynchronousMachine regulates voltage, then the RegulatingControl (with attribute values) and Terminal associations need to be provided.
DER Models from IEEE 1547-2018¶
Figure 15: The DERIEEEType1 class has been added to IEC 61970-302, 2nd edition, for describing the dynamics of DER that complies with IEEE Std. 1547-2018. This data is used to describe smart inverter functions and other DER behavior during time-series power flow in GridAPPS-D. See IEEE 1547-2018, IEEE 1547.1-2020, and IEEE P1547.2/D6.2 (Annex F) for descriptions of the classes and attributes. The 302 classes and attributes generally map to the interoperability tables in the IEEE standards, the main difference being the use of capital letters and underscores in the IEEE tables. Both inverters (Figure 13) and rotating machines (Figure 14) can associate to DERIEEEType1 for supplemental nameplate and rating information in the network model. Preliminary values for these attributes would be available from an application to interconnect DER, and then updated as the project moves through commissioning to operational status. To be determined whether DERDynamics can have cardinality 0..* as shown, or 0..1 like other dynamic functions in 302.
Figure 16: DER compliant with IEEE Std. 1547-2018 shall support at least three control modes for voltage and reactive power. In addition, Category B DER shall support a fourth mode, WattVar, and an active power control mode, VoltWatt. The reactive power control modes are mutually exclusive, so only one of those shown should be enabled, i.e., the unused modes can either be left out of the model, or disabled.
Figure 17: The other settings in IEEE Std. 1547-2018 cover tripping or momentary cessation during voltage and frequency disturbances, response to frequency variations (droop), and power limiting. The parameters in ServiceSettings define the voltage and frequency range within which the DER is allowed to connect, or to reconnect after tripping. The settings and modes in this figure are not mutually exclusive, but there are ranges of adjustability specified in IEEE Std. 1547-2018.
Figure 18: Many DER will respond to voltage and frequency and the point of connection, which would be a Terminal associated to the PowerElectronicsConnection, SynchronousMachine, or AsynchronousMachine shown at the top of Figure 15. Where this is not the case, e.g., the DER responds to voltage and frequency on the other side of a PowerTransformer, the RemoteInputSignal mechanism can be used as shown in this figure. In GridAPPS-D, only the voltage signal is relevant. IEEE Std. 1547-2018 states that voltage functions are based on the average, minimum, or maximum of all applicable voltages. The applicable voltages depend on how the connection appears to the grid, as conveyed in the attributes of DERIEEEType1. Hence, the RemoteInputSignal.phase attribute is not used. Instead, use all applicable voltages from the associated Terminal.
Extension for Houses¶
Figure 19: Houses are used to create 2nd-order thermal models of the building envelope, with internal ThermostatController and heating/cooling systems. The purpose is to introduce realistic load stochastic behaviors that are independent from and faster-moving than data typically available to an electric utility. To enable repeatable simulations, the House data structures have been defined here as a CIM extension. The House must be attached to one EnergyConsumer that incorporates other end-use loads, and connects to the distribution system. The House attributes are the minimum necessary to define a GridLAB-D house model, and during simulation, the house heating/cooling system will add to the ServicePanel loads. Therefore, the application should reduce the nominal value of EnergyConsumer.p in order to “make room” for the heating/cooling load that will switch on and off, responding to the ThermostatController and the weather. The ThermostatController contains the minimum attributes needed for PNNL’s double-ramp, double-auction market mechanism. In the future, this will be harmonized with CIM market structures in the 62235 package.
Extension for Profiles¶
Figure 19a: Optional references to shapes for OpenDSS, and players or schedules for GridLAB-D.
The extension class is EnergyConnectionProfile, each instance of which could have many associations to EnergyConsumer, PowerElectronicsConnection, or SynchronousMachine as appropriate. One use case is to represent residential loads with the House extension class, and commercial spot loads with this extension class. A Python script will add the EnergyConnectionProfile instances, as other scripts presently insert houses, DER, and measurements. Then the CIMHub export function will netlist them as appropriate for OpenDSS and GridLAB-D.
See attribute documentation for applicability. The shapes, players, and schedules are not maintained in CIM, i.e., they must be made available to the simulator from an external source.
dssDaily: Reference to OpenDSS Daily curve, for Load, Storage, PVSystem, Generator, and WindGen power
dssDuty: Reference to OpenDSS Duty Cycle curve, for Load, Storage, PVSystem, Generator, and WindGen power
dssLoadCvrCurve: Reference to OpenDSS CvrCurve, for Load objects
dssLoadGrowth: Reference to OpenDSS Growth curve, for Load objects
dssPVTDaily: Reference to OpenDSS Daily curve, for PVSystem temperature
dssPVTDuty: Reference to OpenDSS Duty Cycle curve, for PVSystem temperature
dssPVTYearly: Reference to OpenDSS Yearly curve, for PVSystem temperature
dssSpectrum: Reference to OpenDSS harmonic current Spectrum, for Load, Storage, PVSystem, Generator, and WindGen power
dssYearly: Reference to OpenDSS Yearly curve, for Load, Storage, PVSystem, Generator, and WindGen power
gldPlayer: GridLAB-D Player for base_power attributes on Load and Triplex_Load objects, real and reactive power for Diesel DG objects, P_Out for Battery objects, and Insolation for Solar objects. Netlisted as player.value.
gldSchedule: GridLAB-D schedule for base_power attributes on Load and Triplex_Load objects, real and reactive power for Diesel DG objects, P_Out for Battery objects, and Insolation for Solar objects.
Extension for Faults¶
Figure 20: Faults include open conductors and short circuits (optionally including ground) on any combination of phases. In GridAPPS-D, every Fault will be an EquipmentFault associated to a Terminal (i.e., we are not using LineFault, which requires a lengthFromTerminal1 attribute). The occurredDateTime supports the scripting of fault sequences. The stopDateTime is optional. If provided, it will be the time at which a sustained fault has been repaired. If not provided, then the fault is temporary and will clear itself as soon as it’s been deenergized.
Typical Queries¶
These queries focus on requirements of the first volt-var application.
Capacitors (Figure 5, Figure 21, Figure 22, Figure 23)
Create a list of capacitors with bus name (Connectivity Node in Figure 1), kVAR per phase, control mode, target value and target deadband
For a selected capacitor, update the control mode, target value, and target deadband
Regulators (Figure 7, Figure 8, Figure 21, Figure 38)
List all transformers that have a tap changer attached, along with their bus names and kVA sizes
Given a transformer that has a tap changer attached, list or update initialDelay, step, subsequentDelay, mode, targetDeadband, targetValue, limitVoltage, lineDropCompensation, lineDropR, lineDropX, reverseLineDropR and reverseLineDropX
Transformers (Figure 6, Figure 9)
Given a bus name or load (Figure 3), find the transformer serving it (Figure 25, Figure 28)
Find the substation transformer, defined as the largest transformer (by kVA size and or highest voltage rating)
List the transformer catalog (Figure 9, Figure 29) with name, highest ratedS, list of winding ratedU in descending order, vector group (https://en.wikipedia.org/wiki/Vector_group used with connectionKind and phaseAngleClock), and percent impedance
List the same information as in item c, but for transformers (Figure 6) and also retrieving their bus names. Note that a transformer can be defined in three ways
Without tanks, for three-phase, multi-winding, balanced transformers (Figure 25 and Figure 26).
With tanks along with TransformerTankInfo (Figure 9) from a catalog of “transformer codes”, which may describe balanced or unbalanced transformers. See Figure 28 and Figure 29.
With tanks for unbalanced transformers, and TransformerTankInfo created on-the-fly. See Figure 28 and Figure 29.
Given a transformer (Figure 6), update it to use a different catalog entry (TransformerTankInfo in Figure 9)
Lines (Figure 2, Figure 10, Figure 21)
List the line and cable catalog entries that meet a minimum ratedCurrent and specific WireUsageKind. For cables, be able to specify tape shield vs. concentric neutral, the WireInsulationKind, and a minimum insulationThickness. (Figure 36)
Given a line segment (Figure 2) update to use a different linecode (Figure 10, Figure 35)
Given a bus name, list the ACLineSegments connected to the bus, along with the length, total r, total x, and phases used. There are four cases as noted in the caption of Figure 2, and see Figure 32 through Figure 35.
Given a bus name, list the set of ACLineSegments (or PowerTransformers and Switches) completing a path from it back to the EnergySource (Figure 3). Normally, the applications have to build a graph structure in memory to do this, so it would be very helpful if a graph/semantic database can do this.
Voltage and other measurements (Figure 1, Figure 11)
Given a bus, attach a voltage solution point (SvVoltage, Figure 39)
List all voltage solution points and their buses, and for each bus, list the phases actually present
For tap changer position (SvTapStep, Figure 40), attach and list values as in items a and b
For capacitor switch status (SvShuntCompensatorSections, Figure 41), attach and list values as in items a and b
Loads (Figure 3, Figure 37)
Given a bus name, list and total all of the loads connected by phase, showing the total p and q, and the composite ZIP coefficients
Switching (Figure 4, Figure 31)
Given a bus name, trace back to the EnergySource and list the switches encountered, grouped by type (i.e. the leaf class in Figure 4). Also include the ratedCurrent, breakingCapacity if applicable, and open/close status. If SwitchPhase is used, show the phasing on each side and the open/close status of each phase.
Given switch, toggle its open/close status.
Object Diagrams for Queries¶
This section contains UML object diagrams for the purpose of illustrating how to perform typical queries and updates. For those unfamiliar with UML object diagrams:
Each object will be an instance of a class, and more than one instance of a class can appear on the diagram. For example, Figure 21 shows two ConnectivityNode instances, one for each end of a ConductingEquipment.
The object name (if specified and important) appears before the colon (:) above the line, while the UML class appears after the colon. Every object in CIM will have a unique ID, and a name (not necessarily unique), even if not shown here.
Some objects may be shown with run-time state below the line. These are attribute value assignments, drawn from those available in the UML class or one of the class ancestors. The object may have more attribute assignments, but only those directly relevant to the figure captions are shown in the diagrams of this section.
Object associations are shown with solid lines, role names, and multiplicities similar to the UML class diagrams. One important difference is that only one way of navigating a particular association will be defined in the profile. For example, the lower left corner of Figure 1 shows a two-way link between Terminal and ConnectivityNode in the UML class diagram. However, Figure 21 shows that only one direction has been defined in the profile. Each Terminal has a direct reference to its corresponding ConnectivityNode. In order to navigate the reverse direction from ConnectivityNode to Terminal, some type of conditional query would be required. In other words, the object diagrams in this section indicate which associations can actually be used in GridAPPS-D.
In some cases, the multiplicities on the object diagrams are more restrictive than on the class diagrams, due to profiling. For example, EnergyConsumer and ShuntCompensator must have exactly one Terminal, not 1..*.
The object diagrams are intended to help you break down the CIM queries into common sub-tasks. For example, query #1 works with capacitors. It’s always possible to select a capacitor (aka LinearShuntCompensator) by name. In order to find the capacitor at a bus, say “bus1” in Figure 12, one would retrieve all Terminals having a ConnectivityNode reference to “bus1”. Each of those Terminals will have a ConductingEquipment reference, and you want the Terminal(s) for which that reference is actually a LinearShuntCompensator. In this CIM profile, only leaf classes (e.g. LinearShuntCompensator) will be instantiated, never base classes like ConductingEquipment. There can be more than one capacitor at a bus, more than one load, more than one line, etc.
Figure 21: In order to traverse buses and components, begin with a ConnectivityNode (left). Collect all terminals referencing that ConnectivityNode; each Terminal will have one-to-one association with ConductingEquipment, of which there are many subclasses. In this example, the ConductingEquipment has a second terminal referencing the ConnectivityNode called bus2. There are applications for both Depth-First Search (DFS) and Bread-First Search (BFS) traversals. Note 1: the Terminals have names, but these are not useful. In some cases, the Terminal sequenceNumber attribute is needed to clearly identify ends of a switch. Note 2: in earlier versions of GridAPPS-D, we had one-to-one association of TopologicalNode and ConnectivityNode, but these are no longer necessary. Note 3: transformers are subclasses of ConductingEquipment, but we traverse connectivity via transformer ends (aka windings). This is illustrated later.
In order to find capacitors (or anything else) associated with a particular “feeder”, Figure 22 shows that you would query for objects having EquipmentContainer reference to the Feeder object. In GridAPPS-D, we only use Feeder for equipment container in CIM, and this would correspond to one entire GridLAB-D model. There is also a BaseVoltage reference that will have the system nominal voltage for the capacitor’s location. However, in order to work with equipment ratings you should use ratedS and ratedU attributes where they exist, particularly for capacitors and transformers. These attributes are often slightly different than the “system voltage”. Most of the attribute units in CIM are SI, with a few exceptions like percent and kW values on transformer test sheets (i.e., CIM represents the test sheet, not the equipment).
Figure 22: All conducting equipment lies within an EquipmentContainer, which in GridAPPS-D, will be a Feeder object named after the feeder. It also has reference to a BaseVoltage, which is typically one of the ANSI preferred system voltages. Power transformers are a little different, in that each winding (called “end” in CIM) has reference to a BaseVoltage. Note that equipment ratings come from the vendor, and in this case ratedU is slightly different from nominalVoltage. All conducting equipment has a Location, which contains XY coordinates (see Figure 1). The Location is useful for visualization, but is not essential for a power flow model.
Completing the discussion of capacitors, Figure 23 provides two examples for single-phase, and three-phase with local voltage control. As shunt elements, capacitors have only one Terminal instance. Loads and sources have one terminal, lines and switches have two terminals, and transformers have two or more terminals. Examples of all those are shown later. In Figure 23, the capacitor’s kVAR rating will be based on its nameplate ratedU, not the system’s nominalVoltage.
Often, the question will arise “what phases exist at this bus?”.There is no phasing explicitly associated with a ConnectivityNode, and we don’t use the Terminal phases attribute in preference to the “wires phase model” classes. For example, thephases at a line segment terminal can always be obtained from the ACLineSegmentPhase instances. To answer the question about bus phasing, we’d have to query for all ConductingEquipment instances having Terminals connected to that bus, as in Figure 21. The types of ConductingEquipment that may have individual phases include LinearShuntCompensators (Figure 23), ACLineSegments, PowerTransformers (via TransformerEnds), EnergyConsumers, EnergySources, PowerElectronicsConnections, and descendants of Switch. If the ConductingEquipment has such individual phases, then add those phases to list of phases existing at the bus. If there are no individual phases, then ABC all exist at the bus. Note this doesn’t guarantee that all wiring to the bus is correct; for example, you could still have a three-phase load served by only a two-phase line, which would be a modeling error. In Figure 23, we’d find phase C at Bus611 and phases ABC at Bus675. Elsewhere in the model, there should be ACLineSegments, PowerTransformers or Switch descendants delivering phase C to Bus611, all three phases ABC to Bus675.
Figure 23: Capacitors are called LinearShuntCompensator in CIM. On the left, a 100 kVAR, 2400 V single-phase bank is shown on phase C at bus 611. bPerSection = 100e3 / 2400^2 [S], and the bPerSection on LinearShuntCompensatorPhase predominates; these values can differ among phases if there is more than one phase present. On the right, a balanced three-phase capacitor is shown at bus 675, rated 300 kVAR and 4160 V line-to-line. We know it’s balanced three phase from the absence of associated LinearShuntCompensatorPhase objects. bPerSection = 300e4 / 4160^2 [S]. This three-phase bank has a voltage controller attached with 2400 V setpoint and 240 V deadband, meaning the capacitor switches ON if the voltage drops below 2280 V and OFF if the voltage rises above 2520 V. These voltages have to be monitored line-to-neutral in CIM, with no VT ratio. In this case, the control monitors the same Terminal that the capacitor is connected to, but a different conducting equipment’s Terminal could be used. The control delay is called aVRDelay in CIM, and it’s an attribute of the LinearShuntCompensator instead of the RegulatingControl. It corresponds to “dwell time” in GridLAB-D.
Figure 24 through Figure 30 illustrate the transformer query tasks, plus Figure 38 for attached voltage regulators. The autotransformer example is rated 500/345/13.8 kV and 500/500/50 MVA, for a transmission system. The short circuit test values are ZHL=10%, ZHT=25% and ZLT=30%. The no-load test values are 0.05% exciting current and 0.025% no-load losses. These convert to r, x, g and b in SI units, from ZLT= Urated* Urated/ Srated, where Sratedand Uratedare based on the “from” winding (aka end). The same base quantities would be used to convert r, x, g and b back to per-unit or percent. The open wye – open delta impedances are already represented in percent or kW, from the test reports.
Figure 24: Autotransformer with delta tertiary winding acts like a wye-wye transformer with smaller delta tertiary. The vector group would be Yynd1 or Yyd1. For analyses other than power flow, it can be represented more accurately as the physical series (n1) – common (n2) connection, with a vector group Yand1. In either case, it’s a three-winding transformer.
Figure 25: A three-winding autotransformer is represented in CIM as a PowerTransformer with three PowerTransformerEnds, because it’s balanced and three-phase. The three Terminals have direct ConductingEquipment references to the PowerTransformer, so you can find it from bus1, busX or busY. However, each PowerTransformerEnd has a back-reference to the same Terminal, and it’s own reference to BaseVoltage (Figure 13); that’s how you link the matching buses and windings, which must have compatible voltages. Terminals have a sequenceNumber, but the PowerTransformerEnd’s endNumber is what establishes correct linkage to catalog data as discussed later. By convention, ends with highest ratedU have the lowest endNumber, and endNumber establishes that end’s place in the vectorGroup.
Figure 26: Power transformer impedances correspond to the three-winding autotransformer example of Figure 24 and Figure 25. There are three instances of TransformerMeshImpedance connected pair-wise between the three windings / ends. The x and r values are in Ohms referred to the end with highest ratedU in that pair. There is just one TransformerCoreAdmittance, usually attached to the end with lowest ratedU, and the attribute values are Siemens referred to that end’s ratedU.
Figure 27: Open wye - open delta transformer banks are used to provide inexpensive three-phase service to loads, by using only two single-phase transformers. This is an unbalanced transformer, and as such it requires tank modeling in CIM. Physically, the two transformers would be in separate tanks. Note that Tank A is similar to the residential center-tapped secondary transformer, except the CIM phases for the secondary would include s1 and s2 instead of A and B.
Figure 28: Unbalanced PowerTransformer instances comprise one or more TransformerTanks, which own the TransformerTankEnds. Through the ends, wdgHi collects phases ABN and busLo collects phases ABCN. Typically, phase C will also exist at wdgHi, but this transformer doesn’t require it. We still assign vectorGroup Yd1 to the supervising PowerTransformer, as this is the typical case. The modeler should determine that. By comparison to Figure 27, there is a possible ambiguity in how endA3 represents the polarity dot at the neutral end of Wdg A3. This is the purpose of TransformerTankEnd.reversed, which should be True for wdgA3.
Figure 29: This Asset catalog example defines the impedances for Tank B of the open wye – open delta bank. This is a 50 kVA, 7200 / 240 V single-phase transformer. It has 1% exciting current and 0.4 kW loss in the no-load test, plus 2.1% reactance and 0.5 kW loss in the short-circuit test. A multi-winding transformer could have more than one grounded end in a short-circuit test, but this is not common. The catalog data is linked with an AssetDataSheet association shown to the left. Furthermore, endNumber on the TransformerEndInfo has to match endNumber on the TransformerTankEnd instances associated to Tank B. Instead of catalog information, we could have used mesh impedance and core admittance as in Figure 21, but we’d have to convert the test sheets to SI units and we could not share data with other TransformerTank instances, both of which are inconvenient.
Figure 30 through Figure 36 illustrate the query tasks for ACLineSegments and Switches, which will define most of the circuit’s connectivity. The example sequence impedances were based on Z1 = 0.1 + j0.8 Ω/mile and Z0 = 0.5 + j2.0 Ω /mile. For distribution systems, use of the shared catalog data is more common, either pre-calculated matrix (Figure 34) or spacing and conductor (Figure 35 and Figure 36). In both cases, impedance calculation is outside the scope of CIM (e.g. GridLAB-D internally calculates line impedance from spacing and conductor data).
Figure 30: An ACLineSegment with two phases, A and C. If there are no ACLineSegmentPhase instances that associate to it, assume it’s a three-phase ACLineSegment. This adds phases AC to bus671 and bus684.
Figure 31: This 50-Amp load break switch connects phases AC between busLeft and busRight. Without associated SwitchPhase instances, it would be a three-phase switch. This switch also transposes the phases; A on side 1 connects with C on side 2, while C on side 1 connects with A on side 2. This is the only way of transposing phases in CIM. Note the Terminal.sequenceNumber is essential to differentiate phaseSide1 from phaseSide2. Also note that LoadBreakSwitch has the open attribute inherited from Switch, while SwitchPhase has the converse closed attribute. In order to open and close the switch, these attributes would be toggled appropriately. See Figure 4 for other types of switch.
Figure 32: This is a balanced three-phase ACLineSegment between bus632 and bus671, 2000 feet or 609.6 m long. Sequence impedances are specified in ohms, as attributes on the ACLineSegment. This is a typical pattern for transmission lines, but not distribution lines.
Figure 33: The impedances from Figure 27 were divided by 609.6 m, to obtain ohms per meter for seqCat1. Utilities often call this a “line code”, and other ACLineSegment instances can share the same PerLengthImpedance. A model imported into the CIM could have many line codes, not all of them used in that particular model. However, those line codes should be available for updates by reassigning PerLengthImpedance.
Figure 34: This is a two-phase line segment from bus671 to bus684 using a line code, which has been specified using a 2x2 symmetric matrix of phase impedances per meter, instead of sequence impedances per meter. This is more common for distribution than either Figure 32 or Figure 33. It’s distinguished from Figure 33 by the fact that PerLengthImpedance references an instance of PerLengthPhaseImpedance, not PerLengthSequenceImpedance. The conductorCount attribute tells us it’s a 2x2 matrix, which will have two unique diagonal elements and one distinct off-diagonal element. The elements are provided in three PhaseImpedanceData instances, which are named here for clarity as Z11, Z12 and Z22. However, only the row and column attributes are meaningful to identify the matrix element. In this example, Z11 and Z22 are slightly different. In order to swap phases A and C, we would swap the sequenceNumber values on the ACLineSegmentPhase instaces. As presented here, mtx604 can apply to phasing AB, BC or AC.
Figure 35: The two-phase ACLineSegment impedance defined by sharing wire and spacing data from a catalog. Each ACLineSegmentPhase links to an OverheadWireInfo instance via the AssetDataSheet association. If the neutral (N) is present, we have to specify its wire information for a correct impedance calculation. In this case, ACN all use the same wire type, but they can be different, especially for the neutral. Similarly, the WireSpacingInfo associates to the ACLineSegment itself via a AssetDataSheet assocation.
Figure 36: The upper five instances define catalog attributes for Figure 30. The WirePosition xCoord and yCoord units are meters, not feet, and they include sequenceNumber assignments to match ACLineSegmentPhase sequenceNumbers. The phaseWireSpacing and phaseWireCount attributes are for sub-conductor bundling on EHV and UHV transmission lines; bundling is not used on distribution. The number of WirePositions that reference spc505acn determine how many wires need to be assigned. Eliminating the neutral, this would produce a 2x2 phase impedance matrix. Although the pattern appears general enough to support multiple neutrals and transmission overbuild, the CIM doesn’t actually have the required phasing codes. When isCable is true, the WirePosition yCoord values would be negative for underground depth. To find overhead wires of a certain size or ampacity, we can put query conditions on the ratedCurrent attribute. To find underground conductors, we query the ConcentricNeutralCableInfo or TapeShieldCableInfo instead of OverheadWireInfo. All three inherit the ratedCurrent attribute from WireInfo. Cables don’t yet have a voltage rating in CIM AssetInfo, but you can use insulationThickness as a proxy for voltage rating in queries. Here, 5.588 mm corresponds to 220 mils, which is a common size for distribution.
Figure 37 illustrates the loads, which are called EnergyConsumer in CIM. The houses and appliances from GridLAB-D are not supported in CIM. Only ZIP loads can be represented. Further, any load schedules would have to be defined outside of CIM. Assume that the CIM loads are peak values.
Figure 38 illustrates the voltage regulator function. Note that GridLAB-D combines the regulator and transformer functions, while CIM separates them. Also, the CIM provides voltage and current transducer ratios for tap changer controls, but not for capacitor controls.
Figure 39 through Figure 41 illustrate how solved values can be attached to buses or other components.
Figure 37: The three-phase load (aka EnergyConsumer) on bus671 is balanced and connected in delta. It has no ratedU attribute, so use the referenced BaseVoltage (Figure 22) if a voltage level is required. On the right, a three-phase wye-connected unbalanced load on bus675 is indicated by the presence of three EnergyConsumerPhase instances referencing UnbalancedLoad. For consistency in searches and visualization, UnbalancedLoad.p should be the sum of the three phase values, and likewise for UnbalancedLoad.q. In power flow solutions, the individual phase values would be used. Both loads share the same LoadResponse instance, which defines a constant power characteristic for both P and Q, because the percentages for constant impedance and constant current are all zero. The two other most commonly used LoadResponseCharacteristics have 100% constant current, and 100% constant impedance. Any combination can be used, and the units don’t have to be percent (i.e., use a summation to determine the denominator for normalization).
Figure 38: In CIM, the voltage regulator function is separated from the tap-changing transformer. The IEEE 13-bus system has a bank of three independent single-phase regulators at busRG60, and this example shows a RatioTapChanger attached to the regulator on phase A, represented by the TransformerTankEnd having phases=A or phases=AN. See Figure 28 for a more complete picture of TransformerTankEnds, or Figure 25 for a more complete picture of PowerTransformerEnds. Either one can be the TransformerEnd in this figure, but with a PowerTransformerEnd, all three phase taps would change in unison (i.e. they are “ganged”). Most regulator attributes of interest are found in RatioTapChanger or TapChangerControl instances. However, we need the AssetDataSheet mechanism to specify ctRatio, ptRatio and ctRating values. These are inherent to the equipment, whereas the attributes of RatioTapChanger and TapChangerControl are all settings per instance. For the IEEE 13-bus example, there would be separate RatioTapChanger and TapChangerControl instances for phases B and C.
Figure 39: In this profile, a solved voltage value attaches to ConnectivityNode in GridAPPS-D. Positive sequence or phase A is implied, unless the phase attribute is specified.
Figure 40: SvTapStep links to a TransformerEnd indirectly, through the RatioTapChanger. There is no phasing ambiguity because TransformerTankEnd has its phases attribute, while PowerTransformerEnd always includes ABC. Units for SvTapStep.position are per-unit.
Figure 41: The on/off value for a capacitor bank attaches directly to LinearShuntCompensator. If the phase attribute is not specified, then this value applies to all phases.
Interoperability Testing¶
CIMHub participated in EPRI’s Grid Model Data Management (GMDM) project, through the CIM Interoperability Tests in Charlotte, NC, June 14-16, 2022. We passed power flow tests with the reference model, and imported planning model assemblies provided by four (out of four attempted) other vendors.
The GMDM CIM profile included some experimental features, and some other choices that differed from CIMHub design choices. In preparation for this test, we adopted some experimental features that are not in standard CIM:
Removed the leading underscore from mRID so it matches
rdf:about. The UML documentation is arguably not clear, but the CIM standard appears to be moving in this direction.Adopted the OrderdPhases attribute for TransformerTankEnd. This attribute removed ambiguity from complicated transformer connections. It should be adopted for TransformerTankEnd, EnergyConsumerPhase, PowerElectronicsConnectionPhase, and possibly others.
We did not make other changes to the internal CIMHub model for this GMDM test. Instead, the incoming GMDM files were converted to a CIMHub schema:
./gmdm/adapt_gmdm.pyimplemented the first pass, outside of BlazegraphCombines incoming XML files from the GMDM planning assembly
Merges the incoming namespaces into
http://iec.ch/TC57/CIM100#Converts kV to volts, and MVA to VA
Converts the experimental WireAssembly to WireSpacingInfo
Correct the spelling of PhotoVoltaicUnit to PhotovoltaicUnit
Swap EquipmentContainer and AdditionalEquipmentContainer, because the incoming GMDM files assign EquipmentContainer to the Substation while CIMHub assigns it to the Feeder. The priority is not clear in CIM UML.
Produced
adapted.xml, which was loaded into Blazegraph
./gmdm/step2.pyimplemented the second pass using SPARQL and BlazegraphMake sure EquipmentContainer is the feeder for Breaker, PowerTransformer, EnergySource, which are inside the Substation for GMDM.
Populate LinearShuntCompensator.sections and bPerSection from phases
Populate Switch.ratedCurrent and Switch.normalOpen from phases on Breaker, Fuse, LoadBreakSwitch, Recloser
Put a base voltage on Breaker and EnergySource
Populate RegulatingControl.enabled as true
Populate RatioTapChanger.normalStep as 0
Populate RatioTapChanger.TransformerEnd from TransformerEnd.RatioTapChanger, i.e., reversing an association
Populate RegulatingControl.RegulatingCondEq from RegulatingCondEq.RegulatingControl, i.e., reversing an association
Implement targetValueUnitMultiplier on targetValue and targetDeadband for TapChangerControl and RegulatingControl. Supported values are
noneork.Convert the GMDM wind, solar, and storage units into a single virtual battery [5], [6] behind the PowerElectronicsConnection. Neither OpenDSS nor GridLAB-D can represent multiple units connected to the DC side of an inverter.
CIMHub may differ from other CIM implementations in these respects:
We use IdentifiedObject.mRID as a database key.
We combine incoming CIM XML files into a single XML file with a single namespace.
We do not employ the CIM unit and multiplier system. Instead, all internal physical units are SI. Multipliers can still be implemented with e3 and e6 in attribute values; the base units are still SI to CIMHub.
We use the CIM Dynamics package to represent DER control functions as defined in IEEE 1547-2018. (This implementation did not play a role in the GMDM test).
We prefer to represent a single-phase, center-tap service transformer with 3 windings, and s1s2 phasing. This allows unbalanced loading on the secondary, which becomes significant at faster measurement data rates. The CIM also supports a simplified single-phase service transformer [7], which would be used with ABC phasing on the secondary side. This may be more convenient for tracing meter associations to primary phases within the CIM. With s1s2 secondary phasing, the application would have to perform such tracing.
CIMHub has not implemented an export to CIM. The CIM export command in OpenDSS uses the CIMHub profile, which does not match GMDM and may not match other profiles. Any future support of CIM import and export should use the adapter approach we used for the GMDM tests. This will insulate CIMHub’s internal schema from changes, including experimental proposals, that may be found in other profiles.
Other CIM Topics¶
Metering Relationship to Loads in the CIM¶
Figure 42 shows how emulated trouble calls will be connected to loads (EnergyConsumers) for test scenarios. The TroubleTicket is associated with Customer, CustomerAgreement and UsagePoint, which can then be associated to Equipment or any of its descendants. Figure 42 shows the linkage to EnergyConsumer or EnergySource, but it can also be linked to RegulatingCondEq (e.g., rotating machine and inverter-based DER). There are many attributes of Customer, CustomerAgreement and UsagePoint that are not yet used in GridAPPS-D, and not shown in Figure 43. These would be important for future metering and customer management applications. For now, the only TroubleTicket attributes to be used are dateTimeOfReport, resolvedDateTime and troubleKind. The PNNLTroubleCallKind was added because the existing troubleCode attribute is a non-standardized String. However, the comment attribute could be used for optional comments on each TroubleTicket.
Figure 42: Trouble Calls route through Metering Usage Points to EnergyConsumers
CIM Enhancements to be Proposed¶
Possible CIM enhancements:
Different on and off delay parameters for RegulatingControl (Figure 5)
Current ratings for PerLengthImpedance (Figure 2). At present, some users rely on associated WireInfo, ignoring all attributes except currentRating.
Transducers for RegulatingControl (Figure 5)
Dielectric constant and soil resistivity (Figure 10)
Add the Fault.stopDateTime attribute
Single-phase asynchronous and synchronous machines.