Power Quality Control in Smart Distribution Gridsusing Power Electronic ConvertersA Mini-project Report inPower Electronics for Renewable EnergyTET 4190

Group Members:

1. Manisha Malla2. Nabina Pradhan3. Nebrom Berihu Araya4. Tigist Atnafseged Adamu

Contact persons:

Astrid Petterteig, Senior Scientist, SINTEFJohn Are Wold Suul, Electric Power Engineering Department

Lecturer:

Professor Tore M. Undeland,Faculty of Information Technology, Mathematics & Electrical Engineering,Norwegian University of Science & Technology

i

Abstract

The existing power grid faces many limitations and challenges in a world that is increasinglydependent on electricity. As a result, some government agencies, utility companies,researchers and engineers in the electric power industry have envisioned of transforming theexisting grid into Smart Grid. Although there is no global and precise definition of theconcept, there is consensus on its major features among most stakeholders. It is expected to bean intelligent, sustainable, resilient and reliable power grid suitable for the 21st centuryeconomy. One of the advancements envisioned in smart grid is power quality control. As oneof the major enabling technologies of smart grid, power electronics will play an important rolein power quality control. In this paper, a brief theoretical background of smart grids andpower electronic devices for power quality control is presented. The advantage ofDSTATCOM (Distribution Synchronous Compensator) in mitigating voltage dip that occursin a distribution network is explained. Simulation of DSTATCOM for voltage dip mitigationwas done in PSCAD/EMTDC. The results are presented in this paper.

ii

Contents

Abstract………………………………………………………………………………………………….i

1. Introduction ......................................................................................................................................... 1

1.1. Smart Grid .................................................................................................................................... 1

1.2 Power Quality ................................................................................................................................ 3

1.2.1 Voltage Dip............................................................................................................................. 4

1.2.2 Voltage Dip Mitigation........................................................................................................... 4

2. Power Quality Controllers based on VSC ........................................................................................... 5

2.1 Volatge Source Converter.............................................................................................................. 5

2.2 Distribution Static Compensator ................................................................................................... 6

2.2.1 Basic Configuration and Operation of D-STATCOM............................................................ 6

2.3 Dynamic Voltage Restorer ............................................................................................................. 8

2.4 Unified Power Flow Controller .................................................................................................... 10

3. Simulations and Results..................................................................................................................... 11

3.1 Simulation circuit ......................................................................................................................... 11

3.2 Control Circuit.............................................................................................................................. 11

3.3 Results ......................................................................................................................................... 14

4. Conclusion ......................................................................................................................................... 15

References ............................................................................................................................................. 16

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1. IntroductionThe electric power grid can be defined as the entire apparatus of wires and machines thatconnects the sources of electricity (i.e., the power plants) with customers and their myriadneeds. [1] Although it has been acclaimed as “the most significant engineering achievement ofthe 20th century”, the existing power grid face various challenges.[5] The demand forelectricity has grown to an extent that transmission networks are being pushed ever closer totheir stability and thermal limits. Loss of system stability, high transmission losses andvoltage limit violations have become major issues. Moreover, it is not only simple supplyreliability that consumers want today ̶ they want high quality supply voltage. Automatedmanufacturing processes, the IT industry, financial institutions, hospitals, electronic consumerproducts are some major areas where high power quality is required. Last but not least, theconcern on climate change has caused a pressing demand for shifting from fossil fuels torenewable energy sources. [2]

The traditional solutions of upgrading electric transmission system infrastructure in the formof new power plants, new transmission lines, substations and associated equipment cannotfully address these big challenges. It is very important more than ever to rethink the features,components and organization of the whole grid system. The shift in the development oftransmission grids to be more intelligent has been summarized as “smart grid,” as well asseveral other terminologies such as IntelliGrid, GridWise, FutureGrid, etc. [3]

The Smart Grids program, formed by the European Technology Platform (ETP) in 2005,created a joint vision for the European networks of 2020 and beyond [4]. Its objective featureswere identified for Europe’s electricity networks as flexible to customers’ requests, accessibleto network users and renewable power sources, reliable for security and quality of powersupply and economic to provide the best value and efficient energy management.

A Federal Smart Grid Task Force was established by the U.S. Department of Energy (DoE)under Title XIII of the Energy Independence and Security Act of 2007. In its Grid 2030vision, the objectives are to construct a 21st-century electric system to provide abundant,affordable, clean, efficient, and reliable electric power anytime, anywhere [3].

1.1. Smart GridA smart grid is an electricity network that can intelligently integrate the behavior and actionsof all users connected to it - generators, consumers and those that do both – in order toefficiently deliver sustainable, economic and secure electricity supplies. [1] Smart Gridincorporates monitoring, analysis, control and communication capabilities into the electricpower grid in order to improve reliability, optimize asset utilization, improve security,increase energy efficiency and allow diverse generation and storage options. Smart Grid alsoallows homeowners and businesses to utilize electricity as efficiently and economically aspossible hence, reduces cost and increases reliability and transparency. [7]

In existing power grids, there is one way power flow from power stations, via transmissionand distribution systems, to customers. Customers are uninformed and non-participative in thepower system, which is dominated by central generation. A smart grid, on the other hand, isdesigned for bidirectional communication between appliances and power grids, to useelectricity more efficiently than ever before which will give benefits to both consumers and

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producers. It will have informed, involved, and active consumers together with demandresponse and distributed energy resources.

The smart features of the envisaged transmission grid [3] are summarized below:

A) Digitalization: The smart transmission grid will employ a unique, digital platform forfast and reliable sensing, measurement, communication, computation, control,protection, visualization, and maintenance of the entire transmission system.

B) Flexibility: The flexibility for the future smart transmission grid is featured in itsexpandability for future development with the penetration of innovative and diversegeneration technologies, adaptability to various geographical locations and climates,and seamless compatibility with various market operations.

C) Intelligence: The smart grid will be capable of sensing system overloads, reroutingpower to prevent or minimize a potential outage, and working autonomously whenconditions require resolution faster than humans can respond.

D) Resiliency: A fast self-healing capability will enable the system to reconfigure itselfdynamically to recover from attacks, natural disasters, blackouts, or networkcomponent failures.

E) Sustainability: The sustainability of the smart transmission grid is featured assufficiency, efficiency, and environment-friendly.

F) Customization: The design of the smart grid will be client-tailored for the operators’convenience without the loss of its functions and interoperability. It will also cater tocustomers with more energy consumption options for a high quality/price ratio.

The major enabling technologies that will be used to achieve the aforementioned smartcharacteristics are summarized as follows [3]:

1) Alternative clean energy sources and new materials. The development of renewableenergy sources will be very important to make the grid sustainable andenvironmentally friendly. New materials will increase power transfer capabilities,reduce energy losses and lower construction costs.

2) Advanced power electronics and devices. Advanced power electronics will greatlyimprove the quality of power supply and flexibility of power flow control.

3) Sensing and measurement. Smart sensing and measurement will serve as the basis forcommunications, computing, control, and intelligence in the grid.

4) Communications. A fast and accurate information exchange in different platforms willimprove the system resilience by the enhancement of system reliability and security,and optimization of the transmission asset utilization.

5) Advanced computing, control methodologies and intelligent technologies. High-performance computing and control technologies will enable real-time modeling andsimulation of complex power systems, and automation of the entire customer-centricpower delivery network. Intelligent technologies will enable fuzzy logic reasoning,knowledge discovery, and self-learning.

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6) Mature power market regulation and policies. The mature regulation and policiesshould improve the transparency, liberty, and competition of the power market. Highcustomer interaction with the electricity consumption will be enabled.

With the above enabling technologies in mind, power electronics will be increasinglyimportant in all major areas of tomorrow’s electric power system:

Generation: The use of distributed generation and renewable energy sources isincreasing from time to time. Power electronics is needed for integration of these tothe grid.

Transmission: The use of distributed and renewable energy resources requires powerflow and quality control. Power electronics devices known collectively as FACTS(Flexible AC Transmission Systems) are being used for this purpose and their use willcontinue to increase. Besides, the promising transmission system known as HVDC(High Voltage DC Transmission) uses power electronics to a large extent.

Distribution: At the distribution level, power quality control, distributed generationand energy storage systems will all be dependent on power electronics.

Consumption: The use of power electronics in industrial and home appliances hasrecently increased. It is expected to rise even more as smart end-user devices becomepart of the electric power system.

1.2 Power QualityThe term power quality is not universally agreed upon but the concept has become a veryimportant aspect of power delivery. Other terminology in use is “quality of power supply” and“voltage quality”. [15]

Interest in power quality has recently increased mainly due to the following factors: Equipment has become more sensitive to voltage disturbances. Equipment causes voltage disturbances. The number of loads fed via power electronic

converters has recently increased. These present a challenge in ensuring power quality. There is a growing need for standardization and performance criteria. The power quality can be measured. Harmonic currents and voltage dips are no longer

difficult to measure.

The quality of electrical power supply is a set of parameters which describe the process ofelectric power delivery to the user under normal operating conditions, determine thecontinuity of supply (short and long supply interruptions) and characterize the supply voltage(magnitude, asymmetry, frequency, and waveform shape). [15]

Power quality phenomena can be divided into two types.

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A characteristic of voltage or current (e.g., frequency or power factor) is never exactlyequal to its nominal and desired value. The small deviations are called voltagevariations or current variations

Occasionally the voltage or current deviates significantly from its normal or idealwaveshape. These sudden deviations are called events.

Power quality events are the phenomena which can lead to tripping of equipment, tointerruption of the production or of plant operation, or endanger power system operation. Thisincludes interruptions, undervoltages, overvoltage, phase angle jumps and three phaseunbalance.

1.2.1 Voltage DipA voltage dip is a short time (10 ms to 1 minute) event during which a reduction in r.m.svoltage magnitude occurs. It is often set only by two parameters, depth/magnitude andduration. The voltage dip magnitude is ranged from 10% to 90% of nominal voltage (whichcorresponds to 90% to 10% remaining voltage) and with a duration from half a cycle to 1 min.Voltage dip in a three phase system affects both phase to phase(line voltage) and phase toground(phase voltage). A voltage dip is caused by a fault in the utility system, a fault withinthe customer’s facility or a large increase of the load current, like starting a motor ortransformer energizing. The most common faults are single phase to ground or phase to phaseshort circuit, which lead to high current. Due to this high current, a voltage drop occurs overthe network impedance. When the fault occurs the voltage in the faulted phase drops to closeto zero and non faulted phases remains more or less unchanged [8,9].

1.2.2 Voltage Dip MitigationVoltage dips in transmission and distribution systems can be mitigated in different ways. Atpresent, a wide range of very flexible controllers, which capitalize on newly available powerelectronics components, are emerging for custom power applications [10, 11]. These devicesare used to control and stabilize voltage in the Power System. These devices consist of staticVAR generator or absorber and a suitable controlling power electronic device.

These devices provide fast-acting reactive power compensation to power system networks.These devices are connected on transmission systems to improve voltage profile and systemstability during both normal and contingency system conditions. The use of these deviceshelps to increase transmission capacity and stabilizes voltage in different buses over a widerange of loads. These devices also compensate the reactive power demand of the widelyvarying loads. If the load in the system is very high, the demand of reactive power is also veryhigh, so there will be high amount of reactive power flow in the system and it causes thevoltage drop in the line. Therefore, the voltage at the receiving end will decrease. Similarly, ifthe load in the system is very low, voltage at the receiving end of the line increases due tocharging current (Ferranti effect). It means that if the generated reactive power is less than theconsumed reactive power in the system, the voltage drops and vice versa. Therefore, thevariation of voltage is because of imbalance in generation and consumption of reactive powerin the system.

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2. Power Quality Controllers based on VSCPower quality control will be one of the issues addressed in smart grids. Power electronics isexpected to be the main enabling technology in this area. The most widely used active powerquality controllers based on power electronics are FACTS devices.

FACTS (Flexible AC Transmission System) are a power electronic based system and otherstatic equipment that provide control of one or more AC transmission system parameters toenhance controllability and increase power transfer capability. The FACTS devices likeUnified Power Flow Controller (UPFC), Unified Power Quality Conditioner (UPQC),Distribution Synchronous Compensator (D-STATCOM), Dynamic Voltage Restorer areexpected to gain widespread use in smart distribution networks for power quality control[2][5] .

Power quality controllers which are based on voltage source converter (VSC) are explained inthe following topics. These controllers are DSTATCOM, DVR and UPFC. The emphasis inthis paper is on the capability of these devices in mitigating voltage dips in a distributionsystem.

2.1 Volatge Source Converter

A voltage-source converter is a power electronic device, which can generate a sinusoidalvoltage with any required magnitude, frequency and phase angle. The converter is normallybased on an energy storage device, which will supply the converter with a DC voltage. Thesolid-state electronics in the converter is then switched to get the desired output voltage. Thecontroller generates the required switching pattern. The VSC is a basic component of devicesused for mitigation of voltage dips and harmonic distortion.[14]

1.0 [uF]

Fig. 1 Voltage Source Converter

Three phase ac outputVdc

Controller

6

2.2 Distribution Static Compensator

The Distribution Static Compensator (DSTATCOM) is a custom power device based on aVoltage Source Converter (VSC) shunt connected to the distribution networks. ADSTATCOM is normally used to precisely regulate system voltage, improve voltage profile,reduce voltage harmonics and for load compensation. However, the D-STATCOM can alsomitigate voltage dips by injecting reactive power to the point of connection with the grid.Active power will be needed if both magnitude and phase angle need to be compensated [12].DSTATCOM uses power electronics converter to synthesize the reactive power output ratherthan using conventional capacitors and inductors combined with fast switches. ADSTACTOM converter is controlled using pulse width modulation (PWM). DSTATCOM haslower rated power, and faster power electronics switches as compared to STATCOM , thusthe PWM carrier frequency used in distribution controller can be much higher than in atransmission controller.

2.2.1 Basic Configuration and Operation of D-STATCOM

A D-STATCOM consists of a VSC, a dc energy storage device, a coupling transformerconnected in shunt with the ac system, and associated control circuits. Fig. 2 shows the basicconfiguration of D-STATCOM.

Fig.2 Basic Buildings Blocks of the DSTATCOM

Vi˂δVDC

Controller

CouplingTransformer

VSCStorage device

AC system bus

Vs˂0

DSTATCOMLoad

I

7

The VSC converts the dc voltage across the storage device into a set of three-phase ac outputvoltages. These voltages are in phase and coupled with the ac system through the reactance ofthe coupling transformer. Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchangesbetween the D-STATCOM and the ac system. Such configuration allows the device to absorbor generate controllable active and reactive power. The controller of the D-STATCOM is usedto operate the inverter in such a way that the phase angle between the inverter voltage and theline voltage is dynamically adjusted so that the D-STATCOM generates or absorbs thedesired VAR at the point of connection.

The VSC connected in shunt with the ac system provides a multifunctional topology whichcan be used for up to three quite distinct purposes: voltage regulation and compensation ofreactive power, correction of power factor, and elimination of current harmonics.(DSTATCOM & DVAR Simulation)

Consider a DSTATCOM connected to the system using a Y-Y coupling transformer. Thesystem voltage is Vs and the output voltage of the DSTATCOM is Vi. The instantaneousvalues of the system phase voltage are given by:

Vsa = √2 VssinωtVsb = √2 Vssin(ωt - 2π/3) (1)Vsc = √2 Vssin(ωt + 2π/3)

where Vs is the rms value of system phase voltage.

The rms line to line voltage output of the three phase inverter is (√3/(2√2))Vdma, where Vd isthe voltage of the DC-link capacitor and ma is the amplitude modulation index. [16]Therefore, the output voltages of the DSTATCOM referred to the primary side of thetransformer can then be given by:

Via = √3/2.n.maVd (sinωt + θ)Vib = √3/2.n.maVd (sinωt + θ - 2π/3) (2)Vic = √3/2.n.maVd (sinωt + θ + 2π/3)

where n is the transformer turns ratio.

To illustrate how a DSTATCOM mitigates voltage dip, consider a simple equivalent circuitshown in Fig. 3.

Fig.3 Simple equivalent diagram illustrating voltage dip mitigation

Source ῀ Load

VL

Ish

8

Assume that the system voltage is 1 pu under normal conditions. Then, voltage dip occurs andvoltage of the system without the DSTATCOM is

Vdip = Vcosφ + jVsinφ (3)

By injecting current from the DSTATCOM, the voltage is brought back to 1 pu. The requiredchange in voltage from the DSTATCOM is

∆V = 1 - Vcosφ – jVsinφ (4)

Current injected by the DSTATCOM is numerically equal to

Ish = P – jQ (5)

where P is the active power flow from the DSTATCOM and Q is the reactive power flowfrom the DSTATCOM to the system.

∆V = Ish.Z = (P – jQ)(R+jX) (6)

where Z is the equivalent impedance seen by the DSTATCOM.

Combining equations (4) and (6), the active and reactive power injected by the DSTATCOMare given by,

P = (R(1 – Vcosφ) – VXsinφ)/(R2 + X2) (7)Q = (RVsinφ + X(1 – Vcosφ))/(R2 + X2) (8)

2.3 Dynamic Voltage Restorer

Dynamic Voltage Restorer (DVR) is a voltage controller having the same building blocks as aDSTATCOM but its coupling transformer is connected in series with the ac system as shownin figure 4. The resulting voltage at the load bus bar equals the sum of the grid voltage and theinjected voltage from the DVR. The converter generates the reactive power needed while theactive power is taken from the energy storage. The energy storage can be different dependingon the needs of compensation. [14]

Fig.4 Standard configuration of a DVR

9

jXshRshVsh

Fig.5 Schematic diagram of a DVR

Figure 5 shows the schematic diagram of a DVR. The thevenin equivalent circuit of thesystem is shown on the left side of the DVR. The system impedance Z

thdepends on the fault

level of the load bus. When the system voltage (Vth

) drops, the DVR injects a series voltage

VDVR

through the coupling transformer so that the desired load voltage magnitude VL

can be

maintained. The series injected voltage of the DVR can be written as,

VDVR

= VL+ Z

thIL

- Vth (9)

whereV

Lis the desired load voltage magnitude,

Zth

is the load impedance,IL

is the load current , andV

this the system voltage during fault condition.

The load current IL

is given by,IL= ((PL+jQL)/VL)* (10)

Considering VL as a reference, equation (1) can be written as

VDVR

‹α = VL‹0 + Z

thIL‹ ( β-φ) - V

th‹ θ (11)

Here α ,β and θ are the angle of VDVR

, Zth

and Vth

, respectively, and φ is the load power factorangle, where

φ = tan-1

(QL/P

L) (12)

The complex power injection of the DVR can be written as,SDVR=VDVR IL*

(13)

Voltage sourceconverter

Energy storage

DVR

Vdvr

VL

PL+jQL

Is

Vth Rth jXth

10

If the injected voltage VDVR

is kept in quadrature with IL, no active power injection by the

DVR is required to correct the voltage. It requires only the injection of reactive power whichis generated by the DVR itself. DVR can be kept in quadrature with I

Lonly up to a certain

value of voltage dip and beyond which the quadrature relationship cannot be maintained tocorrect the voltage dip. For such a case, injection of active power into the system is essentialwhich must be provided by the energy storage system of the DVR.[14]

2.4 Unified Power Flow Controller

The Unified Power Flow Controller (UPFC) is used to control the power flow in atransmission system by controlling the impedance, voltage magnitude and phase angle. Thebasic structure of the UPFC consists of two voltage source converters (VSCs); where oneconverter is connected in parallel to the transmission line through a shunt transformer whilethe other is in series with the transmission line through a series transformer. Both VSCs areconnected to each other by a common dc link including a storage capacitor. The shunt inverteris used for voltage regulation at the point of connection injecting reactive power into the lineand to balance the real power flow exchanged between the series inverter and the transmissionline. The series inverter can be used to control the real and reactive line power flow insertingvoltage with controllable magnitude and phase in series with the transmission line. Thereby,the UPFC can fulfill functions of reactive shunt compensation, active and reactive seriescompensation and phase shifting. [17]

3

0

Fig.6 Basic Configuration of a UPFC

VSC VSC

Is Ir

V1Vs V2Vr

XsXr

Shuntconverter

Seriesconverter

11

3. Simulations and Results

3.1 Simulation circuit

The simulation was done using PSCAD/EMTDC software. A simple distribution system isshown connected to a DSTATCOM in Fig3.1. The supply side is represented by a Theveninequivalent circuit. The distribution voltage level is 11 kV. Two loads, which are designated asload 1 and load 2 are connected to the distribution bus. The DSTATCOM is connected to thesecondary of a Y-∆ coupling transformer. The transformer has a primary voltage of 11 kV anda secondary voltage of 2 kV. The DSTATCOM circuit consists of a three phase invertercircuit with six IGBTs and six diodes used for commutation. The sizing of the DC storagecapacitor is an important part of DSTATCOM design. In this project, the size of the DCcapacitor was not obtained mathematically. But a capacitor size of 300 μF gave good results.

RL

0.1 [ohm]#1 #2

g1

g2

g3

g4

g5

g6

21

300.

0 [u

F]

23

25

24

26

22

ABC->G

TimedFaultLogic

45.0

[ohm

]0.

16 [H

]

150.0 [ohm] 0.5 [H]

VA

35.0 [ohm] 0.2 [H]

TimedBreaker

LogicOpen@t0BRK1

BRK1

System Bus

DSTATCOMLoad 1

Load 2

Fig.7 Simulation Circuit for DSTATCOM

3.2 Control CircuitA control circuit that will generate the required firing pulses is needed so that theDSTATCOM provides the necessary voltage support during voltage dip condition. Thevoltage control circuit used is shown in figure 8.

The actual rms per unit voltage of the distribution bus is measured and provided to the controlcircuit as Vpu. Then a maximum of Vpu and 0.1 is taken. i.e. if the voltage of the bus is too

12

Vref

D +

F

-

I

P

TconstPgain

TIME *

0.1

*57.29578G1 + sT1

1 + sT2

Vpu

Shft

PI parameters

1

0

Tconst

0.1

2

0

Pgain

1.14

1.5

0

Vref

0.9pu

MaxD

E

low, a fixed value of 0.1 is taken. The low pass filter avoids unnecessary high frequencycomponents in the input. The reference voltage Vref is specified as the desired voltage of thebus. For the first 0.1 s, it is actually a ramp voltage so that large error is not generated until thesupply voltage builds up to the nominal level. The error signal, which is the differencebetween the measured and reference voltages, is fed into the lead lag circuit. The PI controllerthen gives the required phase shift that will minimize the error. This is changed to degrees andprovided as the output Shft. The proportional gain and the integral time constant of the PIcontroller need to be adjusted to obtain good results.

Fig.8 Voltage Control Circuit

Sinusoidal Pulse Width Modulation is used to generate the firing pulses that drive the IGBTs.For SPWM, sinusoidal modulating signal and triangular carrier signal are required. The circuitused to generate the sinusoidal signals is shown in fig. 9. The PLL gives six outputs, whereeach output is a linear ramp varying between 0 and 360 degrees. These are shifted by theangle obtained from the voltage control circuit (Shft) plus 30 degrees, which is the phase shiftbetween the primary and secondary of the transformer. Then, the sinusoidal signals forturning on and off of the IGBTs are obtained using a standard trigonometric function. Thevalue of amplitude modulation ratio (ma) used is 1.0.

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Va

Vb

Vc

PLLSix

Pulse 6thetaY

Vnc

Vnb

Vna

45

61

23

1234

56

D +

F

-

30.0

Shift:

66(in-sh)in

sh

SinOn

SinOffSin

Sin

Shft

Fig.9 Sinusoidal waveform generator

Triangular carrier waveforms are generated from the circuit in figure 10.The value offrequency modulation ratio (mf) used is 33.0. It was taken to be high enough to avoidharmonics and it should be a multiple of 3. The proportional and integral gains of the PIcircuit inside the PLL have to be adjusted to get optimum results. The nonlinear transfercharacteristic component is used to generate the triangular waveforms needed for turning onand off the IGBTs.

GiPLLGpPLL

Va

Vb

Vc

PLL theta

Vnc

Vnb

Vna

*

33.0

1234

56

1234

56

TrgOn

TrgOff

AngleResolver

PLL Cntrl

100

0

GpPLL

50

1000

0

GiPLL

500

1 2 3Vna Vnb Vnc

Vn

Fig.10 Triangular Wave Generator

In the generation of the above waveforms, care should be taken so that IGBTs on the same legdo not turn on or off at the same time. This is the reason for interchanging the signal wires infigures 9 and 10. It can also be seen in the switching pulse diagrams of figure 13.

The interpolated firing pulses component takes the sinusoidal and triangular waveforms andgives the required firing pulses. These are shown in figure 11. The DSTATCOM is blockedinitially for 0.1 s until the system reaches steady state operation.

14

TIME

g1

g4

g6

g3

g2

g5

Dblck

6

6

6

6

L

H

HON

OFFL

(1)

(4)

(5)

(6)

(2)

(3)TrgOn

TrgOff

SinOn

SinOff

1

Fig.11 Firing Pulse Generator

3.3 ResultsFirst, simulation was carried out without connecting a DSTATCOM. Voltage dips occurringdue to short circuit fault and increase in the load were simulated. Initially load 1 is connectedto the bus while load 2 is not. Short circuit fault starts at 1 s and goes on for 1.5 s. After thefault is cleared, the load of the system is increased by closing the breaker at 3.5 s andconnecting load 2. The resulting pu rms line to line voltage of the system is shown in figure12. It can be clearly seen that both short circuit fault and load increase cause voltage dip. Forboth cases the voltage goes down from 0.90 pu to 0.69 pu.

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 ... ... ...

0.000.100.200.300.400.500.600.700.800.901.001.101.20

Volta

ge (p

u)

RMS Voltage

Fig.12 System voltage during voltage dip (without DSTATCOM)

15

The DSATCOM was then connected to the system and simulation carried out using the samesettings. The firing pulses that drive the IGBTs were obtained from the control circuit and are,partly, shown in figure 13.

Switching Pulses

0.900 0.925 0.950 0.975 1.000 1.025 1.050 1.075 1.100 ... ... ...

0.000.200.400.600.801.001.20

y

G4

-0.200.000.200.400.600.801.001.20

y

G6

-0.200.000.200.400.600.801.001.20

y

G2

Switching Pulses

0.900 0.925 0.950 0.975 1.000 1.025 1.050 1.075 1.100 ... ... ...

-0.200.000.200.400.600.801.001.20

y

G1

-0.200.000.200.400.600.801.001.20

y

G3

-0.200.000.200.400.600.801.001.20

y

G5

Fig. 13 Switching pulses of the DSTATCOM

The per-unit rms voltage of the bus when the system is connected to a DSTATCOM is shownin figure 14. The simulation shows that the DSTATCOM can successfully mitigate thevoltage dip within about 0.2s. Therefore, the voltage of the system bus is kept at the desiredlevel of 0.9 pu.

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 ... ... ...

0.000.100.200.300.400.500.600.700.800.901.001.101.20

Volta

ge (p

u)

RMS Voltage Vref

Fig. 14 Voltage dip mitigation using DSTATCOM

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4. ConclusionIn this project the concepts of smart grid and power quality have been studied and explained.It has been found that there are currently numerous studies, researches and projects with theemphasis of transforming the existing grids into more advanced smart grids. However, there isno global standard as to what exactly a smart grid is and what its constituents and features are.

Power quality will be of major concern in smart grid of the future. Active power qualitycontrollers based on power electronic converters and advanced control circuits will beindispensable in ensuring the quality of power supply in the near future. This report focusedon voltage dips, which are the most detrimental power quality problems. A DSTATCOMcircuit and its control were designed. While simulation, designing the control strategy forDSTATCOM was an important aspect. The control of flow of reactive power to and from theDC capacitor is done by PI controller. The control circuit includes Phase Lock Loopcomponents to generate switching signals i.e. triangular waves and sinusoidal waves. Toswitch on and off the IGBT’s PWM switching control is used. IGBT’s are used because it iseasy to control the switch on and off of their gates. From the simulation results, theDSTATCOM responded well in mitigating voltage dips caused by a three-phase balancedfault and load fluctuation. This can be clearly seen by comparing figures 12 and 14. Inconclusion, by doing this project, we learned how to simulate the power electronics devices inPSCAD. Though we have used this simulation tool for the first time and we had littletheoretical background in the subject matter, the results obtained are satisfactory.

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References

[1] M. Amin & J. Stringer, “The Electric Power Grid: Today and Tomorrow”, MaterialsResearch Laboratory Bulletin, vol. 33, Apr. 2008,http://www.mrs.org/s_mrs/bin.asp?CID=12527&DID=206424, (accessed Oct. 2010).

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