Method and system for controlling the connection of an electrical machine to an electrical network via a bypass contactor

Abstract

Method and system for controlling the connection of an electrical machine to an electrical network via a bypass contactor Method for controlling the connection of an electrical machine (3) to an electrical network (5) via a bypass contactor (4) and the operation of a converter (2) by which the machine (3) is connected to the network (5), from a state where the machine (3) is connected to the network via the converter (2) without the output voltage of the latter being synchronized with that of the network (5), the method comprising: a) controlling the converter (2) to synchronize the output voltage thereof with that of the network (5); b) controlling the contactor (4) to connect the machine (3) to the network (5) via the contactor (4); c) controlling the converter (2) following detection of the instant when the contactor (4) closes, in order to interrupt its operation. Figure for abstract: Fig. 1

Description

Title of the invention: Method and system for controlling the connection of an electrical machine to an electrical network via a bypass contactor Technical field

[0001] The present invention relates to the field of power electronics, and more particularly relates to a technique for controlling the connection of an electrical machine to an electrical network via a bypass contactor and the operation of a converter by which the machine is connected to the network, hereinafter called a bypass technique. Prior art

[0002] Several bypass techniques exist in the literature for the purpose of saving energy, particularly in order to meet increasingly stringent environmental standards. Each percent improvement in energy efficiency can result in a substantial gain.

[0003] US2012 / 0187886 describes a bypass method in a system comprising a Variable speed drive powering an electric motor, the drive itself being powered by an electrical network, and two contactors: a contactor at the output of the drive and a so-called bypass contactor which connects the motor to the network when it is closed. After synchronizing the output voltage of the drive with that of the electrical network, the contactor at the output of the drive opens and then the bypass contactor closes. There is therefore a "blackout" time of up to 5 seconds during which the motor is not powered by either the drive or the electrical network. When the motor is at its nominal load, a significant loss of torque is possible, which induces a very high current draw when switching from the power supply by the drive to the power supply by the network.

[0004] US8014110 discloses a bypass method which, unlike the method described in US2012 / 0187886, requires only one contactor, the bypass contactor, and is exclusively applicable in the case of a drive with an active rectifier (no thyristor rectifier). This method consists of synchronizing the motor with the network, closing the bypass contactor, then stopping the inverter of the drive.

[0005] Such a method necessarily requires filtering elements at the output of the inverter in order to absorb the switching edges. It cannot be applied as presented to a drive with a mixed passive input bridge (with diodes and thyristors) without oversizing the filtering elements to protect the thyristors from breakage by reducing the voltage edges at the output of the inverter.

[0006] Other bypass methods exist in the literature. Some rely on the installation of contactors upstream of the inverter. This allows the thyristor rectifier to be disconnected from the network before the bypass. However, in order to ensure continuity of service, excessive oversizing of the DC bus is inevitable in order to provide the necessary energy to the motor during the blackout. It should be noted that this does not prevent a high network overcurrent at the time of the bypass due to the short circuit between phases which appears at the inverter output.

[0007] Other methods are based on replacing the mixed bridge with a diode rectifier, known to be more robust against voltage variations over time, with the addition of filtering elements on both sides of the drive (method similar to that described in US8014110). This technique ensures the safety of the drive with a significant additional cost and a modification of the structure of the drive initially designed with a mixed bridge. Statement of the invention

[0008] There is therefore a need to improve the bypass methods of the prior art, particularly in terms of network disruption and / or to reduce the cost of equipment. Bypass process

[0009] The invention aims to meet this objective and has as its subject, according to one of its aspects, a method for controlling the connection of an electrical machine to an electrical network via a bypass contactor and the operation of a converter by which the machine is connected to the network, from a state where the machine is connected to the network via the converter without the output voltage of the latter being synchronized with that of the network, the method comprising: a) controlling the converter to synchronize its output voltage with that of the network; b) the contactor control to connect the machine to the network via the contactor; c) control of the converter following detection of the instant when the contactor closes, in order to interrupt its operation.

[0010] Closing the bypass contactor before the converter stops ensures continuity of service (no blackout time) without any excessive current draw at the time of transition from converter power to grid power, thereby reducing grid disturbance.

[0011] Furthermore, the method according to the invention does not require any filtering at the output of the converter, nor protection of the semiconductor components of the latter by additional passive elements, nor even oversizing of components belonging to the network, such as inductors, which makes it possible to reduce the cost of equipment.

[0012] Synchronizing the converter output voltage with that of the network consists of canceling the phase shift between the converter output voltage and that of the network when the machine frequency equals that of the network.

[0013] Detecting the instant when the contactor closes preferably comprises measuring at least one current flowing through the closed contactor, the instant when the contactor closes being detected when the absolute value of the current intensity is greater than or equal to a predefined threshold, this threshold preferably being greater than the noise floor of the measurement chain.

[0014] Preferably, the electrical network is three-phase.

[0015] The instant when the contactor closes is then preferentially detected when the absolute value of the maximum current intensity in one of the three phases flowing through the closed contactor is greater than or equal to the predefined threshold.

[0016] In one embodiment, the converter comprises an inverter connected to the machine and comprising H-shaped transistor bridges.

[0017] The electrical machine may be a generator.

[0018] Alternatively, the electrical machine is a rotating or passive load, in particular an electric motor or a kinetic energy accumulator.

[0019] The converter may be a variable speed drive comprising a DC rectifier through which the network is connected to the inverter.

[0020] The rectifier can be a mixed passive rectifier with diodes and controlled switches, in particular thyristors.

[0021] Step c) may include blocking the transistors of the inverter, in particular by cutting off the control signals of the gates of these transistors, being for example PWM signals.

[0022] Preferably, the ratio between the processing time of the converter stop order and the switching period of the converter is less than 0.5, better still less than 0.2, even better still less than 0.1; for example the processing time of the converter stop order is less than 50 ps for a switching frequency of the converter less than 10 kHz. Bypass system

[0023] The invention also relates, according to another of its aspects, to a system for controlling the connection of an electrical machine to an electrical network via a bypass contactor and the operation of a converter by which the machine is connected to the network, from a state where the machine is connected to the network via the converter without the output voltage of the latter being synchronized with that of the network, the system being arranged to: - control the converter to synchronize its output voltage with that of the network; - control the contactor to connect the machine to the network via the contactor; - control the converter following detection of the moment when the contactor closes, in order to interrupt its operation.

[0024] Such a system can be used for implementing the method according to the invention as defined above. Bypass installation

[0025] The invention also relates, according to another of its aspects, to an electrical installation comprising the bypass system according to the invention, the converter and the contactor.

[0026] The installation may include the electrical machine.

[0027] The invention also relates, according to another of its aspects, to the use of the bypass system according to the invention for implementing the bypass method according to the invention. Brief description of the drawings

[0028] The invention may be better understood by reading the detailed description which follows, of non-limiting examples of its implementation, and by examining the attached drawing, in which:

[0029] [Fig-1] [Fig.l] is a schematic view of an example of an electrical installation according to the invention;

[0030] [Fig.2] [Fig.2] is a diagram illustrating an example of the progress of step a) of the method according to the invention;

[0031] [Fig.3] [Fig.3] is a diagram of an example of an electronic circuit implementing step c) of the method according to the invention;

[0032] [Fig.4] [Fig.4] is a set of timing diagrams of the signals occurring when the contactor closes;

[0033] [Fig.5] [Fig.5] schematically represents the results of the experimental validation in terms in particular of voltage curves for a load rate of the variator at approximately 20%;

[0034] [Fig.6] [Fig.6] schematically represents the results of the experimental validation in terms of current curves for a load rate of the variator at approximately 20%;

[0035] [Fig.7] [Fig.7] represents two current curves extracted from [Fig.6];

[0036] [Fig.8] [Fig.8] schematically represents the results of the experimental validation rimental in terms of voltage curves for a variator load rate of approximately 50%;

[0037] [Fig.9] [Fig.9] schematically represents the results of the experimental validation ratio in terms of current curves for a drive load rate of approximately 50%;

[0038] [Fig. 10] [Fig. 10] represents two current curves extracted from [Fig.9];

[0039] [Fig. 11] [Fig. 11] schematically represents the results of the experimental validation rimental in terms of voltage curves for a variator load rate of approximately 80%;

[0040] [Fig. 12] [Fig. 12] schematically represents the results of the experimental validation in terms of current curves for a load rate of the variator at approximately 80%;

[0041] [Fig. 13] [Fig. 13] represents two current curves extracted from [Fig. 12]; and

[0042] [Fig. 14] [Fig. 14] illustrates the cycling process in the example used for the va definition of the method according to the invention. Detailed description

[0043] [Fig.l] schematically illustrates an example of an electrical installation 1 according to the invention. This installation 1 comprises a system 20, according to the invention, for controlling the connection of an electrical machine 3 to an electrical network 5 via a bypass contactor 4 and the operation of a converter 2 by which the machine 3 is connected to the network 5, from a state where the machine 3 is connected to the converter 2 without the output voltage of the latter being synchronized with that of the network 5.

[0044] The system 20 is configured to implement the method according to the invention.

[0045] In this example, the electric machine 3 is a motor. This can be a motor asynchronous intended for driving compressors, pumps, fans or for ship propulsion.

[0046] The electrical network 5 is, in this example, three-phase L1, L2, L3, for example 400 V 50 Hz or 60 Hz.

[0047] The bypass contactor 4 may be an electromechanical relay but in a variant, it may be solid-state.

[0048] Converter 2 is, in this example, a variable frequency speed variator (also called VSD) connected at the input to network L1, L2 and L3.

[0049] The converter 2 comprises a DC rectifier 21 (AC / DC converter), a DC bus 23, a conventional bus filter 24 and an inverter 22 (DC / AC converter).

[0050] In this example, the rectifier 21 is a mixed passive rectifier with diodes 211 and thyristors 210.

[0051] The inverter 22 comprises IGBT transistor bridges 220 in H whose gates are controlled by PWM signals generated by the system 20.

[0052] To implement the method according to the invention, the system 20 controls in step a) the variator 2 in order to synchronize the output voltage of the latter with that of network 5. The sub-steps of step a), called pre-bypass, are shown diagrammatically in [Fig.2],

[0053] Several synchronization techniques are presented in the literature. The one adopted in the example consists of starting motor 3 with drive 2 without synchronization. Once the motor frequency is equal to that of the network, a regulation makes it possible to cancel the phase shift between the network voltage and that at the output of the drive.

[0054] When synchronization is performed, the bypass phase can begin.

[0055] In step b), the system 20 controls the contactor 4 to connect the motor 3 di directly to network 5 via contactor 4.

[0056] Once this contactor is closed, a short circuit between the phases of the network tends to appear between one or more IGBTs and one or more freewheeling diodes of different phases of the inverter. The number of semiconductors involved in this short circuit depends on the time of closing of the contactor in relation to the sinusoidal waveform of the output current. If the inverter were not stopped quickly, the current would continue to rise until the IGBTs desaturate and the overcurrent protections of the inverter (hardware and / or software) are triggered.

[0057] No breakage of the inverter would be due to this increase in current. However, the fact that the inverter 22 continues to switch induces excessive stress on the triggers of the thyristors 210 which undergo the cutting of the inverter 22. This cutting naturally induces the breakage of these thyristors.

[0058] The method according to the invention is based on detecting the instant when the contactor 4 closes. This detection makes it possible to stop the switching of the IGBTs 220 before the thyristors 210 break (step c) of the method).

[0059] For this, current sensors 40 are installed on the network cables upstream of the contactor 4. Once the latter is closed, the rise in current is detected by an electronic circuit whose schematic diagram is shown in [Fig.3]. The maximum absolute value of the intensities of the currents ILb IL2, IL3 corresponding to the three phases flowing through the closed contactor 4 is compared to a predefined threshold. When this threshold is crossed, an order to inhibit all PWM commands of the IGBT transistors 220 is given. The level of the predefined threshold is determined so that it is very low (for a fast reaction) but sufficiently high compared to the noise floor of the measurement chain. In the experimental validation whose results are presented later, the predefined threshold was set at 50A for an inverter whose nominal current is approximately 140A.

[0060] The advantage of the method according to the invention is the absence of a blackout time for the electrical machine, with a single contactor, without filtering at the output of the inverter, nor oversizing of filtering at the input of the rectifier. Indeed, the machine 3 continues to see electrical energy arriving from inverter 2 until the bypass contactor 4 has closed. This is possible thanks to the very fast reaction time of the current threshold detection chain which interrupts the operation of inverter 2 as soon as the current rises upstream of the contactor. The number of switchings seen by the thyristors is therefore limited.

[0061] It is appropriate to ensure that the processing time of the PWM signal cut-off order is significantly less than the switching period of inverter 2. For experimental validation (see below), this delay is equal to 42ps for a switching frequency of 3 kHz. The timing diagram of the bypass operation is shown in [Fig.4], Te being the processing time for cutting off the PWM signals.

[0062] When the user wishes to change the speed of the motor, the inverter can take over safely according to methods already published in the literature. Examples

[0063] The experimental validation of the method according to the invention was carried out with a passive mixed bridge drive (diodes and thyristors) as input, the POWERDRIVE MD SMART MD3 75T powering a 90kW LS 400V asynchronous motor. This motor drives a synchronous machine which injects the electrical energy into the network through a POWERDRIVE FX 100T drive.

[0064] This test bench makes it possible to experimentally validate the method according to the invention up to approximately 80% of the nominal load of the equipment under test (MD3 75T).

[0065] The waveforms of the experimental validation results are illustrated in Figures 5 to 13 with different values of load rate of the speed variator: - approximately 20% load for figures 5 to 7, - approximately 50% load for figures 8 to 10, and - approximately 80% charge for figures 11 to 13.

[0066] The load rate of a variable speed drive is defined as the proportion of the nominal power of the motor which is supplied by the drive.

[0067] All experimental validation measurements were performed with respect to the L1 phase of the network.

[0068] Figures 5 to 13 schematically represent the waveforms of different physical quantities concerned by the bypass process.

[0069] Figures 5, 8 and 11 illustrate the curves of the current flowing through the contactor Itng, of the network voltage Uim2, of the motor voltage Um and of the voltage across a thyristor of phase L1 Vthi. Figures 6, 9 and 12 represent the curves of the current flowing through the contactor I^g, the current at the output of the inverter Iond, the motor current Im and the network current of phase L1 Ires. Figures 7, 10 and 13 illustrate the curves of the current flowing through the contactor ItHg and the motor current Im.

[0070] The arrows in Figures 5 to 13 indicate the transition phase from power supply by the inverter to power supply by the network.

[0071] Views (b) and (c) in each of Figures 5 to 11 are an enlargement of views (a) by a factor of 10 and 500 for the time scale, respectively, around said transition phase.

[0072] It can be seen, in particular in views (b) and (c) of each of figures 5 to 13, that the current I^g is zero before the bypass instant. The visual oscillations of the current Itrig before the bypass instant in figures 6(a), 9(a) and 12(a) are due to the precision of the experimental current probes used (Rogowski). In fact, the probe used for measuring the current Itrig of the order of 100A is of caliber 6000A.

[0073] In each of figures 5 to 13, it can be seen, in particular on the curve of the motor current Im, that the motor continues to operate, without there being a blackout time.

[0074] Furthermore, no breakage of the thyristors appears according to the curve of the voltage at the terminals of a thyristor of phase L1 Vthi.

[0075] Indeed, the network is protected in this example with 500A UR fuses and the drive with 350A UR fuses. If one of the thyristors were broken (in closed circuit), a short circuit between network phases would have immediately appeared in the Vthi waveform measured at the terminals of the thyristor, but this is not the case. This result then validates the bypass method according to the invention.

[0076] It is noted here that the load is not affected by the short circuit between phases which appears at the time of the bypass at the inverter level. Indeed, the curves clearly show that, when the current I^g begins to increase (effect of the short circuit) because of the closing of the contactor, the motor current Im, for its part, remains stable and does not undergo any rapid increase in current.

[0077] However, the results obtained also show the existence of a ripple in the motor current at the time of the bypass. It is probably due to imperfections linked to the synchronization of the motor voltage with that of the network as well as to the difference between the effective values of these two voltages due to the presence of a line inductance at the input of the drive. Indeed, the greater the load, the greater the difference between these voltages and the greater the ripple.

[0078] It should be noted that, despite everything, for the different motor current levels, the ripple remains low and only lasts one period thanks to the continuity of the motor power supply which does not undergo any blackout during the bypass.

[0079] The course of the cycling of the method according to the invention follows the flowchart of [Fig.14],

[0080] Cycling consists of verifying the robustness of the bypass method according to the invention. For this, an automated system can be set up in order to carry out a sufficient number of bypasses without any failure in the drive. Each bypass cycle lasts approximately 5 minutes in order to allow the various components of the drive to restore their initial states.

[0081] The choice of the maximum number of bypasses is set based on the following assumption: "Throughout the lifetime of the drive, one bypass per week will be performed." Thus, for a drive lifetime of 40 years, the maximum number of bypasses would be approximately 2000. Once the cycling was completed, no anomalies were detected in the drive. All semiconductors remained functional.

[0082] The invention relates to all industrial applications involving continuous processes and / or relating to the production / conversion of energy.

[0083] The invention is not limited to the embodiment described above. The electrical machine may, for example, be a generator or a kinetic accumulator. Similarly, the power transistors may be of a type other than IGBTs, for example MOSFETs.

Claims

Claims

1. A method for controlling the connection of an electrical machine (3) to an electrical network (5) via a bypass contactor (4) and the operation of a converter (2) by which the machine (3) is connected to the network (5), from a state where the machine (3) is connected to the network via the converter (2) without the output voltage of the latter being synchronized with that of the network (5), the method comprising: a) controlling the converter (2) to synchronize the output voltage thereof with that of the network (5); b) controlling the contactor (4) to connect the machine (3) to the network (5) via the contactor (4);c) controlling the converter (2) following detection of the instant when the contactor (4) closes, in order to interrupt its operation, the detection of the instant when the contactor (4) closes comprising the measurement of at least one current (IL1, IL2, Ils) flowing through the closed contactor (4), the instant when the contactor (4) closes being detected when the absolute value of the intensity of the current (ILb IL2, IL3) is greater than or equal to a predefined threshold, this threshold being preferably greater than the noise floor of the measurement chain.;

2. Method according to the preceding claim, the electrical network (5) being three-phase.

3. Method according to the two preceding claims, the instant when the contactor (4) closes being detected when the absolute value of the maximum intensity of the current (ILb IL2, IL3) in one of the three phases flowing through the closed contactor (4) is greater than or equal to the predefined threshold.

4. Method according to any one of the preceding claims, the converter (2) comprising an inverter (22) connected to the machine (3) and comprising H-shaped transistor bridges (220).

5. A method according to any one of the preceding claims, the machine (3) being a generator.

6. Method according to any one of claims 1 to 4, the machine (3) being a rotating or passive load, in particular an electric motor or a kinetic energy accumulator.

7. Method according to the preceding claim in its attachment to claim 4, the converter (2) being a speed variator comprising a DC rectifier (21) by means of which the network is connected to the inverter (22).

8. Method according to the preceding claim, the rectifier (21) being passive mixed with diodes (211) and controlled switches, in particular thyristors (210).

9. Method according to claim 4 or any one of claims 5 to 8 in its attachment to claim 4, step c) comprising the blocking of the transistors (220) of the inverter (22), in particular by cutting off the control signals of the gates of these transistors, being for example PWM signals.

10. Method according to any one of the preceding claims, the ratio between the processing time of the order to stop the converter (2) and the switching period of the converter (2) being less than 0.5, better still less than 0.2, even better still less than 0.1, and preferably the processing time of the order to stop the converter (2) is less than 50 ps for a switching frequency of the converter (2) less than 10 kHz.

11. System (20) for controlling the connection of an electrical machine (3) to an electrical network (5) via a bypass contactor (4) and the operation of a converter (2) by which the machine (3) is connected to the network (5), from a state where the machine (3) is connected to the network via the converter (2) without the output voltage of the latter being synchronized with that of the network (5), the system (20) being arranged to: - control the converter (2) to synchronize the output voltage thereof with that of the network (5); - control the contactor (4) to connect the machine (3) to the network (5) via the contactor (4);- controlling the converter (2) following detection of the instant when the contactor (4) closes, in order to interrupt its operation, the detection of the instant when the contactor (4) closes comprising the measurement of at least one current (IL1, IL2, Ils) flowing through the closed contactor (4), the instant when the contactor (4) closes being detected when the absolute value of the intensity of the current (ILb IL2, IL3) is greater than or equal to a predefined threshold, this threshold being preferably greater than the noise floor of the measurement chain.;

12. Electrical installation (1) comprising the system (20) according to the preceding claim, the converter (2) and the contactor (4).

13. Installation according to the preceding claim, comprising the electrical machine (3).

14. Use of the system according to claim 11 for implementing the method according to any one of claims 1 to 10.

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