Grid stability analysis for solar substation · Grid Stability Analysis for High Penetration Solar Photovoltaics Ajit Kumar K Asst. Manager . Solar Business Unit . Larsen & Toubro

Grid Stability Analysis for High Penetration Solar Photovoltaics

Ajit Kumar K Asst. Manager

Solar Business Unit

Larsen & Toubro Construction, Chennai

Dr. M. P. Selvan

Asst. Professor

Department of Electrical & Electronics Engineering

NIT Tiruchirappalli

K Rajapandiyan Manager

Solar Business Unit

Larsen & Toubro Construction, Chennai

Co – Authors

Introduction The ever growing global energy needs and the immediate need for an environment friendly

sustainable growth has made us focus on renewable energy sources, especially wind and solar.

But these renewable energy resources when implemented in large scale without any specialized

controls is found to impact the integrity, reliability and stability of the grid.

Solar PV forms a major portion among the utility level renewable energy power plants.

Solar PV power penetration into the grid is on continuous rise and plants of order of hundreds of MW

are coming up in India and at global level.

The large upcoming utility scale solar plants are expected to behave similar to the conventional

plants when it comes to handing grid stability.

Hence it is important to study and analyze the impact of the large scale penetration of solar PV

power into the grid.

2

Problem Statement Till now, the solar PV installations were small in size and quantity and were connected only at

distribution level. But large solar parks of order of hundreds of MW are coming up and will be connected at transmission level.

Solar PV unlike thermal power plant is asynchronously integrated into the grid through inverter. Hence they do not contribute for grid inertia. It is a significant aspect of conventional synchronous generators that helps in the inertial response during frequency control.

At present the solar PV plant’s anti-islanding protection immediately trips the plant when there is a grid fault. When the capacity of the plant is large, it will cause generation-load imbalance.

If the plant capacity is large, the seasonal and weather variations like clouding will heavily impact the grid.

There are several such impacts caused by the increased penetration of solar PV into the power system.

3

Project Objectives and Scope To understand the need and requirement in analyzing the impact of high

penetration PV into the grid,

The basics of grid stability has been studied.

A brief study has been done on the present scenario of Indian power sector to understand the current and future solar PV penetration levels and the policy framework in India to promote solar power.

To understand the performance requirements of large and upcoming solar PV

plants,

Study on the controls existing in a conventional power plant to manage the grid stability

has been done

In order to identify the drawbacks and impact of high penetration PV on the power

system, literature survey has been done.

4

Project Objectives and Scope (Contd.) A standard bus system has been identified and modelled in ETAP software. Solar

PV plant is integrated into the standard bus system. This system has been used for

further analyses.

The impact of increased solar PV penetration on the steady state performance of

the system has been studied.

The impact of large solar PV penetration on the transient stability of the grid has

been studied.

5

Study on Indian power sector scenario Based on CEA and MNRE data, the present solar penetration is about 2.23% (6,762.85 MW of solar among 3,02,833.2

MW in total as on April 2016).

Renewable energy installments being order of the day and government’s thrust in promoting renewable energy

throughout the country the total penetration of solar is set to increase at a rapid pace.

As per CEA projection and MNRE’s JNNSM-2015 target, the penetration would be around 23% by 2022 (100 GW of

solar among 434900 MW in total).

6

Controls existing in a conventional power plant Generation side controls existing in a conventional power plant (Based on the Industrial Visit

done at MTPS, Tamil Nadu)

Frequency stability and active power control

Initial phase – Grid inertial response

Control phase – Primary control (Turbine speed governor system – ALFC & RGMO with speed droop), Secondary control – AGC, Tertiary control, Overall plant control.

Voltage stability and reactive power control

Steady state voltage regulation – AVR in Excitation system with voltage control mode

VAR compensation and support – AVR in Excitation system with VAR mode (Under or over excitation)

Voltage profile improvement by on-load or off-load tap changing transformer

Angle stability

PSS in excitation system to improve small signal angle stability

Transient stability improvement – High speed excitation along with PSS and several other controls.

7

Methodology for analysis Steady state analysis

A standard bus test system integrated with a large solar PV plant has been considered. Through

‘Load Flow Analysis module’ in ETAP software, the impact of large scale penetration of Solar PV

on the steady state performance of the grid is assessed with a specific focus on,

Voltage Variation in all buses

Slack bus power

Line loading effect and system losses

Transient analysis

A standard bus test system integrated with a large solar PV plant has been considered. Through

‘Transient Stability Analysis’ module in ETAP software, the impact on the transient stability

performance of the grid is studied for the following transient events,

Effect due to a Bus Fault

Effect due to Loss of a Transmission Line

Effect on Critical Clearing Time

Effect due to Load Rejection

8

Modelling in ETAP – IEEE 9-bus system

9

Modelling in ETAP: IEEE 9-bus system integrated with solar PV plant

10

Steady state analysis: Effect on steady state bus voltages

96

97

98

99

100

101

102

103

104

0 22 44 66 88 110 132 154 176 199 221 243

Volta

ge in

%

Solar Power injected in MW

Case 1: PV Penetration @ Bus 5

Bus 4 Bus 5 Bus 6 Bus 7 Bus 8 Bus 9 Solar Bus

96

97

98

99

100

101

102

103

104

0 22 44 66 88 110 132 154 176 199 221 243 265

Volta

ge in

%




11

Steady state analysis: Effect on steady state bus voltages

90

92

94

96

98

100

102

104

0 22 44 66 88 110 132 154 176 199 221 243 265

Volta

ge in

%




Similar trend of voltage variation is observed in all three cases as shown in the plot – Bus voltages (except for generator buses) improved with increase in solar penetration till a point and then it started dropping because of increased line drop.

The intensity of variation in voltages varied with the location of penetration. The maximum of the variation in bus voltages observed in all three cases is listed below,

Case 1: 2.5% variation @Bus 5; Case 2: 3.35% @Bus 4; Case 3: 8.35% @Bus 5;

The peak point of the curve also varies with the location of penetration.

it is seen from the study that among the three cases, PV injection at bus 5 was better as it allowed for more penetration with less severe variation in voltages.

12

Steady state analysis: Effect on System loss

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

0 22 44 66 88 110 132 154 176 199 221 243 265

Real

Pow

er (M

W)


Loss in system (MW)

Penetration at bus 5 Penetration at bus 6 Penetration at bus 8

Total system loss (both MW and MVAR) was decreasing initially as the level of penetration was increasing and beyond a point the losses started increasing. Case 3 was severe where the losses started increasing from the beginning.

Optimal penetration level with respect to the system losses can be identified from the system loss profile and also the best location for penetration can also be identified from this analysis.

-150

-100

-50

0

50

100

150

200

0 22 44 66 88 110 132 154 176 199 221 243 265

Reac

tive

Pow

er (M

VAR)


Loss in system (MVAR)

Penetration at bus 5 Penetration at bus 6 Penetration at bus 8

14

Steady state analysis: Effect on Transmission Line Power Flow

-200

-150

-100

-50

0

50

100

150

0 22 44 66 88 110 132 154 176 199 221 243

Act

ive

Pow

er (M

W)


Line P (MW)

Line 1 (4-5) Line 2 (4-6) Line 3 (7-5)

Line 4 (9-6) Line 5 (9-8) Line 6 (7-8)

-30

-20

-10

0

10

20

30

40

50

60

70

0 22 44 66 88 110 132 154 176 199 221 243Reac

tive

Pow

er (M

VAR)


Line Q (MVAR)

Line 1 (4-5) Line 2 (4-6) Line 3 (7-5)

Line 4 (9-6) Line 5 (9-8) Line 6 (7-8)

The variation in loading of the transmission lines was mixed with few lines experiencing increase in power and few lines experiencing decrease in power, as the % penetration increases. Few of the lines experienced power reversal beyond a point. The changes in loading of line 1 is severe of all.

It is very important to consider the impact of solar penetration on transmission line loading parameters while planning the network.

15

Steady state analysis: Summary

Appropriate

location

Maximum

possible

penetration

Based on bus

voltage Bus 5 66 MW

Based on

system loss Bus 5 44 MW

Increase in PV penetration can bring about variation in steady state bus voltage levels and can be really critical and could even contribute to affecting the voltage stability of grid.

It might bring about severe changes into other parameters like steady state real power and reactive power loading of transmission lines and other equipment in the system and also affect the system losses.

It is important in performing such a study, which will help engineers in planning the system with high penetration levels of solar PV power and in identifying the critical PV penetration levels and appropriate location for penetration in a given network.

16

Transient Stability Analysis: Effect on Critical Clearing Time

0,193 0,19 0,187

0,161

0,146

0

0,05

0,1

0,15

0,2

0,25

0 5 10 15 20

Time

(Sec

ond)

Solar Penetration %

Critical clearing time The critical clearing time of the system

continuously decreases as the the solar PV

penetration increases.

For the penetration beyond 20%, the system is

unstable for any clearing time.

Solar penetration (%)

Critical clearing time (second)

0 0.193 5 0.19 10 0.187 15 0.161 20 0.146

> 20 Unstable for any clearing time

17

Transient Stability Analysis: Effect due to a bus fault

The oscillations following the disturbance for the base case with 0% solar PV penetration is smooth and is converging towards a stable value. For the case with 10% solar penetration, the amplitude of oscillations is a bit more and is getting little irregular. Yet it was converging and the system was stable. For 30 % penetration case, the oscillations have been completely irregular and don’t converge to a stable value.

0

20

40

60

80

100

1200

0,31

0,62

0,93

1,24

1,55

1,86

2,17

2,48

2,79

3,09

13,

391

3,70

14,

011

4,32

14,

631

4,94

15,

251

5,56

15,

871

6,18

16,

491

6,80

17,

111

7,42

17,

731

8,04

18,

351

8,66

18,

971

9,28

19,

591

9,90

110

,211

10,5

2110

,831

11,1

4111

,451

11,7

6112

,071

12,3

8112

,691

13,0

0113

,311

13,6

2113

,931

14,2

4114

,551

14,8

6115

,171

15,4

8115

,791

16,1

0116

,411

16,7

2117

,031

17,3

4117

,651

17,9

6118

,271

18,5

8118

,891

19,2

0119

,511

19,8

21

Ang

le (d

egre

e)

Time (Second)

Relative rotor angle of Generator G2

0% solar penetration 30% solar penetration 10% solar penetration

18


0102030405060708090

1000

0,33

0,66

0,99

1,32

1,65

1,98

2,31

2,64

2,97

3,28

13,

611

3,94

14,

271

4,60

14,

931

5,26

15,

591

5,92

16,

251

6,58

16,

911

7,24

17,

571

7,90

18,

231

8,56

18,

891

9,22

19,

551

9,88

110

,211

10,5

4110

,871

11,2

0111

,531

11,8

6112

,191

12,5

2112

,851

13,1

8113

,511

13,8

4114

,171

14,5

0114

,831

15,1

6115

,491

15,8

2116

,151

16,4

8116

,811

17,1

4117

,471

17,8

0118

,131

18,4

6118

,791

19,1

2119

,451

19,7

81

Ang

le (d

egre

e)

Time (Second)

Relative rotor angle of Generator G3

0% Solar penetration 30% Solar Penetration 10% Solar Penetration

Similar trend has been observed for Generator G3.

The system is unstable beyond 20% penetration

19


0

20

40

60

80

100

1200

0,65 1,

31,

95 2,6

3,23

13,

881

4,53

15,

181

5,83

16,

481

7,13

17,

781

8,43

19,

081

9,73

110

,381

11,0

3111

,681

12,3

3112

,981

13,6

3114

,281

14,9

3115

,581

16,2

3116

,881

17,5

3118

,181

18,8

3119

,481

Volta

ge (%

)

Time (Second)

Voltage at Bus 7 (Faulty bus)

0% Solar Penetration 30% Solar Penetration 10% Solar Penetration

40

50

60

70

80

90

100

110

00,

671,

342,

012,

683,

331

4,00

14,

671

5,34

16,

011

6,68

17,

351

8,02

18,

691

9,36

110

,031

10,7

0111

,371

12,0

4112

,711

13,3

8114

,051

14,7

2115

,391

16,0

6116

,731

17,4

0118

,071

18,7

4119

,411

Volta

ge (%

)

Time (Second)

Voltage at Bus 4


The oscillations after the fault for the base case are minute and converging to a stable value. The oscillations for 10%

penetration case was with higher amplitude and was getting irregular. The voltage dips are quite high. But it was getting

stabilized after a while. Whereas for the 30% penetration case the oscillations and voltage dips have been very severe and

was not stabilizing.

As the level penetration increases the system becomes unstable and goes out of synchronism beyond a point.

20

Transient Stability Analysis: Effect due to Load Rejection

3035404550556065707580

00,

310,

620,

931,

241,

551,

862,

172,

482,

793,

091

3,40

13,

711

4,02

14,

331

4,64

14,

951

5,26

15,

571

5,88

16,

191

6,50

16,

811

7,12

17,

431

7,74

18,

051

8,36

18,

671

8,98

19,

291

9,60

19,

911

10,2

2110

,531

10,8

4111

,151

11,4

6111

,771

12,0

8112

,391

12,7

0113

,011

13,3

2113

,631

13,9

4114

,251

14,5

6114

,871

15,1

8115

,491

15,8

0116

,111

16,4

2116

,731

17,0

4117

,351

17,6

6117

,971

18,2

8118

,591

18,9

0119

,211

19,5

2119

,831

Ang

le (d

egre

e)

Time (Second)

Relative Rotor Angle - G2


The oscillations for the base case have been uniform is was converging and settling to a new value. The oscillations for the

10 % case is less in amplitude but little irregular compared to the base case. Yet it is converging to a finite one. Beyond 20%

penetration the system is unstable. The oscillations for the 30% penetration case are severe and completely irregular from

the base case. It is not converging and going out of step.

21

Transient Stability Analysis: Effect due to Load Rejection

95

100

105

110

115

120

125

130

135

140

00,

65 1,3

1,95 2,

63,

241

3,89

14,

541

5,19

15,

841

6,49

17,

141

7,79

18,

441

9,09

19,

741

10,3

9111

,041

11,6

9112

,341

12,9

9113

,641

14,2

9114

,941

15,5

9116

,241

16,8

9117

,541

18,1

9118

,841

19,4

91

Volta

ge (%

)

Time (Second)

Voltage at Bus 5


95

100

105

110

115

120

125

130

135

00,

65 1,3

1,95 2,

63,

241

3,89

14,

541

5,19

15,

841

6,49

17,

141

7,79

18,

441

9,09

19,

741

10,3

9111

,041

11,6

9112

,341

12,9

9113

,641

14,2

9114

,941

15,5

9116

,241

16,8

9117

,541

18,1

9118

,841

19,4

91

Volta

ge (%

)

Time (Second)

Voltage at Bus 9


In the base case voltage undergoes minor disturbance after the disconnection and smoothly settles towards the new

value. The oscillation frequency is quite high for the 10% penetration case but is converging towards the new value. For 30%

penetration case the oscillations are severe and irregular. The frequency is also less and is not converging.

In this case too, the system gets unstable as the level of solar penetration increases.

22

Transient Stability Analysis: Effect due to Loss of a Transmission Line

40

50

60

70

80

90

1000

0,47

0,94

1,41

1,88

2,35

2,82

3,28

13,

751

4,22

14,

691

5,16

15,

631

6,10

16,

571

7,04

17,

511

7,98

18,

451

8,92

19,

391

9,86

110

,331

10,8

0111

,271

11,7

4112

,211

12,6

8113

,151

13,6

2114

,091

14,5

6115

,031

15,5

0115

,971

16,4

4116

,911

17,3

8117

,851

18,3

2118

,791

19,2

6119

,731

20,2

0120

,671

21,1

4121

,611

22,0

8122

,551

23,0

2123

,491

23,9

6124

,431

24,9

0125

,371

25,8

4126

,311

26,7

8127

,251

27,7

2128

,191

28,6

6129

,131

29,6

01

Ang

le (d

egre

e)

Time (Second)

Relative rotor angle G2


The oscillations for the base case is uniform and is converging and settling to a new value. The oscillations for the 10 % case

is very similar compared to the base case and is converging to a finite value. Beyond 20% penetration the system is

unstable in this case too.

The oscillations for the 30% penetration are severe and completely irregular from the base case. It is not converging and is

going out of step. 23

Transient Stability Analysis: Effect due to Loss of a Transmission Line

80

85

90

95

100

1050

0,97

1,94

2,91

3,87

14,

841

5,81

16,

781

7,75

18,

721

9,69

110

,661

11,6

3112

,601

13,5

7114

,541

15,5

1116

,481

17,4

5118

,421

19,3

9120

,361

21,3

3122

,301

23,2

7124

,241

25,2

1126

,181

27,1

5128

,121

29,0

91

Volta

ge (%

)

Time (Second)

Voltage at bus 8


80

85

90

95

100

105

110

00,

941,

882,

823,

751

4,69

15,

631

6,57

17,

511

8,45

19,

391

10,3

3111

,271

12,2

1113

,151

14,0

9115

,031

15,9

7116

,911

17,8

5118

,791

19,7

3120

,671

21,6

1122

,551

23,4

9124

,431

25,3

7126

,311

27,2

5128

,191

29,1

31

Volta

ge (%

)

Time (Second)

Voltage at bus 5


Severe drop in voltage occurs for bus 8 as the line 6 is disconnected from that bus. In the base case, the voltage slowly

recovers and settles to the new value. Heavy high frequency oscillations occur in case of 10% penetration but moves

towards the new value. In case of 30% penetration, irregular low frequency oscillations occur and doesn’t converge to a

stable value.

The system is getting unstable for the loss of a transmission line as the level of penetration increases.

24

Transient Stability Analysis - Summary Thus in all the case studies done, the system was getting unstable for a transient

disturbance as the level of PV penetration is increased.

The bus voltage magnitudes and relative rotor angle and hence synchronism are the most adversely affected system parameters during the transients in the system with high penetration of PV.

It is very important in taking into account the transient performance of the system with high penetration levels of the PV to maintain the stability and integrity of the system following faults.

The total system inertia is very less in systems with high PV penetration leading to severe issue following various system disturbances.

25

Conclusion The preliminary study helped in understanding the drawbacks of high penetration solar

PV into the power system. Further studies revealed the control requirements of upcoming large PV plants. The need and importance in analyzing the impact of the high penetration photovoltaics into the grid has been understood.

The increased penetration of solar PV into the grid without any specialized controls has been proved to affect both the steady state performance and the transient stability of the grid, through the analysis done in ETAP.

Thus suitable control mechanisms are required from the upcoming large solar plants to address such issues and to mitigate the stability issues arising out of increased solar PV penetration.

It is important in performing such a study, which will help in planning the system with high penetration levels of solar PV power and in identifying the critical PV penetration levels for a given network.

26

Future work The impact on the grid due to sudden loss of the solar PV generation due to

problems from plant side like sudden variation of irradiance due to clouding, etc.

can be studied and analyzed.

Control systems and mechanisms can be designed for large solar PV plants like

active power and reactive power controls similar to that of conventional plants,

LVRT capability, etc. and their effectiveness in mitigation of the impact on grid

performance can be analyzed.

27

Thank you

Documents

Grid stability analysis for solar substation · Grid Stability Analysis for High Penetration Solar Photovoltaics Ajit Kumar K Asst. Manager . Solar Business Unit . Larsen & Toubro