www.siemens.com/energy
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
Table of Contents
1
Abstract .................................................................................................................. 3
2
Initial Situation in the Power Plant......................................................................... 4
3
Motivation and Theory........................................................................................... 4
3.1
Operating Principle of a Phase Shifter ................................................................... 6
4
Modification into a Phase Shifter........................................................................... 6
4.1
Important Factors affecting Modification .............................................................. 6
4.2
Feasibility Study..................................................................................................... 7
4.3
Design of New Mechanical Components............................................................... 8
4.4
New Turning Gear (Hydraulic Motor) ................................................................... 9
4.5
New Electrical Components (Startup Frequency Converter)................................. 9
4.6
Modification of Lube and Lift Oil Supply ........................................................... 11
4.7
Evaluation of Potential Faults and Synchronization Conditions.......................... 12
4.8
Startup .................................................................................................................. 13
5
Customer Benefits ................................................................................................ 14
6
Conclusion............................................................................................................ 14
7
References ............................................................................................................ 15
8
Disclaimer ............................................................................................................ 16
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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.
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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.
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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.
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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.
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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.
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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 thrust bearing to
New hydraulic motor
axially stabilize
for turning gear
intermediate shaft
New intermediate
shaft
Fig. 3: New components installed for phase-shift operation [2]
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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 Is
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).
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Overview schematic of phase-shifter components
10 kV auxiliary power buses
Generator line, 10.9 km
Amprion 380 kV
interconnected power
system
10.5 kV/2x2.8 kV
Generator transformers
Startup freq.
converter
100 to 1530 rpm
up to 14 MW (Siemens)
DC intermediate
circuit
Generator power breaker
Line disconnector
ABB
voltage
controller
Unitrol
ABB
Is limiter
feed
New GIS switchgear system
Synchronous generator
27 kV, 1500 MVA
RWE Power
Simplified schematic
Phase shifter
Main exciter
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.
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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
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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]
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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 (180 - 310 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
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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
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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 (2011-08-31):
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 (2012-03-10):
Final report: Biblis A Modification into a Phase Shifter
[4] Siemens Instrumentation, Controls and Electrical, (2012):
The Biblis A Generator Stabilizes the Grid
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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, www.siemens.com and on the SEC’s website, www.sec.gov. 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.
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the property of Siemens AG, its affiliates,
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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..
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