5-2014

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Int. J. Engg. Res. & Sci. & Tech. 2014 ATIGO Kossi Chch, 2014

ISSN 2319-5991 www.ijerst.com

Vol. 3, No. 2, May, 2014

2014 IJERST. All Rights Reserved

Research Paper

DFIG WTG PLANT ELECTRICAL FAULTS ANALYSIS

AND THE ROTOR SIDE CONVERTER OVER-

CURRENT SOLUTION APPROACH

ATIGO Kossi Chch1*

*Corresponding Author: ATIGO Kossi Chch, [email protected]

With the rapid increase of the modern high power wind plants all over the world, a new problemassociated with the response of wind turbines to temporary low-voltages has arisen. Today, thisproblem has become the topic of discussion because of the stability of the power grids at whichthese farms are connected. A majority of wind turbines use voltage source converters with aDC-link, especially those using Doubly Fed Induction Generators (DFIG). When the grid voltageexceeds a certain limit, the current flowing through the converters critically increase, resulting inan instantaneous release of protection equipments to avoid the destruction of the power systeminstallations. One of the widely used protection system in DFIG plant is the crowbar systemwhich when triggered satisfies the over-current problem, but is not enabled to feed the plantwith the required reactive power as imposed in the new (FRT) requirements. To handle suchsituations, special countermeasures are required. This paper identifies and outlines the problemand analyzes a possible measure to ride through the low voltage safely. Additionally, fault currentflow level control is announced without being developed.

Keywords: DFIG, WTG, Wind power plant, Voltage source converter, Doubly fed inductiongenerator, FRT

INTRODUCTION

The renewable energy (hydro, solar, wind,geothermal, bio-power, ocean) represents about21.7% of the worldwide global electricityproduction. According to the Global Wind EnergyCouncil (GWEC), a total of 282,587 MW wasinstalled in 2012 representing about 60% of thetotal renewable energy, excluding hydropower [1].The instability of these plants due to the windflow and the used technology constitute a majorproblem in grid exploitation; also the faults in thegrid influence the plant due to critical variation inthe current, voltage and the torque. Therefore,

1 Master Student in Power System and its Automation, Department of Energy & Electrical Engineering, Hohai University, NanjingChina.

many requirements are imposed on the plantenergy production like fault ride throughrequirements for grid-connected wind plant. Manyprocedures are being used to solve this problembut the efforts are often half rewarded. For a plantusing Doubly Fed Induction Generator (DFIG), asystem called crowbar is often used but this doesnot satisfy 100% of the requirements and is notwithout a negative consequence on the installedelectronics. In this paper, we analyze some criticalfaults of the DFIG wind plant and present a simpleapproach to ride through a fault causing an over-current in the rotor circuit. During a fault, the


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disconnection of the wind power plant helps inprotecting the incorporated power electronicsfrom damage due to thermal overload; howevera quick disconnection is not desirable as it canprovoke the release of other plants in the gridand lead to loss of large amount of energygeneration by cascading effect. The WPP (WindPower Plant) must support the voltage in case ofthree-phase short circuit in the grid. If the fault isclose to the WPP, there should neither beinstability of the WPP nor disconnection from thegrid for voltage-time values exceeding the boldline (see Figure 1). This procedure called FaultRide Through (FRT) generally known as LowVoltage Ride Through (LVRT) and High VoltageHigh Through (HVRT) is now almost a mandatoryrequirement in many grid codes for modern grid-connected WPP. The voltage level and the FRTtime delay is not the same for all the grid codes.

Figure 1: LVRT Requirement for Some Grids

In zone 1, active power generation capacitymust be recovered after the indication of thefault and must be increased at a gradient of20% of the rated power per second.

In zone 2, instead, a short disconnection ofthe WPP from the grid is allowed; however,a quick resynchronization should follow afterthe fault clearing and it must return to thepre-fault standard generation conditions sothat the outage lasts for not more than 10s.Therefore, the plant must return to supplyingactive power within 2s after the fault is clearedat a gradient of 10% of the rated active powerper second.

If the short circuit is far from the plant, itshould not be disconnected from the grid

because fault is generally eliminated by thenetwork protection in 5s. [2]

IMPLEMENTATION OF

OUR SYSTEM

We will simulate a grid side fault with Matlab DFIGdetailed model [3].

During an over current fault it is used to blockout the switches of the IGBT of the rotor sideconverter and used a crowbar system to drivethe current between the crowbar resistor and therotor windings. When the contacts of theconverter IGBT are opened, a very high voltageappears at their pins and they can be damaged[4]. And when all the short-circuit current needsto be directed into the crowbar, a specialdimensioning is necessary. Our idea is to fix thecurrent in the converter and lead the rest to thecrowbar reducing the dimension of the latter.

As meant above, when the circuit is opened,the voltage is so high, and according to the gridrequirements, during a low voltage problem, theplant must still be connected to the grid at leastfor a fixed time and be able to produce a reactivepower to support the grid as indicated in LVRTrequirements [4].

Principle: Fix the rotor side current at amaximum value when the rotor is in over-currentsituation. A resistor will be put in the DC circuitduring the fault to limit the DC bus current thuslimiting the current in the rotor side convertor.When the fault occurs, the DC side resistor andthe crowbar are triggered (Figure 2).

To evaluate the resistances, we will simplifyall the computing (Figure 3).

Figure 2: Current Dividing System with DCSerial Resistor and Crowbar System

Dr.HebaHighlight


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Figure 3: Resistors Computing System

For the rotor side converter circuit

From (1):

...(1)

...(2)

...(3)

...(4)

With Equation (4) we can determine a value forRDC, with I < Itmax, the rotor maximum current in

the steady case, and Vr= Vrfault. Vrfault is theapproximate value for the rotor voltage during thetransient fault.

From Equation (4), we have

...(5)

At the point A,

For the crowbar circuit:

...(6)

...(7)

...(8)

During the fault Ir =Irfault the rotor current duringthe fault, RDC the admitted resistance for the DCbus, Vr=Vrfault

We can then get the curve R(Ir).

Figure 4: Variation of the Crowbar Resistancein the Function of the Rotor Current

VR+ VC=VDC

VDC= Vt cos

VR=RDC*IDC

Ic= IDC

Vt=m*VrIt=m*Ic

m=r

t

VV

RDC* = *m*Vr cos VC

and

It= ( *m*Vr cos VC )

RDC= *( *m*Vr cos VC*m*Vr- VC)

RDCmin ( *m*Vrfaultcos VC )

Vr=VCw

VCw=R*Ij

Icw= Ij

R= * * Vr=


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We will use Matlab DFIG detailed model for oursimulation. We can observe that the voltage ofthe rotor during the fault when the converter isconnected through a resistance and the crowbaris triggered, is about 2 times the nominal value.

Vrfault=2Vr= 1150V and Vc= 1150V.

Estimation of the current:

The power of the plant is 1.5MW,

The rotor nominal voltage is 575V

If we suppose that the rotor is dimensioned for60% of the power during the fault, and if cos =0then the nominal current in the rotor is:

If we fix the current of the rotor circuit at Ir=1500A,we have the converter current:

Ic=1500*23/79=437A

Note: The value of the rotor circuit and theconverters depend on the dimensioning. Here wejust fix a theoretical value for our simulation. Thereare IGBTs which support more than 3000A, thevalue of Ir will depend on the project requirements(cost and power).

With Equation (5),

Let us analyze the rotor current during the fault:

From 0.11S the current has a pick of 5 times, at0.12S it decreases to 3 times and stays at 1.8times from 0.14S till the end of the fault. WithIr=1500A, we have the following table

T(s) 0.201 - 0.21 - 0.22 -

0.21 0.22 0.501

R() 0.344 0.592 1.1

Table 1: Variation of the ResistanceDuring the Fault

For our simulation, as the model of the windplant is not built by us, we do not have all theparameters and nominal values. We also do notchange any of the parameters of the originalsystem (Matlab DFIG wind plant detailed model),except removing the fault which initially wasintroduced in the grid. We simulate a new fault inthe stator circuit to make high current flows inthe rotor circuit. The results of our simulationsare as shown in the following figures. The durationof the simulation is 5s with 3s delay time for faultsimulation:

As the requirement for most of the grids isaround 2s delay for fault simulation, we willextend our fault delay to 3s with a 5ssimulation time. The results are as follows:

Ir=Itmax= *60%=1565A,

m=

RDCmin ( *m*Vrfault*cos VC ) and

cos= * = = 0.43, let fix

cos = 0.4

Rdc 1.84

Let fixe RDC=2 , compute the new

It= ( *m*Vrfault cos VC )

It= = 1379.5A

= 2234

tr

r

II

V

= 22 5.1379115034

rI

R =

= 22 5.1379

2536

rI


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Figure 5: Stator Voltage and CurrentWith 3s fault simulation, we can see goodamelioration of voltage and current in the statorcircuit.

Figure 6: Grid Converter Voltage and Current


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The current in the grid converter which was3times the steady value is ameliorated to 1.75times.

Figure 7: DC Bus Voltage

The DC voltage during a fault oscillatedbetween 1100 and 1200V with starting peakvoltage of 1350V; with our system, the peak islow and the voltage oscillates between 1120 and1180V.

The problem of the rotor voltage remains; it ispractically 2.5 times the steady value. Howeverthe over-current is corrected a little-1.8 timesinstead of 2 times.

Our dimensioning had a limit because we donot have all the characteristics of the model weused, because it was not built by us due to ourlimited knowledge in model-building.

With 3s fault simulation, the power producedwhen the system is triggered is about the halfwithout the system during the fault; however, thereactive power is produced instead of consumingwithout the system application during the fault.

Figure 8: Rotor Converter Voltage and Current


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SUMMARY

The DFIG system applied to wind powergeneration has gained considerable academicattention and industrial application during the past10 years and is still the actual topic in view of thegrowth in wind power in the worldwide energygeneration every year. In the present situation,the wind power is one of the best solutions topalliate the dependence on fossil energies;however, with transient faults, wind power is stillthe black beast for the research field. In our work,we have presented the fault ride throughrequirements and give an approach to ridethrough a low voltage for existing DFIG windturbine. Even though this is not a full solution, itcan lead to deep practical researches. Our workwill continue by implementing a variableresistance in the crowbar system.

BIBLIOGRAPHY

1. Matlab wind farm, DFIG detailed modeldescription.

2. PhD thesis, Conception globale desgnrateurs asynchrones double

Figure 9: Crowbar Voltage and Current

Figure 10: Active and Reactive Power


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alimentation pour oliennes, DAVIDEAGUGLIA, FACULTE DES SCIENCES ETDE GENIE UNIVERSIT LAVAL QUBEC.

3. REN21 renewable Energy policy Network for

the 21st Century, Renewable 2013 Globalstatus report.

4. Wind power technology, Joshua Earnest,ISBN-978-81-203-4778-6.

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