7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 1/8
Sep. 2013, Volume , No. (Serial No. )
Journal of Energy and Power Engineering, ISSN 1934-8975, USA
1
Critical Clearing Time and Voltage Stabili ty of DG
Integration in Lebanon:
A Simulation Using MATLAB/SIMULINK
KifahDaher 1, Student and Maged B. Najjar
2
1. Department of Electrical Engineering, University of Balamand, North Tripoli, Lebanon; [email protected]
, IEEE member
2. Ph.D., Professor; power area, Department of Electrical Engineering, University of Balamand, North Tripoli,
Lebanon, [email protected]
Abstract: These days, the renewable energy is widely participating in the electric energy production worldwide. Different studiesand work have been done concerning the integration of Distributed Generations (DG) with the power grid; optimal sizing and
allocation techniques has shown the appropriate penetration method for superlative beneficial effects. This paper investigates
particularly the integration of DG in existing power systems while assuring the system’s stability under fault conditions. The relative
between the DG especially induction generator type with respect to transient stability is examined. The critical clearing time (CCT) is
an essential criterion for a DG to ride through faults; in this paper a theoretical calculation of CCT is obtained all along with
simulation results. CCT is improved using a STATCOM at distribution level or by the implementation of inverter-based distributed
generations. A simulation was designed to investigate the proposed approaches using MATLAB/SIMULINK.
Keywords: Smart Grid, Distributed generators DG, Renewable, Power electronics, Converter, Power flow, Control, CCT.
1. Introduction
The constantly increasing penetration of distributed
generation (DG); especially the wind farms make it
important to understand the impact of these machines
on a power system. Distributed power generation
integration with the conventional grid can have
positive and negative impacts. The impact of DG on the
stability of a power system depends on the technology
of the DG and on the penetration level. Distributed
generations are widely used after the advancement of
technology in the electrical engineering field especially
smart grid topology. Maintaining the grid stability and
voltage regulation is the challenge facing smart grid
technology and DG integration [1]. Figure 1 shows the
electricity production in Lebanon. Electricity
generation from public sector covers approximately
56% of the total demand which is around 2500 MW
[2]. Private backup diesel generators provide the rest
44%. DG participates largely in the electricity
generation in Lebanon for this reason investigating the
CCT and the ride through faults capability along with
voltage stability is extremely important.
2. Study Case Network - Microgrid
The following section introduces a benchmark
distribution system of medium voltage which will be
used to test the behavior of DGs under fault conditions.
Figure 1: Electricity production in Lebanon [2]
50%
6%
44%
Public thermal
Power Plants
Public hydro power
plants
Private backup
Diesel Generators
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 2/8
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 3/8
Critical Clearing Time and Voltage Stability of DG Integration i n Lebanon
3
Components as capacitors and condensers supply the
generator by the needed reactive power in parallel to
the grid [4]. In our simulation, capacitors will be used
to generate the required reactive power.
Stability of an induction machine can be studied
from the torque Vs speed characteristic curve. If the
induction machine is driven to a speed higher than the
synchronous speed, it becomes a generator and delivers
power. The speed torque characteristic is shown in
Fig.3.
For an unvarying mechanical input, two equilibrium
points are observed in this figure. The point in the
stable region is the steady-state stable speed and the
second one is the critical speed point. If before thegenerator’s critical speed point is reached the fault is
cleared, the mechanical input torque will be less than
the electrical torque. This will lead the rotor to decrease
speed to a value that match the one before the
disturbance. In the other hand when this point is
crossed, the electrical torque will be less than the
mechanical torque which will invokes acceleration in
the rotor speed leading to a runaway slip. Thus, the
generator will be unstable [5].
2.2.2 Wind Turbine Doubly-Fed Induction Generator
The WTDFIG block was used from the
SimPowerSystems library browser to represent some
of the connected DG. Those kinds of DG are connected
to the Grid using control methodology and power
electronics. Fig. 4 represents the operating principle
of the used induction generator.
Figure 3: Generator Torque v/s Speed Stable Region [6]
It is based on an AC/DC/AC converter connected in
series with the induction machine then to the grid. The
converter is formed from a rotor side converter and a
grid side converter. Voltage Sourced Converters (VSC)
uses IGBTs to convert the DC voltage source into an
AC. The DC voltage can be represented by a capacitor.
To smoothen the output, a coupling inductor is
connected to the grid side converter. The rotor
windings are connected to the rotor side converter by
brushes and slip rings, and the stator windings are
connected directly to the grid. Wind power is converted
into electrical one and then is delivered to the grid by
both the stator and the rotor windings. The pitch angle
and the voltage signals are controlled by theimplemented control system in order to manage the
wind turbine output. The power, the DC voltage and
the reactive power or the terminals voltage can be
controlled. Converters can generate or absorb reactive
power. Thus, they are used to control the voltage or the
reactive power. In our simulation, since we need a
fixed DG output voltage, we used a voltage control
model in order to investigate voltage stability issues
[7].
2.2.2 Static Synchronous Compensator (STATCOM)
The STATCOM has been reported to improve the
transient stability margin in power systems with DG
integration of wind type [8]. STATCOM can be
implemented to regulate the voltage as a shunt
compensator for the WTIG.
Figure 4: The WTDFIG model [7]
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 4/8
Critical Clearing Time and Voltage Stability of DG Integration i n Lebanon
4
The shunt connected STATCOM is based on the
vector control principle, the injected reactive current is
controlled and regulated in a way to obtain the
reference grid voltage. The basic STATCOM model is
shown in Fig. 5.
The STATCOM’s main role is to control its
terminal’s voltage by managing the quantity of reactive
power transferred from or to the power system. In the
case if the system’s voltage is low and the STATCOM
is connected, it responds as a capacitive device by
injecting reactive power. In the other hand, if the
system’s voltage is high it absorbs reactive power and
acts as an inductive device.
On the secondary of the coupling transformer shownin Fig. 6, a VSC is connected for the reactive power
variation control. It uses forced-commutated power
electronic devices such as GTO, IGBT or IGCT to
convert the DC into an AC voltage. Fig. 6 reflects the
STATCOM operation principles. It shows the active
and reactive power transfer between the bus voltage
and the voltage generated by the VSC [9].
Figure 5: Basic STATCOM model [8]
Figure 6: Operating Principle of STATCOM [9]
The active and reactive power equations are:
P =V1 V2 sinδ
X , Q =
V1(V1 − V2 cosδ)
X (1)
The angle δ is zero during steady state operation
mode since voltages are in phase. Thus P is zero and
only reactive power will flow. The amount of reactive
power is given by
Q =�V1 (V1 − V2)
X (2)
In the used model, PWM inverters control the IGBT
type VSC. AC voltage is derived from the DC source
by the mean of Pulse-Width Modulation technique.
Using filters harmonic voltages are cancelled. By
controlling the modulation index of the PWM theVSC’s output voltage is varied [9]. Fig.7 shows a block
diagram of the STATCOM (single line) and a
simplified block diagram for the control system.
The control system mainly consists of:
A phase-locked loop (PLL), Measurement systems
measuring the d and q components of AC
positive-sequence voltage and currents and Regulation
loops consist of voltage and current regulators.
To study the impact of the STATCOM on the DG
from a stability approach, it is connected with the wind
DG to the system and transient stability studies are
performed. The impact of the STATCOM on the
oscillations and induction generators stability will be
observed.
Figure 7: STATCOM Block Diagram [9]
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 5/8
Critical Clearing Time and Voltage Stability of DG Integration i n Lebanon
5
3. Simulation and Results
In this section, the critical clearing time (CCT) factor
is reviewed and its impact on the stability of an
induction generator is highlighted. Increasing thestability of an induction machine is achieved by
increasing the CCT factor. For this purpose,
STATCOM and power electronics voltage source
converter impact on the CCT margin is simulated.
3.1 Transient Stability Criteria: the CCT
With the continuous increase in the diffusion of the
wind farms in the electric systems as in Lebanon, the
DGs impact on a power system is important to be
analyzed. The DGs should ride through the fault
according to the new grid codes [10]. During severe
faults as a three phase fault, the electromagnetic torque
decrease when the mechanical torque related to wind
speed is considered to be constant. In this case the rotor
shaft accelerates; hence it is important to analyze the
speed impact. Critical clearing time CCT and critical
speed CS are obviously the main factors on which the
stability of DGs depends.
Analytical calculation of critical clearing time isderived in [11].
TCritical =2H
−Tm∫ dwr =
2H
−Tm (wcs − wss )
wcs
Wss (3)
The slip factor determines the wind generator
stability. During faults, the increases slip point at which
the electromagnetic torque corresponds to the same
amount as before disturbance is known the critical slip.
If the disturbance is cleared beyond the stable point, a
continuous increase in the rotor speed will be
encountered whereas, the electromagnetic torque
decreases. In this case runaway slip is denoted. The
time duration starting from the fault time until the
critical slip point is the CCT. Hence the CCT can be
determined from the torque and rotor speed
characteristic of the induction machine.
For this purpose, the CCT of a single induction
generator wind turbine was simulated using
Matlab\Simulink. As shown in Fig. 8, an induction
generator wind turbine from the SimPowerSystems
library was connected to Bus 7 of the MV test network.
The wind speed is considered constant at a value of
8m/s. The DG delivers 2 MW output power at
steady-state. A three-phase fault was induced at the
terminal of the DG. The STATCOM is disconnected.
The simulation time is 20 seconds which is enough to
investigate the transient behavior of the rotor speed.
Fault is induced at t=2sec and the rotor speed is plotted
in pu for various fault clearance time t= 3.4 s, 3.49 s
and 3.5 s.
Fig. 9 shows the response on the rotor speed for
three-phase fault induced at the terminal of the DG andcleared for different time. It is obvious from the figure
that at the fault clearing t=3.5 s the machine is unable to
remain stable. Thus, the CCT is 1.5 seconds. Above
this critical point the rotor will continue to accelerate
indicating a runaway slip. By this mean the CCT of and
induction generator can be calculated, which will be
useful since we will be focusing on increasing this
transient stability criteria.
Figure 8: DG connected to Bus 7
Figure 9: rotor speed vs. time for various faults clearing time
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 6/8
Critical Clearing Time and Voltage Stability of DG Integration i n Lebanon
6
3.2 STATCOM impact on transient stability margin
Various scenarios are performed on the MV test
Network to investigate the transient stability of the
DGs connected to the network. Four classical WTIG providing each 4MW are connected to buses 4, 7, 9 and
10. The stator of these DGs is in straight connection
with the grid. The total loads connected to the system
are 45 MW and 10.5 MVars.
Part of the power absorbed by the loads will be fed
from the DG when they are connected. To investigate
the stability of the wind turbine for a large disturbance,
a three-phase fault is induced at bus 3. The fault was
cleared at different time and the CCT was calculated.
Not all the DGs have the same sensitivity to the fault;
hence they don’t have the same CCT. However, the
CCT will be the smallest one which ensures that all DG
remain connected and re-establish a normal operating
point after a disturbance.
The first scenario consists of finding the CCT for the
DG of squirrel cage induction generator type connected
to the grid without the STATCOM support. The
simulation is repeated when the STATCOM is
connected in order to investigate its impact on thetransient stability margin [8]. The simulation time for
this scenario is 10 sec. Fig. 10 and 11 represent the
rotor speed versus time during a three-phase fault. The
three-phase fault at bus 3 is induced at t=2 sec, and is
cleared at t=2.9 sec in Fig.10 and t=2.95 sec in Fig. 11.
It reflects the instability issue and a runaway slip is
denoted. Thus, the critical time point is CCT= 0.95 sec
(without STATCOM).
The critical clearing time CCT is directly related to
the critical speed of the rotor. The disturbance should
be cleared before this critical time in order for the DG
to remain stable and succeed in restoring a steady-state
operating point.
The four DGs are capable of re-operating in a stable
mode after the disturbance was cleared at t=2.95 sec.
When the fault clearing time is at t=2.95 sec none of the
generator could re-establish a stable operation as
shown in figure 11. In our simulation platform, the
STATCOM has been connected to bus 3.
To observe the effect of the STATCOM on the
critical clearing time (CCT), the switch was closed,
hence the STATCOM is connected and the three-phase
fault was induced at bus 3. By investigating the fault
clearing time it is found that when the STATCOM is
connected, the CCT becomes CCT=1.15 sec. It is
obvious that the STATCOM had increased the critical
clearing time CCT by 200ms. Hence giving the
machine more time to right through fault and remain
connected and re-establish a stable operating point
after the fault is cleared.
During fault, the STATCOM have the capacity tocontrol the reactive power transfer at its terminal.
Hence achieves a voltage regulation mode. When the
three-phase fault is induced, the system voltage drop,
hence the STATCOM generates reactive power and act
as capacitive device. Fig. 12 shows the voltage at
STATCOM’s terminal (top) and the reactive power
generated by the STATCOM all along the simulation
(bottom).
Figure 10: CCT calculation for clearing fault time t = 2.9 s
Figure 11: CCT calculation for clearing fault time t = 2.95 s
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 7/8
Critical Clearing Time and Voltage Stability of DG Integration i n Lebanon
7
Fig. 13 focuses on the voltage behavior during the
transient time of the simulation. Fig.13 (top) represents
the voltage profile of bus 7 without STATCOM
intervention whereas Fig.13 (bottom) for the
STATCOM connected. We can realize that the
STATCOM’s advantage is re-stabilizing the voltage on
the reference voltage 1 pu following a disturbance.
Therefore assuring the voltage ride through capability
for the DGs.
3.3 Impact of control methodology VSC
In this section a robust control stability enhancement
method is investigated. The DGs are replaced by the
DFIG with AC/DC/AC converter. Rotor and grid sideconverter are VSC power electronic devices (IGBTs).
Voltage control parameter has been picked up for this
block and the reference voltage is selected.
Figure 12: Voltage and generated reactive power by
STATCOM
Figure 13: Voltage transient behavior without and with
STATCOM
The simulation is run, and since the DGs are
responsible of voltage regulation, the STTACOM is
omitted from the platform.
It is realized that DG with power electronics that
uses control methodology are capable of stabilizing the
induction machine, since the active and reactive power
are controlled in a way to maintain the output voltage at
a reference of 1 pu. Fig. 14 shows the response of
doubly fed induction generator wind turbine with
power electronics controllers.
A steady-state operating point is succeeded after
clearing the fault. In what concern the voltage, it
re-stabilizes at 1 pu, the reference voltage. Thus when
the fault is cleared, if none of the standard is violated,the DG with robust control methodology remain
solidly connected to the grid. Figure 15 focuses on the
voltage behavior during the transient time of the
simulation showing the robustness of this strategy.
4. Conclusion
In this paper, voltage stability and critical clearing
time for DG were investigated. The main focus was on
wind turbines used in distributed energy applications.
Solutions for enhancing transient stability margin or
increasing the CCT were proposed. Simulation results
proved that the SATCOM can increase the CCT thus
the fault ride through capability of the DG. Moreover,
the STATCOM contributed in restoring the voltage to
its initial reference point following a disturbance.
However, power electronics converters that use
forced-commutated electronic devices implemented
with DG interconnection succeed in controlling the
voltage and the reactive power remarkably. Hence, two proposed method were investigated to enhance the
fault ride through capability and the voltage stability of
a DG. The output power and voltage control
dynamically achieved by the power electronics
converters integrated in the DG makes it more flexible,
and the stability margin was extended. In a distribution
system where classical wind farms are connected to the
grid, installation of a STATCOM is mandatory.
7/26/2019 Stat Com Sim
http://slidepdf.com/reader/full/stat-com-sim 8/8
Critical Clearing Time and Voltage Stability of DG Integration i n Lebanon
8
Nevertheless, when new DG power electronic based
converters are being used, there is no need for the
STATCOM.
References
[1] C. Lopes, A. C. Nascimento, J. P. A. Vieira, M. V. A.
Nunes, U. H. Bezerra, Reactive Power Control of Direct
Drive Synchronous Wind Generators to Enhance the Low
Voltage Ride-Through Capability, in 2010 IREP
Symposium.
[2] M. Ziade, Technical Challenges to 24/7 electricity in
Lebanon, available online at:
http://www.carboun.com/energy/technical-challenges-to-
247-electricity-in-lebanon/
[3] K. Rudion, A. Orths, Z. A. Styczynski, K. Strunz, Design
of a benchmark of Medium Voltage DIstribution Network
for investigation of DG Integration, in Power EngineeringSociety General Meeting, 2006 IEEE Conference,
Montreal, Que.
[4] S. Heier, Grid Integration of Wind Energy Conversion
Systems, John Wiley & Sons Ltd, 1998, ISBN
0-471-97143-X
[5] Alternative Energy tutorial, Wind Turbines using
Induction Generators, available online at:
http://www.alternative-energy-tutorials.com/wind-energy
/induction-generator.html
[6] J. Vaidya, Advanced Electric Generator & Control for
High Speed Micro/Mini Turbine Based Power Systems, in
PowerGen Conference held in Orlando, Florida in
December 2002.
[7] N. Miller, J. Sanchez Gasca, W. Price, R. Delmerico,
Dynamic modeling of GE 1.5 and 3.6 MW wind turbinegenerators for stability simulations, GE Power Systems
Energy Consulting, IEEE WTG Modeling Panel, Session
July 2003
[8]
M. Molinas, S. Vazquez, T. Takaku, J.M. Carrasco, R.
Shimada and T. Undeland, Improvement of Transient
Stability Margin in Power Systems with Integrated Wind
Generation Using a STATCOM: An Experimental
verification”, in IEEE 2005 International Conference on
Future Power Systems.
[9] N. G. Hingorani, L. Gyugyi, Understanding FACTS;
Concepts and Technology of Flexible AC Transmission
Systems, IEEE Press book, 2000.
[10]
M. Tsili, S. Papathanassiou, A review of grid codetechnical requirements for wind farms, IET Renew. Power
Gener., 2009, Vol. 3, Iss. 3, pp. 308–332
[11] A. Abbaszadeh, V. Mortezapour, A. Zohoori, Analytical
Calculation of Critical Clearance Time of Doubly Fed
Induction Generator, in Environment and Electrical
Engineering (EEEIC), 2011 10th International
Conference, Rome.
Figure 14: Wind turbine behavior with control methodology
Figure 15: Voltage Transient Behavior of DG with Control Strategy