Power electronic converter, electric motor drive and method for operating power electronic converter

Abstract

The invention relates to a power electronic converter, such as an inverter or converter, for electrical applications, such as motor drives, electrolyzers or chargers. The power electronic converter includes a DC link and an active front end (AFE) connected to a grid via a filter and a grid plate. The grid plate has a first surge relay. The first surge relay has a resistive device. The resistive device may be a resistor, more specifically an NTC resistor or a PTC thermistor. The power grid plate further comprises a second surge relay. During a start-up phase of the power electronic converter and during abnormal grid voltage conditions, the at least one surge relay is controlled to turn on or off such that the DC link is charged via or bypassing the resistive device, and the AFE is used to boost the voltage of the DC link to a safety level and in a controlled condition. The invention also relates to an electric motor drive and to a method for operating a power electronic converter.

Description

Technical Field

[0001] This invention relates to a power electronic converter, such as an inverter or converter, for electrical applications (such as motor drives, electrolyzers, or chargers). The power electronic converter includes a DC link and an active front end (AFE) connected to the power grid via a filter and a first surge switch having a resistive device. The surge switch can be any normally-off electronically controlled switch, such as a semiconductor (e.g., MOSFET, IGBT, IGCT, etc.) or an electromechanical switch (e.g., a relay). Since relays are the most common devices, the term "relay" is used hereinafter, although a more general surge switch in the form of a semiconductor switch can be used. The resistive device can be a resistor, more specifically an NTC resistor or a PTC thermistor. The power grid also includes a second surge relay, wherein at least one of the surge relays is controlled to turn on or off during the startup phase of the power electronic converter and during abnormal grid voltage conditions, such that the DC link is charged via or around the resistive device, and that the AFE is used to raise the DC link voltage to a safe level and under controlled conditions. The invention also relates to an electric motor drive according to claim 6, and a method for operating a power electronic converter according to claim 7. Background Technology

[0002] This invention relates to the field of inverters and converters for supplying electrical power from grid power to electrical applications such as electric motors. The invention can relate to low-harmonic drives (LHD) or zero-distortion drives (ZD) and renewable applications such as solar panels, wind turbines, fuel cells, and X-power applications, as well as related fields such as (fast) chargers and energy storage systems. Typically, active front-ends (AFEs) and power-controlled batteries (PFCs) of all power levels can be used in this context.

[0003] Typically, AFEs are equipped with filters, such as LCL filters. LCL filters cause a continuous (reactive) current draw on the surge circuit when the surge relay is open. Under grid fault conditions (e.g., low grid voltage), electrical applications such as motor drives may fail to start because the required DC link voltage is not reached. This level cannot be reduced using conventional startup strategies, as transients in the grid would otherwise result in surge currents that could damage the AFE circuitry. Instead, the motor drive remains in an uncontrolled state where resistive devices (such as surge resistors / thermistors) are loaded by the LCL filter and will overheat, as they are typically designed to carry current for only a very short time. Such events can reduce system reliability, as grid faults can age the AFE or, in some special cases, even damage the electrical application. To prevent this, the same startup and faulting strategies as passive rectifiers can currently be used. Summary of the Invention

[0004] The object of this invention is to overcome this problem. This object is achieved by the power electronic converter according to claim 1, the electric motor driver according to claim 6, and the method for operating the power electronic converter according to claim 7. Advantageous embodiments of the invention are subject to the dependent claims.

[0005] Claim 1 provides a power electronic converter, such as an inverter or converter, for electrical applications (such as motor drives, electrolyzers, or chargers). The power electronic converter includes a DC link and an active front-end (AFE) connected to the power grid via a filter, a first surge relay, and a second surge semiconductor or relay. The first surge relay has a resistive device, such as a resistor, NTC resistor, or PTC thermistor. According to the invention, during the startup phase of the power electronic converter and during abnormal grid voltage conditions, at least one of the surge relays is controlled to turn on or off, such that the DC link is charged via or around the resistive device, and that the AFE is used to raise the DC link voltage to a safe level and under controlled conditions. Specifically, in the first phase, both surge relays can remain open. After an adjustable time interval, the first surge relay can close, and after another time interval, the second surge relay can close to control the total power input to the AFE. The surge relays can be mounted on the grid board or integrated into different boards, coil boxes, or they can be floated as discrete cable connection elements.

[0006] Therefore, the basic idea of ​​this invention is to utilize the AFE (Augmented Front-End) as early as possible to raise the DC link voltage in a controlled manner. In this way, the relay in the surge circuit can close within seconds, and the surge resistor device or surge resistor will never be overloaded.

[0007] During voltage dips under normal operating conditions, the electrical application can remain connected as long as at least one phase has a minimum voltage of, for example, 10% of the nominal voltage. The advantage of this invention is that it significantly reduces the startup process by several minutes and avoids loss of control over the application.

[0008] Generally, this invention improves robustness against power grid failures and thus enhances the reliability of electrical applications. Under normal power grid conditions and with proper installation, the passive sequence of the startup phase that electrical applications rely on can be reduced to a level that is safe for the electrical application under all circumstances. Furthermore, since no additional fuses and third relays are required, the cost and size of the power electronic converter can be reduced.

[0009] Using the proposed invention, relays can close at much lower voltages while increasing robustness against surge events. The proposed invention can also be applied to voltage dips during normal operation. Here, electrical applications can also remain in standby mode during prolonged voltage drops or even power outages, allowing the application to be safely and immediately restarted once the grid returns to normal voltage levels.

[0010] This invention can be implemented entirely at the software level of the power electronic converter. It makes electrical applications more compact, cheaper, and more reliable. Even during grid outages, it increases the uptime of electrical applications.

[0011] This invention utilizes additional options for AFEs, making the additional efforts required for protection and surge circuitry necessary. With this invention, electrical applications become more robust against all challenges related to voltage sag and are also protected from some user misuse, such as connecting drives to a grid with excessively low mains voltage.

[0012] In a preferred embodiment of the invention, the filter is an LCL filter or an LC filter. The filter may include components assigned to a dedicated coil box and / or an AFE power board.

[0013] In another preferred embodiment of the invention, a first surge relay is disposed on a first phase of the power grid connection, and a second surge relay is disposed on a second phase of the power grid connection. The third phase of the power grid connection may include a third surge relay or may not include a surge relay at all, and a direct connection may be provided between the power grid and the filter.

[0014] In another preferred embodiment of the invention, surge relays that are independently controlled by each other via a control panel are provided.

[0015] In another preferred embodiment of the invention, the surge relay is controlled via a relay board that is powered via a DC power connection to the AFE (particularly the AFE's control board).

[0016] The present invention also relates to an electric motor driver including a power electronic converter.

[0017] The present invention also relates to a method for operating a power electronic converter, the method comprising the following steps:

[0018] - Monitor normal grid voltage conditions and grid voltage sags;

[0019] - If a grid voltage sag is detected, the DC link voltage is increased, while the motor-side inverter reduces its output power, thus keeping the DC link voltage constant or increasing; and

[0020] - If normal grid voltage is detected, the motor-side inverter is set to the setpoint.

[0021] In a preferred embodiment of the present invention, the method includes the following steps:

[0022] - During the startup phase of the power electronic converter and during abnormal grid voltage conditions, the surge relay is switched on or off by means of a controller, so that the DC link is charged via or by bypassing the resistor without overloading the resistor, and the voltage of the DC link is increased.

[0023] In a preferred embodiment of the invention, the startup phase includes a normal startup phase having a sequence of the following steps, preferably all of the following steps, and preferably in the following given order:

[0024] - When the application is powered on, the surge relay is open by default, and the AFE and the motor-side inverter are disconnected;

[0025] - Apply the grid voltage to the Lcm terminal;

[0026] - Supply power voltage to the controller via a switch-mode power supply (SMPS);

[0027] - Preferably, the startup phase is paused for a defined period of time to stabilize the voltage of the DC link;

[0028] - Close the surge relay;

[0029] - If the grid voltage is lower than typical, the voltage of the DC link is boosted via the AFE while the motor-side inverter is disconnected; and

[0030] - If the grid voltage is within the limit, the motor-side inverter is turned on, and power is supplied to the power application without power limitation.

[0031] In another preferred embodiment of the invention, the startup phase includes an abnormal startup phase having a sequence of the following steps, preferably all of the following steps, and preferably in the following given order:

[0032] - Before the application is powered on, the surge relay is open by default, and the AFE and motor-side inverter are disconnected;

[0033] - Apply the grid voltage to the Lcm terminal;

[0034] - Supply power voltage to the controller via a switch-mode power supply (SMPS);

[0035] - Preferably, the startup phase is paused for a defined period of time to stabilize the voltage of the DC link;

[0036] - Detect abnormal grid voltage;

[0037] - Close surge relays (1, 2);

[0038] - When the motor-side inverter is disconnected, immediately begin increasing the voltage of the DC link via the AFE;

[0039] - Based on grid voltage conditions, the motor-side inverter is switched on, and power is supplied to power applications with power limitations; and

[0040] - Detects normal grid voltage conditions, connects the motor-side inverter, and supplies power to power applications without power limitations.

[0041] In another preferred embodiment of the invention, when an abnormal grid voltage occurs during normal operation, the protected start-up sequence includes the following steps, preferably all of the following steps, and preferably in the following given order:

[0042] a - The power application is either running or in standby mode depending on its current setpoint;

[0043] b - Detect abnormal grid voltage conditions, such as grid voltage sag;

[0044] c - AFE immediately begins to increase at the maximum current rate limit, while the motor-side inverter reduces power to ensure that the voltage of the DC link remains constant or increases;

[0045] d - When the grid voltage returns to normal (i.e., back to normal), the grid voltage is usually overvoltaged due to the high DC link voltage, and the resulting surge current is limited to a safe level due to the high DC link voltage;

[0046] e - The motor-side inverter can immediately return to the set point. Attached Figure Description

[0047] refer to Figure 1 Further details and advantages of the invention will be described below. Figure 1 A circuit diagram of the power electronic converter of the present invention is shown. Detailed Implementation

[0048] Figure 1 The circuit diagram illustrates a schematic of the power electronic converter of the present invention, wherein the power grid is located at Lcm on the left and the DC link is located on the right, indicated by a bus bar. An AFE (or more precisely, an AFE power board) is connected to the power grid via a filter indicated by a coil box and via the power grid board. The power grid board includes a first surge relay 1 and a second surge relay 2, the first surge relay 1 having a parallel resistive device 3 such as a resistor, an NTC resistor, or a PTC thermistor. The resistive device 3 may be arranged in series with a fuse. At least one of the surge relays 1 and 2 is controlled to turn on or off during the startup phase of the power electronic converter and during abnormal power grid voltage conditions, such that the DC link is charged via or around the resistive device 3. Therefore, an AFE can be provided to raise the DC link voltage to a safe level and under controlled conditions.

[0049] Using a standard startup sequence, all relays are typically opened first, and the DC link is charged through surge resistors. The relays remain open until the DC link voltage reaches its minimum (e.g., 530V, corresponding to a 380V mains voltage via passive rectification). The SMPS and control unit then start up, and the relays close. In the event of abnormal mains conditions (e.g., providing only 424V DC link voltage at a low mains voltage of 300V), the relays remain open to protect the electrical application from any transient events. This is not a problem for the passive front end of the rectifier, as there is no LCL filter cap in the system and the surge resistor is not significantly loaded. Theoretically, the electrical application can remain in this state.

[0050] When using an AFE (Automatic Front-End), filter capacitors are typically chosen to reduce system cost, size, and weight. Since these capacitors are connected to the mains line after the surge circuit, they are permanently charged and discharged due to the AC voltage of the grid. The current obtained through a surge resistor is significantly higher, for example, by a factor of 8 to 30 times, compared to a passive front-end. Because losses in surge resistors increase with the square of the current, their size needs to be, for example, 50 to 1000 times larger than passive rectifiers, to keep the electrical application running continuously at this stage. This is not a practical choice due to loss, size, and cost considerations. Alternatively, the capacitors can be disconnected using a relay, or a third relay can be implemented to completely disconnect the electrical application in this case. However, this approach also negatively impacts cost and size.

[0051] This invention utilizes the fact that the AFE can boost the mains voltage to any DC link voltage, and that the SMPS only activates at 300Vdc (corresponding to a mains voltage of 212Vac). According to the invention, when the mains voltage is, for example, between 212V and 380V, the DC link voltage is boosted to 530V or even 700V. Only at this point can relays 1 and 2 be closed. Even if the mains voltage suddenly drops, it will not cause inrush current through the AFE because the DC link voltage remains at least as high as normal, preventing any inrush current flow caused by the mains voltage. An additional boost to, for example, 700V may be advantageous because in such cases, the voltage typically returns to an excessively high level, and surge events can be prevented or at least significantly reduced, thereby increasing the lifespan of the electrical application. Operating the LHD drive in areas below, for example, 212Vac mains voltage may not be critical because the current through the capacitor and surge circuit 3 is sufficiently low to prevent resistor overheating. As long as the AFE is in boost mode, the motor-side inverter should be disconnected to prevent overload of the AFE switch.

[0052] During normal operation, voltage sags in the power grid can occur in several ways. Short, deep sags to zero volts can occur along the range of 80% of the nominal grid voltage, and can happen anywhere between these extreme cases. In the event of a voltage sag during normal operation, any drive will coast because there is now power to drive the electrical application. Based on the amount of the sag, the electrical application will resume operation immediately after the voltage returns to normal. However, if the DC link voltage drops below, for example, 530V, the electrical application will shut down completely and require a fresh start. With an AFE on at least one phase and at least a very small grid voltage, the electrical application can remain powered and a restart can be avoided. Furthermore, all communications remain active, and grid voltage derating operation with a certain amount of electrical application is possible.

[0053] The following sequence illustrates the operation of the power electronic converter of the present invention.

[0054] A normal startup sequence for a power electronic converter may include the following steps:

[0055] b. Surge relay open circuit;

[0056] c. Apply the mains power supply voltage to the Lcm terminal;

[0057] d. SMPS (Switch Mode Power Supply) provides power voltage to (multiple) control units;

[0058] e. The motor-side inverter remains disconnected;

[0059] f. Optional: Wait a few seconds to allow the DC link voltage to stabilize;

[0060] g. Surge relay closed;

[0061] h. Boost the DC link voltage by one bit (e.g., 2%) via AFE, and disconnect the motor-side inverter;

[0062] i. The motor-side inverter is active, and the motor can be driven without power limitations;

[0063] A protected startup sequence for a power electronic converter may include the following steps:

[0064] a. Surge relay open circuit;

[0065] b. Apply the mains power supply voltage to the Lcm terminal;

[0066] c. SMPS (Switch Mode Power Supply) provides power voltage to (multiple) control units;

[0067] d. The motor-side inverter remains disconnected;

[0068] e. A voltage drop occurs on the power grid;

[0069] f. Surge relay closed;

[0070] g. Increase the DC link voltage via AFE (e.g., 2% above the nominal rectified grid voltage), and disconnect the motor-side inverter;

[0071] h. The motor-side inverter is active, and the motor can be driven without power limitations.

[0072] Under abnormal grid voltage conditions (such as voltage sag), the power electronic converter can operate in the following sequence:

[0073] a. The surge relay closes, and the power electronic converter operates at its set point;

[0074] b. Voltage sag in the power grid;

[0075] c. The AFE immediately starts boosting the voltage by the maximum current rate limit, while the motor-side inverter reduces power to ensure that the DC link voltage is at least constant or even increases earlier;

[0076] d. When the grid voltage recovers, the grid voltage is usually overvoltaged due to the high DC link voltage, and the resulting surge current is limited to a safe level due to the high DC link voltage;

[0077] e. The motor-side inverter can return to the set point immediately. (No drive restart is required).

Claims

1. A power electronic converter for electrical applications such as motor drives, electrolyzers, or chargers, the power electronic converter being such as an inverter or converter, the power electronic converter including a DC link and an active front end (AFE) connected to a power grid via a filter, a first surge switch (1), and a second surge semiconductor or relay (2), the first surge switch (1) being such as a semiconductor or relay having a resistive device (3), the resistive device (3) being such as a resistor, an NTC resistor, or a PTC thermistor; in, During the startup phase of the power electronic converter and during abnormal grid voltage conditions, at least one of the surge relays (1, 2) is controlled to turn on or off, such that the DC link is charged via or around the resistor device (3), thereby enabling the active front end to raise the voltage of the DC link to a safe level and under controlled conditions.

2. The power electronic converter according to claim 1, characterized in that, The filter is an LCL filter or an LC filter.

3. The power electronic converter according to any one of the preceding claims, characterized in that, The first surge relay (1) is set at the first point of the power grid connection, and the second surge relay (2) is set at the second point of the power grid connection.

4. The power electronic converter according to any one of the preceding claims, characterized in that, The surge relays (1, 2) are configured to be controlled independently of each other via a control panel.

5. The power electronic converter according to any one of the preceding claims, characterized in that, The surge relays (1, 2) are controlled via a relay board, which is powered via a DC power connection connected to the active front end, particularly to the control board according to any one of the preceding claims.

6. An electric motor driver comprising a power electronic converter according to any one of the preceding claims.

7. A method for operating a power electronic converter according to any one of claims 1 to 5, Its characteristics include the following steps: - Monitor normal grid voltage conditions and grid voltage sags; - If a grid voltage sag is detected, the voltage of the DC link is increased, while the motor-side inverter reduces its output power, so that the voltage of the DC link remains constant or increases. as well as - If the normal grid voltage is detected, the motor-side inverter is set to the set point.

8. The method according to claim 7, characterized in that, The method includes the following steps: - During the startup phase of the power electronic converter and during abnormal grid voltage conditions, the surge relays (1, 2) are switched on or off by means of a controller, such that the DC link is charged via or by bypassing the resistor without overloading the resistor, and the voltage of the DC link is increased.

9. The method according to claim 7 or 8, characterized in that, The startup phase includes a normal startup phase, which has a sequence of the following steps, preferably all of the following steps, and preferably in the following given order: - Before the application is powered on, the surge relay is open by default, and the active front end and the motor-side inverter are disconnected; - Apply the grid voltage to the Lcm terminal; - The power supply voltage is supplied to the controller via a switch-mode power supply (SMPS); - Preferably, the startup phase is paused for a defined period of time to stabilize the voltage of the DC link; - Close the surge relays (1, 2); - If the grid voltage is lower than the typical value, the voltage of the DC link is boosted through the active front end while the motor-side inverter is disconnected; as well as - If the grid voltage is within the limit value, the motor-side inverter is turned on, and power is supplied to the power application without power limitation.

10. The method according to claim 7 or 8, characterized in that, The startup phase includes an abnormal startup phase, which has a sequence of the following steps, preferably all of the following steps, and preferably in the following given order: - Before the application is powered on, the surge relay is open by default, and the active front end and the motor-side inverter are disconnected; - Apply the grid voltage to the Lcm terminal; - The power supply voltage is supplied to the controller via a switch-mode power supply (SMPS); - Preferably, the startup phase is paused for a defined period of time to stabilize the voltage of the DC link; - Detect abnormal grid voltage; - Close the surge relays (1, 2); - When the motor-side inverter is disconnected, the voltage of the DC link is immediately increased through the active front end; - Based on the grid voltage conditions, the motor-side inverter is switched on, and power is supplied to the power application in a power-limited manner; and - Detect normal grid voltage conditions, turn on the motor-side inverter, and supply power to the power application without power limitations.

11. The method according to any one of claims 7 to 10, characterized in that, When an abnormal grid voltage condition occurs during normal operation, the protected start-up sequence includes the following steps, preferably all of the following steps, and preferably in the following given order: a - The power application is either running or in standby mode depending on its current setpoint; b - Detect abnormal grid voltage conditions, such as grid voltage sag; c - The active front end immediately begins to increase at a maximum current rate limit, while the motor-side inverter reduces power to ensure that the voltage of the DC link remains constant or increases; d - When the mains voltage becomes normal, the mains voltage is usually overvoltaged due to the high DC link voltage, and the resulting surge current is limited to a safe level due to the high DC link voltage; e - The motor-side inverter can immediately return to the set point.

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