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A SGAM-based architecture for synchrophasor applications facilitating TSO/DSO interactions

Authors:
A SGAM-Based Architecture for Synchrophasor
Applications Facilitating TSO/DSO Interactions
Hossein Hooshyar and Luigi Vanfretti
KTH Royal Institute of Technology, Stockholm, Sweden
hosseinh@kth.se, luigiv@kth.se
Abstract— Distribution grid dynamics are becoming
increasingly complex due to the transition of these networks
from passive to active networks. This transition requires
increasing the observability and awareness of the interactions
between Transmission and Distribution (T&D) grids,
particularly to guarantee adequate operational security. As part
of the work carried out in the EU-funded IDE4L project, a
specific use case, containing PMU-based monitoring functions,
has been defined to support the architecture design of a
distribution grid automation system. As a result, the
architecture can accommodate for synchrophasor applications
that provide key dynamic information extraction and exchange
between DSO and TSO. This paper presents the use case and the
portion of the IDE4L architecture that accommodates for
scenarios that exploit synchrophasors for monitoring
applications.
Index Terms—active distribution grid, architecture, PMU,
SGAM, WAMS.
I. INTRODUCTION
To facilitate adequate technical functioning of the overall
electric power system, the interactions between the technical
operation of distribution grids and the main transmission grid
requires to “exchange information” that can help in the
technical functions of both network operators. As of today, it
is challenging for Distribution System Operators (DSOs) to
provide and maintain a network model of their electrical grid,
which if constantly updated, could help in extracting key
information about the network’s state. In addition, DSOs
currently do not have access to nor exchange much
measurement data with the Transmission System Operator
(TSO), and if they do, very little (or non) of these
measurements are shared in hard real-time, nor do they have
high sampling resolution and time-synchronization. This
means that the measurement data available is too limited in
quantity (i.e. locations and signals), and also in “observability”
(i.e. the content of the frequency spectrum from their sampling
resolution) [1].
Therefore, in the current situation, a short-term solution to
enhance “information exchange” would be to make available
new measurement devices that provide real-time, high-
sampled data across operational boundaries from which
information can be extracted. To this end, the work within the
Ideal Grid For All (IDE4L) project, funded by the European
Commission, considered the utilization of time-synchronized
phasor measurements with millisecond reporting rate, i.e. real-
time data from Phasor Measurement Units (PMU) [2]. This
approach is sensible considering the recent trends in North
America and Europe to explore the potential of utilizing
PMUs at the distribution level [3,4]. The implementation of
such PMU-based “information exchange” has to go through a
properly designed and implemented architecture that can
satisfy all application-dependent technical requirements, while
at the same time considering the different actors and
operational boundaries involved.
The IDE4L architecture is built upon the 5-layer Smart
Grid Architecture Model (SGAM) framework that has been
developed by the CEN-CENELEC-ETSI Smart Grid
Coordination Group [5]. The SGAM is the main response to
the EU Mandate M/490 for the development of a framework
to support European smart grid deployment [6]. The main
inputs to the architecture design are the use cases that describe
the architecture requirements, actors and functionalities. The
use of SGAM aids in developing a common understanding
between power grid domain experts and IT experts [7]. The
IDE4L architecture is built from several use cases, one of
which is the “Distribution grid dynamic monitoring” defining
PMU-based monitoring functions that can provide key
dynamic information for both the Distribution Management
System (DMS) of DSOs and the Energy Management System
(EMS) of TSOs.
This paper presents the “Distribution grid dynamic
monitoring” use case and its mapping on the SGAM
framework. This mapping gives the portion of the IDE4L
architecture that can accommodate for the scenarios that
exploit synchrophasor technology for monitoring applications.
The paper begins by presenting the use case in Section II.
Sections III illustrates the use case mapping on different layers
of the SGAM framework. Conclusions are drawn in Section
IV.
II. USE CASE DEFINITION
As indicated in the previous section, the objective of the
“Distribution grid dynamic monitoring” use case is to manage
the complexity and increased inter-dependence between
electric power transmission and distribution grids, that have
This work is supported by the FP7 IDE4L project funded by the
European Commission and the STandUp for Energy Collaboration Initiative.
raised as a result of the ‘energy transition’. The “business
case” considered herein is to support a long-term sustainable
energy system. One strategy to achieve this is through
exchange of key dynamic information between TSOs and
DSOs. Key information extraction and exchange will be
performed by coupling PMU data from different voltage levels
across the operational boundaries of the grids, and exploiting
it coherently in PMU applications that derive actionable
information.
Figure 1 illustrates the use case diagram, mapped on the
Smart Grid Plane. The Smart Grid Plane spans in horizontal
dimension the complete electrical energy conversion chain,
partitioned into five domains of Generation, Transmission,
Distribution, DER and Customer Premises. In the vertical
dimension, it spans the hierarchical levels of power system
management, partitioned into six zones of Process, Field,
Station, Operation, Enterprise (not shown) and Market (not
shown). The plane enables the representation of the zones in
which power system management interactions between
domains or inside a single domain take place. More
information on the domains and zones can be found in [5].
As shown in the figure, synchrophasors are provided by
PMUs distributed on the feeders, installed at the Primary
Substation (PS) or at the Secondary Substation (SS). The
synchrophasors are then collected by the PS-level and the SS-
level Phasor Data Concentrators (PDC) which, in turn, stream
the data through a Wide Area Network (WAN). The data is
transferred either over TCP/IP on IEEE C37.118.2 protocol or
over UDP/IP on IEC 61850-90-5 protocol to a higher level in
the architecture hierarchy. The data is finally delivered to
DMS computers at the DSO for real-time processing and
extraction of dynamic information, performed by newly
developed monitoring applications. The outputs of the
applications are to be used by other DMS functions; however,
some key dynamic information is selected to be sent to TSO to
support the EMS functions.
As Figure 1 indicates, because some of the synchrophasor
applications can be implemented in distributed fashion within
the architecture, data processing and information derivation is
performed at both the Station and the Operation zones.
The actors involved in this use case are transducer (i.e.
instrumentation chain including CTs and VTs), PMU, PDC,
communication interface, DMS, and EMS [8]. The functions
involved in this use case are electrical conversion,
synchrophasor calculation, data acquisition, data concentration
and time-alignment, data exporting, data curation, extraction
of different time-scale components from the PMU data, and
derivation of key information out of the data. Note that the
functions run regularly, i.e. no triggering event is considered
in this use case. As mentioned before, once the use case
description is available, it is possible to realize the use case
mapping onto the SGAM layers.
III. THE SGAM ARCHITECTURE
As mentioned previously, the IDE4L architecture is
constructed based on the SGAM reference model to analyze
and visualize the use cases in a technology-neutral manner.
Figure 2 depicts the three-dimension representation of the
SGAM framework which is established by merging the
concept of the interoperability layers with the Smart Grid
Plane. As the figure shows, the SGAM consists of five
interoperability layers representing business objectives and
processes, functions, information exchange and models,
communication protocols, and components. Note that in
addition to the relations between objects on the same layer
(e.g. physical connection of components on the component
layer), there exist interrelations between objects on different
layers. Business processes, as objects of the business layer, are
realized by functions, as objects of the function layer, which
are in turn executed by components, as objects of the
component layer. The execution of the functions requires the
components to support data models, as objects of the
information layer, and communication protocols, as objects of
the communication layer. More information on the layers can
be found in [5].
This section shows the mapping of the “Distribution grid
dynamic monitoring” use case, defined in Section II, on the
SGAM layers. This mapping gives the portion of the IDE4L
architecture that can accommodate for the scenarios that
exploit synchrophasor technology for monitoring applications.
Figure 1. Diagram of the “grid dynamic monitoring” use case mapped on
the Smart Grid Plane.
A. Development of the Component Layer
The component layer depicts the use case actors in form of
hardware which is used to provide the intended use case
functionalities. As shown in Figure 3, the computers at the
Station and the Operation zones host the PMU-based
monitoring functions. The computers are fed by the PMUs at
the Field zone through the PDCs at the Station zone. In
addition, the Modem+Switch components represent the
connection between the Local Area Networks (LAN) and
WANs. Note that the component layer is beneficial on
evaluating the cost of the components that are to be utilized in
the architecture.
Figure 2. The Smart Grid Architecture Model (SGAM) framework [5].
Distribution
Trans.
Process
FieldStationOperation
PMU
(PS)
Gen. DER Cost.
Pre
m
.
HV MV LV
Transducer
PMU
(SS)
PMU
(distributed)
Modem+
Switch
(SS)
Modem+
Switch
(PS)
Computation
Unit
(SS)
Computation
Unit
(PS)
PDC
(SS)
PDC
(PS)
Modem+
Switch
DMS
Computer
Modem+
Switch
EMS
Computer
Transducer
Figure 3. The use case mapped on the SGAM component layer.
B. Development of the Business Layer
The business layer is intended to host the business
processes, services and organizations, the business objectives,
economic, and regulatory constraints underlying the use case.
However, it is suggested in [7] to move apart the business
aspects of the use case to be documented in a separate use case
definition. This is because a technical use case might be
exploited to achieve multiple business objectives, i.e. there’s
no one-to-one mapping between the technical use cases and
the business objectives. Although the IDE4L project has
adopted the same viewport, the business layer is derived in
Figure 4 for the sake of illustration. As depicted in the figure,
the business layer shows the area which is affected by the use
case and consequently influenced by its underlying business
objective.
Distribution
Trans.
PMU
(PS)
Gen. DER Cost.
Pre
m
.
HV MV LV
Transducer
Transducer
PMU
(SS)
PMU
(distributed)
Modem+
Switch
(SS)
Modem+
Switch
(PS)
Computation
Unit
(SS)
Computation
Unit
(PS)
PDC
(SS)
PDC
(PS)
Modem+
Switch
DMS
Computer
Modem+
Switch
EMS
Computer
Figure 4. The use case mapped on the SGAM business layer.
C. Development of the Function Layer
The function layer is intended to represent the functions,
realizing the use case, and their interrelations with respect to
domains and zones. The position of the functions is inferred
from the use case diagram mapped to the Smart Grid Plane,
shown in Figure 1. The interrelations can be derived from the
list of exchanged information that will be further explained in
the next section. In addition, the function layer provides a
function-to-component mapping which in turn helps in the
definition of software and hardware requirements of the
components.
Figure 5 shows the use case mapped on the SGAM
function layer. The main functions, developed during the
IDE4L project, are briefly explained below. The readers are
referred to the provided references for further details.
Data Curation and Extraction of Components: This
function is based on an enhanced Kalman Filtering
technique that performs both bad data removal (i.e.
eliminating noise, outliers, and missing data) and signal
feature extraction (e.g. steady state components, dynamic
components with different time scales, etc.) from the
PMU measurements in real-time. Hence, both input and
output of this function are synchrophasors [8,9].
Derivation of Key Information: This is actually a family
of functions including Steady State Model Synthesizer [9],
Oscillatory Mode Meter [10], Voltage Stability Analyzer
[11], Sub-synchronous Oscillation Detector [12], and
Feeder Dynamic Rating [13]. The inputs to all of these
functions are curated voltage and current synchrophasors
whereas their outputs consist of different number sets in
floating point format, often having lower reporting rate
compared to that of the synchrophasors.
Distribution
Trans.
PMU
(PS)
Gen. DER Cost.
Pre
m
.
HV MV LV
Transducer
Transducer
Transducer
PMU
(SS)
PMU
(distributed)
Modem+
Switch
(SS)
Modem+
Switch
(PS)
Computation
Unit
(SS)
Computation
Unit
(PS)
PDC
(SS)
PDC
(PS)
Modem+
Switch
DMS
Computer
Modem+
Switch
EMS
Computer
Electrical conversion
Synchrophasor calculation
Measurement acquisition
Data concentration
Data curation and extraction of components
Partial derivation of key informat ion
Data transfer
Measurement acquisition
Data curation and extraction of components
Derivation of key information
Data export
Figure 5. The use case mapped on the SGAM function layer.
D. Development of the Information Layer
The information layer is depicted in two views of Business
Context and Canonical Data Model, where the former one
describes the information being exchanged between the
components and the latter one is intended to show underlying
canonical data model standards which are able to provide
information objects needed for the implementation of the use
case. Specification of the data model standards helps in the
selection of the proper software to be installed on the
components of the architecture.
Figure 6 shows the use case mapped on the SGAM
information layer. Note that the two views of Business Context
and Canonical Data Model are usually shown in two separate
figures; however, due to the space limitations, they are merged
into one figure in this paper. The exchanged information,
shown in the figure, is consistent with the specification of the
inputs/outputs of the functions, explained in the previous
section. In addition, as the figure indicates, IEC 61850-7-4 is
used for the data modeling of the PMU measurements that are
mapped to the logical node MMXU data objects of the IEC
61850 standard [14]. This is consistent with the
communication protocol, used to transfer the synchrophasors,
which will be further discussed in the next section.
Figure 6. The use case mapped on the SGAM information layer.
E. Development of the Communication Layer
The emphasis of the communication layer is to describe
communication protocols and technologies for the
interoperable exchange of information between the
components. In addition, the layer can be used for cost
assessment in the construction and the management of the
required communication infrastructure. Figure 7 shows the use
case mapped on the SGAM communication layer. As the
figure shows, while the synchrophasors are transmitted on IEC
61850-90-5 protocol, the derived dynamic information are
communicated on an arbitrary Web Service which can be any
protocol using the TCP/IP or UDP/IP client-server
mechanism. It is worth noting that a gateway has been
developed in the IDE4L project to transmit the PMU data on
IEC 61850-90-5 protocol [14,15]. The gateway allows to
transfer the PMU data by mapping and encapsulating the
synchrophasors (that are already mapped to the MMXU data
objects) in GOOSE or Sampled Value messages and sending
them over UDP/IP. The gateway sits at the server side (i.e.
PMU or PDC) to generate IEC 61850-90-5 messages and at
the client side (i.e. PDC or computation unit) to parse IEC
61850-90-5 messages, acting as a data mediator for user
applications.
As mentioned before, another aspect of the communication
layer is to assign appropriate technologies for communication
links between the components. Each information exchange
sets specific requirements in terms of transfer time, transfer
rate, synchronization accuracy, and availability on the
communication link through which it’s transmitted. Hence,
proper technologies should be assigned to the links to satisfy
the requirements imposed by the information exchanges. For
instance, while fiber-optic communication is recommended
for PDC-to-PDC and PDC-to-computer links (due to the high
transfer rate and transfer time requirements), LTE and point-
to-point HiperLAN technologies may also be used for PMU-
to-PDC and computer-to-computer links (due to the lower
requirement on transfer rate) [16]. Assuming that the
information exchanges within the use case require a high level
of communication link availability, it is important to consider
some sort of redundancy by for example constructing
communication infrastructure in parallel or utilizing other
communication links to implement a parallel path.
IV. CONCLUSIONS
This paper presented a use case, containing PMU-based
monitoring functions, and its mapping on different layers of
the SGAM framework to support the architecture design of a
distribution grid automation system; such that the architecture
can accommodate for key dynamic information exchange
between TSOs and DSOs. As future work, different metrics
will be defined to evaluate the performance of the designed
architecture.
REFERENCES
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Distribution
Trans.
PMU
(PS)
Gen. DER Cost.
Pre
m
.
HV MV LV
Transducer
Transducer
Transducer
PMU
(SS)
PMU
(distributed)
Modem+
Switch
(SS)
Modem+
Switch
(PS)
Computation
Unit
(SS)
Computation
Unit
(PS)
PDC
(SS)
PDC
(PS)
Modem+
Switch
DMS
Computer
Modem+
Switch
EMS
Computer
IEC 61850-90-5
IEC 61850-90-5
Web Service
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Web Service
Figure 7. The use case mapped on the SGAM communication layer.
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... The controller uses the equation above to compute the bus frequency from the bus voltage data, which is implemented as a Modelica code in the frequency computation blocks inside the simulation set-up that will be described in the next sections. Figure 9 shows the power system model mapped on the component layer of Smart Grid Architecture Model (Hooshyar and Vanfretti, 2017), while it's Modelica implementation presented in Figure 10. Figure 9 is useful to understand re-synchronization would require the coordination between three domains -transmission, distribution, and DER owners, and thus, the proposed resynchronization controller would be beneficial. The figure shows how PMU data is measured at both a transmission and distribution substation, while the re-synchronization controller is located at the DER substation. ...
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