FREQUENCY CONVERTERS

DESCRIPTION

Technical Field

The present invention relates generally to power electronic converters, especially to Active Front End frequency converters (AFE converter) which are used in many industry applications. More specifically the present invention relates to a method of precharging DC link in AFE converter in order to limit charging current slope and avoid grid voltage drop caused by inrush current.

Technical Problem

AFE frequency converter typically includes a rectifier or converter, a DC link, and an inverter. When electric power is applied to the AFE converter, the voltage across the DC link capacitors, referred to as the DC link voltage, rises from zero to a rated value, typically around 600 to 650 V for low voltage AC grid of 400 V. If this rise of the DC link voltage were left to occur naturally, it would happen very quickly by drawing very large electric currents from the input power lines, through the rectifier, and into the DC link capacitors. This large current, referred to as an inrush current, can be damaging to the components of the AFE converter and causes electrical grid voltage drop. Thus, to avoid damage to the AFE converter components and electrical grid voltage drop, the rise of the DC link voltage from 0 V to the rated voltage has to be accomplished in some controlled manner. This controlled raising of the DC link voltage is referred to as a DC link precharge operation.

The present innovation solves the problem of limiting inrush current in DC link precharge operation implementing secondary controllable active rectifying elements connected in series with DC link capacitors, that uses pulse charging in order to minimize inrush current and prevent voltage drop, where secondary controllable active rectifying elements are controlled in such manner to have both DC link capacitor bank ends disconnected during non-charging period of precharging operation.

Previous State of Art

The general information regarding state of the art can be found in: o Sinamics low voltage engineering manual, Siemens AG 2008.

o System for precharging DC link in a variable speed drive, patent application

PCT/US2006/004563, York international corporation

o AC Precharge-circuit, patent application PCT/US2013/01 19903 , Rockwell Automation

Technologies

In practice use in AFE frequency converter, inrush current problem is solved by applying next technical methods:

(I) Introducing bypass switching system that limits inrush current by connecting precharge resistors between input power line and the rectifier

(II) Charging DC link over rectifier consisting (at least partially) of thyristors, also called silicon controlled rectifiers, or SCRs.

(III) Integrating secondary diode rectifier brigde with precharge resistors in parallel to the main rectifier module of the VSD

(IV) Using IGBT transistors or thyristors as antiparallel diodes in standard IGBT inverter for precharging DC-link as described in patent application PCT/US2006/004563

In the first method (I.), a precharge contactor is used to connect precharge resistors between the input power line and the rectifier or, sometimes, between the input power line and the DC link. These precharge resistors limit the inrush current to a manageable level. After the precharge is completed, the precharge resistors are excluded from the circuit by opening the precharge contactor, and the input power line is connected directly to the rectifier by closing another contactor, referred to as the supply contactor. The supply contactor remains closed during the operation of the system. This method is well suited for frequency converters in which the rectifier is a simple diode rectifier, which offers no means for controlling the inrush current. The main disadvantage of this method is in the cost and size of its components, in particular of the supply contactor, which can negatively impact the cost and size of the entire frequency converter.

In the second method (II) main rectifier itself is used to accomplish precharge. The rectifier in this case has at least one SCR in each phase. SCRs are power semiconductors whose current conduction can be electronically controlled. The conduction of the rectifier's SCRs is controlled so as to let only small pulses of inrush current flow during precharge. After the precharge is completed, the rectifier's SCRs are controlled to conduct at all times, i.e., the rectifier after the precharge acts as if it were a diode rectifier.

In the third method (III), diode rectifier with serial resistance as secondary rectifier is connected in parallel to the main rectifier module of the VSD. The inrush current is limited by resistors, that are also main disadvantage of this solution since current is by secondary rectifier is drawn throughout precharge and regular operation therefore causing energy losses due to resistor dissipation.

The fourth method (IV) refers to VSD with IGBT main rectifier where inrush current is controllable by additional IGBT transistor that is placed as replacement of existing IGBT antiparallel diode or as separate IGBT connected in series with the main rectifier. In this method, during pre-charge operation, IGBT antiparallel diode acts as rectifier, while additional IGBT transistor regulates pulses of inrush current flow, therefor limiting the current intake.

While methods (I) and (II) have durability and energy consumption issues due to the usage of resistors for current limitation, methods (III) and (IV) incorporates active switching solutions that operates rather well in the terms of energy consumption and usability.

The present innovation as methods (III) and (IV) also uses active switching solution for precharging, but operates with line to line voltage and acts as low power secondary DC-link charging circuity.

Summary of invention

The present invention solves the problem of precharging DC link in frequency converter applications, lowering the voltage drop (glitch) on grid impedance using power switching elements connected in series with DC link capacitors, and controlled in such manner to have both capacitor bank ends disconnected during non-charging period of precharging operation.

The DC link (200) consists of capacitors (202) and resistors (204) which filter the DC power and store energy from the DC bus. The DC link has positive and negative pole, where we are in series to DC link connecting first controllable active rectifying element between first power line and DC link positive pole, and second controllable active rectifying element between DC link positive pole and second power line.

The controllable active rectifying elements are controlled in such manner to operate pulse charging of DC link capacitors, while in non-charging period of pre-charge operation both DC link poles are electrically disconnected from line to line voltage.

This construction allows quick charging of DC link capacitors while sustaining control of charging current in AFE frequency converters.

Since we have both DC link poles are electrically disconnected from line to line voltage during non-charging period of pre-charge operation, the present innovation is usable in non-floating applications like in AFE frequency converters with RFI filter that has electrical ground connectivity.

In addition, as part of pre-charging circuitry choke can be added in series with DC link capacitor in order to limit the maximum value and slope of charging current pulses.

Brief Description of the Drawings

Figure 1 shows the block structure of general system configuration of the present invention.

Figure 2A shows a circuit diagram of DC link precharging circuit having two controllable active rectifying elements and DC link circuitry.

Figure 2B shows a circuit diagram of DC link precharging circuit having one controllable active rectifying element and DC link circuitry.

Figure 3 shows a circuit diagram of the AFE converter in which the present invention embodied in DC link precharging circuit is implemented. Detailed Description

Figure 1 illustrates generally system configuration of the present invention. An AC power source (10) is connected via circuit breaker (3) to frequency converter (600) which supplies motor (500) in variable speed drive application or generator (500) in some renewable energy sources (wind power systems, cogeneration plants). An AC power source (10) is low voltage three phase AC power grid with line voltages up to 690 V and line frequency of 50 Hz or 60 Hz.

The frequency converter (600) receives/gives AC power with a particular line voltage and fixed line frequency from/to AC power source (10) and provides AC power to the motor or generator (500) at a desired voltage and desired frequency, both of which can be variable. The motor and generator (500) can be of different types (induction, synchronous, permanent magnet), but with capable of being operated at variable speeds.

The frequency converter (600) have three basic stages: Active front end (AFE) converter (100), DC link (200) and inverter (400). The AFE converter (100) converts the fixed line voltage and fixed line frequency from the AC power source (10) into DC power. The DC link (200) filters the DC power and provides energy storage by means of capacitors. Finally, the inverter (400) converts the DC power from DC link (200) into variable frequency and variable voltage AC power for the motor and generator (500).

The RFI filter (70) is placed between AFE converter (100) and contactor (50). It reduces high frequency interference emitted by frequency converter (600) to AC power sources (10). RFI filter is grounded.

Input of DC link precharging circuit (300) is connected to two phases (line to line) of the AC power source (10), and DC link (200) positive and negative pole is connected to DC link precharging circuit. DC link precharging circuit is used to gradually charge capacitors in the DC link (200) and to limit charging current which could damage DC link capacitors.

When DC link charging process is finished, DC link voltage is equal to approximately value of maximum line voltage of AC power source (10) and the contactor (50) connects the AFE converter (100) via the RFI filter (70) to the AC power source (10). Figures 3 shows entire circuit diagram for AFE converter (100), DC link (200), DC link precharging circuit (300), RFI filter (70) and contactor (50).

The AFE converter (100) consists of LCL filter (110) and PWM rectifier (120). The PWM rectifier (120) includes six power switches consisting of IGBT (122, 124) and antiparallel connected diode (121 , 123). The PWM rectifier (120) also includes the corresponding control system (not shown for simplicity) to control the switching of the power switches in order to get the desired output voltages of DC link (200). PWM rectifier (120) with LCL filter (1 10) operates as a boost rectifier to obtain output DC voltage of DC link (200) which is greater than maximum value of the input line to line voltage. Also, the LCL filter (110) provides almost pure sinusoidal current from AC power source (10) with THDi < 5%.

The components of RFI filter (70) are capacitors, chokes and resistors. There are various configurations of RFI filter, and their task is to reduce high frequency interference emitted by frequency converter on the way to carry out them into the ground.

The DC link (200) consists of capacitors (202) and resistors (204) which filter the DC power and store energy from the DC bus. The resistors (204) have function to maintain a substantially equal DC link voltage between capacitors (202). Also, the resistors (204) have the function of discharging capacitors (202) when the frequency converter (600) is disconnected from the AC power source (10).

The DC link precharging circuit (300) in first embodiment consists of two controllable active rectifying elements (301 , 302), choke (303) and two fuses (305), where controllable active rectifying elements can be thyristors or IGBT transistors.

For thyristor used as controllable active rectifying elements, cathode of first thyristor (301) is connected to the positive pole of the DC link (200), while second thyristor (302) is connected to the negative pole of the DC link (200).

This configuration of thyristors provide that both thyristors (301, 302) can conduct precharcing current at the same time during only positive halfperiod of the line to line voltage (UL1 -UL2), while during the negative half period both thyristors are reverse biased and blocked. Thyristors (301) and (302) are controlled changing the firing angle in each positive half period to permit only small charging current impulses during precharging operation. The choke (303) is used to limit the maximum value and slope of charging current pulses. Too high slope of charging current pulses can cause the high voltage drops (glitches) on the grid impedance.

The same current impulse charging and control as described can be achieved by replacing thyristors with IGBT transistors, where we can achieve even faster pre-charging by switching IGBT transistors on and off at higher frequency than in case when thyristors are used as controllable active rectifying elements that are fired once in every other input voltage half- period.

Also the same effect of impulse precharging can be achieved with only one controllable active rectifying element installed, as shown in Figure 2B, where second controllable active rectifying element (302) is replaced with rectifying diode (304). (Figure 2B).

Industrial Applicability

Frequency converters with AFE converter on grid side are used in many industry applications such as regenerative electric drives, variable speed wind turbine, solar power plants, etc. In such application, according to technical standards, it is mandatory to have RFI filter installation. Taking into account the main advantages of the present invention, fast DC link charging without losses, without electromechanical components and with impulse charging current in limited amount and slope, the invention is widely applicable in frequency converter technique, especially in cases where application often requires switching on and off during regular operation as i.e. in the crane drives with remote control.

Abbreviations

AC Alternating current

DC Direct current

RFI Radio frequency interference

AFE Active front end

IGBT Insulated gate bipolar transistor References

10 AC power source

30 Circuit breaker

50 Contactor

70 RFI filter

100 AFE converter

1 10 LCL filter

121 Antiparallel diode

122 IGBT transistor

200 DC link

202 DC link capacitor

204 DC link resistor

300 DC link precharging circuit

301 First controllable active switching element

302 Second controllable active switching element

303 Choke

304 Diode rectifier

305 Electrical fuse

400 Inverter

500 Motor

LI AC voltage first phase

L2 AC voltage second phase

P+ Positive pole of DC link capacitor bank

P- Negative pole of DC link capacitor bank