Cascade machine

The cascade machine is an electrical machine that can be used as a generator in wind turbines . This generator does not require any slip rings , as they are necessary with double-fed asynchronous generators or with synchronous generators , but the cascade machine shows a similar operating behavior as an asynchronous machine with slip rings. Due to disadvantages, this type of generator has so far only been used in a larger system (as of 2008).

General

double stator-fed three-phase cascade
( DDMK , BDFTSIG )

brushless double-fed machine ( BDFM )

In order for a generator to be able to convert mechanical energy into electrical energy, its rotor must be excited . This is done either via a permanent magnet or a rotor winding ( excitation winding ). The rotor winding is excited with direct current ( excitation current ). The winding ends are brought out on slip rings and the excitation current is transmitted by means of carbon brushes . The disadvantage of this type of excitation is the maintenance effort of the slip ring apparatus and the carbon brushes.

With the cascade machine , the problems with the slip rings and brushes can be avoided and still you don't have to forego the advantages of the three-phase asynchronous machines .

There are two types of cascade machines:

the double stator-fed three-phase cascade (DDMK) brushless doubly-fed twin stator induction generator (BDFTSIG)
the self-cascading machine (SKM), which is also known as a brushless double-fed machine (BDFM).

Both the self-cascading machine (SKM) with a common iron circle and the cascade machine with separate systems can be used well in wind power plants.

Structure and function of the cascade machine

Basic circuit of a DDMK in a wind turbine. Two stator windings are shown.
Winding 1 on the mains,
winding 2 on the frequency converter.

The cascade machine is constructed differently than conventional asynchronous generators and consists of a combination of two asynchronous machines. The rotors of this combined machine are both mechanically and electrically connected to form a unit. The rotor has a self-contained winding without external connections. Since it is self-contained, it is very similar to a cage winding. It has a simpler structure than rotor windings with two separate coils, so the losses are lower than with rotors with separate windings . The conductor distribution in the runner must be uneven, if necessary, even some slots in the runner remain unwound. In accordance with the number of basic pole pairs, only asymmetrical windings are used for the rotor. This is a new, completely unconventional rotor winding for coupling the stator windings.

There are two windings with different numbers of pole pairs and in the stator of the cascade generator . Depending on the requirements, these stator windings can be designed as separate windings with different numbers of pole pairs or as pole-changing windings. So that the two stator windings can be accommodated in a laminated core , a suitable number of pole pairs and for electromagnetic decoupling of the windings must be selected. The two stator windings of the cascade machine take on the role of the stator winding and the rotor winding of an asynchronous machine with slip ring rotor. The part of the stator winding with the number of pole pairs takes on the task of the normal stator winding. The part of the stator winding with the number of pole pairs has the role of the rotor winding. The machine has the resulting number of pole pairs . ${\ displaystyle p_ {1}}$ ${\ displaystyle p_ {2}}$ ${\ displaystyle p_ {1}}$ ${\ displaystyle p_ {2}}$ ${\ displaystyle p_ {1}}$ ${\ displaystyle p_ {2}}$ ${\ displaystyle p _ {\ mathrm {res}} = p_ {1} + p_ {2}}$

The two stator windings are magnetically coupled to one another via the rotor. There is no direct, galvanic coupling within the stator. The result is a generator that has the properties of a three-phase induction machine with a slip ring rotor , but does without the disruptive slip rings.

Separate stator windings have decisive advantages for operation on the frequency converter:

Each of the two windings can be provided with the appropriate number of turns independently, which is very often necessary due to the connection voltage of the network and the output voltage of the converter.
The windings are galvanically separated, so in the event of a fault, no direct current can flow from the converter via the cascade generator into the network and thus back to the converter connection on the network side .

All windings are housed in an active part made up of a stator and rotor. It is also possible to design one of the two stator windings for direct current, but the machine would then be operated as a brushless synchronous machine. However, this design is very rare and only a "special model" of these rotary field machines, the asynchronous induction machine is used most frequently.

The converter

When used as a variable-speed drive or as a generator, e.g. B. in a wind turbine, the cascade machine is used together with a frequency converter . This frequency converter is connected to the second stator winding and takes over the active power control as well as the synchronization with the network . The converter can be operated with overmodulation by a zero voltage, since the separate windings prevent a zero current through the generator into the network. The converter has to transmit around 23% of the system output, so you only need a converter with a low output. Most of the regenerative power is fed directly into the network from the first stator winding.

In addition to the active power and the torque, the reactive power of the two windings can also be adjusted by the converter. The machine can draw the inductive reactive power it needs from both stator winding 1 and winding 2. The converter can also be set so that the overall system behaves capacitively and covers the reactive power requirements of inductive consumers. The transfer of the reactive power via the cascade machine, which operates with low slip , increases the reactive power. A large reactive power can therefore be fed into the grid with a small converter power.

Operating behavior

Principle structure: self-cascading machine with separate windings ( SKM )

The rotor winding has the task of coupling the two different fields of the number of pole pairs and the stator winding with one another. It does not have to have a specific number of strands. The absolute values of the voltages induced in it are also not of great importance. The coupling between the two fields must take place in the same way as with separate rotor windings. This means that the currents flowing in the rotor winding simultaneously generate a field for the number of pole pairs and a field for the number of pole pairs . This induces currents both in the stator winding with the number of pole pairs and in the rotor winding. The two stator windings are thus coupled to one another by the rotor currents. With a so-called rotor current observer, the rotor current can be regulated via the stator voltage of the second stator winding. This measure allows the torque of the cascade machine to be set dynamically and precisely to a high quality. ${\ displaystyle p_ {1}}$ ${\ displaystyle p_ {2}}$ ${\ displaystyle p_ {1}}$ ${\ displaystyle p_ {2}}$ ${\ displaystyle p_ {1}}$ ${\ displaystyle p_ {2}}$

Basic structure : double stator-fed three-phase machine cascade ( DDMK )

The stator winding 1 is connected to the network and emits the regenerative power directly into the network. The stator winding 2 is connected to the converter and, depending on the speed, delivers electrical power to the converter or takes power from the converter. The resulting power to the network is: ${\ displaystyle P_ {1}}$

{\ displaystyle \! \ P _ {\ mathrm {ges}} = P_ {1} + P_ {2}}

given, which is equal to the mechanical power if the losses are neglected . The advantage of this design is that the cascade generator only transfers part of the power to the grid via the converter, while most of the power is transferred directly to the grid. This lowers the costs and the structural volume for the converter and reduces problems with harmonic currents in the network. ${\ displaystyle P _ {\ mathrm {mech}}}$

Generator operation

Converter performance
comparison for wind farms with different generator types

Counter arrow definitions

In the case of the cascade generator, the mechanical power on the shaft is output in total via both stator windings. The power flows change depending on the speed of the generator and the frequency of the network.

The mechanical power P _mech with the speed n and the mechanical moment M is:

{\ displaystyle P _ {\ mathrm {mech}} = 2 \ cdot \ pi \ cdot n \ cdot M}

The synchronous speed n _{0 of} the generator is, as with the asynchronous generator, given by the ratio of the frequency f _{1 of} the first winding:

{\ displaystyle n _ {\ mathrm {0}} = {\ frac {f _ {\ mathrm {1}}} {p _ {\ mathrm {res}}}}}

As with the asynchronous generator, the slip s becomes:

{\ displaystyle s = {\ frac {n _ {\ mathrm {0}} -n} {n _ {\ mathrm {0}}}}}

calculates and gives the frequency f _{2 of} the second winding:

{\ displaystyle f _ {\ mathrm {2}} = s \ cdot f _ {\ mathrm {1}}}

Due to the constant frequency of the first winding, it gives the "air gap power"

{\ displaystyle P = 2 \ cdot \ pi \ cdot n _ {\ mathrm {0}} \ cdot M}

from.

When operating above the synchronous speed n ₀ , both stator windings emit power. The first winding with synchronous line frequency feeds its power directly into the AC voltage network, the second winding transfers its power to the converter. This means that the converter only needs to be designed for part of the total power and is therefore smaller than in other constellations. The speed varies, e.g. B. between n ₀ and 2 n ₀ , the power of the converter only needs to be designed for half the mechanical power.

Advantages and disadvantages

Advantages:

Variable speed operation
Active power control for power optimization and limitation
Mains supply with low harmonic content and adjustable reactive power
Low actuator costs, e.g. B. through double-fed generator concept in connection with frequency converter with reduced power
Low maintenance
High reliability of the generator

Disadvantage:

Complicated control structure
Weak damping, especially in the upper speed range and on large machines
"Gap" in the performance curve at synchronous speed
Inverter output greater than that of an asynchronous machine with a slip ring rotor
Lower material utilization with the self-cascading machine thanks to the linear working area

Common generator concepts in wind turbines
Generator type	advantages	disadvantage
permanent magnet synchronous machine	no slip rings high power density variable speed	high price for magnets large converter output (full converter)
DC-excited synchronous machine	high power density variable speed	Slip rings required large converter output (full converter)
Slip-ring asynchronous machine	small converter power variable speed	Slip rings required

Summary

With the double-fed cascade generator, generator systems can be set up that are characterized by low converter costs, low network perturbations and low maintenance. The cascade generator feeds most of the power directly into the grid. Only a small part has to be passed over the converter. The entire power flow can be controlled with the converter current.

Statutory provisions and other regulations

EN 60 034 Part 1 General provisions for rotating electrical machines
EN 60 034 part 8 Terminal designations and direction of rotation for electrical machines
DIN IEC 34 Part 7 Types of rotating electrical machines
EN 60034-5 Degrees of protection of rotating electrical machines
EN 60034-6 Types of cooling, rotating electrical machines
DIN IEC / TS 60034-17 (VDE 0530 Part 17) Inverter-fed induction motors with squirrel cage
DIN IEC / TS 60034-25 (VDE 0530 Part 25) Guidelines for the design and operating behavior of induction motors that are specially dimensioned for converter operation
EMC directive 89/336 / EEC

literature

C. Fräger: New type of cascade machine for brushless speed actuators with low converter costs . Progress Reports Series 21, No. 189. VDI-Verlag, 1995, ISBN 3-18-318921-6 .

F. Bauer: New control method for the double-fed machine cascade . Volume 8, 1985, pp. 275-278 (Etz Archive 7).

Jens Kroitsch: Double column-fed cascade machines as generator systems in maritime wind power plants .

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r Jens Kroitzsch: The brushless double-fed induction machine as a generator in decentralized electrical power generation systems, dissertation . Otto von Guericke University Magdeburg, 2006 ( uni-magdeburg.de [PDF]). The brushless double-fed induction machine as a generator in decentralized electrical power generation systems, dissertation ( Memento from June 9, 2007 in the Internet Archive )

↑ ^a ^b ^c ^d ^e ^f Armin Dietz: The cascade machine as a generator for decentralized regenerative energy generation. Research project of the Technical University of Nuremberg, Nuremberg 2014.

↑ Erich Hau: Wind power plants: Basics, technology, use, economy. Berlin / Heidelberg 2008, p. 380f.

↑ Theodor Buchhold, Hans Happoldt: Electric power plants and grids. Second edition, Springer Verlag Berlin - Heidelberg GmbH, Berlin Heidelberg 1952, pp. 101-124.

^ Klaus Heuck, Klaus-Dieter Dettmann, Detlef Schulz: Electrical energy supply. 7th edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden, 2007, ISBN 978-3-8348-0217-0 , pp. 342-350.

↑ ^a ^b ^c ^d ^e Carsten Fräger: Winding for the rotor of electrical machines for coupling two fields with different numbers of poles . (Patent DE19526440C2).

↑ ^a ^b ^c ^d ^e Edwin A. Sweo: Double- fed brushless induction machines with double rod squirrel cage. Patent dated September 29, 2005, document number DE60019730T2.

Remarks

↑ In electrical machines, the active part is the part of the machine in which the magnetic and electrical processes that are important for energy conversion take place. (Source: Hans-Otto Seinsch: Fundamentals of electrical machines and drives. )

[Quelle_2-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r Jens Kroitzsch: The brushless double-fed induction machine as a generator in decentralized electrical power generation systems, dissertation . Otto von Guericke University Magdeburg, 2006 ( uni-magdeburg.de [PDF]). The brushless double-fed induction machine as a generator in decentralized electrical power generation systems, dissertation ( Memento from June 9, 2007 in the Internet Archive )

[Quelle_3-2] ↑ ^a ^b ^c ^d ^e ^f Armin Dietz: The cascade machine as a generator for decentralized regenerative energy generation. Research project of the Technical University of Nuremberg, Nuremberg 2014.

[Quelle_1-3] Erich Hau: Wind power plants: Basics, technology, use, economy. Berlin / Heidelberg 2008, p. 380f.

[Quelle_7-4] Theodor Buchhold, Hans Happoldt: Electric power plants and grids. Second edition, Springer Verlag Berlin - Heidelberg GmbH, Berlin Heidelberg 1952, pp. 101-124.

[Quelle_6-5] Klaus Heuck, Klaus-Dieter Dettmann, Detlef Schulz: Electrical energy supply. 7th edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden, 2007, ISBN 978-3-8348-0217-0 , pp. 342-350.

[Quelle_5-6] Carsten Fräger: Winding for the rotor of electrical machines for coupling two fields with different numbers of poles . (Patent DE19526440C2).

[Quelle_4-7] Edwin A. Sweo: Double- fed brushless induction machines with double rod squirrel cage. Patent dated September 29, 2005, document number DE60019730T2.

[Anm._Seinsch.-8] In electrical machines, the active part is the part of the machine in which the magnetic and electrical processes that are important for energy conversion take place. (Source: Hans-Otto Seinsch: Fundamentals of electrical machines and drives. )