Download Study and Simulation of Phasor Measurement Unit for Wide Area

Study and Simulation of Phasor Measurement Unit for
Wide Area Measurement System
Ms .Darsana M. Nair
Mr. Rishi Menon
Mr. Aby Joseph
PG Scholar
Assistant Professor
Principal Engineer
Dept. of EEE
Dept. of EEE
Power Electronics Group
Saintgits College Of Engineering
Saintgits College of Engineering
C-DAC
Kottayam, Kerala
Kottayam, Kerala
Trivandrum, Kerala
[email protected]
square values of the currents and the voltages. A
Abstract
synchrophasor
based
WAMS,
consists
of
Power system is now operating in a more
geographically placed Phasor Measurement Units
complicated situation and is facing more challenges
(PMUs), which provide the measurements of both the
than ever before. Synchronized phasor measurements
magnitude and the phase angles of the voltage and
have become a prominent technology for real time
the current signals. These measurements are time-
monitoring of power system. At present PMU is the
synchronized using Global Positioning System (GPS)
most sophisticated time-synchronized tool available
with an accuracy of 1 ms [5].
to power engineers and system operators for wide
area applications. It will improve visibility and
The major applications of the phasor
provide real time monitoring, protection, and control
measurements
of the system. This tool has been made possible by
monitoring, protection and control. The phasors,
advancements
and
measured at the same time instant, provide a snapshot
availability of GPS signals. This paper describes the
of the power system network and by comparing the
modeling and testing of PMU in MATLAB/Simulink.
snapshots of the consecutive time instants, not only
Keywords: - Global positioning systems (GPS),
the steady state but also the dynamic states of the
Phasor measurement units (PMU), Wide Area
critical nodes in the network can be tracked. The
Monitoring
Wide Area Monitoring System (WAMS) is primarily
Systems (WAMS), Coordinated Universal Time
used for managing the grid reliability by continuously
(UTC).
monitoring the status of the grid through PMU
1. INTRODUCTION
deployed at specific location in the transmission
in
computer
technology,
include
real-time
power
system
The power system networks are recently
network. The goal of wide area monitoring systems is
being equipped with the synchrophasor based Wide
to make the power system less immune to
Area Monitoring System (WAMS). The conventional
catastrophic failures and to reduce the severity of
Remote Terminal Units (RTUs), forming part of the
such failures when they do occur.
Supervisory Control And Data Acquisition (SCADA)
system, provide only unsynchronized root mean
PMUs are the main building blocks in the
synchrophasor based WAMS. They provide phasor
And is commonly represented as the phasor as shown
in Equation (2):
X = (Xm/√2) e jΦ
data with faster refreshment rate. The information
(2)
provided by them, i.e., the voltage and the current
phasors,
are
Concentrators
processed
(PDCs)
to
at
the
extract
Phasor
the
= (Xm /√2) (cos Φ + j sin Φ)
Data
relevant
= Xr + j Xi
information about the system operating condition.
PMUs are devices able to sample high
In the above equation, ω is the angular
speed, time-stamped snapshots of voltage and current
velocity θ is the initial angle between a reference
in phasor format, frequency and rate of change of
point and the positive peak. Xm is the peak amplitude
frequency. Being synchronized to Coordinated
of
Universal Time (UTC) through Global Positioning
phasor equals to the root mean square (RMS) value
System (GPS) signal, these devices are able to
of the waveform which is (Xm/ √2). The subscripts r
provide real time measurements recorded from
and i signify real and imaginary parts of a complex
different parts of the power system, giving this way
value in rectangular components. The value of Φ
the potential to time-align these measurements and
depends on the time scale, particularly where t = 0. It
have a precise and representative view of the power
is important to note this phasor is defined for the
system. Phasor Data Concentrators (PDCs) are
angular frequency ω; evaluation with other phasors
devices able to concentrate measurements from
must be done with the same time scale and
different PMUs, time-align them and communicate
frequency.
the
waveform.
The
magnitude
of
A phasor representation corresponds to a
them as a single stream to other PDCs or to
monitoring and control devices. In this paper
pure
following sections were presented. Section II
situations, ac signals are typically distorted by the
describes Phasor and PMU functions. Section III
presence of harmonics. As the analysis of a signal is
focuses the simulation of PMU to perform WAM
always focused on specific frequency components,
functions.
voltage
the extraction of the component of interest is
monitoring, Phase angle monitoring, Frequency
important. In a digital measurement system, this is
monitoring, Rate of Change of Frequency monitoring
usually realized by the “Discrete Fourier Transform”
etc.
(DFT) or the “Fast Fourier Transform” (FFT). A time
WAM
functions
include
sinusoid.
However,
in
real
world
span for the measurement is selected, which is known
II. PHASOR AND PMU FUNCTIONS
as the “time window”. PMUs continuously sample
A phasor is a vectorial representation of an
ac signal with sinusoidal waveform. It is well known
that a sinusoid can be written using the equation:
X (t) = Xm cos (ωt + Φ)
(1)
the waveform using a moving time windows and
update the value of the phasor that is output on a
continuous basis.
A phasor is a complex number that
represents both the magnitude and phase angle of the
sine waves found in electricity. Phasor measurements
Fig. 1.Convention for synchrophasor representation
that
occur
at
the
called
derived from a scaled signal from voltage and current
“synchrophasors”, as are the PMU devices that allow
transformers are initially passed through anti-aliasing
their measurement. In typical applications phasor
filters. A PMU may collect data from different
measurement
widely
locations in the system on a simultaneous basis and
dispersed locations in the power system network and
normally requires data from all three phases to
synchronized from the common time source of a
extract the positive sequence component, which is
global positioning system (GPS) radio clock.
what is normally of interest and contains information
Synchrophasor technology provides a tool for system
that can be used to assess the state of the power
operators and planners to measure the state of the
system.
units
same
are
time
sampled
are
from
electrical system and manage power quality.
Under this definition, Φ is the offset from a
cosine function at the nominal system frequency
synchronized to UTC. A cosine has a maximum at t =
0, so the synchrophasor angle is 0 degrees when the
maximum of x(t) occurs at the UTC second rollover
(1 PPS time signal), and –90 degrees when the
positive zero crossing occurs at the UTC second
rollover (sin waveform)[2]. Figure 1 illustrates the
phase angle/UTC time relationship.
Phasor Measurement Unit
There is no uniform structure adopted for
commercially available PMUs as several companies
provide such offerings. However, the functional
blocks of a typical PMU are generic, and the
common components are shown in figure 2. As
shown in Figure.2, analogue input signals, which are
PMUs are synchronized by satellites through
a GPS receiver. The time accuracy of such system is
typically in the order of a few hundred nanoseconds.
Time stamps are created by the GPS receiver as a
label of measurement and for future comparison of
measurements. The other important function of the
GPS receiver is that it can generate a one pulse-persecond signal to a phase-locked oscillator to
synchronise and lock the phase of the sampling clock.
Synchrophasors
measure
voltages
and
currents at principle intersecting locations (critical
substations) on a power grid and can output
accurately time-stamped voltage and current phasors.
Because these phasors are truly synchronized,
synchronized comparison of two quantities is
possible, in real time. These comparisons can be used
to assess system conditions-such as; frequency
Fig 2.Functional block diagram of PMU.
The
III. SIMULATION OF PMU.
For the study of functionality of PMU, its
model is simulated in MATLAB/ Simulink platform.
system
was
modeled
in
[3][4].MATLAB/ Simulink model of
LABVIEW
PMU is
developed as shown in figure 3.
Fig. 3. MATLAB/ Simulink Model for WAMS.
SYSTEM DESCRIPTION
1. Voltage Phasor Measurement Block:
Here a three phase source (415V, 50 Hz) is
This subsystem receives the three phase voltage
connected to a three phase load. PMU model is
signal and extracts each phase voltages Va, Vb and
placed between the source and load. It measures the
Vc. Then it converts this voltage signal to its phasor
voltage phasor, current phasor, frequency and rate of
representation, i.e., in magnitude and phase angle
change of frequency.
form. This is shown in figure.3.
2. Current Phasor Measurement Block:
frequency can shows the change in impedance of
This subsystem receives the three phase
power system components. We are able to observe
current signal and extracts each phase current Ia, Ib
stability of system through rate of change of
and Ic. Then it converts this current signal to its
frequency monitoring.
phasor representation, i.e., in magnitude and phase
angle form. This is also shown in figure.3.
3. Frequency Measurement Block:
This subsystem basically contains a phase
locked loop which can be used to synchronize on a
set of variable frequency, three-phase sinusoidal
signals. It outputs the measured frequency in Hz (ω
/2pi) and ramp ω.t varying between 0 and 2*pi,
synchronized on zero crossings of the fundamental
(positive-sequence) of phase A. This signal is the
given to ROCOF block.
4. Rate of Change of Frequency Measurement Block:
The output signal from the frequency
measurement block is differentiated to obtain the rate
of change of frequency (ROCOF). Figure 4a and b
shows the subsystem for magnitude and phase angle
measurement.
Fig.4b. Subsystem for phase angle measurement
According
to
the
convention
for
phasor
representation phase angle φ is the offset from a
cosine function at the nominal system frequency
synchronized to UTC [2]. Thus a three phase
balanced sinusoidal voltage waveform represented as:
Va= Vm sin ωt
Vb= Vm sin (ωt-120°)
(3)
Vc= Vm sin (ωt+120°)
Can be represented as phasor form (according to
IEEE synchrophasor convention) as below.
Va= Vm /√2 < -90°
Vb= Vm /√2 < 150°
Vc= Vm /√2 < 30°
Fig.4a. Subsystem for magnitude measurement
We can measure frequency and rate of
change of frequency through PMU. Change in
(4)
Fig. 5. Amplitude variation with change in frequency from 50 Hz to 49.95 Hz
Effect of change in frequency
angle difference between any pair of buses. With this
Here the nominal frequency of the source is
we can monitor angle separation or rate-of change of
50 Hz. If there is any change in the frequency from
angle separation between two buses or two parts of a
its nominal value there will be a corresponding
grid to determine stress on the system. Another
change in magnitude and phase angle of voltage. For
important application of phase angle monitoring is
verifying the effect of change in frequency, the
during restoration. The phase angle value of an
frequency of input wave is changed to 49.95 Hz. The
opened tie line or an opened circuit breaker would
variation in magnitude is shown in figure 5.
help an operator in circuit breaker closing. Closing
PMUs create a picture showing the stability
status of the nodes in the monitored area. PMUs take
would take place only if the phase angle was below a
preset threshold.
this picture at the same reference time. Using real-
For the study of this, a two bus system is
time information from PMUs and automated controls
modeled in MATLAB/Simulink. Figure 5 shows the
to predict, identify, and respond to system problems;
model of such a system. Here PMUs are placed at bus
a smart grid can automatically avoid or diminish
1 and 2 to measure the voltage phasors at nodes 1 and
power outages, power quality problems and supply
2 respectively.
disruptions.
Va1, Vb1, Vc1 and Va2,Vb2,Vc2 are the three phase
The relevance of phase angle monitoring comes into
voltages at bus 1 and 2 respectively. Under normal
picture only when we are able to compare the phase
conditions the phase angles of two buses are as we
angles of two different nodes in a network. For this
obtained in the simulation study. If there is any
we have to place two PMUs at two critical points in a
abnormality in any of the buses, there will be a
power system network. Then the difference in phase
corresponding change in the phase angles of the
angles obtained from these two points will give a
affected bus.
clear idea about the stability of the monitored area.
By continuously monitoring, phase angle monitoring
enables access in real time to the accurate phase
Fig. 6. Simulink Model of Two bus system with PMUs placed at each bus
[2] IEEE Standards for Synchrophasors for Power
Systems, IEEE Power & Energy Society, Sponsored
IV.CONCLUSION
Performance of PMU is simulated and
verified
in
MATLAB/Simulink.
The
model
by the Power System Relaying Committee, IEEE Std
C37.118.1™-2011.
developed gives phasor information i.e., information
[3] Vipin Krishna R, S. Ashok, Megha G Krishnan, “
about both magnitude and phase angle of the input
Synchronized
Phasor
waveform. The time stamped information obtained
International
Conference
from the
Controls and Computation (EPSCICON), 8 – 10
PMU
can be
integrated
with the
Measurement
on
Power,
Unit”,
Signals,
conventional SCADA system to make the operation
January 2014.
of the grid smarter and efficient. Also this
[4] S. Mondal, Ch. Murty D. S. Roy, D. K. Mohanta,
information can be used for the proper selection and
“Simulation of Phasor Measurement Unit (PMU)
coordination of relays to make WAM protection and
Using Labview”. 14th International Conference on
Control.
Environment and Electrical Engineering (EEEICREFERENCES
2014), Krakow pp-164-168, 10-12 May 2014.
[1] Ranjana Sodhi, S.C.Srivastava, “A simple scheme
[5] A.G. Phadke and J.S.Throp, “Synchronized
for wide area detection of impending voltage
Phasor
instability,” IEEE Transactions on Smart Grid, Vol.
Springer, 2008.
3, No. 2, June 2012.
Measurements
and
their
applications”,