Mechanical and electrical rebuilding of a turbine generator for phase-shift operation

Transcription

1 Mechanical and electrical rebuilding of a turbine generator for phase-shift operation POWER-GEN Europe 2013 Vienna, Austria June 04-06, 2013 Authors: Detlef Frerichs Anastassios Dimitriadis Maren Wiese Siemens AG Energy Sector Service Division

2 Table of Contents 1 Abstract Initial Situation in the Power Plant Motivation and Theory Operating Principle of a Phase Shifter Modification into a Phase Shifter Important Factors affecting Modification Feasibility Study Design of New Mechanical Components New Turning Gear (Hydraulic Motor) New Electrical Components (Startup Frequency Converter) Modification of Lube and Lift Oil Supply Evaluation of Potential Faults and Synchronization Conditions Startup Customer Benefits Conclusion References Disclaimer Page 2 of 17

3 1 Abstract In its report on the effects of the phaseout of nuclear power on transmission grids and supply security, the German Federal Network Agency for Electricity, Gas, Telecommunications, Post and Railways (Bundesnetzagentur) stated that stability of the German grid could be affected by a fluctuating renewable energy supply and the phaseout of nuclear power, especially during the fall and winter seasons. A stable grid requires the regulation of reactive power. A deficit in available reactive power in the grid can cause a voltage drop, potentially resulting in a power failure. This regulation is supported by large conventional power plants. The disconnection of nuclear power plants from the grid results in a reduction in supply which cannot be compensated by large wind power plants as they hardly supply any reactive power. A potential solution for this problem is the modification of power plant generators. The generator produces reactive power in zero-load operation which is required to support the grid voltage. This operating mode is also known as phase-shift operation. After the subsequent disconnection of both units of Biblis nuclear power plant, additional reactive power was required in the Frankfurt area, as the reactive power supply was too low. This deficit is now largely compensated following modification of the synchronous generator of Biblis A into a synchronous motor or phase shifter. The Biblis synchronous phase shifter now automatically supplies inductive reactive power to the grid and thereby contributes significantly to grid stability. If the voltage is high, the condenser "sucks" reactive power - comparable with an inductance. Siemens, together with RWE Power and Amprion, have managed the retrofit within only five months. This paper describes why and how the 1640 MVA generator of the non-nuclear part of Biblis A became what is currently the world's largest synchronous motor. Page 3 of 17

4 2 Initial Situation in the Power Plant The Biblis power plant is one of the largest power generation sites in Germany. The two 1640 MVA units were constructed over the period from 1974 to 1976 and operated for over 400,000 hours supplying electrical energy for electric power generation in Germany. The generators in this series excel due to their high performance, primarily as a result of the water cooling developed by Siemens for both the generator stator (471 t) and rotor. The generator rotor was designed as a 4-pole rotor with a total dynamic mass of 228 t and a length of 20.5 m and operates at a speed of 1500 rpm. The exciter in these systems is equipped with a rotating rectifier set and consists primarily of a three-phase AC pilot exciter, a three-phase main exciter, a rectifier wheel and the exciter set coolers. In operation, the exciter set produces an excitation current of over 11 ka and is an extremely reliable system. A water pump impeller is flange-mounted behind the exciter for water cooling. This ensures the requisite throughput of cooling water in the generator and the coolers. 3 Motivation and Theory The power generation picture in Germany is changing as a result of the expansion of renewable energy. The German Federal Network Agency published a report in this regard on the effects of the phaseout of nuclear power on the transmission grids and supply security in August of 2011 [1]. Based on this report, the newly developing north/south gradient places stress on the stability of the high-voltage grids. Voltage drops can result on the high-voltage grid side, especially in the Rhine-Main and Rhine-Neckar areas (see Fig.1). In the worst case, failure of a portion of the interconnected power system could cause the failure of further grid sections and thus a cascading failure reaction, which in the worst-case scenario would result in a large-scale blackout. Critical situations must be anticipated, especially during the winter months. A grid failure of this type would result in a significant commercial loss. Reactive power generation plays an important role here, as reactive power serves to stabilize the grid voltage. In normal operation, the generators in the power plants not only supply the active power, but also always provide a portion of the necessary reactive power for the grid. Page 4 of 17

5 Fig. 1: Investigations in March of 2011 of the anticipated grid voltage profiles in the (n-1) case (example)[1] Pure reactive power can be supplied to the grid as additional energy in essentially two ways. One possibility is through stationary banks of capacitors, although these are not suitable for the continuous supply of reactive power due to their regeneration times. Rotating synchronous generators are more advantageous here. By virtue of their rotating mass and the downstream system, these can be connected essentially steplessly and with no time limitations with regard to their availability for flexible operation. The rotating phase shifter is thus a high-performance, automatically and dynamically controllable reactive power source. A further advantage of the rotating phase shifter is operation in the under-excited range, as this enables the removal of excess reactive power from the grid in order to prevent undesirable voltage increases on the grid side, as is often the case on weekends, for example. For these reasons, a feasibility study was initiated in June of 2011 in agreement with the Federal Network Agency by Amprion and RWE Power with the objective of investigating the potential modification of the synchronous generator in Biblis unit A to a phase shifter (synchronous motor) [2]. The modification affected only the conventional section of the power plant and thus also received the approval of the responsible authorities. Page 5 of 17

6 3.1 Operating Principle of a Phase Shifter For a better and easier understanding of the operating principle of a generator as a phase shifter, we can simply look at mechanical engineering examples. The flywheel or pressure equalizing tank are also examples of energy reserves which can be called upon if necessary to support a dynamic mechanical system. In simplified electrical terms, it could be said that the synchronous generator acting as a phase shifter is a kind of pump for reactive power which feeds the interconnected power system with its reactive power and has a stabilizing effect. Unfortunately, this reactive power cannot be transported over long distances. 4 Modification into a Phase Shifter 4.1 Important Factors affecting Modification At the start of the project it was necessary to determine the factors affecting modification of the synchronous generator into a phase shifter. This was initially based on customer grid requirements as well as the power balance of the system. The question of the drive type for the rated speed quickly developed into the key point of the investigations. This was followed by component questions such as removal of the turbine components, modification of the oil supply systems, foundation modifications, axial support of the shaft train, cooling on startup, modification of the protection systems or shaft train calculations and disturbance case studies. This demonstrates the prototype nature of this project and is an indication of the difficulty of the challenges. In addition to the technical task definition, however, the time constraints for implementation of the project also constituted a major challenge for all participants. These challenges could therefore only be overcome by the unique establishment of an entirely new project organization which had to perform many activities in parallel. Page 6 of 17

7 Generator Exciter Rotating Rectifiers Fig. 2: Remaining Biblis A turbine generator set before modification 4.2 Feasibility Study The generator studies were the first essential part to enable implementation of the project. At the start of the project Generator Engineering examined feasibility and the new required components. The electrical calculations regarding the modified startup procedure as well as the new operating mode were investigated for this purpose. It was calculated that no critical conditions would be reached with regard to heating of the exciter, rotor and stator during runup [3]. The anticipated axial loads result from displacement of the rotor from the magnetic center and the downward force calculated for the design of the thrust bearing. The necessary exciter current and the associated rotor current at reduced speed were also determined. This yields the induced synchronous internal voltage of the stator for field detection of the startup converter (see section 4.5). A further important aspect of the design was to determine the necessary initial acceleration power and the available torque. These parameters could be used to determine the run-up time for startup. The correct run-up time is an important factor for successful startup of the generator in view of the limited generator cooling. Page 7 of 17

8 4.3 Design of New Mechanical Components The most complex part was the technical design of the new components required for the modification. As elimination of the turbine components also removed the thrust bearing for the remaining generator, an entirely new concept had to be developed here. It is absolutely essential for startup that the generator rotor be held in its axial position and that the thrust forces on the shaft be accommodated by a thrust bearing. The integration of such a thrust bearing in the remaining shaft train required a new intermediate shaft, which first had to be designed and constructed. Furthermore, a new speed monitoring system had to be integrated and a new turning gear unit connected to break away the remaining shaft train. These new components had to be accommodated in the existing bearing housing between the turbine (LP3) and generator (see Fig. 2 and 3), accounting for the overall alignment of the remaining shaft train. Siemens Engineering succeeded in transferring the theoretical information obtained to the development of these new components in a very short time. New hydraulic motor for turning gear New thrust bearing to axially stabilize intermediate shaft New intermediate shaft Fig. 3: New components installed for phase-shift operation [2] Page 8 of 17

9 4.4 New Turning Gear (Hydraulic Motor) The new turning gear was selected based on calculation of the machine data, the prototype test investigations from the factory and on-site testing. This enabled the inclusion of an adequate safety factor in rating the new turning unit (hydraulic motor) to overcome the breakaway torque of the new 228 t turbine generator set and to bring it up to sufficient speed. Calculations indicated that this criterion is satisfied at a speed of approx. 180 rpm. The hydraulic motor was selected such that the breakaway torque could already be overcome at a pressure of 120 bar. The remaining acceleration was achieved at a pressure of up to 160 bar. The actual acceleration time of the new turbine generator set proved to be shorter than that predicted by the conservative calculations. The hydraulic motor was equipped with an overrunning clutch which connects and disconnects at defined speeds in order to ensure reliable operation. Turning speed is determining for the field detection of the electrical startup frequency converter. The evaluation focused primarily on the four operating conditions of shaft standstill, turning operation, runup and rated speed. 4.5 New Electrical Components (Startup Frequency Converter) The operating principle of the startup frequency converter is sufficiently well known from its use for smaller generators. The main differences for this project are the modification of an existing system and the size of the generator while using the existing rotating exciter. Accounting for these requirements and the extremely short implementation time, an initial rough calculation was determining for rating the startup converter. The interfaces with Generator Engineering were especially important for the detailed calculation of the necessary components of the startup converter in order to achieve an adequately sized and feasible solution. The main components to be installed and designed included the 14 MW mediumvoltage startup converter with two air-cooled transformers as well as the gas-insulated 30 kv medium-voltage switchgear system through which the plant was connected with the 27 kv generator iso phase bus (see Fig. 5). The startup frequency converter is protected by an I s limiter (current limiter). The tasks to be completed also included generating the single line diagram as a schematic of the startup system (see Fig. 4). Page 9 of 17

10 Overview schematic of phase-shifter components 10 kv auxiliary power buses Generator line, 10.9 km 10.5 kv/2x2.8 kv Startup freq. converter 100 to 1530 rpm up to 14 MW (Siemens) DC intermediate circuit Amprion 380 kv interconnected power system Generator transformers Generator power breaker Line disconnector ABB I s limiter ABB voltage controller Unitrol feed New GIS switchgear system Synchronous generator 27 kv, 1500 MVA RWE Power Phase shifter Main exciter Simplified schematic Exciter winding Rotating diodes for DC excitation of synchronous generator Fig. 4: Single line diagram for modification into a phase shifter [2] In the initial phase of the generator study, it was also investigated whether startup of the generator from standstill is possible with a smaller startup frequency converter. However, the results exhibited an excessive degree of uncertainty in the determined values, with the result that this approach was dropped. All configurations were elaborated in close cooperation with RWE Power and the Biblis power plant project team under the technical supervision of Georg Schneider. Responsibility for subsections such as the electrical cabling or assembly of the setdown areas for the startup frequency converter was thus also assumed by local personnel. Siemens provided support in I&C matters and for the synchronization process. I&C integration as well as assembly of the new unit protection system were performed by the power plant's own personnel and by Amprion. Page 10 of 17

11 Transformer 1 Transformer 2 Startup converter Medium-voltage switchgear system Fig. 5: Generator after modification into a phase shifter [2] 4.6 Modification of Lube and Lift Oil Supply The remaining necessary lube and lift oil supply for the plant had to be modified as part of the phase shifter modification. As mentioned above, the existing turning gear was also included in the modification as it was no longer usable. As a result, the exciter bearing, the EE generator bearing, the TE generator bearing, the new thrust bearing and the new turning gear hydraulic motor had to be supplied with lift and lube oil. As the oil flow supplied by the existing pumps was too high after elimination of the turbine components, the excess oil had to be run through a bypass. The existing oil lines in the power plant were used and were combined with resized oil lines. The system was also equipped with new shutoff, check and control valves. As the existing turbine generator bearing housing could be used for implementation of the new parts, some of the existing connections were also reused here. The lube and lift oil supply thus developed by Siemens Turbine Engineering was then tested and adjusted in multiple simulation steps in order to determine the settings for the system. The simulation thus served as the basis for the measurements during the commissioning phase of the oil system. Following a standard series of tests, the oil pressure and flow rates were set based on the specified values and were adjusted to the system. It proved here that the plant Page 11 of 17

12 behaves as predicted in the calculations, and all of the systems could be set with no difficulties. This was possible thanks to the many years' experience of the power plant personnel and Siemens commissioning engineers. 4.7 Evaluation of Potential Faults and Synchronization Conditions Finally, potential electrical faults were investigated with the support of Prof. Kulig at the University of Dortmund. The starting parameters for this were specified by Martin Lösing (Amprion). Eigenfrequency calculations, final rotor dynamic analyses and calculations of transient torques on the coupling connections were performed for all components in order to ensure reliable operation of the system. The synchronization conditions, protection systems and cable dimensions were investigated and implemented primarily by grid operator Amprion and by RWE. In the framework of the project, Amprion performed dynamic simulations, stability calculations and short circuit current calculations for the modification and connection of the phase shifter to the 380 kv grid. The results from this investigation formed the basis for the settings of the unit protection system, the synchronizing unit and the voltage controller [2]. Fig. 6: Rotor dynamic eigenfrequency analyses after modification [3] Page 12 of 17

13 4.8 Startup For startup, the generator is disconnected from the turbine and is set up in the new configuration with the new components. Breakaway torque is overcome by the new hydraulic motor and the generator shaft is accelerated to a turning speed of 180 rpm. The thrust bearing ensures correct axial positioning of the generator shaft in this step. The new speed detection system measures and monitors current speed. The generator is supplied via a static exciter to establish a rotating DC field in the rotor. This rotating DC field generates a rotating stator field. The existing rotating exciter is unable to generate sufficient rotor current at this point. The startup frequency converter starts detecting the 3-phase AC field in the generator stator above a speed of 180 rpm. Once this 3-phase field is clearly detected, the rotating field of the startup converter is synchronized and is connected to the generator stator. The startup frequency converter accelerates the rotor from this point up to an overspeed of 1,530 rpm (51 Hz). As a very high starting current is necessary for the lower speed range ( rpm), the 27 kv transformer on the output side of the startup frequency converter is initially operated in bypass mode. The hydraulic motor is automatically disengaged by the centrifugal clutch (above 400 rpm). Appropriate changes have been made to the settings in the unit protection program in line with the new requirements for correct generator runup. The Referenz-Elektrotechik Measures here include deactivation of the underfrequency protection and switching to a sensitive-setting definite time overcurrent protection of the synchronous machine. [4] For synchronization of the generator with the grid, the startup frequency converter is disconnected from the generator at a speed of 1530 rpm, the normal unit protection system is reactivated, the rotating exciter connected and the system synchronized with the grid within the synchronization time window with decreasing speed. Starting from this time, the generator operates as a motor in the grid. The generator modified in this way can provide reactive power over a range from -400 Mvar (underexcited) to +900 Mvar (overexcited). The resulting average active power consumption is approx. 5 MW. The phase shifter has been operating stably on the grid since commissioning. The total range of reactive power generation has been utilized in this operation. The effectiveness of the Page 13 of 17

14 reactive power input has already been proven by Amprion in several grid disturbances. This phase shifter thus provides Amprion's 380 kv with a high-capacity and effective reactive power source, which can be controlled either by operating personnel or automatically, to accommodate situations involving low (overexcited operation) or high grid voltage (underexcited operation) [2]. 5 Customer Benefits The new operating mode of the generator results in many customer benefits. Assessments focus especially on the changed grid stability situation in Germany. However, it must be assumed that still further changes in this perspective will result in the near future. The most important benefit is the general stabilization of the grid for voltage support over long power transmission routes as well as in local or industrial grids. The rotating phase shifter can supply the reactive power for fast balancing dynamic peak demands of the grid. This form of reactive power generation generally prevents voltage peaks such as can result in static switching operations. These benefits are further augmented by the retention of local jobs in the region as well as by the option of a commercial return on the reactive power for the operator. As a result of the existing infrastructure at the site, a cost-effective modification was achieved within a very short completion time. 6 Conclusion I wish to thank all of the participants from Amprion, RWE, the University of Dortmund, manufacturers and the departments of Siemens AG. It was only possible to complete this project within the short time frame allowed as a result of the optimum cooperation by all involved. It has proven that technical achievements in power generation and distribution are highly valued in Germany. Furthermore, this collaborative effort has produced a feasible and effective solution within the tight time constraints to continue the cost-effective supply of electric power within Germany for the future. The assumption at the onset of the project was that the phase shifter would contribute significantly to grid stability [1]. This statement from the German Federal Network Agency has been fully confirmed following the successful conclusion of the project and the verifiable stabilization of the grid over the past year. This success is further confirmed by the decision Page 14 of 17

15 of RWE to continue operating the generator as a phase shifter for grid stabilization for longer than initially planned. Moreover, this project has highlighted the successful technical feasibility of continuing to use generators as phase shifters and is already regarded as a reference project in many European countries. Initial investigations in other plants have shown that this reference is a milestone in technical understanding and in the development of customer-oriented solutions at Siemens. "An investment in knowledge always pays the best interest" Scientist and businessman Benjamin Franklin 7 References [1] Bundesnetzagentur ( ): Report on Effects of Phaseout of Nuclear Power on Transmission Grids and Supply Security [2] Marin Lösing (5/2012): Modification of Biblis A Synchronous Generator into a Phase Shifter / VGB Power Tech [3] Siemens Energy ( ): Final report: Biblis A Modification into a Phase Shifter [4] Siemens Instrumentation, Controls and Electrical, (2012): The Biblis A Generator Stabilizes the Grid Page 15 of 17

16 8 Disclaimer These documents contain forward-looking statements and information that is, statements related to future, not past, events. These statements may be identified either orally or in writing by words as expects, anticipates, intends, plans, believes, seeks, estimates, will or words of similar meaning. Such statements are based on our current expectations and certain assumptions, and are, therefore, subject to certain risks and uncertainties. A variety of factors, many of which are beyond Siemens control, affect its operations, performance, business strategy and results and could cause the actual results, performance or achievements of Siemens worldwide to be materially different from any future results, performance or achievements that may be expressed or implied by such forward-looking statements. For us, particular uncertainties arise, among others, from changes in general economic and business conditions, changes in currency exchange rates and interest rates, introduction of competing products or technologies by other companies, lack of acceptance of new products or services by customers targeted by Siemens worldwide, changes in business strategy and various other factors. More detailed information about certain of these factors is contained in Siemens filings with the SEC, which are available on the Siemens website, and on the SEC s website, Should one or more of these risks or uncertainties materialize, or should underlying assumptions prove incorrect, actual results may vary materially from those described in the relevant forwardlooking statement as anticipated, believed, estimated, expected, intended, planned or projected. Siemens does not intend or assume any obligation to update or revise these forward-looking statements in light of developments which differ from those anticipated. Trademarks mentioned in these documents are the property of Siemens AG, its affiliates or their respective owners. Page 16 of 17

17 Published by and copyright 2013: Siemens AG Energy Sector Freyeslebenstrasse Erlangen, Germany Siemens Energy, Inc Alafaya Trail Orlando, FL , USA For more information, please contact our Customer Support Center. Phone: / Fax: / (Charges depending on provider) [email protected] All rights reserved. Trademarks mentioned in this document are the property of Siemens AG, its affiliates, or their respective owners. Subject to change without prior notice. The information in this document contains general descriptions of the technical options available, which may not apply in all cases. The required technical options should therefore be specified in the contract.. Page 17 of 17