ANCILLARY SERVICES SUPPLIED TO THE GRID: THE CASE OF THISVI CCGT POWER PLANT (GREECE)

Transcription

1 Claudio Cavandoli, Alessandro Crippa EDISON S.p.A., Engineering Department ANCILLARY SERVICES SUPPLIED TO THE GRID: THE CASE OF THISVI CCGT POWER PLANT (GREECE) 1

2 ANCILLARY SERVICES SUPPLIED TO THE GRID: THE CASE OF THISVI CCGT POWER PLANT (GREECE) - GENERAL DATA Combined Cycle Power Plant, MWe Registered Capacity Multi-Shaft arrangement, 1+1 Configuration 33 MWth Supplementary Firing Designed for Dual Fuel operation (emergency) Air Cooled Condenser ANSALDO ENERGIA V.94.3A4 275 MW NOOTER ERIKSEN 3 Pressure Levels + RH ANSALDO ENERGIA MT 146 MW GEA (21 fans) 2

3 ANCILLARY SERVICES SUPPLIED TO THE GRID: THE CASE OF THISVI CCGT POWER PLANT (GREECE) GREEK ELECTRICAL GRID CODE REQUIREMENTS Each Generation Unit must have primary frequency control capability; The operating reserve shall be not less than 3% of the registered capacity in the range between 50% and 97% of the registered capacity; The operating reserve in the range between 97% and 100% shall be not less than that indicated by a straight line with fixed slope from 3% of registered capacity at 97% output and 0% at full load; The production license holder must be capable of activating, within 30 seconds, the total primary operating reserve and of maintaining supply for at least 15 minutes; The primary operating reserve must be available again 15 minutes after activation. 3

4 GREEK ELECTRICAL GRID CODE REQUIREMENTS Load variation Vs plant load Greek Grid Code Requirements: 10 [MW/30 s] Grid required load variation MW/30s 12.6 MW/30s from minimum load to 97% registered capacity Linear decrease to zero between 97% and 100% registered capacity % 96% 97% 98% 99% 100% Power Plant Load [%] Gas Turbine Load Ramps (AEN V.94.3A.4): 1. Normal Operation: 13 MW/min 60% - 98% Base Load 6.5 MW/min 98% - 100% Base Load 2. Frequency Regulation: 24 MW/min 60% - 98% Base Load 13 MW/min 98% - 100% Base Load 4

5 GREEK ELECTRICAL GRID CODE REQUIREMENTS IMPLEMENTED CONTROL STRATEGY (1/3): The HP steam header is controlled in modified sliding pressure throttling the Steam Turbine HP Control valve (fixed pressure increase of 10 bar with respect to natural sliding pressure, following steam flow-rate) ST HP SECTION - NATURAL SLIDING Vs. MODIFIED SLIDING PRESSURE HP ST INLET PRESSURE [barg] HP STEAM FLOW-RATE [t/h] The ST reacts to Grid frequency disturbs greater than 20 mhz (dead-band), in order to avoid unnecessary depressurization of the headers and fluctuations. The ST Active Power response will be proportional to the frequency disturbance, according to the defined droop control. 5

6 GREEK ELECTRICAL GRID CODE REQUIREMENTS IMPLEMENTED CONTROL STRATEGY (2/3): The bridge valve located at the IP superheater outlet is used to keep slightly pressurized the IP steam drum in order to prevent steam back-flow during the transient (which would limit the contribution of the Steam Turbine IP section). A pressure drop of 2 bar is maintained across the bridge valve during normal operation; the bridge valve is fully opened when the ST contribution to primary frequency regulation is requested. 6

7 GREEK ELECTRICAL GRID CODE REQUIREMENTS IMPLEMENTED CONTROL STRATEGY (3/3): The Supplementary-firing load will follow GT load, from 0%, with GT at 70% load, to 100%, with GT at cold base load. Note: post firing maximum load is limited to its maximum load (MWth) or maximum exhaust gases temperature ( C), whichever is reached first. Supplementary firing load[%] GT load[%] This configuration avoids plant load ramp rate reduction while during operation at fired load. As far as HRSG burners are operated according to GT load, the relevant impact on plant load variation rate is negligible (the flexibility is equivalent to an unfired power plant). 7

8 GREEK ELECTRICAL GRID CODE REQUIREMENTS IMPLEMENTED CONTROL STRATEGY (SUMMARY): % GT load Up to 70% From 70% to 93% From 93% to cold base load Post-firing Off On On GT control mode ST control mode Post-firing control mode Steam pressure control mode Load control + frequency participation Pressure control - Load control + frequency participation Pressure control Open loop as a function of the GT load Load control + frequency participation Pressure control + frequency participation Open loop as a function of the GT load From cold to hot base load On at maximum load Load control + frequency participation Pressure control + frequency participation On at maximum load Natural sliding Modified sliding Modified sliding Constant pressure 8

9 GREEK ELECTRICAL GRID CODE REQUIREMENTS CONTROL SYSTEM SCHEMATIC DIAGRAM: 9

10 GREEK ELECTRICAL GRID CODE REQUIREMENTS DYNAMIC SIMULATION A dynamic simulation study of the plant has been developed together with Politecnico di Milano University and ACT Solutions, in order to evaluate in advance the plant response to a frequency disturbance and with the aim to optimize the above described procedure as well as the involved control system logics and parameters. The following aspects have been considered: process model of the relevant parts of entire BOP (water, steam, flue gas, PF combustor); GT model, with the aim of reproducing its power generation response so as to present realistic boundary conditions at the post-firing chamber inlet (on the plant model side) and to the control system; ST model with the aim of reproducing its power generation response and main process parameters; boundary conditions impressed by the electric system, limiting the scope to the behaviour of the generator frequency; model of control system, integrating all the involved features of GT/ST controllers and the Distributed Control System of the plant. 10

11 GREEK ELECTRICAL GRID CODE REQUIREMENTS DYNAMIC SIMULATION RESULTS [kg/s] [MW] [MW] 11

12 GREEK ELECTRICAL GRID CODE REQUIREMENTS TESTS RESULTS The following tests have been carried out in order to assess Thisvi Power Plant compliance with the Greek Grid Code requirements in terms of Primary Frequency Contribution: - Plant at 97% load: Frequency error injection of ± 200 mhz maintained for 15 min - Plant at 90% load: Frequency error injection of ± 200 mhz maintained for 15 min - Plant at 80% load: Frequency error injection of ± 200 mhz maintained for 15 min - Plant at 60% load: Frequency error injection of ± 200 mhz maintained for 15 min Plant Load [%] Frequency Error [mhz] Primary Frequency Contribution [MW] 30 seconds 15 minutes 97 % % % % % % % %

13 GREEK ELECTRICAL GRID CODE REQUIREMENTS - CONCLUSIONS 1. As a general statement, the ST participation to the frequency regulation can be considered an effective method to share the regulating contribution between the GT and the ST, thus reducing the thermal stresses on the GT and the necessary load reduction, as well as the consequent ST load reduction in combined cycle operation. 2. In Thisvi case moreover, the ST participation is necessary to meet the Greek Grid Code requirements, due to the high contribution requested and to the GT allowable loading gradient. 3. The foreseen control strategy (properly pre-tuned using dynamic simulation tools and finally fine-tuned in site) is an effective method to meet the Greek Grid Code requirements in terms of primary frequency regulation. 13

14 ELECTRICITY FROM RENEWABLE ENERGY SOURCES (RES-e): IMPACT ON TRANSMISSION SYSTEM OPERATION 1. Greece has a photovoltaic and wind potential comparable to central-southern Italy. 2. The development feasibility evaluation needs a detailed analysis on how typical daily PV and wind generation expected profiles. 3. Unpredictability can comply with the load diagram of a typical day. 4. The lack of firmness of PV and wind generation (i.e. the intermittency) and the lack of forecast precision (on day ahead and on day ongoing) play the most important role in the balancing energy of the system. 14

15 LOAD DIAGRAM OF A TYPICAL DAY IN GREECE (source: ) peak load at h 21-22: ~ 6GW [MW] first peak h base load ~ 3GW: about 50% of peak load [h] 15

16 TYPICAL DAILY PHOTOVOLTAIC GENERATION PROFILE IN GREECE (source: Development of future scenarios and EU network data collection SIEMENS WORKPAPER ) peak generation at h NO generation from h to h 6-7 peak load at h

17 TYPICAL DAILY WIND GENERATION PROFILE IN GREECE (source: Development of future scenarios and EU network data collection SIEMENS WORKPAPER ) daily variability: ~ 60% of Pmax peak load at h

18 ELECTRICITY FROM RENEWABLE ENERGY SOURCES (RES-e): PHOTOVOLTAIC AND WIND GENERATION PROFILES vs LOAD DIAGRAM Greek s load diagram has deep fluctuations and two peaks. PV generation profile (in its typical shape) covers part of the first load peak around h; however the highest load peak is between h and h when PV production is certainly equal to zero. Wind generation profile is extremely variable (both in time and in magnitude) and unpredictable. Therefore it is completely unmatched in comparison to load diagram. Variability and uncertainty (difficult to forecast accurately) introduce power imbalances, lower levels of firm generation capacity and loss of services from displaced generation. The challenge is to maintain stable operation of an electrical grid with high penetration of RES. 18

19 ELECTRICITY FROM RENEWABLE ENERGY SOURCES (RES-e): IMPACT ON TRANSMISSION AND DISTRIBUTION SYSTEMS OPERATION Traditionally, the amount of balancing energy, or reserve, provided by controllable thermal or hydro generation had to be sized to balance variations in demand or forced outages of the largest production units. Large penetration of intermittent and in particular wind and photovoltaic generation introduces additional requirements for balancing energy. The reasons are twofold: since RES generation has limited predictability, in order to cope with the forecast error, larger amounts of flexible sources are necessary; also considering that the predictability of RES will improve in the future, however, even with perfect forecasting, PV and wind generation will remain intermittent, non-controllable and very variable from one hour to another. Therefore, additional flexibility is required. 19

20 ELECTRICITY FROM RENEWABLE ENERGY SOURCES (RES-e): IMPACT ON TRANSMISSION AND DISTRIBUTION SYSTEMS OPERATION The evolution in the composition of the optimal portfolio changes with increasing RES-e: - Some of the less flexible base-load conventional plants will be forced out of the markets. - Peak load and Medium load plants will cope with the variability of intermittent sources. - Consequently, the grid needs more and more flexible plants like hydro, pump storage, OCGT and CCGT, as well as high capacity wind power plants technically able to participate in frequency control and the provision of reserve power capacity; - Every MW of wind capacity requires 1 MW of backup firm capacity, to ensure 90% availability. 20

21 ELECTRICITY FROM RENEWABLE ENERGY SOURCES (RES-e): IMPACT ON TRANSMISSION SYSTEM OPERATION - CONCLUSIONS 1. Electricity systems with high penetration of wind and photovoltaic generation have a higher exposure to problems related to grid stability. 2. There is an increasing need for flexible generating units, as a backup firm capacity able to supply ancillary services, for dealing with intermittency and RES-e forecast errors. 3. Ancillary services markets should be developed so that producers could offer such flexibility to system operators and other market participants. 4. There should be a level playing field for balancing responsibility which applies to all producers, including RES ones, in order to stimulate all market participants to carry out thorough proper scheduling and forecasting, limiting system costs. 21