Study on development of Grid code and operation scheme of Jeju Island with high wind penetration

Seung-Il Moon*, Gi-Chan Pyo*, and Jin-Woo Park* *School Of Electrical Engineering and Computer Science, Seoul National Univ, Seoul, Korea

Abstracts – Lots of large-scale projects on wind generation are currently being commissioned, with more planned for the near future in Jeju Island of Korea. The large-scale wind generation, however, could have adverse impacts on stable operation of power system. Accordingly for the stable operation of Jeju power system, the necessity to limit the installation is brought up recently and new large-scale wind power projects are restricted for now. To overcome the limit and increase the wind penetration in Jeju Island, solutions which ensure the stable operation of the Jeju power system as well as increase of wind penetration is presented in this paper; Establishment of grid code for wind generation and development of coordinated operation schemes of Jeju power system.

1. Introduction

Recently, due to international restriction on CO2 emission and fossil fuel exhaustion problem, various efforts to reform the energy production structure are continued in Korea. Investment of Korean government in the research and development of will be come to about 11 billion dollars up to 2030 and various political supports are planned in the future. According to long-term strategy that will determine the direction of national energy policy, fossil fuels will account for 61 percent of total energy consumption by 2030, down from the current 83 percent, while the use of renewable energy will increase to 11 percent from 2.4 percent in 2007. Among these, wind generations are planned to be installed about total capacity of 2,250MW until 2012 and quite a few portions of total capacity are concentrated in Jeju Island. Wind generation is one of the most attractive renewable energy in many countries due to its technical and economic feasibility. Accordingly, many advanced European countries of wind generation have relied on it for the considerable part of their own energy demand in practice. The large-scale wind generation, however, could have adverse impacts on stable operation of power. First of all, drastic changes of active power output according to the change of wind speed make it difficult to maintain the active power balance of the system. In addition, wind generation tends to be interconnected to the point of weak-grid, such islands, mountain areas, and seaside areas, because the average wind speed determines its economic feasibility. These wind generation predisposes the system to disturbance such a fault. It means that increase of wind generation regardless of the supports to the power system has limits. Many countries in which wind generation is increasing rapidly have experienced this problem and Jeju Island is one of the example cases[1]. In Korea, Jeju Island has the highest average wind speed among the promising sites for wind power generation. Many companies have shown interest in the wind power business of Jeju Island due to favorable wind conditions. As a result, the total capacity of wind power installed in Jeju system reached 94MW in the year of 2009. It is a considerable quantity compared to the minimum load of 300MW and enough to threaten the stable operation of system. Further, considering that the wind generation planned to be installed is more than 200MW, stable operation of the Jeju power system can be difficult in the near future. Thus the necessity to limit the wind power installation in Jeju Island is brought up recently and several researches related to maximum penetration limit are performed. Finally, new large-scale wind power business or projects in Jeju Island are restricted for now[2][3]. To overcome the limit and increase the wind penetration in Jeju Island, solutions which ensure the stable operation of the system in spite of high wind penetration can be provided in two respects; Establishment of grid code for wind generation and development of coordinated operation schemes of Jeju power system. Thus the technical considerations which are discussed to establish the grid code for wind generation of Korea and relevant regulations are presented and briefly discussed in the first section of this paper. Then coordinated operation schemes of Jeju power system considering wind generation and HVDC is proposed in the second section of the paper.

2. Establishment of Grid Code for wind generation

The wind power generations recently spread over many countries; numerous large-scale wind projects are currently being planned under the political supports of governments. However, the grid code for wind power generation is not established yet in most countries which are in the early stage of wind power introduction. There are few limitations or qualifications of system operator to the interconnection of wind turbines or wind farms in power system. Moreover, the wind power generations are considered to impact rarely on stable operation of entire power system until now, because most wind turbines are connected to distribution system on a small-scale and their penetration is relatively low comparing with conventional generations. There is a limit to increasing the wind generations while maintaining the system stability in this situation. To increase wind penetration, wind turbines should provide proper supports to system operation, like the conventional generator. It is necessary to utilize the various control functions, such as active and reactive power control, of wind turbine which uses the advanced power-electronics technologies[4]. Major wind manufacturers of the world recently tend to produce the wind turbines which have these functions or provide system in which the functions are included. However it may require additional cost and accordingly the voluntary investment of each individual wind power provider is hard to expect. Thus system operator should enact the grid codes for interconnection and operation of wind generation and demand technical requirements which need to maintain the stable operation of the system according to operating condition. The grid codes of advanced countries in wind generation and various regulations included can be a good example[5]. The biggest barrier against vigorous driving of large-scale wind power business in Jeju Island is also the absence of grid codes for wind farm interconnection and operation. There are only interconnection standards and operation requirements for the general , but grid codes being directly applicable to wind turbine or wind farm are not established yet. However many researches to enact the grid codes for interconnection and operation of wind generation are being executed, and grid codes that reflects the characteristics of Korean power system, especially Jeju power system will be established sooner or later. To develop the principal requirement in grid codes various cases of advanced countries are investigated, and a lot of system studies are performed to apply the characteristics of Korean power system into detailed regulations. The principal requirements of grid codes discussed currently are as following

- Active power regulation - Reactive power regulation - Fault ride through - Communication - Central dispatch of System operator

1) Active power regulation

Active power regulation facilitates the wind turbines or wind farms to receive dispatch orders from system operator and operate by them. This regulation can contribute to increase the installation of large-scale wind farm in small power system, like Jeju Island. Thus European countries, such as Denmark, Norway, Germany, and Ireland, with high wind penetration in general have various regulations related to active power control[6][8][9][11]. In the case of Denmark- Eltra and Elkraft, especially, system operators require detailed and strict rules on active power control to maintain the system stability against increase of wind generation. In addition to absolute power, delta, and ramp rate control, wind power turbines are regulated to have several active power control functions. As result, Denmark can be the developed country in which wind power penetration is over twenty percent of total domestic equipments despite the country has small power system. In Korea, as the wind penetration level of main land is insignificant comparing with scale of power system and the existing generation until now, particular active power regulations are rarely required in mainland. However, the wind penetration of Jeju-Island is increasing rapidly due to favorable wind conditions and the necessity to limit the wind power installation in Jeju -Island is brought up recently. Since Jeju Island also has the plan to increase wind power penetration more than Denmark, to solve the wind penetration limit in Jeju-Island, active power regulations for wind generation should be considered. Thus such active power regulations are expected to be included in grid code being developed in Korea. Considering the characteristics of Jeju power system, detailed regulations in active power control requirement and their substantial roles are as following: a) Active power controllability It regulate that wind farms or wind turbines should be capable to limit the active power production in the range of 20…100 percent of its rated capacity according to dispatch order. This regulation is not to be applied when the wind farm or wind turbine isn’t able to produce active power as ordered due to low wind velocity. It means that wind turbine or wind farm should have the proper equipment and control system which enables each control, such as absolute power, delta, and ramp rate control, defined in grid code. b) Wind farm control system Wind farms should be equipped with control system which ensures the remote and autonomous control of each wind turbine included in according to requirements described in grid code. The example that this regulation is applied actually can be found in Denmark case[6]. According to Danish grid code, for each wind farm the function called "farm controller" is to be implemented and ensure that regulating orders to the wind farm's total production are met in the connection point. The farm controller shall enable ordering of the various types of regulation as total orders which can be given both locally and via remote control and considering lots of wind turbines as just one wind farm. Fig. 1 shows the farm controller schematically.

Fig. 1. Wind farm supervisory control system c) Absolute power limitation Absolute power limitation is used as a means that restricts maximum power output of wind generation systems according to occasion demands such that the expectation of wind power output exceeds the maximum penetration limit. Wind farm or wind turbine should have the function which decreases the output power to reference value. When the regulation is applied to actual operation, spinning reserve to compensate the variation of wind generation can be decreased and instabilities of active power balance due to sudden increase of wind generation can be prevented. The wind penetration limit of isolated power system is also to be calculated simultaneously considering the corresponding state of loads and generations at that time not minimum load condition. It means that installation of wind generation need not to be suppressed by wind penetration limit any more. Actual output power of wind generations is to be limited only by operating condition regardless of total installation capacity. Fig. 2 shows the application of this regulation in practice. d) Ramp rate limitation For stable operation, this regulation limits the power gradient of wind generation to a set point defined by system operator when the output power of wind generations increases rapidly. The commitment of conventional generators decrease as the wind penetration largely increases. In this case, conventional generator committed may not decrease its output power according to rapid increase of wind generation simultaneously. It causes the frequency problem and unstable operation of power system in the worst-case. To prevent the frequency problem, this regulation limits the increasing speed of output power when the changes of load and wind generation are getting larger than total ramp rate capacity of conventional generators. Thus a set point of this regulation should be assessed considering the load following capability of main thermal plants. In the case of Nordel and AESO, system operator of Europe and Canada, only 10% of upward change per minute of wind generations is allowed in grid code. In Denmark, system operator determines and instructs the power gradient limits base on system condition. e) Delta control Delta control which limits the wind power output below the available power by fixed amounts defined by system operator is necessary to secure the spinning reserve in small and isolated power system. It means that wind generations have to operate with its own reserve, like conventional generator. Thus this regulation can be used to solve the problems caused by decrease of spinning reserve of conventional generation and increase of controlling power for wind generation, similarly as ramp rate limitation. The delta control as well as ramp rate limitation in fact can be selective options excepting for in isolated power system with extremely high wind penetration. For example, Jeju power system is basically isolated but it has interconnection to mainland with HVDC system. The HVDC system performs important roles to maintain the system frequency in Jeju power system. The HVDC operated in frequency control mode can damp the frequency deviation caused by wind power fluctuation quickly and also provide sufficient reserve. It means that these regulations may be rarely needed for the time being although its wind penetration is relatively high.

Fig. 2. Practical application of active power regulations; Absolute power limitation, Ramp rate limitation, and Delta control in order

As the wind penetration is largely increased, active power regulations mentioned above at least must be included in grid cod. They are, however, to be applied according to operating condition of the power system and restrict the wind generation if needed only. Thus the losses resulted from the regulation are expected to be relatively small while active power regulations enable the increase of wind generation over the wind penetration limits in the cases, like Jeju Island in which the additional wind projects are restricted to maintain the system stability. Although the investment to prepare the equipments for active power regulations will be necessary, these active power regulations will enable and encourage a lot of wind projects in Jeju Island consequently.

2) Reactive power regulation

To maintain the voltage of overall system stably, reactive power compensators should be equipped sufficiently and controlled continuously according to change of reactive power demand. The characteristics of reactive power consumption of entire power system can be changed largely according to erratic changes of wind generation and it may not be fully compensated due to decrease of conventional generation if the wind penetration is increased. Therefore wind turbines are required to have the reactive power compensation capability and the related equipments which come close to that of conventional generation recently. The related regulations can be founded from the grid code of many countries. In the case of isolated power system, such a Jeju power system that has low SCR(), strict reactive power regulations should be applied to improve voltage stability. Thus it is important to use the DFIG type generators or direct drive type generator of the newest technology which are expected to have a controllability required in wind projects hereafter. It is also needed to improve the control system of existing wind generator to satisfy such various regulations in grid codes for wind generation. a) Reactive power controllability The wind farms and wind generators should be capable to perform the reactive power control or voltage control at the PCC (Point of Common Connection) according to system opertor’s order if the power system is in normal operating condition. Because the existing wind generator of constant speed type needs the additional reactive power compensation to voltage control at the PCC, it was standard to operate the wind generators under the constant regulation, especially unit power factor, until the recent past. However as the wind generators of DFIG type which is able to control reactive power continuously as well as active power using power electronic converters are used in the most wind farms recently, the importance of voltage control is being emphasized. Voltage control ensures the wind generators use its reactive capability efficiently and contribute the recovery of entire power system during and after fault. Therefore, the regulations for voltage control are included or planed to be included in grid code of advanced countries of wind generation, such as Denmark, Nordel, Canada, Ireland, and Germany[6][7][8][9][11].

b) Range of reactive power control This regulation defines the reactive power capability of wind generator and wind farms to apply the reactive power control. The wind generator and wind farm should be capable to control the reactive power in the range defined. The ranges of reactive power control required in most grid codes have similar shape and characteristics. Then the range of reactive power control which is expected to be required in grid code being developed in Korea is presented in Fig. 3.

Fig. 3. The range of reactive power control

3) Dimensioning frequency

As mentioned above, increase of wind generation in isolated system such a Jeju Island make it difficult to control the system frequency stably due to decrease of conventional generators which are responsible to maintain the system frequency. Sudden change of demand or wind generation can cause frequency problems in this situation. Thus the wind generations are required to increase or decrease its output power immediately according to changes of system frequency and maintain the connection to grid for a certain time even if the system frequency changes dramatically. Such countries as Ireland, Germany and Denamrk with high wind penetration have related regulation in grid code and demand wind generation to participate in the frequency control of system actively[6][8][9].

Fig. 4. Dimensioning frequency a) Dimensioning frequency This regulation defines the region of frequency in which the wind generator and wind farms should increase, decrease or maintain its active power output. Fig. 4 shows active power regulation related dimensioning frequency which is expected to be included in grid code being developed in Korea. In this figure, the operating point A, B, C and D should be determined based on consultation between the wind provider and system operator and approved by system operator consequently. The operation schemes of each region are as following; first, the region between A and B means that the wind generators and wind farm should increase its output power to available power like the droop operation of conventional generators according to drops of system frequency. Next, wind generation is to operate with spinning reserve for operation of above region as if the delta control is applied in the region between B and C point. On the other hand, the wind generation is to decrease its output power in proportional to the increase of system frequency in the region between C and D. As a result, the wind generation can participate in frequency control of entire system automatically. Then wind generators and wind farm can be tripped immediately to protect its power electronic equipments when the system frequency varies out of the range between A and D. In the case of Germany, regulations which define active power regulation according to frequency variation are included in grid code, and Ireland also has the same regulation as described above. b) Low and Over frequency ride through Sudden trip of wind generation due to changes of the system frequency, especially drops, causes further frequency drops and it can make the entire system unstable. It is necessary to keep the operation of wind generator like conventional generator when the system frequency even changes largely in the system with high wind penetration. Thus the related regulations in the most grid codes of advance countries of wind generation are stipulated. This regulation is to be included in the grid code of Korea. Then the wind generators and wind farms should be capable to maintain the normal operation continuously when the system frequency varies in the range of 60±1.5Hz and during 20 seconds at least even if the frequency drops to 57.5Hz. Here, the range of frequency variation is same as that of conventional generator which is defined in the standard for Reliability of Power Supply and Quality of Korea. As the grid codes for wind generation are tend to strengthen, the various regulations which require the wind generation to participate the frequency control actively are getting larger importance. Accordingly, many studies on coordinated operation of wind generator and wind farm related to frequency control are researched in Korea. Recently, researches on frequency supports of wind generation utilizing its wide control range and functions mainly studied.

4)Fault Ride Through capability

It was common practice that the wind generators are disconnected immediately to prevent the damage of its equipment and system when the system voltage decreases under the certain level during fault. Trip of wind generator and the accompanying loss of generation, however, can disturb the recovery of system and cause additional frequency problems as the wind penetration increases over a certain level[12]. To prevent these undesired trips of large wind generation due to under voltage and unstable operation, the regulation which requires the wind generator and wind farm to maintain the operation during the fault is being included in grid codes of many countries. This regulation is called as ‘Fault Ride Through(FRT)’ or ‘(LVRT)’. The most grid codes demand the wind generator to equip these FRT capabilities and make them support recovery of the system after the fault cleared applying the additional requirements. Moreover, this regulation is tend to strengthen to the Zero Voltage Ride Through(ZVRT) in most countries.[13] Fault Ride Through regulations of various countries are presented in Fig. 5 and they have a lot in common defining the residual voltage, fault duration, and voltage recovery to maintain the operation of wind generation. Thus wind generators which are being connected to grid should maintain the operation and not be tripped even though low voltage is applied at the PCC during the fault if residual voltage and fault duration are upper side of the graphs. On the other hand, if the residual voltage becomes lower and fault duration is longer than FRT capability, wind generators can be tripped to protect its equipment.

Fig. 5. Fault Ride Through regulations of various countries

Table 1. presents the main parameters, such as residual voltage, fault duration, and voltage recovery, of Fault Ride Through capability in grid codes modified recently and shows that Zero Voltage Ride Through are required in most grid codes. In the countries which do not require the Zero Voltage Ride Through, Spain and Ireland define the relatively lower voltage and longer duration.

Table 1. Main parameters of Fault Ride Through capability

Countries Voltage dip(P.U.) Duration(ms) Recovery(ms) England & Wales 0 150 2500 (85%) Scotland 0 100 1200 (80%) Nordel 0 150 750 (90%) USA(WECC) 0 150 1750 (90%) Denmark 0 150 700 (60%) Australia 0 120 150 (90%) Germany 0 150 Type Canada 0 150 3000 (90%) Spain 0.2 500 1000 (80%) Ireland 0.15 625 3000 (90%)

Considering characteristics of Jeju Island in which severe low voltages are applied during the fault in general, the Fault Ride Through regulation should be include in the grid code being developed. Fig. 6 shows the FRT capability curve expected to be established. In this figure, the detailed parameters are determined through a lot of studies on voltage profile and dynamic stability of Jeju power system during the fault although they can be modified. The fault duration is defined based on setting of voltage relay installed in Jeju power system. Then the residual voltage and voltage recovery are determined considering the voltage profile of Jeju power system during the fault so that the wind generators can maintain the operation for almost all possible faults. This FRT capability curve can be relaxed if considering only the power system of main-land in which the SCR and the voltage profiles are quite high during the fault. This capability curve, however, is likely to be applied to FRT regulation almost as it is, because the wind penetration is highest in Jeju Island and expected to increase largely in the foreseeable future.

Fig. 6. FRT capability curve expected to be established in Korea

5) Communication and Supervisory Control of wind generator

For successful active and reactive power control of wind turbines and wind farms, communication between system operator and wind farm operator as well as technical abilities of wind turbines is very important. The dispatch orders and reactive power controls from system operator are given to each wind generator or wind farm using the communication system. Thus technical abilities of wind generations described above cannot be utilized properly without the communication system well defined. The remote and supervisory control system which operates the wind generators in wind farm according to dispatch order from system operator is also essential. It makes the wind generations work as such conventional generators being controlled by system operator on real-time demands. Communication system also enables the wind generator transmit the various information of each wind farm and wind generator to system operator. Then the system operator determines the dispatch orders efficiently using this information in addition to operating condition of the entire system. The most grid codes define the regulation related to communication and information delivery system for wind generation with active and reactive power regulations recently. In the case of Denmark especially, detailed regulations about the external control and metering of wind turbines are stipulated in grid code. In newly developed Korean grid code for wind generation, specifications for communication protocols and systems will be also included. a) Communication and information delivery Available active and reactive power of wind generation in controllable range can change continuously, as their operating condition changes according to wind speed. Thus the system operator should know about specific information respecting the operating conditions of wind farm or wind generator including wind speed in addition to general information required to conventional generators, when the system operator dispatches wind generations. The regulations for communication and information delivery are included or expected to be included in the grid code of Denmark, Ireland, Canada, and USA[6][7][9][10]. The following table specifies the data which shall be transferable between wind farms (or wind generators) and system operator. The overview is indicative, and details shall be agreed for the individual wind farm and wind generator with the system operator.

Table 2. Summary data overview for the wind generation Summary data overview for the wind generation Operating condition of wind farm: Active and reactive power measurement at PCC [MW, Mvar] Voltage measurement at PCC [V, RMS] Available active and reactive power estimation [MW, ±Mvar] Active power limitation [MWh/5min] Operating condition of wind generators: Number of turbines stopped due to low and high wind Number of turbines stopped due to maintenance and forced outage Total number of turbines out of operation Number of turbines with limited capacity Circumstances information of wind farm: Wind speed [m/sec] Wind direction Temperature [°C] Atmospheric pressure [atm]

In practical application, these regulations require that the SCADA system for wind generation is constructed. Then the EMS of the system should be modified to utilize the information from wind generation in the procedure to determine dispatch orders of entire system. Thus it is also necessary to define the protocol and requirements for communication between system operator and wind farm (or wind generator). b) Active power prediction and notification Wind farms or wind generators should be equipped with the prediction system for active power estimation of the next day and notify active power prediction to system operator in advance. Then the predictions should be updated for certain time periods continuously to ensure the reliable market operation of power system. As the wind penetration increase, wind generation prediction is getting larger importance. If predictions for wind generation are not considered, power system should always prepare the reserve for entire wind capacity installed as well as the load change. It costs additional expenses and makes the system operate inefficiently. Thus in the system with high wind penetration, predictions for wind generations should be notified to system operator in advance so that system operator operates system and makes the plans efficiently. In Denmark, Ireland, and Canada, the regulations for active power prediction and notification are included in their grid codes and applied actually. Moreover, for accurate prediction of wind generation, the technical developments and various researches on the prediction of wind speed are being preceded actively. In the case of Ireland, especially, the regulation require that wind generator and wind farm perform the prediction for active power generation on a daily basis for the following 48 hours for each 30 minute time-period and notify it to system operator. 3. Coordinated control scheme in power system of Jeju Island

The maximum load on Jeju is about 600MW and the minimum is approximately 300MW. Jeju power system is mainly fed by diesel plant, thermal plant and gas turbine plant. Total cumulative capacity of conventional unit is 685MW. Jeju is interconnected with mainland by 2 poles of HVDC links which capacity of 300MW; the 12 pulse bi-polar HVDC system normally transmits 150 MW from main land to Jeju power system, corresponding to 60% of the total load demand in Jeju Island [2]. HVDC is used only for sending cheap electricity from mainland to Jeju, thus control system of Jeju HVDC is designed for uni-directional operation. Meanwhile, another HVDC will be installed in 2012. New HVDC was originally planned only to operate in one direction: from the mainland to Jeju Island. However S/O has modified recently the operation scheme in order to allow a change in the direction of the power flow, so it will be possible to transmit the surplus power to the mainland. The number of wind turbines reached 40, with a total installed capacity of 80MW. Because average wind speed of Jeju Island is the highest among several candidates and it means the highest business earning rate, many business proprietors have the intention of wind power generation in this area. However, there is a risk of too high wind power penetration level compare with the size of the system. Therefore S/O restricted wind power penetration below 20% (120MW) of the maximum load and additional business licenses were withheld [14]. Recently, there are various efforts to increase wind power penetration limit. As mentioned before, development of grid code for the wind power interconnection can help the stable and steady increase of wind power penetration. And efforts to develop the coordinated control scheme of installed devices – wind power generation, conventional generation and HVDC – are now in progress. In this chapter, the overall coordinated control scheme of Jeju power system is presented. The following figure shows the conceptual diagram of coordinated control scheme of Jeju Island.

Fig. 7. Coordinated control scheme of Jeju Island

1) Coordinated control scheme of conventional unit

Operation scheme of conventional generator will not be changed basically. But the utilization factor of the conventional generator will be reduced significantly. At 2012, two HVDC transmissions and few other generators will be needed only to supply the Jeju system load. However stability of DC-AC system must be considered. For satisfactory performance, the ac system should have a minimum inertia relative to the size of the dc links. Thus two diesel power plants and four thermal plants should operate to secure the inertia of AC system. Generators turned on is to be operated at minimum operating point, overall efficiency of generators is expected to decrease.

2) Coordinated control scheme of HVDC a) Operation range and control mode of HVDC #1, #2 It is hard to change the control system of existing HVDC #1. Thus the operational constraints of HVDC #1 will be maintained. Maximum transmission capacity of HVDC #1 is to be limited as 150MW considering N-1 contingency. Minimum transmission capacity is 40MW. N-1 contingency will also be applied to new HVDC #2 thus maximum transmission capacity will be 200MW; the rated capacity of HVDC #2 will be 400MW. Overall maximum transmission capacity of HVDC #1 and #2 will be 350MW. Next figure shows the above concept. When one pole of HVDC #2 is tripped, then another line of HVDC #2 will increase transmission capacity immediately by pole transfer function. After that operating point of each HVDC will be rearranged by S/O. There is frequency control loop in control logic of HVDC #1 thus the frequency of Jeju power system has been controlled by HVDC #1 until now on. However this role of frequency control is more suitable for the HVDC #2, because HVDC #2 can control the flow at the range from +200MW to -200MW. Thus HVDC #1 should be operated in power control mode. And HVDC #2 should be operated in frequency control mode. System operator should determine the active power reference of both HVDC considering the system condition.

Fig. 8. Coordinated control during HVDC 1 pole trip event b) Coordinated control scheme of HVDC To increase the efficiency of the system operation, coordinated control scheme of both HVDC is needed. Each HVDC has different control scheme and S/O must consider this characteristic. HVDC #1 is only able to send electricity from mainland to Jeju Island, but HVDC #2 can send surplus power to the mainland. Without coordinated control scheme, inefficient system operation may be occurred. For example HVDC #1 may receive more electricity from mainland during HVDC #2 send surplus power to mainland. To prevent circulating power problem, frequent change of operating point and haunting, coordination control scheme is needed. Following figure shows the coordinated control scheme of both HVDC.

- State 1 (Both HVDC receive power from mainland) In situations with low wind speed and high local load, both HVDC should receive more than 100MW (HVDC #1: 40MW, HVDC #2: 60MW) from mainland. In this state, each HVDC is to operate according to dispatch order which is distributed dividing total transmission power in proportional to its maximum capacity respectively.

- State 2 (HVDC #1: Minimum power, HVDC #2: Frequency control) When receiving power reduces below 100MW, HVDC #1 should mode to constant power control and maintain minimum power. Instead HVDC #2 reduces receiving power and changes power direction. Frequency control is continuously performed by HVDC #2. In this case, power circulating is occurred, however HVDC #1 should be operated to avoid frequent shut down and start up of HVDC #1.

- State 3 (HVDC #1: Shut down, HVDC #2: Frequency control) When system load of Jeju Island decreases and active power generation of wind power increases continuously and reverse transmission of HVDC #2 is expected for long period. In this case, HVDC #1 should be shut down to prevent power circulating loss. With the expected significant increase of wind power on Jeju Island in the near future, the island will export power during considerable hours. Especially this coordinated control scheme of HVDC will accelerate the increase of wind power in Jeju Island.

Fig. 9. Bidirectional power transmission using coordinated control

3) Coordinated control scheme of wind power generator

There is no grid code for wind power interconnection in Korea until now. Thus even the grid code for wind power interconnection is established in near future, the existing wind farms will not be applied by the grid code and don’t have any motive to enhance their controllability. Different operating scheme should be applied to existing wind farms and planned wind farms accordingly. There are already 6 wind farms (Hangwon, Hankyoung, Sinchang, Weoljung, Samdal) which capacity of 79MW in Jeju Island. And 3 more wind farms (Sungsan#2, Sammu offshore, Nansan) which capacity of 48.5MW have received the permission of construction. These wind farms are not obliged to reject more production than the one dictated by the wind power penetration limit by the grid code. It means that total amount of uncontrollable wind farms in Jeju Island will be reached to 127.5MW which is huge amount compared with size of the Jeju power system. Therefore these wind farms should have monitoring and control system to secure the safe operation of power system. And they have to cooperate with S/O in emergency condition.

Fig. 10. Wind farm supervisory control system Wind farms which will be installed after the establishment of grid code should have abilities to control the active and reactive power. They will also be equipped protection devices including fault ride through capability. It means these wind farms have same control capability with conventional generators. In general, wind power generation has a higher priority than conventional generator. Thus active power control such as downward regulation will only be used in rare instances. S/O will order the wind turbines to reduce or suspend their production if the remaining transmission grid malfunction or the total production is larger than wind power penetration limit. Voltage control is only needed if wind farm is connected at the weak point of the grid. However wind farms can provide proper supports to system operation by these control abilities and it will contribute to allow more wind power generation in Jeju Island. These wind farms should communicate with system operator in real time. All of information about wind farms such as P, Q, voltage, wind velocity and operating status should be provided. And they have to control each turbine generator in wind farm. Supervisory control system is needed to enable these tasks. The fig.10 shows the concept of the wind farm supervisory control system. Wind farm supervisory control system mainly consists of communication module, wind farm controller and individual turbine controller.

4. Conclusion

The power system of Jeju Island is basically weak and small. Thus the increased penetration of wind power in Jeju Island may raise a number of operational concerns due to the intermittent nature of wind and lack of stability of wind turbine. S/O of Jeju Island wanted to set the maximum wind power penetration limit. S/O has withheld additional business license of wind farm. In this paper, various ways to increase the wind power penetration are presented. Establishment of grid code for wind power generator is most important in the case of Jeju Island. Various requirements such as active power control, reactive power control, fault ride through and etc., will be included in the Korean grid code. Technical innovations of the wind turbine generator will be needed to meet the requirements in the grid code. Coordinated control scheme of Jeju Island is also presented in this paper. More flexible and stable operation of Jeju power system will be possible with the coordinated control scheme even if the wind power penetration level is extremely high.

5. References

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