Two area load frequency control for DFIG based wind turbine system using modern energy … · 2018-03-15 · Two area load frequency control for DFIG based wind turbine system using

Two area load frequency control for DFIG based

wind turbine system using modern energy storage

devices

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

In this paper, energy storage devices like super conductor magnetic energy

storage system (SMES) and thyristor controlled capacitor storage phase

shifters (TCPS) and FACTS device like static synchronous series

compensator (SSSC) are used to damp oscillations in a power system. In

general, with increase in number of wind generators connected to grid,

penetration issues slowly increases. Due to this, if there is a sudden change

in load in one area, frequency deviation in all areas takes place which leads

to electro-mechanical oscillations in the system. To damp out these

oscillations tie-line based frequency controllers (TLFC) were generally used

for DFIG systems. Hence for effective damping of oscillations, SMES, TCPS

and SSSC are chosen for DFIG based wind turbine system. It is to find a

suitable device among TLFC, SMES, TCPS, SSSC to work in coordination

to control frequency regulation and tie-line power for area DFIG based

systems. Simulation results prove that oscillations damping can be effective

if coordinated SMES and TCPS or with SSSC installation in both areas are

better options.

Keywords: SMES, TCPS, SSSC, inter-area oscillations and damping, tie-

line power, TLFC, DFIG, wind generator penetrations, frequency control,

and automatic generation control.

1. Introduction

Now days, renewable energy resources like wind are getting importance as

conventional power plants are not alone sufficient to convene the rising 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

International Journal of Pure and Applied MathematicsVolume 114 No. 9 2017, 113-123ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

113

other wind generators [1]. The understanding of DFIG during load changes is very

important for improving the stability of the system [2-5]. These papers describe

that, due to sudden load changes, tie-line frequency and real power from DFIG and

conventional plants deviates from normal value. During this process, weak system

starts to deviates and will trip. This makes the next weaker generator to trip and

soon and finally leads to frequency collapse. Hence, load frequency control for DFIG

wind generator set is necessary similar to conventional power plants. In

synchronous generator system, governor control helps in frequency regulation. In

DFIG based system rotor and grid side controller plays a vital role in regulating

frequency and real power output from the stator.

Apart from load deviations, wind speed variations also affect the frequency

output of DFIG. For this, primary and secondary frequency regulators are adopted

to restore to normal values during perturbations [6-10]. Automatic generation

control (AGC) is recently demonstrated on different wind generators with droop,

deloading or inertia based as primary frequency control. Auxiliary power system

based control as secondary frequency control to achieve quicker response during

load deviations. Since decoupled active power control for DFIG is adopted

generally, electromechanical dynamics is very difficult during large frequency

deviations. This makes, DFIGs are very difficult to participate in frequency

regulation. The above primary and secondary frequency regulations are alone not

sufficient to restrict frequency regulation. Hence FACTS devices are helpful in

achieving quicker and better frequency stability and better load delivery to tie-

lines.

To overcome the effect of load variations, FACTS devices [11-15] like

thyristor based phase shift regulators (TCPAR) or static synchronous series

compensators (SSSC) or energy storage devices like battery or superconducting

magnetic energy storage system (SMES) are used. Coordination of FACTS and

energy storage devices are used recently for conventional power plants to reach

better frequency stability margin. These devices help in sharing real power

between the tie-lines and generators by controlling the generator phase angle

jumps during load deviations. Hence better results of frequency regulation,

maximum power transfer and enhanced reactive power support with power system

stability is achieved with coordinated FACTS and energy devices.

Hence in this paper, coordinated FACTS- FACTS and FACTS- SMES are

checked using MATLAB simulation to find the better device and coordinated device

for frequency regulation for DFIG based two area systems. The devices are

designed to provide negative oscillations during disturbance and thereby frequency

and real power oscillations are mitigated.

2. Area generation control modeling A. Two area power system model

The simulation diagram using transfer function of interconnected two area

power system is shown in Fig.1. It has a conventional steam based synchronous

generator and DFIG based wind energy conversion system in both areas. The

behavior of DFIG and a conventional system with sudden change in load in area is

investigated and frequency regulation plots are analyzed. PNC is added to the

system, as shown in Eq. (1):

International Journal of Pure and Applied Mathematics Special Issue

114

, (1)

where

,

Fig.1 MATLAB based DFIG system with basic primary, secondary and tertiary

control

B. DFIG Modeling

For wind generators based AGC with frequency regulation, the fixed-speed wind

turbines (WT) like PMSG and asynchronous generators circumvent from getting

maximum available power to preserve a reserve margin for regulation of frequency.

With the sophisticated control, stored kinetic energy (KE) in the wind turbine (WT)

mechanical system is extracted with DFIG using primary and secondary frequency

regulation techniques. The DFIG system produces power for irregular mechanical

WT speed and extracts KE for the primary frequency control. The power output

from DFIG is restricted as chosen by the operator. The factors affect the active

power sharing with the grid by DFIG are the wind speed, dynamic mechanical

power output control to a definite level with hoard mechanical energy like flywheel

etc. When the system frequency decreases, the torque set-point are increased, the

rotor speed decreases, and KE is unrestricted. PNC has two components: power

and frequency deviation using conventional inertial control and ΔPref* is

dependent on optimal turbine speed using wind speed is:

(2)

where Kdf is a frequency deviation derivative constant, and Kpf is the frequency

deviation constant, Δf is the frequency deviation after a high-pass filter. The

equivalent DFIG set improves the optimal speed formerly the transient in

frequency is ended. A power reference point Pω, making the speed to track a

preferred speed reference, is given by equation (2) as

(

) ∫( ) (3)

where Kωp and Kωi are the PI controller constants to get quick transient speed

variation and better speed recovery. This makes DFIG system to provide the

desired active power to fewer divergences. The total DFIG power injection to tie-

line is given by ΔPNC as under equation (4):

(4)

The DFIG inertia interprets into H as in SG based inertia control when this DFIG

unit also supplies to inertia of the system. This inertia is restricted by Kdf, whereas

Kpf presents the system damp, given by equation (5a) as:

(

)

( ) (5a)

where (

)

and ( ) The DFIG to system inertia part with

improved inertial control is given in (5b) equation as


115

(

)

( ) (5b)

The equation (5b) is based on the deviation in the frequency using a washout filter

time constant Tω, which depends on a SG based primary regulation behavior

during transient. The reference point is given as

( ) (6)

Here ΔX2 is the frequency change measured and R is the conventional droop

constant, for the wind turbine generator coupled to the grid. The injected active

power by the DFIG is PNC. The injected power is compared with PNCref to obtain

the optimal power output for getting rotor reference speed, where optimal power is

obtained as in Fig. 1. The mechanical power captured by the wind turbine is given

by Eq. (7):

h (

)

(7)

3. Frequency Regulation Frequency control or regulation has three levels of control like primary, secondary

and tertiary control. Primary control refers to an AGC, governors and turbine, with

operation depends on respective loads switching. This combined effect of above

three on frequency is said to be natural frequency. Area Control Error (ACE)

minimization helps in balancing area’s generation for inter-tie systems which

promises the frequency regulation and real power are distributed in each area so

that coordinate control of frequency deviation is possible. Area tie control has three

major functions like frequency control, local load variation deviation control, and

supervision with natural frequency retort to the variation in remote load. For areas

A and B of a two-area system, ACE in each area is generally represented as in

equations (8a and 8b) as 010A Aa As AACE T T B f f (8a) 010B Ba Bs BACE T T B f f (8b)

The summation of all available tie line deviation terms in a AGC system must be

ideally zero, like, TAa-TAs=-(TBa-TBs). Now summing and manipulating

equations (8a) and (8b), we have

010

A B

A B

ACE ACEf f

B B

(9)

The equation (9) says that the error in system frequency is proportional to the sum

of every area ACEs and inversely proportional to area frequency balancing. If any

power deviation in load (ΔPL) in area A takes place, then ACE in A (ACEA) under

AGC strategy is expressed as:

0

0

10

( ) 10

10

10

A a s A

L GA LA A

L A A

L A A

ACE T T B f f

P P P B f f

P f B f

P B f

(10)

and

1,GA

A

P fR

LA AP D f is the increase in DFIG generation and decrease in load

parameters respectively in area A with primary frequency regulation to steady-

state frequency deviation ∆f given by ∆f=-∆PL/βS and


116

101A A

A L

s

BACE P

(11a)

similarly, 10B B

B L

s

BACE P

(11b)

where

1A A A

A

B DR

,

1B B B

B

B DR

,

1 1s A B A B

A B

D DR R

.

If 10 A AB , 10 B BB , then A LACE P and 0BACE which states the setting

frequency prejudice to equal frequency response coefficient. In other words, AGC in

area A will take care of its own power disparity while AGC in area B is liable in

allocating the system frequency control through LFC and governor response.

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 is improved with reactive control

and frequency control is with real power control. The SSSC block diagram is shown

in Fig.2a in red color box. 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 stabilizes the frequency and real

power deviations during wind speed or load change. Similarly, the thyristor control

phase angle stabilizer (TCPS) is a real power device for damping power and

frequency oscillations like SSSC. Here the speed deviation is sensed and gives

command to Δω1 control signal to TCPS. Then there will be shift in phase angle

produced by TCPS which controls the real power flow.

For SMES based energy storage system, with abrupt rise in load demand, the

stored energy is nearly instantly released during DFIG WECS to the grid. The coil

instantaneously gets charged to its full value based on the design on converter 1

and 2. Thus absorbing portion of surplus energy in the system and released and the

coil current gets its regular value. The SMES is also a second order lead/ lag

cmpensator like SSSC in frequency regulation mode.

Fig. 2a (i) Block diagram of SSSC Fig.2a (ii) Block diagram representation

of TCPS


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Fig. 2b (i) transfer function based control of SSSC Fig.2b (ii) tf control of SMES

Fig.2b (iii) transfer function based control of TCPS The TCPS structure as a frequency controller function is shown in Fig. 4. The per

unit rotor speed deviation (Dxi, i = 1, 2), which gives the details of each mode of

concern, is used as the input controller signal. There are two parameters called

stabilization gain Ku and TCPS time constant TPS to be optimized for the better

operation of the TCPS frequency controller.

)(11

1

1

11

4

3

2

1 ssT

K

sT

sT

sT

sTP

SSSC

SSSCSSSC

(12a)

)(1

112 ssT

KKTP

TCPS

fTCPS

(12b)

)(11

1

1

11

4

3

2

1 ssT

K

sT

sT

sT

sTKP

SMES

SMESfSMES

(12c)

The blocks design of the SSSC, TCPS and SMES shown in Fig.2 is designed based

on the equations 12a, 12b and 12c. The choice of time constants and controller

constants helps to damp the oscillations and improve the sustainability during load

disturbances or wind perturbations.

5. Improving LFC offered by WG using SMES,

TCPS and SSSC The complete block diagram of two area load frequency control for DFIG based WT

system using different FACTS devices like SSSC is shown in Fig. 2a (i) and with

TCPS is shown in Fig.2a (ii). The internal control diagram of SSSC, SMES and

TCPS are shown in Fig. 2b (i), (ii) and (iii). The parameters of the LFC DFIG

system with different FACTS and SMES devices is given in the appendix.


118

Fig. 3a MATLAB diagram of FACTS and SMES representation Fig. 3b

MATLAB diagram of coordinated FACTS- SMES

The Area 1 with 1800MW is in the top and the Area 2 with 1200MW is represented

in the bottom of the above figures with conventional and DFIG based wind farms.

The DFIG wind turbine is controlled with pitch angle control and generator is

having primary, secondary and tertiary frequency regulators as in Apart from

these, the DFIG system is provided with FACTS devices alone as shown in Fig. 3a

and with coordination as FACTS- FACTS or FACTS-SMES is shown in Fig. 3b.

The performance of the DFIG system is tested without FACTS, but with basic

protection and with FACTS and coordinated devices. The damping and settling

behaviour is tested for frequency deviation and power regaining capability which is

discussed in the next section.

6. Simulation results The simulation results for the test system with FACTS alone is shown in Fig. 3a

and with coordinated FACTS and SMES is in Fig. 3b. A 10% change in load

command is given in area 1, frequency deviation at 5s is observed in both areas 1

and 2 with FACTS or SMES alone and with coordination is shown. The blue color

line is with DFIG basic protection without FACTS. It is observed that, the

frequency deviation reaches -0.26 Hz and slowly settles after 30s in area 1, reaches

-0.22Hz and settles nearly in the same time. With SSSC, frequency deviation in

area 1 reached -0.15Hz and settles at 35s and reached -0.21Hz at area 2 and settled

at 35s. With SMES shown with red has a deviation of -0.05 Hz and settles in 15s in

area 1 and with deviation of -0.1Hz and settled within 10s in arae 2. The deviation

with TCPS is nearly flat without much deviation and is having ideal behaviour in

both the areas 1 and 2. Hence, among all the FACTS and SMES devices, TCPS is

abetter device for DFIG based system.

Now considering the same system with coordinated FACTS-FACTS or FACTS-

SMES devices in area 1 and area 2 with same disturbance in area 1 at 5s. The

coordinated FACTS-FACTS or SMES refers to application of two or more in same

area in coordination to damp oscillations. It is observed that without coordinated

FACTS, the frequency deviation is same without FACTS devices. With coordinated

FACTS, it is observed that, the frequency deviation in both areas 1 and 2 are

nearly constant at zero value with 10% increase in the load. With SMES-SMES,

the deviation is high as with pink color markings, with SMES - TCPS coordination


119

as wit red color data, the deviation is better than SMES-SMES. with SMES –

TECPS (with green color marking), the deviation is very small and system is

completely stable and is better than other twio cases or with TCPS alone.

Fig 4a(i) with FACTS device, change in frequency in area 1, Fig. 4a (ii) area 2

Fig 4b(i) coordinated FACTS-SMES, change in frequency in area 1, Fig. 4b (ii) area 2

7. Conclusions The load frequency control (LFC) with DFIG based wind turbine system with

FCATS and SMES is studied in this paper. The LFC and area control error (ACE)

behavior of DFIG system is performing in the similar way as with a conventional

system. The paper studied the behavior of DFIG frequency regulation system with

load deviation in area 1 of the two areas using different FACTS devices like SSSC

and TCPS and energy storage device like SMES in one case and with coordinated

FACTS and SMES in other case. With basic primary, secondary and tertiary

frequency control mechanism, the DFIG frequency reached to normal value after

few oscillations. With SSSC, frequency settling is better than with FACTS. But

TCPS is better than SMES and is better than SSSC in controlling real power and

frequency deviation. With coordinated FACTS- SMES, SMES and TCPS behavior

is best, SMES-SSSC is better and SMES-SMES is good than without FACTS or

SMES. Hence, along with basic frequency regulation, application of full rated

TCPS or with half rated each with coordinated SMES-TCPS is a better option for

LFC.


120

Appendix He1=3.5; He2=3.5; Kagc1=0.05; Kagc2=0.05; Ta1=0.2; Ta2=0.2; Kp1=12; Kp2=12;

Kwi1=0.1;Kwp1=1.58; Kwi2=0.1; Kwp2=1.61; R1=3; R2=3; Th1=0.1; Th2=0.1;

Tr1=0.1; Tr2=0.1; Tw1=6; Tw2=6; Tp1=10; Tp2=15; Tt1=1; Tt2=1; Wmax=1.4;

Wmin=0.0; T0=0.07; B1=1.1; Pmax=3; Pmin=0; K=20.1378; T1=1.5025; T2=0.5386;

T3=0.06268; T4=0.05075; Tsm=10.50; Tsm=10.50; K1=20.2188; T11=0.587;

T21=0.158; T31=0.0575; T41=0.2316; Tsm1=0.2151; Tsm1=10.2151; Kfi=1.5;

Tps=0.1; T12=0.866;

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Two area load frequency control for DFIG based wind turbine system using modern energy … · 2018-03-15 · Two area load frequency control for DFIG based wind turbine system using

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