Greece has a large wind potential, however, only 1.7% of the total energy demand of 2003 was met by wind generation [20]. The mainland of Greece is served by the Hellenic Interconnected Transmission System with installed capacity close to 11000 MW. In the Interconnected system 405 MW of wind generators have been installed up to 2005. Due to the geographical distribution and their size (15 to 30 MW installed capacity) the wind farms are mostly connected to the high voltage network through HV/MV substations.
In many islands that are not interconnected to the mainland, the wind penetration is more significant. For example, Crete is the largest autonomous system in Greece with 660 MW of installed thermal capacity running on imported oil fuel. The installed wind farms capacity is
81 MW and their production accounts for the 10% of the annual energy demand. The instantaneous power penetration has reached 39% [20].
After the Kyoto Protocol, Greece, as a member of the European Union follows the European policies concerning greenhouse gas emissions. The state target for Greece is 20.1% of renewable energy source penetration in electricity production and considerable incentives are provided to these renewable generation sources, such as satisfactory fixed feed-in tariffs, investment subsidies, guaranteed access to the grid, long-term contracts, etc. [21]. As a result, applications by Independent Power Producers (IPPs) for more than 13,000 MW of wind farms located in the Greek mainland have been filed to the Regulatory Authority for Energy. More than half of the applications (total 6,700 MW) refer to three specific areas of high wind potential in the Greek mainland [22], namely the Evia island, the Southeast of the Peloponnese peninsula and the Thrace region, as shown in Figure 2-13.
Thrace 0.6 GW Evia 3.6 GW Peloponnese 2.5 GW
Figure 2-13: Geographical distribution of applications for wind farms in Greece.
The areas of interest share some similar characteristics: they have significant local thermal generation and they are connected to the bulk transmission system through congested transmission corridors of the 150 kV transmission network. Although there are specific plans for reinforcing the network in order to alleviate constraints and accommodate future wind farms, new transmission projects face environmental and financing constraints resulting in considerable delays. Also, environmental constraints and some contradictions of the legal framework are causing further delays in the installation of new wind farms. Thus, even though there are about 3260 MW of wind farms approved by the Regulatory Authority, the actual penetration is still very limited.
Special Operating Practices for Wind Power In Greece
In Greece, the Hellenic Transmission System Operator (HTSO) has adopted a special operational practice in order to increase the wind power penetration into the windy area of Thrace in the North East part of Greece. The main concept is the introduction of “interruptible contracts” and the continuous monitoring and control of the power flow through the congested corridors by issuing a setpoint to each wind farm to reduce its production, whenever system security is endangered. This practice implies both regulatory and technical amendments [22].
Figure 2-14 depicts the transmission system in the region. The existing generation comprises a combined cycle power plant at Komotini. The maximum capacity of this plant is 480 MW while its technical minimum is 280 MW. The region of Thrace is connected to the transmission system through four overhead transmission lines through the boundary bus of
Iasmos; the thermal limit of each line is in the order of 170 MVA during summer and 200 MVA during winter. The wind penetration is limited by the available transfer capacity from Thrace to the system; static security is the limiting factor. The Komotini power plant is usually bidding successfully in the power market and therefore Thrace is usually an exporting area. Moreover, for large portions of time, it is a “must-run” unit since it is necessary to provide local voltage support. Under these conditions, the possibility of adjusting local thermal power generation, in order to increase wind penetration is not considered, because of the impact this would have on the electricity market (increased uplift costs). Furthermore, the Komotini power plant contributes to the Automatic Generation Control and the provision of this ancillary service is sometimes very important for the quality of the entire system operation.
Figure 2-14: Schematic diagram of system configuration
HTSO should guarantee the absorption of all the power produced by the wind farms installed in the country. According to the existing planning practices, the maximum wind power penetration in the area should not exceed 100MW. The application of “interruptible contracts” allowed HTSO to double this limit without compromising system security, or the IPP’s economic feasibility.
From the regulation point of view, the new practice is based on “interruptible contracts” for wind generators; these contracts allow the system operator to curtail the power production of wind farms when security constraints are violated. In this framework a new Ministerial Decree has been issued in 2003 which allows HTSO to violate the “priority in dispatch” rule for renewable energy sources by curtailing power output of wind farms with “interruptible contracts” when necessary; it also sets the mutual obligations between the HTSO and IPPs. The control is based on the continuous measurement of the total power flow from Thace to the bulk transmission system through the corridor. The control concept is applied according to the following rule: “the power flow through the interconnecting lines is not allowed to exceed a predefined security limit”. This limit is calculated so as to respect the N-1 security criterion according to the Grid Code regulations. If this limit is violated, the controller sends setpoints to the wind farms with interruptible contracts to reduce their production and consequently the power flow through the congested corridor by the necessary amount. These setpoints represent the upper limit of the power output by each wind farm that can be securely injected into the grid. The necessary power reduction is to be shared by all wind farms with interruptible contracts.
The control scheme [23] is implemented using an autonomous system comprised of Programmable Logic Controllers (PLCs) communicating to each other through two different and independent telecommunication lines. Two independent PLCs (main and backup) are installed at the boundary substation to monitor the operating status and the power flow through the interconnection lines and implement exactly the same algorithm. One PLC is
BULK TRANSMIS- SION SYSTEM Thrace region M ~ : power flow Trans- mission corridor Controller M IASMOS Paranesti Keramoti Xanthi Zarkadia KOMOTINI TPP S : breaker status (ON/OFF) S
installed at each substation where wind farms with interruptible contracts are connected, in order to provide the necessary interface to the wind farm supervising control system. In each wind farm the respective PLC collects real time data, transmits the limiting setpoints (if any) to the wind farm supervising control system and communicates with the PLCs installed at the boundary substation. In addition to the above described autonomous control system, the supervision and control of the wind farm by the Energy Control Center is always enabled via the existing Remote Terminal Units.
The operation of the control system will be monitored by the Control Center through the SCADA for security and settlement purposes. Whenever a wind farm does not comply for reduction according to the setpoint issued by the control system, the dispatchers are alerted with an alarm and they should communicate (via telephone) with the authorized wind farm personnel to execute manually the command. In cases of an emergency the wind farms that did not comply with the commands will be disconnected from the grid using the remote control facilities of the SCADA.
In all cases the control system operation and the wind farm response is recorded in the EMS databases for settlement purposes. The wind farms that did not comply with the reduction commands are penalized and furthermore, the power injected to the grid above the issued setpoints is not remunerated. Also, the curtailed energy by each wind farm is recorded since it must not exceed 30% of wind farms annual potential according to the Ministerial Decree.