Demonstration tests of microgrid systems using renewable energy for small remote islands

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1 21, rue d Artois, F-758 PARIS C1_116_212 CIGRE 212 http: // Demonstration tests of microgrid systems using renewable energy for small remote islands K. ISHIDA Y. SHIMOGAWA Y. SATO K. TAKANO T. IMAYOSHI Kyushu Electric Power Co., Inc. T. KOJIMA Fuji Electric Co., Ltd. Japan SUMMARY Recent years have seen heightened awareness of environmental issues on a global scale, which has led to proactive introduction of renewable energy such as photovoltaic power (PV) and wind turbine (WT) generation. This trend is expected to continue with the Japan government implementing various measures such as a surplus PV power purchase system. Considering this situation and the weatherdependent nature of renewable energy output, it is necessary to examine appropriate measures to ensure power system stability. Kyushu Electric Power Company(KEPCO) supplies electricity to many remote islands, whose power systems are not connected to that of the main island of Kyushu. On such islands, electricity is generated only by diesel generators, posing important issues of generation cost reduction. Furthermore, decreased dependency on oil is desirable in order to avoid risks associated with projected increases in fuel costs and from an energy security standpoint. Most generators on small remote islands are characterised by their small capacity of several tens of kva. The small inertial energy of such generators makes power systems susceptible to PV output fluctuation and load fluctuation. To resolve these issues surrounding renewable energy introduction into such remote islands and verify power system stabilization control technologies developed by KEPCO, we have built microgrid systems, incorporating storage batteries for control, for demonstration purposes. For demonstration tests, the following three system control technologies are under verification: 1) Compensation for PV output fluctuation: By compensating for short-cycle PV output fluctuation using storage batteries, output fluctuation at a grid-connected point can be smoothed. 2) PV output levelling: By compensating for all PV output fluctuations, through the use of storage batteries, output at a grid-connected point can be maintained at a fixed level. 3) Output shift of PV surplus power: By storing surplus power generated by PV and WT in storage batteries, stored power can be output at demand peak through output-shift. This can contribute to utilization of renewable energy as well as reductions in fuel consumption and CO2 emissions by high-efficiency operation of diesel generators. KEYWORDS Microgrid - Renewable energy - Photovoltaic generation - Battery - Small remote islands [email protected]

2 1. Background Recent years have seen heightened awareness of environmental issues on a global scale, which has led to proactive introduction of renewable energy such as photovoltaic power (PV) and wind turbine (WT) generation, with continued expansion anticipated for the future. Considering this situation and the weather-dependent nature of renewable energy output, there is concern that with expanded introduction into the power system as a whole the following problems will occur: Insufficient load frequency capability and drop in generation reserve capability due to fluctuations in renewable energy, Surplus power in off-peak periods, and Voltage rises due to reverse power flow in distribution lines. To resolve these kinds of problems, it is necessary to consider appropriate power system stabilization, with one possibility being the use of power system storage batteries, however there are hardly any operation results for actual systems as yet. Within KEPCO s service area are many remote islands where power generation cost is high, and from the point of view of reducing risk from future fuel price increase, and ensuring energy security, there is the issue of reducing dependence on oil. Against this background, KEPCO has constructed microgrid systems on remote islands that incorporate PV and WT renewable power together with storage batteries. Demonstration testing of these facilities has started with the objectives of reducing remote island fuel costs and CO2 emissions, together with the understanding of the effectiveness of system storage batteries and operational phenomena that need to be considered when each remote island is taken as a trunk electric power system. Demonstration testing started in April 21 and is scheduled to continue until March Outline of demonstration testing 2.1 Issues related to small remote islands not connected to mainland power supply On small remote islands with a demand of several hundred kw, several diesel generators have been installed according to system capacity, and output control and the number of generators in operation follow the load pattern. Many generators on small remote islands are characterised by their small capacity of several tens to several hundreds of kva. The small inertial energy of such generators means that even slight load fluctuations cause frequency fluctuations. On small remote islands with these kinds of power system characteristics, when weather-dependant renewable energy is interconnected with such systems, it is difficult to maintain operation with existing facilities alone. Table 1 shows issues associated with the introduction of renewable energy onto small remote islands [1]. Table 1. Issues associated with renewable energy introduction onto remote islands Issue Supply-demand balance Large frequency and voltage fluctuations Surplus power Description Constant supply-demand balancing is necessary (allowable fluctuations must be within governor follow-up capability for existing diesel generators). Small system capacity makes a system susceptible to frequency and voltage fluctuations caused by renewable energy or load fluctuation. Large PV output during light load period might result in surplus power or/and lowered fuel efficiency due to low output operation of diesel generators, causing uneconomical system operation. 2.2 System stabilization technology For demonstration tests, to address the issues in Table 1, the following three control functions have been developed and are now being tested [2]. 1

3 (1) Compensation for PV output fluctuation: By compensating for short-cycle PV output fluctuation using storage batteries, output fluctuation at a PV system grid-connected point can be smoothed, and the effects on diesel generator operation mitigated. Stabilization control to achieve this is being verified. (2) Output leveling: By compensating for both short-cycle fluctuation mentioned above and long-cycle fluctuation, PV output is used to charge storage batteries, and unstable PV output is output according to an operation plan. (3) Output shift of PV surplus power: Verification is being carried out on the effective use of renewable energy by storing surplus power generated by PV and WT in storage batteries and shifting output to demand peak, as well as on reductions in fuel consumption and CO2 emissions by high-efficiency operation of diesel generators. 3. Performance testing facilities For demonstration testing sites, to verify system control technology in actual systems it was decided to carry out verification on small remote islands where verification is possible with small-scale renewable energy sources, and six islands from the villages of Mishima and Toshima in Kagoshima Prefecture located in the southern part of Kyushu, Japan were chosen. Of the six islands, Kuroshima Island has the largest installed demonstration facility, and its PV output has been set at 6kW, which accounts for 31% of the island s maximum demand. This PV introduction ratio is equivalent to the ratio which would need to be achieved against maximum power in Japan if the 23 target for PV introduction is to be met [2]. For Kuroshima Island, a 1kW wind turbine has also been installed to evaluate the effects of wind power output fluctuation. In addition, lead-acid batteries as well as lithium-ion batteries, whose use is expected to grow, have been selected for storage batteries. Table 2 shows installed system capacity of existing power generation facilities and newly introduced facilities. Table 2. Installed system capacity Island site Existing diesel generators, etc. PV power (ratio) Batteries WT power Kuroshima 24kW 6kW (31%) 322kWh 1kW Takeshima 19kW 7.5kW (9%) 33kWh - Nakanoshima 253kW 15kW (8%) 8kWh - Suwanosejima 16kW 1kW (13%) 8kWh - Kodakarajima 11kW 7.5kW (11%) 8kWh - Takarajima 2kW 1kW (8%) 8kWh - Note: PV ratio is in relation to maximum power demand of each island. The configuration of the remote island microgrid system structure installed on Kuroshima Island is shown in Figure 1, and a photograph of the facilities is shown in Fig. 2. This system is interconnected with the existing 3.3kV distribution system. From the view point of system redundancy, each system is installed with a ±5kW bidirectional inverter. A supervisory control device detects output from PV and WT, and output from existing diesel generator operation, then carries out a series of control procedures, and issues effective power control commands to the two charge/discharge inverters for storage battery systems [3]. In addition, because Kuroshima Island is a remote island located far to the south off the Kyushu mainland, it is difficult to dispatch personnel quickly to the site. For this reason a remote monitoring system is installed and through a wireless network, it is possible to carry out monitoring from our 2

4 research laboratory as well as from the Kagoshima Branch Office and the Kagoshima Customer Service Office, which are both responsible for power supply. Existing power station 3G Diesel (1kW) 2G Diesel (7kW) 1G Diesel (7kW) Supervisory control device ~ WT (1kW) ~ ~ Li-ion batteries PV (6kW) (66kWh) 4V line Remote monitoring system Wireless Network Monitoring PC VPN Charge / discharge Branch inverter Office PC (5kW x 2) Research Customer Laboratory Service Lead-acid batteries PC Office PC (256kWh) ~ Existing distribution system 3.3kV line 3.3kV line Figure 1. Configuration of microgrid system installed on Kuroshima Island 4. Results of testing 4.1 Output fluctuation compensation testing Figure 2. Kuroshima Island system On Takeshima Island, Nakanoshima Island and Takarajima Island, demonstration testing is centered on compensation control of steep output fluctuation using batteries. An example of test results for Takarajima Island is shown in Figure 3. At around 9:58 a steep change in PV output can be seen, and in the space of 5 seconds output changed by approximately 5.3kW (from 1. to 6.3), meaning a speed of change of 1kW/sec. At this time the battery inverters supplied reverse current power, which was equivalent to the amount of short-cycle fluctuation in order to maintain output fluctuation at a certain 3

5 fixed rate or under. As a result, in accordance with the time constant, output at the grid connected point rose slowly to 4.3kW over a period of approximately 2 seconds. 4.2 Output leveling testing On Suwanosejima Island and Kodakarajima Island, demonstration testing is centered on compensation for all PV output fluctuations using storage batteries, and scheduled operation control for power output leveling at the grid connected point. Sample data for Kodakarajima Island is shown in Figure 4. Under output leveling, output at the grid connected point is started at a constant level at a preset time, and either at a preset time or when the state of charge (SOC) of storage batteries has become low, output is stopped. In Figure 4, over the 4 hours between 9: and around 13:, 3kW constant output control was in operation. From 13: output at the grid connected point was maintained at zero, and all PV output was stored in the storage batteries for use the following day. Takarajima Island 1 8 PV Connected point Active power(kw) kW -4 Batteries -6 9:5 9:55 1: 1:5 1:1 Time Figure 3. Output fluctuation compensation testing Active power(kw) Kodakarajima Island Connected point PV SOC 1 Batteries Time Figure 4. Output levelling testing Amount of charge stored in battery(%) 4

6 4.3 PV surplus power output shift testing (1) Outline of control: The objective of output shift control is to shift power generated by PV and WT using storage batteries and thereby make effective use of renewable energy and realize reductions in fuel consumption through highly-effective operation of diesel generators. Specifically, control is carried out over charging of storage batteries with PV and WT generated power, and then over discharging to suppress peak demand level. On small remote islands, because demand peak is during the evening for lighting while demand is light during the daytime when there is PV output, surplus power may be generated during the daytime. At these times surplus power is used to charge storage batteries and output at grid connected point is maintained at zero. During peak demand, output from diesel generators is controlled so as not to exceed a set value, while requirements over the set value are supplied from storage batteries. (2) Operation results: The graph in Figure 5 shows changes in output over a day operating with a value for peak-cut set at 1kW, the same as the rated output of the largest capacity diesel generator. As shown in Figure 5, during the PV output period of around 8: to around 16: control is carried out to charge storage batteries, and maintain output power at the grid connected point equivalent to that of the station service power. During the period from 17: to 22: when lighting is required, control is carried out to supply power from storage batteries when demand exceeds the set value (1kW). On this day, the amount of power shifted and the amount of PV and WT power generated were approximately the same. In addition, WT output changed throughout the day, but with storage battery fluctuation compensation control, there was no effect observed at the grid connected point. The above results confirmed expected behaviour of the peak shift control function. It should be noted that on the day illustrated below, 2 diesel generators were operated and PV output was limited to 3kW. 7 Island power demand 14 PV output (kw) WT output (kw) Battery output (kw) Diesel generator WT WT Peak-cut PV Li-ion batteries Lead-acid batteries Diesel generator output (kw) Island power demand (kw) Discharging -1-2 Connected point Charging Time (hours) Shift Figure 5. Example of power output shifting (3) Control effects: Figure 6 shows the results of trial calculations for fuel consumption reduction effects realized under peak shift control illustrated in Figure 5. Additional start-ups and shutdowns for diesel generators were calculated assuming demand of 1kW or more for a remote island and based on generator fuel characteristics shown in Figure 7, when renewable energy was used and when operation of the largest capacity generator, generator #3 (3G), alone was maintained. The calculation was made with the 5

7 provision that station service power of the existing power station, microgrid system and auxiliary generators, and efficiency of charging and discharging were to be ignored. In this demonstration testing, the combined effect of the amount of power generated by PV and WT and demand peak shift control, has been a reduction of approximately 3% in the amount of electricity generated by diesel generator. Further, the effect of the reduction in fuel cost, including the effect of greater efficiency due to avoidance of additional start-ups of a second generator (1G), has been 6% [4]. 15 Power generated by diesel generator Fuel consumption Reduction rate (%) G+3G (BASE) 3G+RE (RE = renewable energy, 3G = #3 generator, 1G = second generator) Figure 6. Effects on amount of power generated and fuel consumption cost Fuel consumption(%) 2% 18% 16% 14% 12% 1% 8% 6% 4% 2% % 3G+1G 3G High-efficiency operation Generator output(kw) Figure 7. Generator fuel characteristics (calculated assuming rated output time of #3 generator to be 1%) 5. Conclusions The introduction of renewable energy is a worldwide trend, which is likely to further accelerate in the future due to fossil fuel supply and demand constraints. In the future it will be necessary for the overall picture of power systems to evolve, with the objective of making effective and highly efficient use of various power sources. 6

8 On remote islands that have no supply of electricity from the mainland, from the point of view of system stability and reliability, the extent to which renewable energy can be introduced is uncertain. For introduction to take place it is necessary to establish control technologies. In addition, on small remote islands where large scale plants such as nuclear and LNG power plants are not possible, the practical convenience of shipping and storing fuel means that for the time being there is little choice but to rely on internal combustion generation using oil. However, to just burn oil, an expensive resource, which has so many uses from clothing and plastics to medical supplies, is very wasteful. Oil is a valuable resource and to the extent possible should be left to future generations. The demonstration testing described in this paper is still in its early stages. We want to continue verification and by evaluating various operation patterns find ways to reduce fuel consumption and CO2 emissions. Based on the results of this demonstration testing, we will approach challenges such as the configuration of next-generation power systems, which will take into consideration voltage fluctuations caused by PV generation, and advancements in distribution line voltage regulation, and thereby prepare for the expected expanded introduction of renewable energy into the power system. BIBLIOGRAPHY [1] T. Imayoshi, K. Takano, et. al. System Constitution for Renewable Energy Introduction to the Small Scale Island, 29, IEEJ B Section Conference, No. 161 [2] K. Ishida, Y. Shimogawa, et. al. Verification of Microgrid System in Small Islands (Part1) - Overview of Project, 21, IEEJ B Section Conference, No. 22 [3] K. Takano, T. Imayoshi, et. al. Verification of Microgrid System in Small Islands (Part2) - System Structure, 21, IEEJ B Section Conference, No. 221 [4] T. Imayoshi, K. Takano, et. al. Test with Power Utility of Microgrid System in Small Islands, 211, IEEJ National Conference, No