Design of a Phasor Data Concentrator for Wide Area Measurement

Design of a Phasor Data Concentrator for Wide Area Measurement
Design of a Phasor Data Concentrator for Wide
Area Measurement System
Kedar V. Khandeparkar, Nitesh Pandit, A.M. Kulkarni
V.Z. Attar, S.U. Ghumbre
Dept. of Electrical Engineering, Indian Institute of
Technology, Bombay
{kedark, anil}@ee.iitb.ac.in, panditnitesh@gmail.com
Dept. of Computer Science and Information Technology,
College of Engineering, Pune
{vahida, shashi}@coep.ac.in
Abstract—Wide area measurement system is playing a crucial
role in improving the reliability of power system. Deployment
of Phasor Measurement Units in Wide Area Measurement
System has resulted in generation of a large amount of data at
high rate across the PMU network. We propose a design and
implementation of a Phasor data concentrator (named as
iPDC) that can collect the data from different devices and
direct it to real time applications, other iPDCs and also
perform local archival for post processing.
point, in order to suitably evaluate them there, as a
monitoring instrument. The PMUs send the calculated
phasor values by means of the standardized protocol, IEEE
C37.118.
Keywords-Global Positioning System (GPS), Wide Area
Measurement System (WAMS), Phasor Measurement Unit
(PMU), Synchrophasor standard, iPDC, Simulator.
I.
INTRODUCTION
use of synchrophasors for monitoring and
THE
improving the stability of power transmission networks
is gaining in significance all over the world. The aim is to
monitor the system state, to increase the awareness of system
stability and to make optimal use of existing lines. This way,
system stability can be improved and overall transmission
performance can be increased. The data from so many PMUs
and PDCs needs to be collected and directed to proper
channels for its efficient use. Thus, there is a need to develop
an efficient data concentrator that can serve this purpose.
Besides accepting the data from PMUs, PDC should be able
to accept the data from other PDCs as well. We have
designed such a PDC (iPDC) that accepts data from PMU
and PDC that are IEEEC37.118 standard compliant.
The rest of the paper is organized as follows. Section 2
gives a brief introduction to WAMS components. Section 3
discusses the issues in WAMS. Section 4 describes the iPDC
data and configuration frame format. Section 5 describes the
iPDC design and implementation. Section 6 describes the
PMU data storage in database. Section 7 describes the PMU
simulator. Section 8 discusses the results. Section 9
concludes the paper and outlines our future research work.
II.
COMPONENTS OF WAMS
Phasor Measurement Unit
The synchrophasors are recorded by the sensor devices
called Phasor Measurement Units (PMUs). Phasors are time
stamped using the GPS, and synchrophasors are gathered
from a widely distributed transmission network at a central
Phasor Data Concentrator(PDC)
The job of PDC is to collect the data from PMUs and or
PDCs, time correlate them and feed them as a single stream
to other application [1]. It provides additional functions as
well. It performs various quality checks on the phasor data
and inserts appropriate flags into the correlated data stream.
It checks disturbance flags and records files of data for
analysis. It also monitors the overall measurement system
and provides a display and record of performance. It can
provide a number of specialized outputs, such as a direct
interface to a SCADA or EMS system.
III.
ISSUES IN WAMS
Delay
A PDC receives data streams from PMUs and other
PDCs and correlates it in real-time into a single data stream
that is transmitted to a computer via an Ethernet port. The
propagation delays associated with communication links
from a PMU to a PDC depends on the medium and the
physical distance separating these components. In addition,
there is a fixed delay associated with processing,
concentrating, multiplexing, and transducers, and is
independent of the communication [2].
Wait Time
PDCs also have a maximum wait time, typically of 1-4
seconds, to allow for all the PMU data to come in before
aggregated data is outputted by the PDC. If the data from all
the PMUs reach the PDC within this wait-time, it outputs
the aggregated data right away. However, in the extreme
case that the data from one of the PMUs is indefinitely
delayed, and then the PDC will wait up to its pre-defined
wait-time (i.e. 1-4 seconds) before the data is outputted by
the PDC. Hence, the PDC can also introduce an additional
delay equal to its wait-time if one of the PMU channel stop
transmitting data to the PDC. Various details induced at
PMU side are given in [3].
1
Vulnerability in WAMS Security
WAMS systems today utilize several common
communication infrastructures - analog microwave,
synchronous optical network (SONET), or virtual private
network (VPN). Each of these communication methods
contains vulnerabilities that can be used to interrupt
communication or otherwise compromise the WAMS. Like
most of today's SCADA systems, WAMS operate in an
environment of complete and implicit trust. Neither
C37.118 nor IEEE 1334 supports an authentication method.
Without an authentication mechanism, a WAMS could be
influenced by injected traffic being accepted and enacted by
the PDC, state estimator, or other application [4].
IV.
DATA FORMAT OF IPDC
The latest PMU/PDC protocol is the IEEE C37.118 [5]
that was developed in the last few years and approved late
2005. It will replace the IEEE 1344 synchrophasor protocol
which has been in use as the PMU standard since its
development in 1998. Before these standards were
developed, the defacto standard for PMU to PDC
communication has been the Macrodyne type 1 and type 2
protocols developed by Macrodyne Corporation. Some of the
PDC to PDC protocols include the PDC data exchange
format, the PDC stream, second level PDC using NTP time
and the PDC stream, second level PDC using native time.
These standards address issues like synchronization of data
sampling, data to phasor conversions, and formats for timing
input and phasor data output. As specified in IEEE C37.118
Synchrophasor Standard, there are four frame types,
command, data, configuration and header. All these frames
have the following fields in common- SYNC, FRAMESIZE,
IDCODE, SOC, FRACSEC and CHK.
The PDC which has been designed (iPDC) follows the
same standard [5] while combining the frames it receives.
iPDC can receive(data and configuration) frames either
from a PMU or a PDC. On receiving the frames, iPDC timealigns them into a single frame. Those frames having same
SOC and FRACSEC are combined into a single frame in the
order of IDCODEs of PMUs/PDCs. A list of IDCODEs of
PMUs/iPDCs is maintained. The IDCODEs in the list are in
the order in which the iPDC receives configuration frames
from PMU or other iPDCs. Thus the data frames that arrive
out of order need to be aligned in the order of IDCODEs
maintained in a list. These combined frames are then
transmitted on a request from other iPDC. If iPDC receives
data from other iPDC, then internal data of that frame is used
for combining with main frame. This data would contain
multiple PMU data. “Fig. 1” and “Fig. 2” show the combined
configuration and data frames respectively.
Figure2. Combined Data Frame
IPDC
WAMS Architecture
“Fig. 3” shows the WAMS architecture with iPDC and
PMU at different levels. This architecture enables iPDC to
receive data either from a PMU or other iPDC. Both PMU
and iPDC from which the data is being received should be
IEEE C37.118 synchrophasor standard compliant. As shown
in the “Fig. 3”, iPDC at level 0 (substation PDC) receives
data from PMUs and send the combined data to iPDCs at
level 1 (central PDC). Level 1 iPDC accepts the data from
iPDCs at level 0 and so on.
Figure 3. WAMS Architecture
iPDC Design
The client server architecture is common in networks
when two peers are communicating with each other. Of the
two peers (PMU and iPDC) that are communicating with
each other in WAMS, one acts as a client and the other as a
server. Since PMU serves the requests coming from iPDC by
sending data or configuration frames, it is a server. It listens
for command
frames
from iPDC. PMU-iPDC
communication can be either over TCP or UDP
communication protocols. On receiving command frames,
PMU replies to the iPDC with data or configuration frames
according to the type of request.
iPDC functionality is bifurcated as server and client.
iPDC as a Client - When iPDC receives data or configuration
frames its acts as a client. When acting as a client, it creates a
new thread for each PMU or a PDC from which it is going to
Figure 1. Combined Configuration Frame
2
receive data/configuration frames. This thread would
establish connection between the two communicating
entities. It handles both TCP and UDP connections. The first
frame that the server (PMU/iPDC) would receive is the
command for sending the configuration frame. When the
server replies with the configuration frame, iPDC (client)
would generate another request to start sending the data
frames. On receiving such a command frame, the server
starts sending the data frames. If there is some change in the
status bits of data frame which the client (iPDC) notices, it
would take an action. For example if it notices a bit 10 has
been set, it would internally send a command to server to
send the latest configuration frame.
FIFO policy, the first arrive data frame need to be dispatched
to make a space for newly arrived data frame.
Before dispatch operation, the frames need to be sorted
so that they are sent in the proper order to the destination.
Selection sort is used to sort the data frames as per their
IDCODEs. This order of the IDCODE is the same as the
order in which the first configuration frame arrives from
each PMU/iPDC. After sorting, a combined frame is formed
from the all the frames having the same time-stamp.
Combined frame is then dispatched to other iPDC or a real
time application. “Fig. 4” shows the details.
Security in WAMS
Security in WAMS is very important. As a minimal
security, connection tables have been maintained at iPDC
that authenticate each incoming packet. More work need to
be done in this area to ensure cyber security. Data integrity
is assured by performing checksum calculation of the
received packets and verifying it with the checksum in the
packets.
V.
DATABASE DESIGN
MySQL and PostgreSQL Comparison
Among the widely popular open source databases
MySQL and PostgreSQL, MySQL database is chosen for
PMU data storage. PostgreSQL is a unified database server
with a single storage engine. MySQL has two layers, an
upper SQL layer and a set of storage engines. The most
commonly used storage engines in MySQL are InnoDB.
They provide full ACID support and high performance on
large workloads. Applications can combine multiple storage
engines as required to exploit the advantages of each. In
PostgreSQL there is no built-in mechanism for limiting
database size. This is the main reason why most of the web
hosting companies use MySQL. More details and
comparison can be obtained from [6].
Figure 4. iPDC Design
iPDC as a Server- When iPDC receives command
frames from another PDC it would acts as a server. There
would be two reserved ports, one for UDP and another for
TCP on which the iPDC would receive command frame
requests. Thus PDC now plays the role of PMU waiting for
command frames.
Time Alignment and Sorting
A circular FIFO array of fixed number of time-stamp
buffers is maintained. Each received data frame with a timestamp not present in the array is allocated a buffer from this
array. All the subsequent data frames having the same timestamp as arrived frame are then linked to the array index of
the buffer. In this way a linked list of all data frames having
the same time-stamp is maintained. If all buffers are full and
a data frame with a new time-stamp arrives then as per the
iPDC Database
When iPDC receives data/configuration frames, it would
direct the frames to a database server. The server may be on
the local or remote machine. The database server process
would have a IEEEC37.118 parser to parse configuration
and data frames and create objects in memory. After parsing
the data it would make entries in the configuration and data
tables of the iPDC MySQL database. If a configuration
frame comes for a newly added PMU it would be inserted in
the configuration tables. If configuration frame for a
previously added PMU arrives, then the previous entry in
the tables is updated. The data frames are inserted as they
come. This data which is stored in the tables can then be
used for later analysis. The data from the database is
archived periodically. This can be used for post analysis.
“Fig. 5” shows the data storage process.
3
Figure 5. PMU data storage
VI.
PMU SIMULATOR
PMU Simulator has been designed and
implemented to test iPDC application. It generates all the
frames given in [5]. PMU Simulator has been successfully
tested with the PMU Connection Tester [7] and other
PMU devices. It can emulate multiple PMUs on a single
machine with different ports for UDP and TCP. The user
can configure different parameters like phasor voltage,
frequency, analogs, digitals etc. The data rate can also be
varied. We can simulate multiple PMUs at different data
rates and variable number of parameters. User has been
provided the option to select the data from CSV file and
generate the data frames.
VII.
RESULTS
A setup was established with iPDC installed at two
layers in WAMS topology. At layer 1 we had two iPDCs,
each receiving data from 10 simulated PMUs. The
combined frames from each of these iPDCs were then sent
to another iPDC installed at layer 2. This iPDC was made
to receive data from a PMU simulator. iPDC at both
layers functioned well. The data from layer 2 iPDC was
sent to MySQL database server. The query results from
the database tables were verified with the configurations
of simulated PMUs.
VIII.
CONCLUSION AND FUTURE WORK
iPDC design proposed in the paper will serve to be
the basic building block in the design of any other PDC.
In future, we plan to test the iPDC and PMU Simulator in
Real time operating systems like RTLinux, RTai etc., to
check the
performance of iPDC in real time. We also need to ensure
that the system meets the hard real time guarantees. There
is a scope for improvement in database design that would
enable data storage from multiple devices.
ACKNOWLEDGMENT
The authors thank Prof. S.A. Soman, Mr. Ken Martin,
Mr. Gopal Gajjar, Mr. Anupam Chomal, Mr. Kalyan
Dasgupta for their timely suggestions and help.
REFERENCES
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[6] MySQL,
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[1]
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