A Platform for Wide Area Monitoring and Control System ICT Analysis and Development Davood Babazadeh, Moustafa Chenine, Kun Zhu, Lars Nordström Department of Industrial Information and Control Systems, School of Electrical Engineering, KTH- Royal Institute of Technology, SE 100-44 Stockholm, Sweden. E-mail: davoodb, moustafac ,zhuk, larsn{@ics.kth.se} Abstract— PMU-based Wide Area Monitoring and Control (WAMC) system is introduced to improve the monitoring of power grid across large geographic areas and control the grid using more efficient and smart applications. The performance of WAMC applications in real power system scenarios and impact of their supporting Information and Communication Technology (ICT) on the data quality can be quantified and analyzed by pseudo-real co-simulation test beds. The purpose of this study is to propose and develop a WAMC testing platform to facilitate the real-time simulation of dynamic power grid, the ICT infrastructure that overlays the grid and WAMS applications. The platform consists of OPNET, a powerful communication network emulator, connected to a real-time power system simulator through virtualized PMU device. The end point stations such as Phasor Data Concentrator or PMUbased applications are also linked to the platform through OPNET’s real-simulation gateway called SITL (System-In-TheLoop). To assess the performance of the platform architecture, a case study has been performed with five PMUs which collect the data from a power model and deliver to PMU-based modeestimation application over a typical communication network. In this study, the results explicitly intend to quantify the effect of network protocols on data delay. Index Terms-- Communication Network Emulator, CoSimulation, Opal-RT, PMU, System-In-The-Loop, Wide Area Monitoring and Control System
I.
INTRODUCTION (HEADING 1)
The Electric Power System is a large complex system in terms of monitoring and operation due to its dynamic behavior and diversity in components from generation level to loads level. The inter-connection of national grids to neighboring national grids, the integration of renewable energy sources and the restrictions in transmission lines expansion lead to operating the power grid closer to its parameters limits. In this situation, a small disturbance in one area can easily be shifted into cascaded and widespread fault affecting the whole system. To ensure the secure operation of power system, wide area situational awareness is essential. This work is supported by the Swedish Power Industry via ELEKTRA Project 36084 and KIC InnoEnergy INTINCT initiative.
Ahmad Al-Hammouri Department of Network Engineering and Security, Jordan University of Science and Technology Irbid, 22110, Jordan E-mail:
[email protected]
Technologies developed to improve the monitoring of power grid across large geographic areas and accordingly controlling grid in more efficient and intelligent way are referred as wide area monitoring and control system (WAMC). WAMC is in turn built on Phasor Measurement Units (PMUs), these devices are capable of providing realtime high resolution, Global Positioning System (GPS) time synchronization measurement enabling a whole range of possible applications, such as power oscillation monitoring, and faster and more accurate state estimation [1]. This in turn can help to grid reliability and controllability for both realtime operations and offline planning. The design, testing and implementation of such real-time and critical applications would greatly benefit from development of co-simulation test beds that reflect the characteristic of both the ICT system and the physical power process, as well as systems that interface them. The dependency between the synchrophasor driven applications and their supporting ICT systems calls for a need to develop co-simulation platforms where interaction between the power systems and ICT systems can be assessed in a cost efficient manner. Several studies have been carried out to develop co-simulation platforms to study complex interactions between the power system simulations and communication systems simulators such as [1], [3],[4],[5],[6],[7], and [8]. EPOCHS is an early attempt to combine simulations of power system and communication network [1]. It federates three offthe-shelf simulators: Positive Sequence Load Flow (PSLF) to simulate power system steady state; PSCAD/EMTDC for power system transient; and Network Simulator 2 (ns-2) for modeling of communication network. Mediator software is developed as an interface which enables the simulators to exchange data periodically. GECO presented in [9], uses the same framework as EPOCHS, but the time synchronization of simulators are more accurate in this platform. In another approach called the DEVS method [8], the continuous simulations should be transferred into discrete events by using different state event detection mechanisms such as zero
crossing. Recently, the real-time co-simulation is one of the popular methods among researchers. The real-time power simulators are able to assess the electric power phenomena in the range of millisecond which makes it possible to implement hardware-in-the-loop tests. By adding communication network emulator to the power real-time simulator, the challenges of other approaches, such as the complication of federating or synchronization of the involved components can be reduced. Instead, it is required for the simulators to indicate that the simulations run in real-time or over run. The purpose of this paper is to present a co-simulation platform for wide area monitoring and control system analysis and development (WAMCAD). The WAMCAD platform makes it possible to assess the impact of supporting ICT systems on PMU-based applications and system. This is achieved by combining and implementing components that mimic equivalent components in such systems. The platform described in this paper is based on real time simulation, which also allows integration of real–world applications with these simulations. It is hoped that the WAMCAD platform can offer realistic insight into the interdependency of power system and the corresponding ICT system models. The performance of proposed architecture is assessed by running different scenarios. The results explicitly intend to quantify the effect of communication parameters on the network metrics of data delay. A. Outline The rest of the paper is structured as follows: Section II provides with a brief overview of Wide Area Monitoring and Control (WAMC) systems and their basic components. Section III introduces the platform by providing a generic architecture of the different parts it captures. This section also describes the specific technologies used to implement the components of the WAMCAD platform. Section IV presents a set of proof of concept scenarios implemented on the platform and their results. Finally, the paper is concluded in section V. II.
WIDE AREA MONITORING AND CONTROL SYSTEMS
In the centralized architecture paradigm, PMU measurements are collected from various locations in an interconnected power system. These measurements are then communicated to a central location where they are used for monitoring purposes or by assessment applications that raises alarms and calculates metrics [13]. The alarms raised and computed metrics by these monitoring systems are in turn used to provide operator support to determine corrective actions and to control devices in the power network. The ICT system that supports control applications designed based on global information also manifest as a centralized architecture. Alternative architectures, such as using a few remote Phasor Measurement Unit (PMU) signals for specific control or protection applications, are also possible [14]. As illustrated, Figure 1, the basic components of any WAMC system are the following: PMUs, Phasor Data Concentrators (PDC), a PMU-
based application system (which itself is comprised of different information technologies and underlying algorithms), and finally communication networks that link the different components.
Figure 1. Wide Area Monitoring and Control System
The WAMC system interfaces with the power system through the PMUs connected to substation bus-bars or power lines. PMUs have high resolution measurements which are time-stamped by Global Positioning Satellites (GPS) pulses. This ensures a system independent time source. Then PMU measurements are transferred to PDC in order to be collected and sorted as synchronized according to their time stamps. At the final stage, the processed data are forwarded to the WAMC applications where alarms and control commands are generated. In this structure, receiving high rate and accurate data from PMUs makes it possible for applications to improve their information about network status which in turns helps the system to be controlled and monitored in the more real time condition. It is noteworthy that the data flowing among all these components are enabled by a cluster of communication networks. III.
THE WIDE AREA MONITORING ICT TEST PLATFORM
The performance and reliability of a WAMC system can be verified in several ways. One method is to deploy the developed application in real scenarios and test the accuracy and performance of it through trial and error. This method is not always practical since power system as a vital and costly infrastructure, which is expected to be in-service with high availabilities. A more realistic alternative is to establish a test platform which simulates the components and processes within the WAMC system. This method allows the concept of applications to be tested before the costly deployment. In order to reflect this cross research discipline system consisting of several interconnected and interdependent components in cyber environment, each part of the whole system should be virtualized or modeled in corresponding simulator. The architecture of such platform is presented in this section.
A.
Generic Virtual WAMC Simulation Architecture As shown in Figure 2, two separate simulators can be considered for modeling of power process and communication network as the core of the WAMC system. Power system simulator is expected to mimic the continuous time variant dynamics of the grid, while the communication network modeling is a discrete event-based simulation. The main challenging issue in the integration of the simulators is the difficulties with the nature of computations in different simulators and the time synchronization between them. In our general architecture, this problem can be avoided by using the real time simulators for both communication network and power system modeling. In reality, the PMU devices are sampling the required process variables with certain projected rates from running power process, the measured data will be time-stamped with the global clock and then delivered to the communication network. With the same structure, the proposed platform can use a power simulator to emulate the process in real-time. The virtualized or real PMU can then read data from the simulator with specific rates and add timestamps to the measurements.
simulation environment or facilities are selected for each component of the platform as explained below.
Figure 3. Components in the WAMCAD platform
1) Power System Simulator: Simulink and OPAL-RT There are several simulators to model and simulate the electrical power system. As mentioned, the main requirement of the platform is that the simulator must be able to run the simulation in real time. In this study, OPAL-RT simulator can be selected to emulate the power grid. The compatibility of OPAL-RT with MATLAB/Simulink makes it possible to use the PMU measurement block developed inside this simulator in Simulink as well to run small-scale for academic research purposes. 2) Interface: SoftPMU
Figure 2. General WAMC platform
Once the data is sampled, computed and time-stamped by the virtual PMU, it is communicated over a communication system emulator to the receiving end system. The role of communication network emulator is to mimic the behavior of real network for the data packets in real time framework. Therefore, the internal process of communication emulator is not affected by the time reference of power system. Finally, the time reference of end systems i.e. PDC or WAMC application that are consuming and processing this data, should be time-synchronized with the GPS or devices’ time reference to avoid data loss due to lag or lead in references. In short the timing issues in the system are mitigated by ensuring that each component in the system has an independent timesource corresponding to the time in the real world. B. Components of the Platform The platform consists of three main components: Power Emulator, Communication Network Emulator as well as WAMC Application. The measurement devices, i.e. SoftPMUs, can be considered as internal features of power emulator or as a separate cyber environment linked to it. In order to test the general platform architecture, specific
SoftPMU is a software-based synchronized phasor measurement unit designed and developed to be run in the OPAL-RT simulator. The SoftPMU is a virtualized device that can collect real time data generated by the simulators, compute real-time phasors of signals, and send them to communication networks emulator. As shown in Figure 4, SoftPMU consists of two separate blocks. The first block is running in the real time simulator to collect the signal, compute the phasors and send them outside the simulator. The second block called PMU Daemon is developed to receive the calculated phasors from the first block. Afterwards, the raw data is further packed according to C37.118 standard [17] and sent out the packets over TCP or UDP to phasor data concentrator. The detailed description of the PMU Daemon together with its flow chart is presented [18]. The original version of Daemon block was able to send out the packet just over TCP. In this paper, the Daemon block is improved to use both common transport protocol i.e. TCP and UDP.
Figure 4. The softPMU architecture
3) Communication Systems Simulator: OPNET There are numerous simulation software available for the analysis of the performance and behavior of communication networks such as OPNET, OMNET or ns-2. In this study, some important factors are considered to choose the communication simulation environment: 1) the ability to perform the hardware-in-the-loop (HIL) simulations, 2) an accurate and powerful tool which reflects the available networking components in real world as detailed as possible while still keeping the simplicity of modeling and extendibility, 3) the ability of simulation in real time. The detailed comparison of available network simulators and their performance in different scenarios presented in [19], [20], and [21] suggests that OPNET is the most suitable candidate software that fulfills our requirements. Therefore, OPNET is chosen to model the communication network in our WAMCAD platform. OPNET Modeler provides a comprehensive development environment in order to model communication networks and distributed systems. Both behavior and performance of modeled systems can be analyzed by performing discrete event simulations. The OPNET System-in-the-Loop module (SITL) [22] makes it possible to connect the simulation model to live network hardware. SITL module provides the ability of exchanging packets with the external hardware via an Ethernet link while the simulation is running in real-time. Each SITL module inside the simulation environment is assigned to a specific network adapter on the machine to insert the packets received on that NIC into simulation.
Figure 5. SITL interface configuration with the SoftPMU virtual device
The assigned network adapter can be a real interface or virtual one. In a typical configuration of OPNET, each SITL gateway is assigned to a physical network interface. In this platform, in order not to be limited to the number of physical NICs installed on OPNET machine and increase scalability of the platform, a certain number of virtual network interfaces are created to be connected into SITL gateways. In order to connect more SITL gateways on OPNET to SoftPMUs, these virtual NICs are built on top of one physical interface. As shown in Figure 5, the number of SoftPMUs, the number of corresponding virtual interfaces on SoftPMU machine, the number of SITL gateways, and the number of virtual interface on OPNET’s NIC should be equal.
4) Application: openPDC and the KTH PowerIT Platform The application component of the WAMCAD platform consists of openPDC [23] as the phasor data concentrator and the KTH PowerIT [25] as the application hosting platform that connects to openPDC to receive the synchronized measurements. KTH PowerIT makes use of many of the libraries associated with the openPDC, for example the Tennessee Valley Authority (TVA) Code Library [26] and openPDC’s Phasor Protocol Library [23]. Several applications have been implemented in PowerIT platform, such as average frequency visualization and electro mechanical mode estimation. IV.
PROOF OF CONCEPT USING THE PLATFORM
To show the ability of the platform to evaluate the performance of the WAMC system, different parameters such as number of PMUs, sampling rates, network protocols or components availability can be chosen. In this regard, a model of power system with PMU measurements nodes and the corresponding communication network are developed in simulators. Then several scenarios are carried out to: 1) show the capability of platform in evaluation of WAMC system; 2) validate the platform by comparing the simulation results with real data. A. Communication Network Model As shown in Figure 6, communication network model considered for this case study consists of seven subnets. Sub1 to sub5 represent five substations. The WAN subnet includes the core routers of the network and one subnet is considered for control center. Five substations are connected to WAN via 2Mbit optic lines. The number of lines depends on the number of routers assigned for related substations. The control center is connected to WAN via 2 lines of 2Mbit [25]. The Wide Area Network (WAN) as the core of communication network consists of four 3COM core routers which are connected as a ring. These four core routers are linked to each other via 2Mbit. Each substation consists of two devices: 1) one PMU gateway and 2) one video stream source. Video stream profile is added to show the periodic transfer characteristic of the traffic sent from substation. The video devices are placed for surveillance purposes. Video streams can also be considered as background traffic [32]. In this study, the video stream is simulated inside the OPNET. It is also possible to use real device to pass the real stream through the simulator. As shown in Figure 6, totally five SITL gateways are assigned to receive the live PMU streams from SoftPMU machine. Inside the OPNET model, PMU gateways are connected to routers through sitl_virtual_eth_link. The SoftPMU machine broadcasts the five PMU streams to OPNET on five different IP addresses to show that PMUs are spread geographically in different places. Since we use just one physical network interface on OPNET machine, any SITL gateway in OPNET
is able to receive all the packets received on physical NIC. To receive measurement data from proper PMU with the related IP address, SITL gateways should be configured properly to filter other packets and receive only the relevant packets.
of WAMC application. The scenario includes a simple power system model running on SIMULINK. Five PMU instances are considered on SoftPMU to collect voltage phasors of the buses from power process in real-time and send PMU stream packed in C37.118 format to the communication network simulator. Two of five PMU streams (PMU2 and PMU5) are sent over UDP and rest of them over TCP. In addition to live PMU streams coming from SoftPMU, five video streams are simulated inside OPNET as background traffic of each substation which are sent over UDP. The communication network simulator was set to run in real-time also. On the end point, mode estimation and average frequency monitoring applications receive the measured data in real time, calculate the oscillation modes and show average frequency, respectively. The specification of data streams is summarized in the following TABLE I. It is noteworthy that the scenario was set to reflect the real network of Swedish TSO as close as possible. TABLE I. Devices
Figure 6. General Subnets of Network model
The control center consists of one router, one simulated server node and one output gateway i.e. SITL_OUT. The simulated server device is placed to receive the simulated streams generated by the video nodes inside substations and SITL_OUT gateway is developed to pass all PMUs data to PDC or Application in other machine. Note that we try to model the communication network as close as possible to real case used by the Swedish TSO. B. Power System Model In order to test all the components of the proposed platform, it is necessary to run electric power process on power system simulator. Since this study focuses more on development of the entire platform, this study does not aim to model the power system in detail. But for the sake of testing the platform, a simple dynamic power system consisting of five PMU measurements has been modeled and executed in SIMULINK which provides inputs to Soft PMU blocks. OPAL-RT has not been used at this stage, but the compatibility of OPAL-RT with Simulink and previous tests are documented in [29]. C. Scenarios The purposes of tested scenario are: 1) To show a whole picture of wide area monitoring and control system virtualized as promised; 2) To demonstrate the ability of the platform to examine the impact of the parameters in power process, communication configurations or devices on the performance
Network Protocol
SPECIFICATION OF SCENARIO Data Rate
Payload
Routing Protocol
PMU 1
TCP
30 packet/sec
32 byte
OSPF
PMU 2
UDP
30 packet/sec
32 byte
OSPF
PMU 3
TCP
30 packet/sec
32 byte
OSPF
PMU 4
TCP
30 packet/sec
32 byte
OSPF
PMU 5
UDP
30 packet/sec
32 byte
OSPF
Video stream
UDP
200 packet/sec
1024 byte
OSPF
D. Results The result of end-to-end delay is collected for all five PMUs streams and shown in Error! Reference source not found.. The end-to-end delay is selected from the SITL_OUT node of the network model which is the connection point to the application machine. This end-to-end value shows the difference between the times that live packet comes to OPNET and the time that the packet leaves the OPNET. Therefore, the delay of phasor calculation is not included in this value. Apparently, the result starts at the specific point in the figure. This time shift in result of simulation is due to the time that routers need to create their routing tables. After the connection is established, there is traffic congestion at the beginning of the simulation, but the value of end-to-end delay is stabilized over the time. As expected, the results show that packets sent over TCP are slower than the packets over UDP. The delay for UDP streams are in the range of 6-7 milliseconds and for TCP, this value goes to around 8-11 milliseconds. In a real network, it has been observed that the PMU delay means are in a range between 9-15 milliseconds for the data stream over TCP [31]. Such a comparison between empirical delay data and the simulation result can validate the performance of WAMCAD
real time platform. The quantified delay results can be used to evaluate the reliability of WAMC applications given the scenarios of the supporting ICT system as carried out in [30]. In addition to result of end-to-end delay, in order to show that the whole platform is running as expected, snapshots of the mode estimation and average frequency monitoring applications are presented in Figure 8.
monitoring application has been tested as end users of the PMU data, but in the case of using time-sensitive applications, e.g. applications to perform closed-loop control, the real-time power simulator that creates time-stamped PMU data should use the same time reference as PDC/end applications. At present time, the PDC/end application uses local clock as time reference which is enough for monitoring purpose. In addition to time-synchronization improvement, some studies have been planned in order to implement advanced protocol algorithms such DCCP or IEC61850. The next step is also to testing the different Quality of Services for WAMC applications using the platform. REFERENCES [1] [2]
[3] Figure 7. End-to-End delay of PMU data stream
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Figure 8. A snapshot of KTH PowerIT (Mode Estimation and Average frequency monitoring Applications)
V.
[10] [11]
CONCLUSION
In this paper a real-time co-simulation test-bed has been proposed. This test-bed can examine different communication architectures and their impact on wide area monitoring and control applications. The WAMCAD platform using SITL module which runs in real time gives the significant opportunity of HIL experiments to test the performance and interoperability of the components in the system. To validate the performance of platform architecture, a pseudo-real case study from the Swedish TSO has been run on the platform and the simulation result and empirical data has been compared. Although different scenarios have been presented and examined using WAMCAD platform, there are still some issues needs to be further investigated. In this platform the
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