grid stability analysis for solar...
TRANSCRIPT
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
%
Solar Power injected in MW
Case 2: PV Penetration @ Bus 6
Bus 4 Bus 5 Bus 6 Bus 7 Bus 8 Bus 9 Solar Bus
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
%
Solar Power injected in MW
Case 3: PV Penetration @ Bus 8
Bus 4 Bus 5 Bus 6 Bus 7 Bus 8 Bus 9 Solar Bus
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)
Solar Power injected in MW
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)
Solar Power injected in MW
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)
Solar Power injected in MW
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)
Solar Power injected in MW
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
Transient Stability Analysis: Effect due to a bus fault
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
Transient Stability Analysis: Effect due to a bus fault
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
0% Solar Penetration 10% Solar Penetration 30% Solar Penetration
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
0% Solar Penetration 30% Solar Penetration 10% Solar Penetration
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
0% Solar Penetration 10% Solar Penetration 30% Solar Penetration
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
0% Solar Penetration 10% Solar Penetration 30% Solar Penetration
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
0% Solar Penetration 10% Solar Penetration 30% Solar Penetration
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
0% Solar Penetration 10% Solar Penetration 30% Solar Penetration
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
0% Solar Penetration 10% Solar Penetration 30% Solar Penetration
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