performance of dfig during symmetrical and...

Performance of DFIG during symmetrical and

asymmetrical grid faults with damping controller

based SSSC

D.V.N.Ananth1, G.V.Nagesh Kumar2, D.Deepak Chowdary3,

K.Appala Naidu2 1DADI Institute of Engg. & Technology, Anakapalli,

Visakhapatnam, Andhra Pradesh, INDIA, [email protected],

ph: +91-8500265310 2Vignan’s Institute of Information Technology, Visakhapatnam,

Andhra Pradesh, INDIA, [email protected] 3Dr. L. Bullayya Engg. College for Women, Visakhapatnam, Andhra

Pradesh, INDIA

[email protected]

Abstract

The renewable energy resources like wind with doubly fed induction generator (DFIG) is

playing a vital role in meeting the ever-increasing load demand. Most of the industrial and

commercial loads are sensitive to fault, as surge current damages the system. The DFIG

wind turbine set is very sensitive when grid fault occurs, which damages the stator and

rotor winding and also the converters and the capacitor. To overcome these effects, DFIG

grid connected system is equipped with damping controller based static synchronous series

compensator (SSSC) based series FACTS device with a new control scheme for oscillations

damping and quicker voltage injection technique. Voltage damping circuit is provided in

the outer control loop of SSSC for improving voltage profile of stator and rotor. The inputs

for the damping circuit are rotor speed and stator real power and controller is designed

with cascaded 2nd order lead-lag compensator. The results are presented for single line,

two lines and three lines to ground faults and system behavior is examined.

Keywords: DFIG, static synchronous series compensator (SSSC), damping lead

lag compensator, symmetrical and asymmetrical faults

1. Introduction With the latest trends in renewable energy resources, wind turbine based power

generation is getting importance as conventional synchronous generator based

power plants are not alone sufficient to convene with the ever growing load

demand. The DFIG based wind generators are getting popular as real and reactive

power sharing, load withstanding capability, low cost converters are better than

other wind generators. Faults are inevitable for any power system and are very

dangerous for DFIG wind turbine system [1]. Based on this paper, if fault current

is not controlled, the inrush current will damage the converters, dc capacitor, stator

and rotor winding. Crowbar type of protection is used to divert the fault inrush

International Journal of Pure and Applied MathematicsVolume 114 No. 9 2017, 125-135ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

125

current, thereby damage can be prevented [2-4]. This type of protection has a major

drawback that, it will draw huge reactive power from the grid as the DFIG now

runs like squirrel cage induction generator. Therefore many recent works deal with

new control strategies to overcome different types of faults. Improved

demagnetizing, feed-forward transient current control, current-reversely-tracking

control (CRTC) etc [5, 6] are recently proposed techniques for DFIG to improve

fault ride through and the authors are almost successful.

New controllers like PI + resonant (PIR) or P+ Resonant (PR), internal model

control (IMC) [7], sliding mode control (SMC), fuzzy and many are used to improve

the speed of operation instead of conventional PI controller. Metaheuristic control

techniques like bacterial forging, particle swarm optimization etc techniques are

also used for tuning of PI parameters under different fault conditions. Feed forward

regulator, magnetization current compensation, LQR, impedance based high

frequency resonance etc are new techniques to improve performance when DFIG

tied to a week grid.

The FACTS devices like dynamic voltage restorer (DVR), STATCOM [8, 11],

UPFC, dual STATCOM [9], fault current limiters (FCL) [10], energy storage

devices like SMES are extensively used in the literature to overcome any type of

faults and to have better performance of DFIG. Among all these devices, DVR,

STATCOM and FCL are more promising and help in maintaining nearly flat

profile during severe faults. In this paper, static synchronous series compensator

(SSSC) is used to overcome different types of fault occurring near the grid. In this a

24 unit DFIG equivalent grid connected system is considered. The performance of

the DFIG under single line, two lines and three lines to ground faults are observed

with SSSC using MATLAB/ SIMULINK.

2. Rotor and grid side converter design The direct and quadrature (d and q) axis DFIG rotor voltage equations and the

rotor and stator windings double d and q axis fluxes are given by

( )

( )

( )

( ) ( ) ( )

(1)

From the basic equations of DFIG [9], the rotor direct and quadrature axis

voltages are expressed as

(

)

(2a)

(

)

( ) (2b)

where is synchronous speed and is rotor speed.

The block diagram of RSC in Fig.1a is based control circuit for better

performance under LVRT problem. The sub-circuit is EFOC technique using the

equations 2a and 2b. These equations if rewritten as decoupled parameters as in

equations 3a and 3b are developed for RSC controller.

International Journal of Pure and Applied Mathematics Special Issue

126

( ) (3a)

( ) (3b)

The rotor speed in general is and the stator synchronous speed.

However, this is varied from to a new synchronous speed during abnormal

conditions is or simply At ideal situations,

which is reference stator d-

axis vector flux is zero in magnitude and q-axis flux . These holds good for stator

flux magnitude at a given back emf and the rotor speed. The rotor dq axis

transient current are represented in the equations 4a and 4b as

(4a)

(

)

(4b)

The rotor reference voltages in two axis Park’s dq transformation is rewritten

with the help of equations 3a and 3b are given below. This is the output rotor

windings voltage during normal and transient conditions are

(

(

) ) (5A)

(

(

) ) (5B)

The direct (d-axis) and quadrature (q-axis) axis are two axis rotating frame of

reference. Now again, the stator d axis and q axis currents in equations are written

in terms of stator voltage and rotor currents

dr

s

mdr

s

m

s

dsds i

X

X

Xs

Vsi

L

L

Li

(6a)

,qr

s

mqs i

X

Xi

(6b)

The grid two axis voltage in GSC current, voltage, grid resistance and

inductance forms as

1dgqgs

dg

dggdg VLidt

diLiRV

(7a)

1qgdgs

qg

qggqg VLidt

diLiRV

(7b)

The dynamic dc voltage across the dc link capacitor is given in equation (8) as a

function of grid side converter (GSC) and rotor side converter (RSC) and dc link

voltage.

dc

rgdc

CV

PP

dt

dV

(8)

So, the change in DC link voltage across the capacitor depends on rotor voltage

and stator voltage. Hence based on the above discussion, the GSC and RSC control

schemes are developed which is explained in the next section. It is observed that if

rotor voltages are controlled effectively using proper control technique and PWM

operation, the rotor current flow is controlled. When the stator voltage magnitude

increase or decrease from normal value, consequently the rotor currents will be

affected and vice-versa. This means, if there is a decrease or increase in the stator


127

voltage, if rotor currents are adjusted, the stator voltage magnitude can be

improved. In the similar way, with d and q axis control of rotor voltages, the

electromagnetic torque (EMT) can be controlled. Also, it is observed that, the stator

voltage is like a quadratic function with square of voltage term and a single stator

voltage term. If stator voltage increase or decrease suddenly, it leads to an

oscillations and change in magnitude respectively in the EMT. The leakage factor

can be stated as .

3. RSC and GSC design and operation The RSC refers to rotor side converter and GSC is for grid side converter. The

RSC operation is to manage and preserve the speed of rotor during abnormal

conditions like faults thereby stator frequency connected to grid must not deviate

by using d-axis current control scheme. It also helps in regulating the grid reactive

power using q-axis current control method as in Fig. 1a. The outer loop of RSC

consists of speed and reactive power control loops and inner control loops has d and

q two axis current control loops. The reference speed of rotor is derived using

lookup table method from the wind turbine optimal power output PmOpt and grid

power demand. Based on the value of Pm,gOpt, rotor is rotated at optimal speed to

draw optimal (maximum) power from DFIG WECS set. The Speed error is

minimized with PI controller to zero value and the output is the product of stator

flux (Fs) and ratio of stator and rotor inductances (Ls and Lr) to obtain reference

rotor q-axis current (Iqr*). The output from each PI controller is controlled with a

disturbance voltage VdqR to get reference pulse generation voltage. It is to note

that, the pulses are synchronized at slip frequency of RSC rather at fundamental

nominal grid frequency. The synchronizing slip frequency is converted back using

inverse Park’s transformation to get abc stationary rotor reference PWM voltage

parameters as in the figure.

Fig. 1a Rotor side converter (RSC) control scheme for DFIG,

rs

m

LL

L2

1


128

Fig. 1b Grid side converter (GSC) control scheme for DFIG

The GSC basic block diagram is depicted in Fig.1b. For a given wind speed, the

turbine reference control power is predictable with a lookup table. The stator real

power (Pstator) is calculated and the power error is the difference between two

powers (dP) which are preserved near the zero value by using PI controller. The PI

controller output is multiplied with constant (Kp) called real power constant gives

actual convenient power after interruption. The difference in square of reference

capacitor voltage across dc link (Vdc*) and square of actual dc link voltage (Vdc) is

[Vdc*-Vdc] controlled using PI controller to get reference controllable real power.

The error in the reference and actual controllable power is divided by using 2/3Vsd

to get direct axis (d-axis) reference current near grid terminal (Igdref). Difference in

Igdref and actual d-axis grid current is controlled by PI controller to get d-axis

voltage. But to achieve better response to transient conditions, decoupling d-axis

voltage is added as in case of separately excited DC motor. This decoupling term

helps in controlling steady state error and fastens transient response from DFIG

during low voltage ride through (LVRT) or during sudden changes in real or

reactive powers from/ to the system.

The block diagram of GSC control circuit is shown in Fig. 1b and RSC is

designed to get better performance for LVRT issues at (PCC) point of common

coupling. During normal conditions like steady average wind speed and good

ambient temperature, the reactive power will be zero or very low and hence stator

power pumped to the grid will be high. This power control can do use the outer

control loop of GSC. The reference power is obtained from the characteristic lookup

table with characteristics of DFIG adopted for desired operation. This reference

power is compared to actual power and is maintained using the PI control of GSC

as shown in Fig.1. During faults, the stator power varies based on the reactive

power demand, which will is supplied by GSC through the capacitor at the back to

back converters. As reactive power demand increases, stator power changes

accordingly, and hence the terminal voltage at GSC change respectively and

thereby direct axis current injecting at PCC changes. During steady sate, stator

rms voltage and reactive power are constant. But when the fault occurs, the stator

voltage changes, hence reference rms stator voltage changes. This will make the

quadrature component of GSC current to vary. This total mechanism is fast and

can work for symmetrical as well as asymmetrical faults.


129

4. Design of FACTS and energy storage devices

for LFC The FACTS and energy storage devices are being used in power system for

many applications like voltage mitigation, power quality improvement, power

transfer capability improvement, power oscillations damping, frequency regulation

etc. Among many FACTS devices SSSC is an excellent series FACTS device used

for real and reactive power control. Voltage stability will be improved with reactive

control and frequency control is with real power control. The block diagram of

SSSC is shown in Fig.2a and with transfer function based control design in Fig. 2b.

The SSSC produces three phases voltage in quadrature with the line current follow

an inductive or capacitive reactance based on the current flow in the transmission

line. The magnitude and polarity of Vq decides the compensation to be inductive or

capacitive to stabilize the frequency and real power deviations during wind speed

or load change.

Fig. 2a Block diagram of DFIG with SSSC device

Fig. 2b transfer function based control of SSSC

5. Result Analysis For the DFIG network in Fig.2a, the simulation results are presented in this

section. A fault is situated near the PCC at 0.3s and cleared at 0.5s. The behavior of

DFIG system and the compensation of SSSC for different faults under cases three,

double and single phases to ground are analyzed.

5.1 Case A: Three phases to ground or symmetrical fault


130

For the symmetrical fault at PCC with 0.002Ω resistance between 0.3 and 0.5,

the waveforms are given in Fig.3. The voltage at grid is decreased from 1pu to

0.5pu (per unit) and the current shrink from 1pu to 0.4pu as shown in Fig. 3a.

When the fault is suppressed at 0.5s, the voltage of grid raised to 1pu slowly than

the current with surge of 1.1pu. It is due to the fact that sudden change in load

impedance leads to current increase in the network. The similar behavior is

observed with stator as it is also linked to the same phases directly. The rotor

voltage and current are shown in the figure 3b. The rotor voltage is almost constant

and there is a small dip in the current from 0.55pu to 0.4pu. The RSC and GSC

helps in maintaining the voltage and SSSC helps in still maintain at a better

voltage profile by mitigating the surge currents. So, voltage and current profiles are

maintained with converters and SSSC.

Cur

rent

(p.u

.)

V

olta

ge (p

.u.)

Time (S)Time (s)

Cur

rent

(p.u

.)

V

olta

ge (p

.u.)

Fig. 3a Grid voltage (top) and current (bottom) in pu Fig. 3b rotor voltage (top)

and current (bottom) in pu for three phases to ground fault

Time (S)

Torqu

e (p.u

.) ro

tor sp

eed (

p.u.)

Cap

. Volt

(p.u.

)

Time (S)

Vol

tage

(p.

u.)

Fig. 3c dc caapcitor voltage, rotor speed and electromagnetic torque (EMT) in pu Fig. 3d SSSC injecting voltagein pu for three phases to ground fault

From the Fig 3(c), the capacitor voltage is nearly constant dc voltage magnitude

during and after the fault. The ripples decreased due to reversal of current in the

RSC and GSC towards rotor and grid PCC. The dynamics of these are given by the

equations (3a) to (8) as given in section 2. The rotor speed is almost constant at

1.2pu i.e., during the fault from 0.3 to 0.5s and maintained constant then at 1.22pu.

The electromagnetic torque (EMT) is initially at 0.6pu at 14m/s wind speed and

reached to 0.05pu during fault without ripples and when fault is cleared, the EMT

attained its pre-fault value. The oscillations in torque are damped because of the

RSC control scheme with better control strategy and with SSSC damping control

nature proposed in the paper. The SSSC injecting voltage is shown in Fig. 3(d). At

the instant of fault, the voltage dip will be very high and surge currents increases.

The dc capacitor voltage between the RSC and GSC of DFIG is increased. To


131

mitigate them all, voltage injection of SSSC has to be high as shown in this figure

3d. Then during the fault, because of voltage injection, the voltage profile of SSSC

is maintained at 0.28pu for compensation and then decreased to a smaller value

after the fault is cleared. The fifth order lead lag compensator helps to maintain

generator speed and active powers from oscillations. This is done when the inputs

are given command to the outer control loop of SSC. The inner control loops helps

in controlling the current parameters, thereby with proper tuning of PI controllers,

the voltage injection will be quicker and accurate. So, system performance during

three phase to ground fault is improved with proposed SSSC control scheme and

RSC and GSC control, schemes of DFIG.

5.2 Case B: Two phases to ground fault

The same system behavior with two lines to ground asymmetrical fault in

phases A and B is shown in Fig.4. It is observed that A and B phases voltage

magnitude decreased while phase C magnitude is having better voltage profile. But

voltage surges are produced in the faulty phases during the fault. The grid current

in two faulty phases increased and the healthy phase decreased to a smaller value

as in Fig.4a. such type of fault is less severe than symmetrical fault for

conventional synchronous generator plant. But, it is very dangerous for wind

generator and too for DFIG as it has low rating converter.

Curre

nt (p

.u.)

Vol

tage

(p.u

.)

Time (S)

Curre

nt (p

.u.)

Volt

age

(p.u

.)

Time (S)

Fig. 4a Grid voltage (top) and current (bottom) in pu Fig. 4b rotor voltage (top) and current (bottom) in pu for two phases to ground fault

Time (S)

Torq

ue (p

.u.)

rot

or s

peed

(p.u

.) C

ap. V

olt (

p.u.

)

Time (S)

Vol

tage

(p.

u.)

Fig. 4c dc caapcitor voltage, rotor speed and electromagnetic torque (EMT) in pu Fig. 4d SSSC injecting voltagein pu for three phases to ground fault

The rotor voltage and current with SSSC control scheme proposed is shown in

Fig. 4b. It is observed that voltage and current profile of SSSC are improved. The

voltage of rotor is almost constant and the current is having ripples without surges


132

and rise in magnitude and has harmonic nature during the fault. the sudden

change in flux do not decay instantly in the stator. The rotor mutual flux exchange

with oscillating and damping nature of RSC makes it to have the waveform like

this. If current limiting control wit faster decay control helps in improving the

profile but the control scheme becomes little complicated. However, the behavior is

better than the earlier system behavior without two times surge. In fig. 4c, the dc

voltage across the capacitor between the back to back converters is having small

oscillations between 1 and 0.9pu. The rotor speed is nearly constant at 1.2pu

during and after the fault. The EMT is having oscillatory nature with the two

phases to ground fault. It is not decaying to zero or reversing the polarity as with

conventional systems. The SSSC voltage injection is shown in Fig. 4d, the faulty A

and B phase voltages increased while the healthy phase voltage injection is like

normal value during and before the fault. Hence, voltage injection of SSSC is better

with the proposed control scheme.

6. Conclusion In this paper, DFIG grid connected system is analyzed with two and three

phases to ground fault in two cases. The performance is better with proposed RSC

and GSC control scheme and further improved with proposed SSSC control scheme

than with the literature survey. The voltage and current profile of rotor is improved

and dc voltage across the back to back converters is nearly constant without swell

during the fault. The EMT is having lesser swings than conventional system with

symmetrical or asymmetrical fault without changing the polarity of torque to

negative. The rotor speed is almost constant. The SSSC injecting voltage is quick

and accurate as damping controller based fifth order transfer function is newly

proposed in this paper. The RSC and GSC control schemes are proposed with

lookup table based technique to have better reactive power to rotor speed profile.

hence our proposed scheme is better than conventional control schemes with DVR

or SSSC arrangement with easier control scheme, faster and accurate and holds

good for symmetrical or asymmetrical fault with any decrease in the grid voltage.

Appendix The simulation parameters of DFIG used are, Rated Voltage = 690V, Rated

Power = 1.5MW, Stator Resistance Rs = 0.0049pu, , Stator Leakage Inductance Lls

= 0.093pu, rotor Resistance Rrӏ = 0.0049pu, Rotor Leakage inductance Llr1 =

0.1pu, Number of poles = 4, Mutual Inductance Lm = 3.39 pu, Inertia constant =

4.54pu, DC link Voltage = 415V, DC link capacitance = 0.2F, Grid Voltage = 25 KV,

Grid requency = 60 Hz. nominal wind speed = 14 m/sec. Grid side Filter: Lfg =

0.6nH, Rfg = 0.3Ω, Rotor side filter: Lfr = 0.6nH, Rfr = 0.3mΩ, wind speed

variations considered here in seconds: 8, 15, 20 and 10 at 15, 25 and 35s. variation

in grid voltage: 0.8 to 1 and to 1.2pu at 20 and 30s, Reactive power variation: -0.6

to 0 and +0.6pu at 20 and 30s.

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133

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