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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





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]




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]




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




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

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.




8

Nevertheless, when new DG power electronic based

converters are being used, there is no need for the

STATCOM.

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Figure 14: Wind turbine behavior with control methodology

Figure 15: Voltage Transient Behavior of DG with Control Strategy

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