FI123771B

FI123771B - Transformer control

Info

Publication number: FI123771B
Application number: FI20125224A
Authority: FI
Inventors: Jan Stefan Strandberg; Juhamatti Korhonen; Arto Sankala
Original assignee: Vacon Oyj
Priority date: 2012-02-29
Filing date: 2012-02-29
Publication date: 2013-10-31
Also published as: FI20125224A

Description

TRANSFORMER CONTROL

FIELD OF TECHNOLOGY

5 This invention relates to power conversion equipments where a multi-winding transformer is used for galvanic isolation between primary and secondary circuits. The invention particularly relates to a method and apparatus for preventing saturation of a multi-winding transformer of a power conversion apparatus provided with power cells on the primary and secondary 10 sides.

PRIOR ART

US Patent Application 12/825,619 discloses a power converter for 15 converting electric power between e.g. a medium-voltage grid and an AC motor. The converter comprises a multi-winding transformer and low-voltage power cells connected in cascade on both the primary and secondary sides of the transformer.

20 Multi-winding transformers are substantial parts of such power converters where they are applied together with the power electronic components, e.g. for galvanic isolation between primary and secondary circuits. The term multi-winding in this context stands for a transformer with more than one primary winding, which primary windings transfer active power through the 25 transformer to the secondary windings.

Phenomena such as unmatched turn-on/turn-off times, ° semiconductor forward voltage drops, gate driving signal delays or pulsating g load, may cause differences in the positive and negative volt-seconds applied to cvj 30 the transformer. This results in a DC-voltage component at the transformer x terminals, which causes an undesired DC magnetic flux density component in

CL

the transformer iron core. A DC component in the flux means asymmetric c\j magnetization, which may lead to core saturation, high current spikes in the g winding current and even destruction of power components.

O 35

C\J

Several methods have been published for preventing the saturation of multi-winding transformers in power converters.

2 E.g. in R.Patel: ’’Detecting Impending Core Saturation in Switched-Mode Power Converters,” in Proc. of the 7th POWERCON conference, voi. B3, March 1980, is presented a method where an external air-gapped core leg is used for indicating when the un-gapped main magnetic flux path has been 5 saturated.

In G.Stumberger et al: ’’Prevention of Iron Core Saturation in Multi-Winding Transformers for DC-DC-converters”, IEEE Transactions on Magnetics, voi 46, No 2, February 2010, is presented a method where the magnetic flux of the core is measured and the measuring result used for power ίο switch control in order to prevent saturation.

In G. Ortiz et al: “’’Magnetic Ear-Based Balancing of Magnetic Flux in High Power Medium Frequency Dual Active Bridge Converter Transformer Cores”, 8th International Conference on Power Electronics - ECCE Asia May 30-June 3, 2011, is presented a method where an auxiliary core is attached to 15 the main core so that they share a part of the magnetic path. By this way the flux of the main core can be sensed via an auxiliary winding around the auxiliary core, making it possible to prevent the incipient saturation in an early phase.

The drawback of the known methods is that they need additional 20 sensors and/or magnetic core pieces with attached electronics, which increase the complexity and costs of the system.

SUMMARY OF THE INVENTION

25 The object of the present invention is to provide a novel method and apparatus to prevent the saturation of a multi-winding transformer of a power conversion apparatus, preferably without any additional magnetic core t? parts or flux sensors. The above mentioned disadvantages will be avoided and ^ a symmetric flux operation ensured, with quick recovery e.g. from external

LO

o 30 asymmetry-causing impulses. The objective is achieved by a method and cm apparatus according to the invention, characterized by what is stated in the g independent claims. Other preferred embodiments of the invention are

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disclosed in the dependent claims.

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CM

m ™ 35 In the present invention the saturation of a multi-winding ° transformer in a power conversion apparatus is prevented by controlling the power switches of those power cells supplying active power to the transformer.

3

The magnetizing balance is ensured by adjusting to the duty cycle of all input H-bridges supplying power to the transformer.

In a preferred embodiment of the present invention, two parallel controllers are used, working at different time levels.

5 The task of the first controller, working at faster time level, is to maintain the magnetizing balance in the iron core. The feedback information from the core magnetization state for this anti-saturation controller can be arranged in several ways: - e.g. by integrating the voltage of one winding, preferably the 10 voltage of an additional winding as shown in FIG. 4B.

- measuring the sum current of all windings using a common transformer or separate winding-specific transformers - naturally also a flux sensor can be used

The controller output affects simultaneously the duty cycle of every power cell 15 supplying power to the common multi-winding transformer. This change of the duty cycle is able to correct the saturation unbalance, but it may cause differences and unbalance to the winding current values. The operating frequency of this magnetic balance controller is preferably the same as the switching frequency of the power cells connected to the transformer.

20 The task of the second controller according to the invention is to correct the unbalance of those winding currents which are supplying power to the common multi-winding transformer. This controller works at a slower time level, and the input value for it is calculated by averaging the winding current during at least one external load cycle time (grid cycle time on primary side and 25 motor/generator cycle time on secondary side). Each winding has its own controller which affects the duty cycle of the power cell supplying current to that winding. The operating frequency of this current balance controller is preferably £ at least ten times slower than the operating frequency of the magnetic balance ™ controller.

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9 30 oj Exemplary controller time levels, i.e. frequencies at which the £ controller functions are performed in the system program, may be e.g. 12 kHz

CL

for the faster balance controller and 1.2 kHz for the average controller, when the c\j switching frequency of the power cells supplying power to the multi-winding m ™ 35 transformer is 12 kHz. According to the simulated operation at these exemplary ^ conditions, an abrupt magnetic flux unbalance step, which may be caused e.g.

by a step-like error in the pulse width instruction affecting on the switching moment of each power cell switch, will be corrected into a balance in less than 4 20 ms time. The correction of the unbalance of winding currents takes a longer time, but in less than 50 ms also the currents are balanced.

The asymmetry problem of a transformer is usually caused by 5 differences in power switch conduction voltage-drops or switching times. By using the invention the negative effects of this kind of component value dispersions can be minimized and thus components with larger characteristic value tolerances can be used, which improves the reliability and lower the costs of the power electronics system.

10

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and additional objects, features and advantages of the present invention will be more clearly understood from the following detailed 15 description of preferred embodiments of the present invention, taken in conjunction with accompanying drawings, in which: FIG. 1 presents a power converter with a 6-winding transformer, FIG. 2A and 2B present power cells in a power converter, FIG. 3 presents the block diagram of the controller according to the 20 invention, FIG. 4A and 4B present alternatives for measuring the magnetizing current, and FIG. 5 presents the operation of the controller.

25 DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 presents the known topology of a power converter which can ^ convert electric power between a medium-voltage grid and an AC motor, S1 described e.g. in US Patent Application 12/825,619 and which can be applied in

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9 30 the present invention. It comprises low-voltage power cells connected in a SI cascade on both the medium-voltage network MV (frequency 50 / 60 Hz, phases L1, L2, L3) and the medium-voltage electric machine M/G (frequency

CL

adjustable, phases U, V, W) sides, and high-frequency transformers connect

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SI the power cells. The converter includes several serial-connected groups

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™ 35 Gi...Gn, which all include a six-winding transformer Ti...TN and six (6) power S cells (e.g. C11...C16 in group Gi), each power cell connected either to one primary winding WPi or to one secondary winding Wsi- The serial-connected groups Gi...Gn form a multistep output voltage waveform to both external 5 connections (primary grid Up connection L1, L2, L3 and secondary grid, e.g. motor M connection U, V, W). A control unit CU, connected e.g. by a serial data link to the cell internal control units (not shown in FIG. 1), takes care of the appropriate upper level control of the converter.

5

Fig. 2A presents a basic circuit of a power cell in the converter according to FIG. 1. The power cell Cn includes two similar so-called H-bridges, i.e. power switch connections which comprise of four (4) controllable switches, e.g. IGBTs V1 - V4, correspondingly V5 - V8 and four (4) free- io wheeling diodes D1 - D4, correspondingly D5 - D8. The H-bridges are connected via a common DC-voltage link DCn having a capacitor C. The input bridge Hm connects the DC-link voltage via the single-phase input connection IN11 to the transformer primary winding WPn, by changing the polarity normally at 50% duty cycle. Correspondingly, the output bridge Hon can connect the 15 DC-link voltage to the output connection OUTn for forming a part of the converter output voltage.

Because the input bridge works at 50% duty cycle it is possible to replace half of the switches in bridge Hm with two capacitors C1, C2 as in bridge Hm2i according to FIG. 2B, as is well-known for a person skilled in the 20 art.

Fig. 3 presents the block diagram of an anti-saturation controller according to the invention. The controller of Fig. 3 refers to group Gi in Fig. 1, but every group has its own similar controller. The controller affects those 25 power cells supplying active power to the transformer, in this exemplary case to the cells in the grid side (Cu, C12, C13). In power flow direction change the cells under the anti-saturation control will be changed accordingly “on the fly”.

The controller according to the invention actually includes two ™ separate controllers, working at different time levels.

LO

9 30 The first controller, CONmi, is the faster anti-saturation controller, c\j aiming to maintain the magnetizing balance in the transformer. It works on the basis of the average of the measured transformer magnetizing current, imag, ave,

CL

which is compared to the reference value 0 in the summing unit ΣΜι. The sum vj £] value is the input to the Pl-regulator PImag-i, the output of which is an input to ™ 35 the summing units Em... Im. The preferred calculating interval for this ° controller is the same as the switching frequency of the H-bridges connected to the transformer.

6

The other controllers, CON011...CON013, are slower phase-current averaging controllers, aiming to keep the power cell output current average values iouth, ave···Ιουτΐ3, ave at 0. Every cell supplying active power to the transformer has its own controller, in this exemplary case they are cells 5 C11...C13 in Fig. 1. In the controllers, the average values of the output currents (iouTu, ave··· 10UT13, ave) are first compared to the reference value 0 in the summing units Σ011...Σ013· The summing unit outputs are then led via the Pl-regulators ΡΙηοιι···ΡΙηοι3 to the duty cycle summing units Σ111...Σ113. The reference value of these summing units is 50%, which is thus also the final duty 10 cycle reference DHih...Dhii3 for the modulators ΜΗιιι...ΜΗιΐ3 of the input H-bridges Hm... H113 in case of either the transformer anti-saturation controller CONmi or any of the cell output current averaging controllers C0Non --C0NOi3 do not disturb the balance. The preferred calculating intervals for the averaging controllers are longer, e.g. 10 times the calculating interval of the anti-saturation 15 controller.

The basic operation principle of all the regulators CONmi and CON011...CON013 is thus to change the duty cycle of the input bridges so that the average value of the transformer magnetizing current and the cell output currents stay at value 0.

20

Fig. 4A and 4B present two examples of possibilities for measuring the magnetizing balance of a multi-winding transformer.

In Fig. 4A another wire of all input H-bridges H111...H113 are led through a common current transformer core Tc. The sum current, measured by 25 a coil around the core (not shown in Fig. 4A), is thus the same as the magnetizing current. As a person skilled in the art knows, the waveform of the magnetizing current in this kind of a converter is triangular, and the average t? value of it (needed for the anti-saturation controller in Fig. 3) can be calculated ™ e g. as the sum of the positive and negative peak values. A more complete 9 30 method for the average value calculation, which method gives a correct result c\] also in case of core saturation, is to integrate the current area during one operation cycle.

In Fig. 4B there is an additional winding W1 in the multi-winding c\] transformer Ti as presented in Fig. 1. As a person skilled in the art well knows,

LO

^ 35 the waveform of the winding voltage uwi in this kind of a converter is ° rectangular, and the average value of it (needed for the anti-saturation controller in Fig. 3) can be calculated e.g. by integrating the voltage area during one operation cycle.

7

Fig. 5 presents an example, in the topology of a converter presented in Fig. 1, of how the regulator according to the invention works. In the figure, the duty cycle references DHih - - - DHm3 of all input Fl-bridges Hm... Hm3 5 are shown, as well as the average values of the output currents ioirm, AVE··· loUT13, AVE-

At time instant ti (5.0 ms) an external impulse pushes the duty cycle of Hu from the balance value 50% to about 50.6%. This causes the antisaturation controller to wake up and give a counter-push backwards to all three ίο duty cycle signals Dhih...Dhm3 at time instant t2. This in turn causes the average values of output currents to start drifting out of the balance. Within about 3 ms time (by time instant 8 ms), the faster anti-saturation controller restores a new magnetic balance, where the duty cycles of all phases are pushed away from 50% value. The slower controller notices change in the 15 phase current average values at about 10 ms time instant and begins to push the duty cycles back to 50% balance value. At time instant 20ms the duty cycles are back in 50%, but the average values of phase currents still differ from zero. The slow controller continues to control the duty cycles as long as the average value differs from zero, and finally at about 50 ms time instant the 20 average values are 0 in this exemplary case.

While the invention has been described with reference to the previous embodiment, it should be recognized that the invention is not limited to this embodiment, and many modifications and variations will become apparent 25 to persons skilled in the art without departing from the scope and spirit of the invention, as defined in the appended claims. As described above the invention can be applied in converters transmitting power between a polyphase electric 2 machine (motor or generator) and a power transmission network, which can be ^ either a polyphase alternating-current (AC) network or a direct-current (DC) 9 30 network. The invention can also be applied to power transmission between

SI different electric systems, such as e.g. from a DC network to a polyphase AC

network, or between AC networks (grids) of different voltages and different

CL

frequencies. Especially the invention can be applied in a medium-voltage SI environment, in which both the electric machine and the power transmission

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£! 35 network (grid) or networks (grids) are medium-voltage, o

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Claims

1. The method prevents a multi-winding transformer from being saturated in a power converter apparatus 5 consisting of a multi-winding transformer (T-ι) and power cells (Cu-C16, Cni-Cn6) connected to the primary and secondary sides of said transformer consisting of H1121, Hon), each with controllable switches (V1 to V8) and bridges connected to each other by a DC link, which cells are connected to their separate transformer windings via an incoming H-bridge, which normally operates at a certain pulse rate, e.g. 50% pulse rate, and which cells are connected to the external electrical system via an outgoing H-bridge, characterized in that the method comprises the steps of: 15. measuring the magnetization state of the transformer core and winding currents in the windings supplying electric power to the transformer; pulse ratios of the bridges to maintain the magnetization equilibrium in the transformer;

The method of claim 1, wherein the pulse ratio of the input bridges is set such that the average of the transformer excitation current and the cell output currents is 0. S?

The method of claim 1 or 2, wherein in the? Magnetization setup, the pulse rate is simultaneously adjusted in each power cell supplying a LO 9 30 power to a common multi-winding transformer. CVJ CVJ

The method of claim 1, 2 or 3, wherein the setting of the CL excitation mode operates at the first time level and the winding current setting £ 1 at the second, slower time level. LO ^ 35

The method according to any one of claims 1 to 4, wherein the measurement data of the magnetization state of the core is calculated by integrating the voltage of one winding, preferably the voltage of the auxiliary winding. 12

The method according to any one of claims 1 to 4, wherein the measurement data of the magnetization state of the core is arranged by measuring the total current of all windings. 5

The method of any one of claims 1 to 4, wherein the measurement of the cardiac magnetization state is provided using a flow sensor. ίο

A method according to any one of the preceding claims 1 to 4, wherein the winding currents are set separately for each winding.

9. A system to prevent saturation of a multi-winding transformer in a power converter apparatus having control means (CU) 15 consisting of a multi-winding transformer (T-ι) and power cells (Cu-C16, Cni-Cnö) connected to the primary and secondary halves of said transformer consisting of two H-bridges (Hm H1121, Hon), each with controllable switches (V1-V8) and bridged by a DC link, which cells are connected to their separate transformer windings via an incoming H-bridge, which normally operates at a certain pulse rate , e.g., at 50% pulse rate, which cells are connected to the external electrical system via an outgoing H-bridge, characterized in that the system comprises: 25. means for measuring the magnetization state and winding current of the transformer core in the coils supplying electrical power to the transformer; matched t? to determine the pulse ratios of the power supply input bridges to the transformer based on the measurement of the magnetization state to maintain the magnetization equilibrium LO 9 30 in the transformer, and c \ J to adjust the pulse ratios of the electric power supply input bridges C \ 1 C \ 1 LO ™ 35

The system of claim 9, wherein the control means is adapted to adjust the pulse ratio of the input bridges such that the transformer excitation current and the cell output currents have an average of 0. 13

The system of claim 9 or 10, wherein the control means comprises an excitation state setting controller for setting a pulse ratio simultaneously in each of the power cells supplying the power to the common multi-winding transformer. 5

The system of claim 9, 10 or 11, wherein the control means comprises an excitation state setting controller common to all transformer active power supply cells, and a winding current setting controller for each cell supplying active power to the transformer, wherein the magnetization state setting controller operates at the layout controls operate at another, slower time level.

A system according to any one of the preceding claims 9-12, wherein the transformer is provided with additional windings so that measurement information of the cardiac magnetization state can be calculated by integrating the voltage of the additional winding.

A system according to any one of the preceding claims 9 to 12, wherein the measurement data of the core magnetization state is arranged by measuring the sum current of all windings.

A system according to any one of the preceding claims 9 to 12, wherein the measurement data of the cardiac magnetization state is provided using a flow sensor. 25

A system according to any one of the preceding claims 9 to 15, wherein the operating frequency of the magnetic balance controller 'is the same as the switching frequency of the power cells connected to the transformer. C \ J i LO 9 30

A system according to any one of the preceding claims 9 to 16c, wherein the operating frequency of the current balance controller is at least ten times slower than the operating frequency of the magnetic balance controller CL. CVJ CVJ m CVJ δ 35 CVJ