mechanical and electrical rebuilding of a turbine generator for phase

Authors: Detlef Frerichs Anastassios Dimitriadis Maren Wiese Siemens AG Energy Sector Service Division

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

POWER-GEN Europe 2013 Vienna, Austria June 04-06, 2013

www.siemens.com/energy

AL: N; ECCN: N © Siemens AG 2013. All rights reserved. of 17

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


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.


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.


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.


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.


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.

Generator Exciter

Rotating Rectifiers


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.

Fig. 3: New components installed for phase-shift operation [2]

New hydraulic motor

for turning gear

New thrust bearing to

axially stabilize

intermediate shaft

New intermediate

shaft


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 medium-

voltage 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).


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.

Overview schematic of phase-shifter components10 kV auxiliary power buses

Amprion 380 kV interconnected power system

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

Line disconnector

Generator power breaker

ABB Is limiter

New GIS switchgear system

Synchronous generator 27 kV, 1500 MVA RWE Power

Phase shifter

ABB voltage controller Unitrol

feed

Simplified schematic

Exciter winding

Main exciter

Rotating diodes for DC excitation of synchronous generator

Generator transformers


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

Transformer 1 Transformer 2

Startup

converter

Medium-voltage

switchgear

system


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]


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


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


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


8 Disclaimer

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mechanical and electrical rebuilding of a turbine generator for phase

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