Chapter 4. Analysis of reserve in 10-minute increments
B. Estimation of the output variation rate of renewable energy using weather data
As explained in Chapter 3, to ensure consistency between the two analytic models that this study used to predict solar and wind power generation, the 16 solar PV and 16 wind power installation locations that were selected to conduct the reserve analysis by hour using the MPSOPF model are used again here for the 10-minute reserve analysis.
To estimate short-term variability by 10-minute increment, the Weather Data Open Portal22 was used to obtain one-minute weather data for the 16 locations in 2017. To estimate the variation rate of output by source of power generation, instead of the correlation between solar PV and wind power, solar radiation (MJ/m2) and wind speed (m/s) data, excluding temperature (°C), were used, from among the input variables used in Chapter 3 of this study. As for solar PV, one-minute solar radiation data for 15 locations were obtained, excluding one location for which data were not available.23 As for wind power, one-minute wind speed data for 15 locations were obtained, except for one offshore location for which data were not available.24 The capacity of the offshore location that was excluded was allocated to the other 15 locations.
Based on these weather data and using the same method presented in Chapter 3, one-minute generation data corresponding to the renewable energy capacity target for 2030 were created. For the 10-minute analysis, the one-minute generation data were summed up by 10-minute increment to obtain 10-minute generation data. Based on the 10-minute generation data, the variation rate of renewable energy per 10 minutes was calculated. These procedures are summarized in Figure 4-2.
Figure 4-2. Method of estimating the variation rate of renewable energy
Source: author
단계1. 재생에너지 보급용량 및 계획용량에 기반한 Stage 1. Select locations, according to deployed renewable energy capacity and planned capacity.
22 https://data.kma.go.kr, accessed on Oct. 24, 2018.
23 s6, ASOS 152, Ulsan.
24 w15, AWS 22106, offshore, Pohang.
관측지점 선정
태양광 및 풍력 각각 16개 관측 지점 선정 Select 16 solar PV and wind power locations.
단계2. 기상자료개방포털로부터 기상 데이터 취득 Stage 2. Obtain weather data from the Weather Data Open Portal.
선정된 관측 지점에서 1분 단위 일사량(MJ/m2) 및 풍속(m/s) 데이터 취득
Obtain one-minute solar radiation (MJ/m2) and wind speed (m/s) data for selected locations.
단계 3. 재생에너지 발전량 데이터 생성 Stage 3. Create renewable energy generation data.
일사랑과 풍속데이터로 1분 단위 발전량 데이터 생성
생성된 데이터를 10분 단위로 합산하여 10분 단위
발전량 데이터 생성
Create one-minute generation data using solar radiation and wind power data.
Sum up the one-minute generation data per 10 minutes to obtain 10-minute generation data.
단계 4. 재생에너지 발전량 변동률 계산 Stage 4. Calculate variation rate of renewable energy generation.
생산된 재생에너지 발전량의 10분 단위 변동률 계산
Calculate 10-minute variation rate of the renewable generation data.
In the case of the downward variation of renewables, the supply to the power system becomes insufficient, requiring the procurement of an appropriate amount of reserve to prevent load shedding. Conversely, in the case of the upward variation of renewable energy generation, oversupply occurs, calling for means of addressing the oversupply problem. While some countries maintain an operating reserve in consideration of both downward and upward variation, Korea maintains an operating reserve only for downward variation, because any upward variation of output can be addressed by cutting back on non-base load generation; and if that is not feasible, renewable energy generation can be curtailed. Hence, this study analyzes only the variation rate in the case of a downward variation of renewable energy generation.
The renewable variation rate used for this analysis was estimated with a certain confidence interval. As the maximum value may include unsuitable data, such as outliers and measurement errors, various confidence intervals, including 2-sigma (95%), 2.5-sigma (98.8%), and 3-sigma (99.7%), were considered to find values, with this study adopting a confidence interval of 2.5-sigma (98.8%).
Figure 4-3. Example of probability distribution under different confidence intervals
Source: author
Under the Renewable Energy 3020 Implementation Plan, the installed capacities of solar PV and wind power in 2030 are expected to be 33,530MW and 17,674MW, respectively, totaling 51,204MW. For the installed capacity of 2030, the data of Tables 3-2 and 3-3 were used.25 Based on this and the wind speed and solar radiation data of 2017, the generation of solar PV and wind in 2030 were
25 As one-minute weather information for the offshore location of Pohang (w15, AWS 22106) was not available, the installed capacity of four locations (excluding Yeongdeok) among the five offshore wind installation locations in Table 3-3 (Saemangeum, Sinan, Yeosu, and Moseulpo) was divided by 3,000MW to establish the offshore installed capacity of 2030.
estimated, and the output variation rate estimated. The 10-minute downward variation rates of solar PV and wind generation are listed in Table 4-2.
Table 4-2. Estimation of 10-minute variation rates for solar PV and wind generation
Installed capacity (MW) 10-minute downward variation rate
(Lower CI = 98.8%)
Solar PV: 33,530 -9.94%
Wind power: 17,674 -4.65%
Source: author.
The 10-minute variation rates of solar PV and wind power at the lower CI of 98.8% (2.5-sigma) are estimated at negative 9.94%
and negative 4.65%, respectively. However, when solar PV and wind power are operated simultaneously, each does not generate power independently. For example, when solar PV generation rises, wind generation may decrease, and when solar PV generation decreases, wind generation may rise, leading to a smoothing of variation overall. Therefore, a simple sum of the variation rates of solar and wind could overestimate the variation rate.
Therefore, to consider the smoothing effect of solar PV and wind power, this study integrated solar and wind generation and estimated the installed capacity of solar PV and wind power in 2030 and the variation rate of output. As a result, the 10-minute downward variation rate of renewable energy generation in 2030 (lower CI= 98.8%, 2.5-sigma) was found to be negative 6.29%.
Table 4-3. Estimation of 10-minute rate of change of renewable energy in 2030 Installed capacity of renewables in 2030 (MW) 10-minute rate of change
(lower CI= 98.8%) Solar PV
33,530
Wind 17,674
-6.29%
Total installed capacity 51,204
Source: author.
2. Estimation of 10-minute reserve
A. 10-minute reserve reflecting renewable energy variability
According to the current domestic reserve standard, reserve that must be able to fully respond within 10 minutes is contingency reserve. Currently, the installed capacity of renewable energy generation plants is smaller than that of non-base load generation plants, and thus contingency reserve for renewable energy resources is not considered. However, with the construction of large-scale generation complexes by 2030, the installed capacity of renewables will grow to 50GW. Therefore, the reserve of renewable energy in 2030 needs to include contingency reserve. In this case, the contingency reserve to be established for renewable energy resources is the maximum capacity of a single renewable energy generation complex, as with the calculation of the contingency reserve for non-base load generation.
Meanwhile, as the capacity of renewable energy generation rises, so does the variation of renewable energy generation. Sudden, short-term output variations may be treated as accidents, because they involve large capacities and are difficult to predict.
Hence, the minimum 10-minute reserve required is the largest among: ① renewable energy variation, ② accident at the largest single renewable energy generation complex, and ③ accident at a non-base load generation plant. In this study, the reserves of these three cases were calculated and the largest of the three was selected as the 10-minute reserve.
Table 4-4. Estimation method of 10-minute reserve
Purpose of reserve operation
1) Cope with sudden, short-term changes (10 minutes) in renewable energy generation output
2) Cope with the sudden suspension of a single renewable energy generation facility (due to weather events such as high winds, typhoon, etc.)
3) Cope with unexpected suspension of a generation facility
Estimation method
The largest of the following is selected as the 10-minute reserve:
1) Total installed capacity of renewable energy generation x 10-minute downward variation rate (lower CI=98.8%)
2) Maximum capacity of a single renewable energy generation plant 3) Maximum capacity of a generation facility
Available
resources Capacity of non-base load generation plants in operation that can be integrated into the grid within ten minutes.
Source: author.
1) Required reserve to cope with sudden, short-term changes in the output of renewable energy generation As mentioned above, because the current installed capacity of solar and wind generation is small compared to the size of the total power system and change in demand, renewables are not subject to reserve. However, in 2030, the increased penetration of renewable energy will increase variation, which must be counted when calculating the required reserve.
Excluding the effect generated by integrating the generation of solar PV and wind, the respective variations of solar PV and wind generation are listed in Table 4-5. The 10-minute variation rates at the lower CI of 98.8% for 2017 are 9.94% for solar PV and 4.65%
for wind. If these rates are applied for the installed capacity in 2030, the variation of solar PV and wind power are 3,300MW and 820MW, respectively, totaling 4,120MW.
Table 4-5. Variation without smoothing effect Installed capacity of renewable energy
(MW)
10-minute downward variation rate
(lower CI= 98.8%) Variation (MW)
Solar PV 33,530
9.94%
of installed capacity 3,300
Total 4,120 Wind
17,674
4.65%
of installed capacity 820
Source: author
Table 4-5 assumed that solar PV and wind power are independent resources, ignoring the smoothing effect. As a result, variation estimated based on the respective rates of the two resources could be too high. Accordingly, to estimate the total variation rate considering both solar and wind, total generation was calculated by summing up solar PV and wind power generation over a certain period of time. Using the same method to estimate the respective variation rates for solar PV and wind in 2030, the variation rate of total generation in 2030 was estimated. The 10-minute downward variation rate (lower CI=98.8%) of the estimated variation rate was found to be 6.29%. Using this rate, the variation of VRE in 2030 is 3,200MW (Table 4-6).
Table 4-6. Variation of output due to the variability of renewable energy
Installed capacity of renewable energy (MW) 10-minute downward variation rate
(lower CP= 98.8%) Variation (MW)
Solar PV 33,530
Wind
17,674 6.29%
of installed capacity 3,200
Total installed capacity 51,204
Source: author
The estimates in Tables 5-5 and 5-6 are only examples. When estimating actual reserve, the actual installed capacity and variation rate of renewable energy at the relevant time need to be applied.
2) Required reserve to cope with accidents at renewable energy power plants
The required reserve needed to cope with accidents in 2030 can be estimated for the largest single generation complex. As an example of a construction plan for a future renewable energy power plant, the Saemangeum Renewable Energy Project26 was referred to. The size of a single generation complex is based on a substation, and 2.1GW is the capacity of a single generation complex. This value can be adjusted as the penetration of renewable energy increases and additional information, such as power plant construction plans, is released.27
3) Required reserve to cope with the unexpected suspension of operation of a unit of a power plant The power plant with the largest capacity among non-base load power plants currently in operation is Singori Unit 3, a nuclear power plant with a capacity of 1,400MW. Thus, the maximum capacity of a non-base load power plant in 2030 is set at 1,400MW.
Figure 4-4. Example of estimation of 10-minute reserve for 2030
Source: author
태양광 및 풍력 발전 변동성 Solar PV and wind generation variability
재생에너지 발전 변동량 Renewable energy generation variation
재생에너지 발전단지의 최대용량 Maximum capacity of renewable energy generation complex
새만금 재생에너지 사업 계획 Saemangeum Renewable Energy Project 발전기 단위기 1기의 최대용량 Maximum capacity of a unit of a power plant 신고리3호 Singori Unit 3
최대값 Maximum value
2030년 10분 예비력 10-minute reserve in 2030
4) Estimation of 10-minute reserve for renewable energy
In sections 1) to 3) above, the variation of renewable energy generation, maximum capacity of a renewable energy generation complex, and maximum capacity of a unit of a power plant are estimated to be 3,200MW, 2,100MW, and
26 Saemangeum is a leading renewable energy development in Korea that is expected to have a solar PV capacity of 2.4GW and offshore wind capacity of 0.6GW by 2022.
① Naecho Substation (0.4GW), ② New substation (2.1GW), ③ Outer side of Saemangeum (0.5GW) (Source: Saemangeum Development and Investment Agency/Jeollabuk-do, 2018, a diagram on p. 4 of press release of October 29, 2018, “Saemangeum Renewable Energy Vision Proclamation Ceremony held”).
27 For example, the MPSOPF model used in this study assigned 2.4GW to each of the five offshore wind locations. However, this is an assumption that was made to simplify the model, and the scale of an actual offshore wind power complex cannot be determined.
1,400MW, respectively. The largest of these (3,200 MV) is the 10-minute reserve for 2030.