The present invention relates to a system for controlling an electrical load.
With a view to increasing the power supplied to the same electrical load, it is known to place several power converters in parallel. The main problem with this configuration lies in the fact that a current flows from one power converter to the other. The result is that the sum of the currents on the three output phases of a power converter is not zero whereas it should be. For minimizing this problem, one known solution is to synchronize the PWM (Pulse Width Modulation) controls of the inverter stages therebetween. Such solutions are, for example, disclosed in patents U.S. Pat. Nos. 7,327,111, 8,188,694 or 6,917,271. They use a centralized control unit which generates the PWM signals intended for the inverter stages of all the power converters connected in parallel. On the other hand, with a separate control unit for controlling each inverter stage, synchronizing the PVVM signals is no longer sufficient since it does not guarantee that each control unit generates the same output voltage. From one power converter to another, fluctuations appear, linked, for example, to variations in the parameters, gains or measurements of currents.
The aim of the invention is therefore to provide a control system using multiple power converters connected in parallel by the DC bus on the one hand, by the motor voltages on the other via an inductance circuit, for controlling an electrical load, said control system being arranged for balancing the output currents between the power converters connected in parallel.
This aim is achieved by a system for controlling an electrical load, said system including:
According to another feature, the first power converter and the second power converter each comprise a DC power supply bus applying a DC voltage to their inverter stage and the first power converter and the second power converter are interconnected by their DC power supply bus.
According to another feature, the DC power supply bus of the first power converter and the DC power supply bus of the second power converter each comprise a power supply line with a positive electrical potential and a power supply line with a negative electrical potential.
According to another feature, the output of the first power converter is connected to the output of the second power converter.
Other features and advantages will appear in the following detailed description, which makes reference to the accompanying drawings in which:
The inventive control system is intended for controlling an electrical load, such as, for example, an electric motor M. The control system has the feature of comprising multiple power converters connected in parallel to the same electric motor.
Each power converter is, for example, of the variable speed drive type. In a known way, as represented in
As represented in
In the rest of the description, the index a is used to represent each of the three output phases.
The inventive control system also comprises multiple control units UCk (k ranging from 1 to n), a separate control unit being assigned to the control of the inverter stage INVk of each variable speed drive VVk.
In a known way, a control unit UCk comprises a main control module M1_k (k ranging from 1 to n) receiving as input an estimate or measurement of the output current of the controlled variable speed drive and determines, according to one or more setpoints and the parameters of the electric motor, an output voltage to be applied to the electric motor. More precisely, the output current iσk obtained is multiplied by the number n of variable speed drives of the system so as to determine the motor current iσMeqk. This motor current iσMeqk is then used for determining the motor voltage vσMeqk to be applied by implementing a known control law LC (not forming the subject matter of the invention). This motor voltage vσMeqk then corresponds to the output voltage vσk.
In the inventive control system, each control unit UCk autonomously determines an output voltage vσk to be applied to the electric motor M and each variable speed drive VVk is autonomously controlled for applying this output voltage vσk to the electric motor M.
In other words, for each variable speed drive of the system, the following relationships apply, forming a system S1:
In which:
The inductors are preferably selected as being identical.
The above relationships do not guarantee that the currents are balanced when the physical components used differ from one variable speed drive to another.
Accordingly, the objective of the invention is to control the variable speed drive currents while taking account of the current to be applied to the electric motor. This objective is expressed as follows:
iσ1+iσ2+ . . . +iσn=iσM
iσ1=iσ2= . . . =iσn
For guaranteeing that the currents between the variable speed drives are balanced, the invention consists in ensuring that at least one of the output currents of a variable speed drive is known to the control units of the other variable speed drives of the system. For this, the simplest way is to select a variable speed drive called the ‘master’ the output current of which is sent to the control units of the other variable speed drives of the control system.
The ‘master’ variable speed drive is, for example, the variable speed drive of rank 1 associated with the control unit of rank 1. The ‘slaves’ variable speed drives are then those of ranks 2 to n, each associated with a respective control unit of rank 2 to n.
In the system thus formed, the control unit of the ‘master’ variable speed drive of rank 1 comprises a main control module M1_1 as described above, for determining the output voltage vσ1 to be applied at the output of the ‘master’ variable speed drive according to the output current iσ1 of the ‘master’ variable speed drive VV1.
According to the invention, the control units of ranks 2 to n each comprise a main control module M1_k (k ranging from 2 to n) as described above, which is arranged for determining the output voltage of the controlled variable speed drive and a secondary control module M2_k (k ranging from 2 to n). This secondary control module is arranged for determining a correction voltage Δvσk to be applied to the output voltage vσk determined by the main control module M1_k. In each control unit of rank 2 to n, the secondary control module M2_k receives, as input, the output current iσk of the controlled variable speed drive and the output current iσ1 of the variable speed drive VV1 of rank 1. The secondary control module M2_k is arranged for determining the difference between the output current iσk of the controlled variable speed drive VVk and the output current iσ1 of the variable speed drive of rank 1 and for injecting said difference into a proportional action corrector or a proportional-integral action corrector PI. From the difference injected as input, the corrector determines a correction voltage to be applied to the output voltage determined by the main control module. For each variable speed drive, this correction voltage thus reflects the difference to be corrected between the output current of said variable speed drive and the reference output current of the variable speed drive selected as ‘master’.
The secondary control unit thereby implements a control algorithm expressed by the following relationships, for a ‘slave’ variable speed drive k (k ranging from 2 to n):
With:
By reusing these expressions in the system S1 expressed above, the following are obtained:
Which shows an effective control of the current deviation, i.e.:
In a control system with two variable speed drives, the principle of the invention is thus to ensure that the output current of one of the two variable speed drives is supplied to the other variable speed drive. If the system comprises more than two variable speed drives connected in parallel, the same principle may be used so that one of the variable speed drives supplies its output current to the other variable speed drives of the system.
The following demonstration illustrates a generalization of the solution. This general solution consists in stabilizing the following system (1):
in which the currents follow the relationship:
iσ1+iσ2+ . . . +iσn=iσM
The objective is then to balance the currents between the various variable speed drives:
σ1=iσ2= . . . =iσn
To simplify writing, it is considered here that the inductances Li are all identical and equal to L. The difference between the inductances may be handled as an interference in the system thus simplified. In matrix writing, the relationships (1) become:
The voltages supplied by each variable speed drive may be resolved as the sum of two quantities:
vσi=vσMi+vEσ1
A first quantity vσMi is used to control the motor.
A second quantity vEσ1 is used to control the balance of the variable speed drive currents.
To simplify writing, it is considered here that the components of the voltages vσMi used to control the motor are all identical and equal to vσm
P the invertible change-of-coordinates matrix is defined such that:
Then the currents iΣ1 for i from 1 to n−1 are defined such that:
P having the form:
where Q must be an invertible matrix.
If the vectors un of dimension n (rows)×1 (column) and un−1 of dimension N−1 (rows)×1 (column) are defined
P is written as follows::
The inverse of P has the following form:
R is the matrix defined by:
R=n×(ln−1+un−1·un−1
This transformation is applied to the voltages. Then the voltages vΣ1 for i from 1 to n−1 are defined such that:
In these new coordinates, the relationship (2) becomes:
The currents iΣ1 are of the form:
Row 1 of system (3) relates to the motor control. It can be considered that each variable speed drive provides a component for
Now only rows from 2 to n are considered, dealing with the issue of controlling the flowing current, giving:
At this stage, the control voltages should be defined by the following relationships:
The function f has the property of stabilizing system (4). Two examples are cited and, without loss of general application, this is followed by the example of the proportional action corrector.
As a first example, the case of a proportional action corrector may be defined:
The matrix K has the property of having all these eigenvalues with strictly negative real parts. This property ensures an exponential convergence towards zero of the current deviations in the case of system (3).
As a second example, the case of a proportional and integral action corrector may be defined.
W is defined as the state of the integrators:
With system (4), the following is obtained:
The matrices Kp and Ki have the property of stabilizing the system thus composed, i.e. the matrix KPI has the property of having all its eigenvalues with strictly negative real parts, where
Where IN−1 is the identity matrix of dimension N−1, and 0N−1 is the zero matrix of dimension N−1.
Returning to the voltages to be delivered by each variable speed drive in the case of a proportional action correction, the following relationship is obtained:
According to the matrix G=Q−1×K×Q, the currents must be shared between the various variable speed drives.
Case 1: selection of Q=IN−1 (identity matrix), and K a diagonal matrix.
Selecting vEσ1=0, gives G=K, and:
This requires that the current of variable speed drive 1 is shared with the other variable speed drives.
Case 2: selection of
and K a matrix
Selecting VEσ1=0, gives
This then requires that the current of the variable speed drive i−1 is shared with the variable speed drive i (for i ranging from 2 to n).
Number | Date | Country | Kind |
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13 58733 | Sep 2013 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2014/069389 | 9/11/2014 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2015/036479 | 3/19/2015 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
5446645 | Shirahama | Aug 1995 | A |
6917271 | Zhang et al. | Jul 2005 | B2 |
7327111 | Rastogi et al. | Feb 2008 | B2 |
8188694 | Tallam et al. | May 2012 | B2 |
20080073978 | Ollila | Mar 2008 | A1 |
20080158917 | Chen | Jul 2008 | A1 |
20090116266 | Lai | May 2009 | A1 |
20120013283 | Tallam | Jan 2012 | A1 |
Number | Date | Country |
---|---|---|
41 05 868 | Sep 1991 | DE |
2008-086127 | Apr 2008 | JP |
Entry |
---|
International Search Report issued Feb. 5, 2015, in PCT/EP2014/069389 filed Sep. 11, 2014. |
International Search Report dated Feb. 5, 2015, in PCT/EP2014/069389 filed Sep. 11, 2014. |
Panov, Y. et al., “Analysis and Design of N Paralleled DC-DC Converters with Master-Slave Current-Sharing Control”, Twelth Annual Applied Power Electronics Conference and Exposition, Conf. 12, (Feb. 23, 1997), pp. 436-442, XP000736214. |
Number | Date | Country | |
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20160211766 A1 | Jul 2016 | US |