Automatic Generation Control and Ancillary Services

Transcription

1 GRC Transactions, Vol. 37, 2013 Automatic Generation Control and Ancillary Services Josh Nordquist, Tom Buchanan, and Michael Kaleikini Ormat Technologies, Inc. Keywords Hawaii, Puna, Puna Geothermal Venture, Ormat, Organic Rankine Cycle, ORC, Ormat Energy Converter, OEC, Ormat Geothermal Combined Cycle, GCC, Geothermal Combined Cycle Unit, GCCU, Integrated Combined Cycle, IGCC, Integrated Combined Cycle Unit, IGCCU, modular, geothermal, dispatchable power, dispatchable generation, Hawaii Electric Light Company, HELCO, Hawaii Electric Company, HECO, bottoming cycle, integrated two-level unit, ITLU, droop Introduction Puna Expansion Facility In 2005, initial discussions for a proposed geothermal energy expansion between the Hawaii Electric Light Company (HELCO) and the Puna Geothermal Venture (PGV) representatives took place. PGV desired to increase generation from the existing 30 MW contract to a proposed 38 MW contract. In 2008, Governor Linda Lingle, Hawaii Electric Company (HECO), the U.S. Department of Energy (DOE) and the Hawaii Department of Business, Economic Development and Tourism (DBEDT) signed an MOU launching the Hawaii Clean Energy Initiative (HCEI). This initiative set goals at 70 percent of Hawaii s energy to be from clean energy by the year Renewable energy would comprise 40 percent and the remaining 30 percent would be derived by efficiencies (HCEI, 2013). An 8 MW expansion agreement was reached between HELCO and PGV in early 2011, representing the first agreement for a fully dispatchable geothermal power plant. A geothermal project in the Puna area of Hawaii began in the mid-1970s with the development of a geothermal well, HGP-A, in the lower Kilauea East Rift Zone on the southeast side of the Big Island. An experimental power plant was brought online in the early 1980s by the U.S. Department of Energy, producing 3 MW. This power plant was shut down in the late 1980s. During the mid- 1980s efforts began to develop a larger project, based on the success of the experimental unit, but focused on utility scale generation. Driven by the Public Utility Regulatory Policies Act (PURPA), enacted in 1978, which promoted the use of domestic renewable energy, Constellation Energy, with funding from investors, mainly Credit Suisse, teamed up with OESI to develop the project. The joint venture was named the Puna Geothermal Venture. The PGV power plant was commissioned in It was a 25 MW power plant comprised of ten Ormat Geothermal Combined Cycle Units (GCCU). These GCCUs were the first of their kind, patented by Ormat, to integrate both a unique back pressure steam turbine and Organic Rankine Cycle into a modular power unit. Additionally, all of the GCCUs were air cooled, requiring no water for operations. In 1995, PGV successfully negotiated with HELCO to increase the Power Purchase Agreement from 25 MW up to 30 MW. The geothermal resource at PGV was, and still remains, one of the hottest in the world. Geothermal wells at PGV flow steam and brine at temperatures of 600 F (315 C) at a high pressure of 1,430 psi (100 bar). In 2004, after 11 years of successful operation, PGV was purchased by Ormat Technologies, Inc. (the same Ormat who manufactured the GCCUs at PGV) and added to Ormat s growing fleet of operating geothermal power plants. Ormat, now the household name in geothermal energy, began focusing on the benefits of clean, reliable energy over four decades ago. In the early 1970s Ormat commercialized the Organic Rankine Cycle technology for the application of remote power solutions, manufacturing small (in today s standards) power units in Massachusetts. In the early 1980s, Ormat ventured into geothermal, commercializing low temperature geothermal power in the U.S. As low temperature geothermal power generation began to grow in the US in the early 1990s and Ormat s technology was proving to be the primary choice, Ormat began to expand the application for its OEC to offshore platforms and waste heat recovery installations. In the late 1990s Ormat expanded to owning and operating geothermal projects that generate revenue through electricity sales. Today, with over 611 MW of geothermal and Recovered Energy Generation power plants, and with over 1,600 MW of installed OEC capacity worldwide (Ormat, 2013), Ormat has firmly planted itself in the development and support of clean energy, and has been able to prove, where many have failed, that there is a long-term, reliable, solution for the world s energy crisis. 761

2 Puna Geothermal Venture 8 MW Expansion Project In February 2011, after the expansion project agreement was reached between HELCO and Ormat, HELCO submitted the proposed Power Purchase Agreement (PPA) to the Hawaii Public Utilities Commission (PUC) for approval. In December 2011, the Hawaii PUC approved the PGV 8 MW Expansion project. Reducing energy rates to HELCO s customers, reducing Hawaii s dependency on fossil fuels, increasing reliability and optimizing the existing geothermal resource are just four of the immediate benefits the PGV facility and HELCO would experience. Additionally, HELCO would have the ability to direct the net output of the PGV facility remotely by the System Operator. This ability required definition of many technical requirements for the new expansion and existing facility that may or may not be currently present. For example, droop settings were required to be at 4 percent without a deadband. Ramp rate was required to be 2 MW per minute along with a quick load pick up feature of 3 MW in 3 seconds. Net output control was to be between 22 MW and 38 MW. Over and under voltage would be applied at levels HELCO experiences and responds to currently. Ormat engineers would be required to provide solutions to meet these detailed requirements for replacing oil-fired units for an island grid system. Ormat committed to these solutions and specific operating requirements. State of Renewables in Hawaii In 2012, Hawaii achieved 13.9 percent of energy needs from renewable energy, well on the way to achieving an intermediate goal of 15 percent renewable generation by When combining renewable energy and energy efficiency mechanisms, Hawaii has achieved 28.7 percent of energy from clean energy sources (the goal for 2030 is 70 percent) (HCEI, 2013). On the island of Hawaii, 40.9% of all energy produced in 2012 was from renewable facilities with PGV accounting for 22.8% of the total. wind development, intermittent sources of energy. High growth in solar and wind development drove equipment prices lower and, combined with the lower risk of development, accelerated the development of these resources throughout the U.S. While the accelerated growth of renewables is a major achievement to reduce dependency on fossil fuels, it created an issue with utilities. Intermittent renewable resources are hard to predict and out of their control, while their electricity demand is known and must be met. In order for utilities to react to intermittent resources they need dispatchable power resources. For a utility, this is not something new and there are a number of fossil fuel-based dispatchable solutions available. Geothermal was not considered a dispatchable renewable technology, until today. The situation in Hawaii was similar to the U.S. mainland. There has been considerable growth in solar and wind resources over the past years. Also, as Hawaii s electrical grid is generally smaller and isolated, fluctuations in the hour to hour load that the utility needs to provide is relatively greater. Hence the value added of a renewable power generation resource that could be fully dispatchable. Ormat was the innovator that found a solution and Hawaii (HELCO) provided the trust that Ormat could do it. The adaptation of the base load power plant to fully dispatchable was no easy task. While many qualities were physically present, a number of changes, new technology, and testing were required. To add to the challenge, PGV is still required to produce base-load energy until the dispatchability was achieved. Technical Aspects of the 8 MW Expansion Developer s Perspective Prior to the expansion the PGV power plant, the geothermal wellfield consisted of multiple artesian production wells deliver- Another First for Ormat and the Geothermal Industry As mentioned prior, the expansion of the PGV facility is the first fully dispatchable geothermal power plant. For the last three decades, geothermal developers have been focused on selling geothermal energy as a base-load renewable energy product. PURPA created a market for this energy in the 1980s and 1990s through Standard Operating #4 (SO4) contracts. The growing need for renewable energy through Renewable Portfolio Standards (RPS) developed by states sparked renewable energy development beginning in the new century and continuing today. The RPSs also promoted the growth of solar and Figure 1. Diagram of an Ormat Geothermal Combined Cycle (GCC). 762

3 ing two phase geofluid to a single flash steam separation unit. The steam from the separation unit was supplied to 10 Ormat GCCUs. The separated geothermal brine was sent directly back to the wellfield for injection. Ormat Geothermal Combined Cycle (GCC) technology converts the steam energy into electrical power by expansion of the steam through a back pressure steam turbine and then condensation of the steam in a vaporizer of a secondary working fluid. The condensate is returned to the wellfield for injection. The vaporized working fluid is then supplied to a second turbine for power generation and then passed to a condenser and cycle pump for a closed-loop Organic Rankine Cycle (ORC) (Figure 1). In this arrangement the separated geothermal brine still carried significant enthalpy, but was simply returned to the wellfield for injection. In order to take advantage of this enthalpy source, two Ormat Energy Converter (OEC) bottoming units were added through the power plant expansion to improve overall recovery and optimization of the resource. The integration of two bottoming cycle OECs converted the PGV facility into an Integrated Geothermal Combined Cycle (IGCC) technology arrangement (Figure 2). Figure 2. Diagram of an Ormat Integrated Geothermal Combined Cycle (IGCC). The additional bottoming units provide several resource benefits. Brine is used to increase power generation by 8 MW without increasing geothermal fluid production resulting in the optimization of the geothermal resource. Cooler injection fluids results in an increase in injection capacity due to an increase in density and, therefore, inertia. With the additional capacity of the two bottoming units, only nine of ten GCCU s need to operate in order to reach full output. The tenth spare GCCU allows maintenance on the steam units to be performed with minimal impact on overall output. Utility/Grid Perspective In addition to the added 8 MW of capacity, the expansion project was tasked to provide dispatchable generation. Historically geothermal power generation has been considered base-load power, that is, the geothermal facility produces all the power it can generate and other generators on the grid adjust their generation level to match overall grid demand. On large grids where geothermal generation is a small portion of total generation this base-load approach works well. On an isolated island grid, or other grids where renewable generation is a larger portion of total generation, the key to increasing the penetration of renewable generation resources will be the ability for any renewable resource to provide the technical aspects that were typically only provided by fossil-fueled generation units. This allows participation in the grid s Automatic Generation Control (AGC). AGC from a renewable generation resource provides the utility the ability to remotely dispatch the facility, 24 hours a day. AGC is a computerized control system used by grid system operators to control multiple generators connected to the grid to closely match generation-to-load demand. An electrical grid must closely match generation to the continuously changing load demand on the system. This requires frequent adjustments to the power output of the various generators. In simple terms, the AGC watches the frequency of the grid, if it is increasing this means there is more generation coming into the grid than the load is consuming and, vice-versa, if frequency is decreasing there is not enough generation to match the load. AGC then automatically requests adjustments to the generation being contributed by all of the connected generators. Before AGC, a single large generator was operated in isochronous (fixed speed) mode to set the frequency of the grid and all other generators connected to the grid would operate in droop speed mode to adjust their output to balance the frequency of the grid. AGC allows more flexible participation of load balancing for all generators connected to the grid. Remote control capability is accomplished through communication between the HELCO system operator AGC and the PGV facility System Control and Data Acquisition (SCADA) system. By allowing communication between these two computerized control systems, the network system operator can automatically request adjustments to the plant generation to match grid demand while in coordination with other generation facilities connected to the grid. The communication also allows the plant SCADA system to update the system operator on available capacity and spinning reserves. In order for the PGV facility to participate in the HELCO AGC, the power plant must be capable of operating over a wide range of power generation. It must be able to adjust its power output quickly in response to the AGC (Ramp Rate) and it must maintain its frequency within close tolerance of the grid (percentage droop). This is an unusual task for a geothermal power plant since the heat source, especially artesian wells, naturally do not respond quickly to changes in demand, yet power generation must be managed to quickly respond to the AGC generation request. 763

4 This requirement of quick response to changes in power generation while maintaining stable operation of the geothermal resource became the first challenge for the design. The requirement is to quickly turn down generation and quickly turn up generation within reasonable ranges. The solution Ormat settled on was to maintain geothermal fluid flows from the wellfield at relatively steady rates and find ways to provide bypass for fluid or heat around the generation equipment as needed, governed by the power demand from the AGC. Bypassing geothermal fluid around some of the generating units to balance the generation with demand works but the response times are slower than what is usually required by the contract ramp rates. In order to improve response times and provide a level of spinning reserve required by HELCO, Ormat chose to provide bypass for some of the heat input to a particular OEC or GCCU by using a turbine bypass. This would allow some of the heat absorbed by the organic working fluid to be passed around the turbine and dumped directly into the condenser. The final solution required a coordinated and orderly response to changes in power generation demand and must be balanced between the two new bottoming units and the ten existing GCCU s. This control must be coordinated for all twelve of the generators within the PGV facility and is the brains of the dispatchable solution. Description of the AGC Response Solution Definition of Requirements and Constraints Each unit was evaluated for its safe stable maximum and minimum operating capacity. The sum of the unit minimums defines the overall plant minimum generation. The sum of the unit maximums defines the overall plant maximum generation. The AGC must limit its generation request between these overall plant maximum and minimum generation. By contract these limits were set at 38 MW maximum and 22 MW minimum. An allocation and priority of generation dispatch was established for all generating units. It was determined that, based on several variables and considerations, the two bottoming units would be dispatched first down to their defined minimum stable operating capacity, further dispatch would come from the older GCCUs. Lastly, in the event of emergency over frequency situation, the steam turbines bypasses will be used to quickly reduce generation and therefore over frequency. The plant overall generation needs to respond quickly to AGC requirements. By contract the response rate (ramp rate) up or down was set at 2MW/min. In order to meet the ramp up rate, a certain amount of spinning reserve must be maintained. This spinning reserve would be achieved by maintaining excess flow of organic vapor from each units vaporizer. The excess flow would be bypassed around the turbine directly to the condenser. In the case of ramp up requirement the turbine injection valves would respond by opening and the turbine bypass valve would respond by closing to maintain pressure in the vaporizer. By contract the base value of spinning reserve is required to be 3 MW. For the droop speed mode of control, each generating unit must maintain an allowed droop frequency to work with the AGC. The required droop was established at 4 percent. In addition to grid frequency control, the grid voltage must be maintained by the AGC. The voltage required by the grid would be controlled by a standard voltage regulator on each generating unit. AGC Inputs to the PGV SCADA System The AGC provides continuous input to the PGV SCADA system for the following parameters: Required Net Power This is net delivered power to the grid. Grid Frequency This is the currently required frequency of the grid. Grid Voltage This is the currently required voltage of the grid. The PGV SCADA System Feedback to the AGC The PGV SCADA system provides continuous feedback to the AGC about its generating capabilities and includes the following parameters: Current Actual Spinning Reserve This is the currently available spinning reserve in MW from all units. Current High Limit Available Dispatch This is the current plant net generation plus the current actual spinning reserve from all units. Low Limit for Available Dispatch This is the sum of the minimum stable generation for each operating generating unit. Control Philosophy Net Delivered Power The HELCO AGC determines net required power from the PGV facility and provides the Required Net Power set point to PGV. This set point must be within the constraints of High and Low limits of dispatch control. The control scheme for Net Power control follows the simplified diagram in Figure 3. Spinning Reserve Management In addition to maintaining the AGC required net power, the plant must maintain a minimum spinning reserve. Under most conditions the actual spinning reserve will exceed the 3 MW. Spinning reserve is created by adding excess heat to the vaporizer and then diverting some of the vapor flow around the turbine directly to the condenser. An optimum vaporizer pressure set point is established for each unit. This vaporizer pressure is maintained by relieving the excess flow though the turbine bypass valve. The 764

5 Figure 3. Control schematic for adjusting net power output to match required net power provided by the AGC. The PGV controller prioritizes and distributes speed set point signals to all OEC units at the facility. Figure 4. Control schematic for adjusting net power output of an OEC while maintaining both speed set point derived from the GC Net Power Required while also maintaining the required amount of spinning reserves. control philosophy was chosen to allow power generation of any given unit to vary but not at the expense of the required spinning reserve. Therefore, the control mechanism limits power generation if available spinning reserve is less than required spinning reserve. The control scheme for maintenance of spinning reserve follows the simplified diagram in Figure 4. The Finished Product During 2012, PGV commenced commercial operation of the expansion project. This was achieved by extensive acceptance testing associated with all aspects of the PPA between Ormat and HELCO, proving the full dispatchability of the project. While, especially here, the solution is simplified and summarized for understanding, the actual work involved was extremely 765 detailed, elaborate, and challenging. In a project like this, as with many projects, the devil is in the details. For this project, there were a lot of details. First, there was developing a geothermal technology that could operate both as base load and dispatchable. This did not only involve development of equipment (OECs) but also very involved work within the control of this equipment. Then, there was advancing existing equipment, which had been operating for 20 years, to also operate as base load and dispatchable. This effort can be more challenging on both the equipment and control as new physical equipment may need to be made and older control systems, with minimal capabilities, need to be fully replaced. Finally, the development of an overall control philosophy that can receive requests from AGC, and enable the whole PGV facility to respond accordingly, while staying within the operating limits of the equipment and the wellfield, was challenging. It s important to restate that there are 12 power units at the PGV facility, ten that have been operating at the site for over 20 years. While the power agreement between HELCO and PGV includes minimum base load output, the PGV facility is inherently fully dispatchable.in the end, it was the determination and commitment by Ormat that brought this solution to reality. The details in such a project were tremendous, and at times insurmountable, but the goal was worth the effort; the first fully dispatchable geothermal facility in the world. This new project not only increases the renewable energy capacity in Hawaii and decreases the dependency on fossil fuel sources, but ultimately proves that renewable resources and particularly geothermal can replace the fossil fuel-based energy production that is used today, both base load and dispatchable.

6 Figure 5. One of the expansion OECs at PGV that use geothermal brine for power generation and are fully dispatchable. References Hawaii Clean Energy Initiative, Hawaii Clean Energy Initiative, Hawaiian Electric Companies hit new high in renewable energy use in org, April,