Analysis of System Impact Caused by Wind Power in Taiwan
Analysis of System Impact Caused by Wind Power in Taiwan Shu-Wei Liu, Yung-Ming Su, Wen-Jyh Yan Energy & Environment Research Laboratories Industrial Technology Research Institute Hsinchu, Taiwan, R.O.C. Tel.: +886 03 5916275 Fax: +886 03 5820030 E-mail: [email protected] Summary Taiwan, an abundant wind resource area, was not well developed in wind power till 2000. The first wind power system was demonstrated and constructed by the promotion of government with the capacity of 2.64 MW in 2000. The value of installation capacity was nearly to 200 MW on April, 2007. Taiwan Power Company (TPC), a dominator of Taiwan’s power system, showed its concern over grid connection effect. The “Technical Guidelines for The Connection of Renewable Energy Generating to The Grid” was formulated by TPC in 2002. This paper is focus on the research of wind power grid connection analysis. It was simultaneously fulfilled by Energy & Environment Research Laboratory of Industrial Technology Research Institute in the case of local government BOT project.
Key words: wind power, power quality, grid connection 1 Introduction Recently, wind power has become the significant role in the renewable energy. The wind power installation capacity is 2159 MW in 2010, and it is indeed an ambitious goal. Industrial Technology Research Institute was commissioned by Bureau of Energy, Ministry of Economic Affairs to promote the wind energy. The tasks are wind velocity measurement, wind farm assessment, application of demonstration systems, promotion strategies formulation etc. After five years, it has reached to a satisfactory accomplishment. Taiwan’s wind condition was deeply influenced by the north-east direction wind flow in winter season. It is also benefited from the effect of Taiwan Strait. The average full load hours on the west coast from Taipei county to Chiayi county is nearly 2600-3000. It is no doubt a better region to compare with other countries. During the period of 2000-July, 2004, the demonstration subsidies for private sectors installing wind turbine was NT$ 16,000 dollars per kW. The result of three demonstration systems separately located in Mailiao, Zhongtun and Hsinchu (shown in Fig. 1, Fig.2 and Fig. 3). The total capacity was 8.54 MW. In order to facilitate the utilization of renewable energy, TPC announced the “Taipower Renewable Energy Purchase Scheme” on November 11, 2003. A favorable renewable power purchase agreement (ppa) of GBP 0.035/kWh (NT$2.0) is offered for the purchase of renewable electricity. The ppa is guaranteed for at least 15 years. Total capacity offered for renewable electricity under this scheme is limited to 600 MW. The wind power promotion changed to the period of electricity generation purchasing from July, 2004. The scheme will remain effective until the passage of the “Renewable Energy Development Bill” or at the time of Privatization of TAIPOWER, as is cited in the “Draft Revisions of the Electricity Act”.
Figure1. Mailiao Demonstration Systems (2.64 MWVestas)
Figure2. Zhongtun Demonstration Systems (2.4 MWEnercon)
Figure3. Andante Demonstration Systems (3.5 MWVestas) Apart from three demonstrative wind farms, the other wind farms belong to Taiwan Power Company and Infra-Vest GmbH in Taiwan. Energy & Environment Research Laboratories of Industrial Technology Research Institute to promote the wind power for local government for speeded up the development of wind power. There are three local governments have been promoted at present, that including Kaohsiung County, Chiayi County and Ilan County. This paper uses the example of Chiayi County to explain the evaluation of system impact caused by wind generation and the regulations of Taiwan Power Company in Taiwan.
2 Chiayi Budai proposal To ensure wind farm won’t affect the power system and result the effect on power quality after wind farm connects to the power system. It is necessary to analyze the impact on the power system and get the connection permission from Taiwan Power Company.
2.1 Locations of wind farm The wind farm is located at Budai Township, Chiayi County. This wind farm is planned to install n=13 wind turbines (See Figure4) of the same type. Capacity of each turbine is 2.0MW. The total capacity will be 26MW. According to “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company”If the total capacity of the Generator Equipment exceeds 20MW, the Generator Equipment has to connect with extra-high-voltage system, and the connection’s voltage will depend on each case’s situation. Therefore, this wind farm shall be connected to the Xinwen substation 69kV bus. The distance between Xinwen substation and the closest wind-turbine is 200M.
BudaiTownship
Taiwan
Chiayi County
Xinwen Substation
Figure4. Map of wind farm location
2.2 Point of common connection For the point of common connection the following data are available: Table.1 Network site data District Point of common connection PCC: Rated voltage Rated apparent power at PCC Maximum 69kV 3-phase short-circuit current Minimum 69kV 3-phase short-circuit current
Chiayi Xinwen substation Vgrid rated = 69kV 143MVA ISC max = 8.21kA ISC min = 7.22kA SkV max = 3 × Vgrid rated × ISC max
Maximum 69kV 3-phase short-circuit power
= 3 × 69kV × 8.21 kA =981.189MVA SkV min = 3 × Vgrid rated × ISC min
Minimum 69kV 3-phase short-circuit power
= 3 × 69kV × 7.22 kA =862.873MVA
Grid reactance to resistance ratio Grid impedance angle Circuit breaker rated short-circuit current
6.47 ψk max = arctan(6.47) = 81.21° 25kA
2.3 Wind turbine power quality data We choose three types of wind turbines that are able to use in this project to evaluate the impact of grid connection. The evaluation is mainly depended by the measurement report of IEC-61400-21. The data of the three types of wind turbines are in the Table.2. Notice that, the Grid impedance angle is 81.21° , but the power quality measure report of wind turbine only provides flicker coefficient and voltage change factor when wind turbine is running at ψk = 30°, 50°, 70°, and 85°. So we choose to use the data when ψk is 85° to calculate. Simultaneously, suppose the annual average wind speed is 8.5m/s.
Table.2 Data of the three types of wind turbines Wind turbine type Data Number of turbine n Effective power PnG Relative effective power maximum for 1 min P1 min Relative effective power maximum for 10 min P10 min Power factor (at max. power) λ Phase angle φ Rated voltage VWT rated Flicker coefficient c(ψk,va) Voltage change factor ku(ψk) Minimum short-circuit current IKE
Type A
Type B
Type C
13
13
13
2MW
2MW
2MW
1.00
1.08
0.96
1.00
1.02
1.00
0.9991
0.9965
0.9965
2.43°
4.79°
4.79°
400V
20kV
20kV
2.8
5.72
4.04
0.19
0.06
0.12
3179A
306A
289.5A
3 Power quality assessment 3.1 Calculation of the network equipments The network equipments (e.g. connection cable) are to be dimensioned for the 10-minutes average value of the apparent power of the whole SA max 10 min. Together with Table.1 and Table.2 we will be able to calculate the value of SA max 10 min. The equation is showed below as equation (1). The results are showed below in Table.3. From the results, we know that the existing grid equipments are dimensioned sufficiently for the maximum power of the wind farm since the rated apparent power at PCC is 143MVA. (1) SA max 10 min = n × SE max 10 min = n × PnG × P10min / λ Table.3 the 10-minutes average value of the apparent power of wind farm SA max 10 min Type A SA max 10 min = 13 × 2M ×1.00 / 0.9991 = 26.02MVA Type B SA max 10 min = 13× 2M ×1.02/ 0.9965 = 26.61MVA Type C SA max 10 min = 13 × 2M ×1.00 / 0.9965 = 26.09MVA
3.2 Steady state voltage change The calculation of the steady state relative voltage change U of the grid at PCC is based on the 1-minute average value of the wind farm total apparent power SA max 1 min. The equation is presented as equation (2). The calculation result is showed in Table.4. The voltage changes are below the specified limit of 2.5% (According to the specification of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company” Item 5 No. 2). ∆U=SA max 1 min × cos(ψk min + ϕ ) / SkV min Table.4 value of steady state voltage change U Type A ∆U=26.02M × cos(80.88 + 2.43 ) / 862.873M = 0.351% Type B ∆ U=28.18M × cos(80.88 + 4.79 ) / 862.873M = 0.246%
Type C ∆U=25.05M × cos(80.88 + 4.79 ) / 862.873M = 0.219%
(2)
3.3 Voltage change caused by switching The voltage change caused by the switching of the wind turbine generators during cut-in of the wind farm can be expected to be rather small, as the turbines will not start up at the same time. In normal situations the wind farm operation system will ensure a consecutive cut-in of the single wind turbine, two or more wind turbines will not be switched on simultaneously. The calculation of the relative voltage dk is based on the voltage change factor ku(ψk) at start-up of the wind turbine at rated wind speed. The cut-in of the wind turbine at rated wind speed is the highest value occurring for switching operations and thus a worst case assumption for the wind turbine under consideration. Accordingly, the maximum voltage change caused by switching of single system condition is:
dk =k u (ψk ) ×
PnG Sn = ku (ψk ) × SkV min λ × SkV min
(3)
Table.5 value of voltage change dk Type A d k = 0 .1 9 × 2 M /(0.9 9 9 1 × 8 6 2 .8 7 3M ) = 0 .0 4 4% Type B
d k = 0 .0 6 × 2 M / (0 .9 9 6 5 × 8 6 2 .8 7 3 M ) = 0 .0 1 4 %
Type C
d k = 0 .1 2 × 2 M /(0 .9 96 5 × 86 2 .8 7 3M ) = 0 .02 8 %
These are far below the maximum grid voltage change limit of 2.5%. The control system could even allow all the wind turbines of the wind farm to cut-in at the same time, even then the total relative voltage change of 0.572% (0.0044%*13) , 0.182% (0.0014%*13) , 0.364% (0.0028%*13) would not exceed the limits(According to the specification of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company” Item 5 No.2).
3.4 Voltage flicker The flicker distortion caused by continuous operation of the wind farm can be assessed from the flicker coefficient for continuous operation c(ψk,) if a single wind turbine, given at the grid impedance angel ψk and at average mean wind speed va at the site. The long-term flicker is assessed by equation (4). The calculation result is showed in Table.6. Plt = n × c(ψk ,v a ) ×
PnG Sn = n × c(ψ k ,v a ) × SkV min λ × SkV min
(4)
Table.6 value of long-term flicker Plt Type A Plt = 13 × 2.8 × 2M/ (0.9991 × 862.873M ) = 0.02342 Type B
Plt =
Type C
Plt = 13 × 4.04 × 2M/(0.9965 × 862.873M) = 0.03388
13 × 5.72 × 2M /(0.9965 × 862.873M ) = 0.04797
From IEC 61400-21, we know that when wind turbine is running, long-term voltage flicker Plt is equaled to short-term voltage flicker Pst. Researches in real experiments show the ratio between Pst and V10 is between 2 and 4, the average is about 3. Because of that, V10 can be found by using the equation (5), the results are showed in Table.7. All the results are lower than 0.45% which is also the limitation of Taiwan Power Company.
∆V10 =
pst plt ≈ 3 3
(5)
Table.7 value of voltage flicker V10 Type A Type B Type C
∆V10 =0.02342/3=0.00781% ∆V10 =0.04797/3=0.01599% ∆V10 =0.03388/3=0.01129%
3.5 Short circuit current at the PCC After wind turbines and grids are connected together, there must be influences on power system’s fault current. Before calculating short-circuit current, (not considering the reducing short-circuit current caused by transformers) we need to first figure out the short-circuit power SSC of the wind farm. Then combine short-circuit power SSC and maximum short-circuit power SkV max of the wind farm to calculate the new contact’s short-circuit power of the grid at PCC SkV Res. We can calculate the value of short-circuit current ISC Res at the PCC of 69KV grid, the equation are showed below as equation (6), (7), and (8). Table 8 displaces the maximum short-circuit current at the PCC of 69KV grid. The maximum short-circuit current at the PCC of 69KV grid is lower than circuit breaker rated short-circuit
current of 25kA. According to the specification of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company” Item 5 No. 1.
SSC =n × 3 × VWT rated × I KE
(6)
SkV Res =SkV max + SSC
(7) (8)
ISC Res =SkV Res /( 3 × Vgrid rated ) Table.8 value of short-circuit current ISC Res at the PCC of 69KV Type A
8.450kA
Type B
9.363kA
Type C
9.301kA
3.6 Harmonics The harmonic currents shall be limited to the degree needed to avoid unacceptable harmonic voltages at the PCC. The demand from Taiwan Power Company of harmonics current is regarded from the standard harmonics limitation, IEEE 519-1989. IEC 61000-3-6 gives guidance for summation harmonic current distortion from loads. Applying this, the harmonic current at the PCC is due to a wind turbine installation with a number of wind turbines may be estimated applying equation (9). And the result is showed in Table.10.
Ih∑ = β
∑( n
I h ,i
n
i =1
)β
(9)
i
where n is the number of wind turbine connected to the PCC; IhΣ is the h’th order harmonic current distortion at the PCC; ni is the ratio of the transformer at the i’th wind turbine; Ih,I is the h’th order harmonic current distortion of the i’th wind turbine; β is the exponent given in Table.9. Table.9 Specification of exponents according to IEC 61000-3-6 Harmonic order
β
1.0
1.4
2.0
Table.10 harmonics current Harmonic Wind current I2 turbine type
Order I3
I4
I5
I6
I7
I8
I9
I10
I11
I12
I13
I14
I26
I28
I32
I34
I35
ITHD
Type A
0.200 0.100 0.100 0.144 0.048 0.240 0.048 0.048 0.048 0.083 0.028 0.028 0.028 0.047 0.044 0.047 0.033 0.031 0.342
Type B
0.250 0.530 0.240
Type C
0.200 0.240
harmonics limitation
-
-
0.139
-
0.058
-
-
-
-
-
-
-
-
-
-
-
0.651
0.230
-
0.125
-
-
-
-
-
-
-
-
-
-
-
-
0.418
0.500 2.000 0.500 2.000 0.500 2.000 0.500 2.000 0.500 1.000 0.250 1.000 0.250 0.075 0.075 0.075 0.075 0.150 2.500
4 Verification result from Taiwan Power Company The results of grid’s impact analysis showed above were sent to Taiwan Power Company to precede a verification of connection impact. Taiwan Power Company agreed to issue permission to connect. But the maximum short-circuit current at the PCC of 69KV grid needs to lower than 8.96kA. Since the fault current of this impact analysis does not include the effect from wind turbine transformer, step-up transformer, and impedance etc. If add the consideration above, wind turbine short-circuit current at the PCC of 69KV grid should be able to lower than 8.96kA.
5 Conclusions The results of grid’s impact analysis of the wind farm at Chiayi County are all conformed to the regulations of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company.” There are no large effects on the power quality pollution of all three wind turbines. The main reason is that the total capacity of the wind farm is not large compared to 69kV system capacity, and there are the features of IEC proof of wind turbines. Without proof from IEC, wind turbine is almost unable to sell. That’s why there is no large effect on pollution on power quality of all three types of generators used in this project. But the verification result from Taiwan Power Company requires short-circuit current at the PCC of 69KV grid lower than 8.96kA. Furthermore, researcher can investigate into whether this limitation is appropriate or not in the future.
6 References International Standard IEC 61400-21, Wind turbine generator systems – Part 21Measurement of assessment power quality characteristics of grid connected wind turbines, 2001. [2] Energy Bureau of Ministry of Economic Affairs ,” Demonstration and Promotion of Wind Energy Program”, the whole course of implement report, Energy Bureau of MOEA, Taipei, 2005. [3] Measurement and assessment of power quality characteristics of grid connected wind turbines, International Electrotechnical Commission (IEC), TC 88: wind turbine systems, working group 10, CDV. [1]
Key words: wind power, power quality, grid connection 1 Introduction Recently, wind power has become the significant role in the renewable energy. The wind power installation capacity is 2159 MW in 2010, and it is indeed an ambitious goal. Industrial Technology Research Institute was commissioned by Bureau of Energy, Ministry of Economic Affairs to promote the wind energy. The tasks are wind velocity measurement, wind farm assessment, application of demonstration systems, promotion strategies formulation etc. After five years, it has reached to a satisfactory accomplishment. Taiwan’s wind condition was deeply influenced by the north-east direction wind flow in winter season. It is also benefited from the effect of Taiwan Strait. The average full load hours on the west coast from Taipei county to Chiayi county is nearly 2600-3000. It is no doubt a better region to compare with other countries. During the period of 2000-July, 2004, the demonstration subsidies for private sectors installing wind turbine was NT$ 16,000 dollars per kW. The result of three demonstration systems separately located in Mailiao, Zhongtun and Hsinchu (shown in Fig. 1, Fig.2 and Fig. 3). The total capacity was 8.54 MW. In order to facilitate the utilization of renewable energy, TPC announced the “Taipower Renewable Energy Purchase Scheme” on November 11, 2003. A favorable renewable power purchase agreement (ppa) of GBP 0.035/kWh (NT$2.0) is offered for the purchase of renewable electricity. The ppa is guaranteed for at least 15 years. Total capacity offered for renewable electricity under this scheme is limited to 600 MW. The wind power promotion changed to the period of electricity generation purchasing from July, 2004. The scheme will remain effective until the passage of the “Renewable Energy Development Bill” or at the time of Privatization of TAIPOWER, as is cited in the “Draft Revisions of the Electricity Act”.
Figure1. Mailiao Demonstration Systems (2.64 MWVestas)
Figure2. Zhongtun Demonstration Systems (2.4 MWEnercon)
Figure3. Andante Demonstration Systems (3.5 MWVestas) Apart from three demonstrative wind farms, the other wind farms belong to Taiwan Power Company and Infra-Vest GmbH in Taiwan. Energy & Environment Research Laboratories of Industrial Technology Research Institute to promote the wind power for local government for speeded up the development of wind power. There are three local governments have been promoted at present, that including Kaohsiung County, Chiayi County and Ilan County. This paper uses the example of Chiayi County to explain the evaluation of system impact caused by wind generation and the regulations of Taiwan Power Company in Taiwan.
2 Chiayi Budai proposal To ensure wind farm won’t affect the power system and result the effect on power quality after wind farm connects to the power system. It is necessary to analyze the impact on the power system and get the connection permission from Taiwan Power Company.
2.1 Locations of wind farm The wind farm is located at Budai Township, Chiayi County. This wind farm is planned to install n=13 wind turbines (See Figure4) of the same type. Capacity of each turbine is 2.0MW. The total capacity will be 26MW. According to “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company”If the total capacity of the Generator Equipment exceeds 20MW, the Generator Equipment has to connect with extra-high-voltage system, and the connection’s voltage will depend on each case’s situation. Therefore, this wind farm shall be connected to the Xinwen substation 69kV bus. The distance between Xinwen substation and the closest wind-turbine is 200M.
BudaiTownship
Taiwan
Chiayi County
Xinwen Substation
Figure4. Map of wind farm location
2.2 Point of common connection For the point of common connection the following data are available: Table.1 Network site data District Point of common connection PCC: Rated voltage Rated apparent power at PCC Maximum 69kV 3-phase short-circuit current Minimum 69kV 3-phase short-circuit current
Chiayi Xinwen substation Vgrid rated = 69kV 143MVA ISC max = 8.21kA ISC min = 7.22kA SkV max = 3 × Vgrid rated × ISC max
Maximum 69kV 3-phase short-circuit power
= 3 × 69kV × 8.21 kA =981.189MVA SkV min = 3 × Vgrid rated × ISC min
Minimum 69kV 3-phase short-circuit power
= 3 × 69kV × 7.22 kA =862.873MVA
Grid reactance to resistance ratio Grid impedance angle Circuit breaker rated short-circuit current
6.47 ψk max = arctan(6.47) = 81.21° 25kA
2.3 Wind turbine power quality data We choose three types of wind turbines that are able to use in this project to evaluate the impact of grid connection. The evaluation is mainly depended by the measurement report of IEC-61400-21. The data of the three types of wind turbines are in the Table.2. Notice that, the Grid impedance angle is 81.21° , but the power quality measure report of wind turbine only provides flicker coefficient and voltage change factor when wind turbine is running at ψk = 30°, 50°, 70°, and 85°. So we choose to use the data when ψk is 85° to calculate. Simultaneously, suppose the annual average wind speed is 8.5m/s.
Table.2 Data of the three types of wind turbines Wind turbine type Data Number of turbine n Effective power PnG Relative effective power maximum for 1 min P1 min Relative effective power maximum for 10 min P10 min Power factor (at max. power) λ Phase angle φ Rated voltage VWT rated Flicker coefficient c(ψk,va) Voltage change factor ku(ψk) Minimum short-circuit current IKE
Type A
Type B
Type C
13
13
13
2MW
2MW
2MW
1.00
1.08
0.96
1.00
1.02
1.00
0.9991
0.9965
0.9965
2.43°
4.79°
4.79°
400V
20kV
20kV
2.8
5.72
4.04
0.19
0.06
0.12
3179A
306A
289.5A
3 Power quality assessment 3.1 Calculation of the network equipments The network equipments (e.g. connection cable) are to be dimensioned for the 10-minutes average value of the apparent power of the whole SA max 10 min. Together with Table.1 and Table.2 we will be able to calculate the value of SA max 10 min. The equation is showed below as equation (1). The results are showed below in Table.3. From the results, we know that the existing grid equipments are dimensioned sufficiently for the maximum power of the wind farm since the rated apparent power at PCC is 143MVA. (1) SA max 10 min = n × SE max 10 min = n × PnG × P10min / λ Table.3 the 10-minutes average value of the apparent power of wind farm SA max 10 min Type A SA max 10 min = 13 × 2M ×1.00 / 0.9991 = 26.02MVA Type B SA max 10 min = 13× 2M ×1.02/ 0.9965 = 26.61MVA Type C SA max 10 min = 13 × 2M ×1.00 / 0.9965 = 26.09MVA
3.2 Steady state voltage change The calculation of the steady state relative voltage change U of the grid at PCC is based on the 1-minute average value of the wind farm total apparent power SA max 1 min. The equation is presented as equation (2). The calculation result is showed in Table.4. The voltage changes are below the specified limit of 2.5% (According to the specification of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company” Item 5 No. 2). ∆U=SA max 1 min × cos(ψk min + ϕ ) / SkV min Table.4 value of steady state voltage change U Type A ∆U=26.02M × cos(80.88 + 2.43 ) / 862.873M = 0.351% Type B ∆ U=28.18M × cos(80.88 + 4.79 ) / 862.873M = 0.246%
Type C ∆U=25.05M × cos(80.88 + 4.79 ) / 862.873M = 0.219%
(2)
3.3 Voltage change caused by switching The voltage change caused by the switching of the wind turbine generators during cut-in of the wind farm can be expected to be rather small, as the turbines will not start up at the same time. In normal situations the wind farm operation system will ensure a consecutive cut-in of the single wind turbine, two or more wind turbines will not be switched on simultaneously. The calculation of the relative voltage dk is based on the voltage change factor ku(ψk) at start-up of the wind turbine at rated wind speed. The cut-in of the wind turbine at rated wind speed is the highest value occurring for switching operations and thus a worst case assumption for the wind turbine under consideration. Accordingly, the maximum voltage change caused by switching of single system condition is:
dk =k u (ψk ) ×
PnG Sn = ku (ψk ) × SkV min λ × SkV min
(3)
Table.5 value of voltage change dk Type A d k = 0 .1 9 × 2 M /(0.9 9 9 1 × 8 6 2 .8 7 3M ) = 0 .0 4 4% Type B
d k = 0 .0 6 × 2 M / (0 .9 9 6 5 × 8 6 2 .8 7 3 M ) = 0 .0 1 4 %
Type C
d k = 0 .1 2 × 2 M /(0 .9 96 5 × 86 2 .8 7 3M ) = 0 .02 8 %
These are far below the maximum grid voltage change limit of 2.5%. The control system could even allow all the wind turbines of the wind farm to cut-in at the same time, even then the total relative voltage change of 0.572% (0.0044%*13) , 0.182% (0.0014%*13) , 0.364% (0.0028%*13) would not exceed the limits(According to the specification of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company” Item 5 No.2).
3.4 Voltage flicker The flicker distortion caused by continuous operation of the wind farm can be assessed from the flicker coefficient for continuous operation c(ψk,) if a single wind turbine, given at the grid impedance angel ψk and at average mean wind speed va at the site. The long-term flicker is assessed by equation (4). The calculation result is showed in Table.6. Plt = n × c(ψk ,v a ) ×
PnG Sn = n × c(ψ k ,v a ) × SkV min λ × SkV min
(4)
Table.6 value of long-term flicker Plt Type A Plt = 13 × 2.8 × 2M/ (0.9991 × 862.873M ) = 0.02342 Type B
Plt =
Type C
Plt = 13 × 4.04 × 2M/(0.9965 × 862.873M) = 0.03388
13 × 5.72 × 2M /(0.9965 × 862.873M ) = 0.04797
From IEC 61400-21, we know that when wind turbine is running, long-term voltage flicker Plt is equaled to short-term voltage flicker Pst. Researches in real experiments show the ratio between Pst and V10 is between 2 and 4, the average is about 3. Because of that, V10 can be found by using the equation (5), the results are showed in Table.7. All the results are lower than 0.45% which is also the limitation of Taiwan Power Company.
∆V10 =
pst plt ≈ 3 3
(5)
Table.7 value of voltage flicker V10 Type A Type B Type C
∆V10 =0.02342/3=0.00781% ∆V10 =0.04797/3=0.01599% ∆V10 =0.03388/3=0.01129%
3.5 Short circuit current at the PCC After wind turbines and grids are connected together, there must be influences on power system’s fault current. Before calculating short-circuit current, (not considering the reducing short-circuit current caused by transformers) we need to first figure out the short-circuit power SSC of the wind farm. Then combine short-circuit power SSC and maximum short-circuit power SkV max of the wind farm to calculate the new contact’s short-circuit power of the grid at PCC SkV Res. We can calculate the value of short-circuit current ISC Res at the PCC of 69KV grid, the equation are showed below as equation (6), (7), and (8). Table 8 displaces the maximum short-circuit current at the PCC of 69KV grid. The maximum short-circuit current at the PCC of 69KV grid is lower than circuit breaker rated short-circuit
current of 25kA. According to the specification of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company” Item 5 No. 1.
SSC =n × 3 × VWT rated × I KE
(6)
SkV Res =SkV max + SSC
(7) (8)
ISC Res =SkV Res /( 3 × Vgrid rated ) Table.8 value of short-circuit current ISC Res at the PCC of 69KV Type A
8.450kA
Type B
9.363kA
Type C
9.301kA
3.6 Harmonics The harmonic currents shall be limited to the degree needed to avoid unacceptable harmonic voltages at the PCC. The demand from Taiwan Power Company of harmonics current is regarded from the standard harmonics limitation, IEEE 519-1989. IEC 61000-3-6 gives guidance for summation harmonic current distortion from loads. Applying this, the harmonic current at the PCC is due to a wind turbine installation with a number of wind turbines may be estimated applying equation (9). And the result is showed in Table.10.
Ih∑ = β
∑( n
I h ,i
n
i =1
)β
(9)
i
where n is the number of wind turbine connected to the PCC; IhΣ is the h’th order harmonic current distortion at the PCC; ni is the ratio of the transformer at the i’th wind turbine; Ih,I is the h’th order harmonic current distortion of the i’th wind turbine; β is the exponent given in Table.9. Table.9 Specification of exponents according to IEC 61000-3-6 Harmonic order
β
1.0
1.4
2.0
Table.10 harmonics current Harmonic Wind current I2 turbine type
Order I3
I4
I5
I6
I7
I8
I9
I10
I11
I12
I13
I14
I26
I28
I32
I34
I35
ITHD
Type A
0.200 0.100 0.100 0.144 0.048 0.240 0.048 0.048 0.048 0.083 0.028 0.028 0.028 0.047 0.044 0.047 0.033 0.031 0.342
Type B
0.250 0.530 0.240
Type C
0.200 0.240
harmonics limitation
-
-
0.139
-
0.058
-
-
-
-
-
-
-
-
-
-
-
0.651
0.230
-
0.125
-
-
-
-
-
-
-
-
-
-
-
-
0.418
0.500 2.000 0.500 2.000 0.500 2.000 0.500 2.000 0.500 1.000 0.250 1.000 0.250 0.075 0.075 0.075 0.075 0.150 2.500
4 Verification result from Taiwan Power Company The results of grid’s impact analysis showed above were sent to Taiwan Power Company to precede a verification of connection impact. Taiwan Power Company agreed to issue permission to connect. But the maximum short-circuit current at the PCC of 69KV grid needs to lower than 8.96kA. Since the fault current of this impact analysis does not include the effect from wind turbine transformer, step-up transformer, and impedance etc. If add the consideration above, wind turbine short-circuit current at the PCC of 69KV grid should be able to lower than 8.96kA.
5 Conclusions The results of grid’s impact analysis of the wind farm at Chiayi County are all conformed to the regulations of “Technical Guidelines For The Connection of Renewable Energy Generating System To The Grid of Taiwan Power Company.” There are no large effects on the power quality pollution of all three wind turbines. The main reason is that the total capacity of the wind farm is not large compared to 69kV system capacity, and there are the features of IEC proof of wind turbines. Without proof from IEC, wind turbine is almost unable to sell. That’s why there is no large effect on pollution on power quality of all three types of generators used in this project. But the verification result from Taiwan Power Company requires short-circuit current at the PCC of 69KV grid lower than 8.96kA. Furthermore, researcher can investigate into whether this limitation is appropriate or not in the future.
6 References International Standard IEC 61400-21, Wind turbine generator systems – Part 21Measurement of assessment power quality characteristics of grid connected wind turbines, 2001. [2] Energy Bureau of Ministry of Economic Affairs ,” Demonstration and Promotion of Wind Energy Program”, the whole course of implement report, Energy Bureau of MOEA, Taipei, 2005. [3] Measurement and assessment of power quality characteristics of grid connected wind turbines, International Electrotechnical Commission (IEC), TC 88: wind turbine systems, working group 10, CDV. [1]
