CH-02 ACS 5000A Hardware (G-1)

Objective:

Upon completion of this part, you can describe

1. Air cooled ACS 5000 main circuit diagram
2. The function of the power hardware.
3. The communication between the power cell and COU.
4. Protection function

Upon completion of this module, you will be familiar with the air-cooled ACS 5000 hardware configuration and be able to describe and explain to others the main circuit diagram. In addition, you will be able to identify the hardware components and explain their function.

Air Cooled ACS 5000A:

The air-cooled ACS 5000 is a general-purpose frequency converter for the control of standard induction and synchronous motors. The air-cooled ACS 5000 is based on well-proven air-cooled ACS 1000 building blocks and combined with the Voltage Source Inverter Multilevel-Fuse less topology. It was developed to specifically meet the requirements of the growing number of applications driven by standard motors up to 6.9 kV and covers the power range from 2 to 7 MW. Because of this solution, the air-cooled ACS 5000 has a low parts count, high system efficiency, high power density and small footprint. In addition, the simplified installation, commissioning and maintenance reduces the costs for the customer. The air-cooled ACS 5000 consists of one Control Unit, 3 identical power cell units and a Capacitor Bank Unit.

ACS5000 A Main Circuit Diagram:

Here is the main circuit diagram of an air-cooled ACS 5000. The 3 identical power cells are used to supply the three motor phases. Each power cell can be divided in four parts: The rectifier, the DC bus, the inverter and the output filter. The rectifier transforms the AC line voltage with constant frequency and constant amplitude in a DC voltage. The standard version of the ACS 5000 has a 36-pulse input rectifier. The DC link contains mainly the DC link capacitors, which smooth the DC link voltage and store electrical energy. The inverter recreates a 3-phase system at the motor terminals with variable frequency and variable voltage. The inverter switches used are IGCT’s, this semiconductor type combines the strength of the latest GTO and IGBT technology. The freewheeling diodes are integrated together with the IGCT on the same wafer. That results in a very compact design. The output LC filter provides a nearly sinusoidal output waveform. This allows the use of standard motors without de-rating.

Diode Rectifier Bridge:

The rectifier transforms the AC input voltage to DC voltage and delivers energy to the inverter. The standard ACS 5000 has a 12-pulse rectifier in each phase. Two 6-pulse diode bridges are connected in series to achieve the 12 pulses per phase. The three rectifier bridges result in a quasi-36-pulse configuration. The harmonic performance is better compared to an ideal 12-pulse rectifier, but not as good as a real 36-pulse solution.

Snubber:

In order to prevent harmful voltage, rise across the diodes when diodes turn off, snubber circuits are installed at the rear side of the power cell. Each phase has two snubber circuits. They absorb high voltage transients during normal commutation and reduce the stress on the rectifier diodes.

Di/Dt Limiting Choke:

A choke is installed in the positive DC bus to protect the diodes against excessive rate of rise of the current. This choke has a robust mechanical construction, which allows it to limit and withstand high currents and current rate of rise in case of a fault.

Diode Failure Supervision:

The Earth Fault board EAF is used to supervise the rectifier diodes. If the measured voltage between the neutral point of the rectifier and the neutral point of the inverter is bigger than a certain value, a diode failure signal will be sent to the control system through fiber optics.

DC Capacitors:

The main characteristic of the voltage and current from the diode rectifier is the low frequency ripple superimposed, and this ripple also shows up on the DC link. DC link capacitors are used to keep the DC link voltage smooth and constant. They also decouple the rectifier from the inverter. The capacitor bank is located on the right-hand side of each phase module. Two DC link connection methods are used in the air-cooled ACS 5000: the hard coupled DC link and the soft coupled DC link. The soft coupled DC link consists of a capacitor bank connected to the DC bus bars via a small current limiter choke with a damping resistor in parallel. To clamp the Neutral Point of the soft coupled DC link, another resister is connected between the middle of the soft coupled DC link and the NP potential. During protection firing or a short-circuit fault, the peak current is limited by the soft coupled DC link. On the other hand, the existence of this circuit makes it more difficult to keep the DC link voltage steady during modulation. The hard coupled DC link capacitors are directly connected to the DC bus bars. One capacitor module has two capacitors inside. One for the soft coupled DC link, the other one for the hard coupled DC link.

Balancing Resistors:

The DC link balancing resistors are connected between the DC link bus bars. They are used to clear static unbalances in the system during modulation. If there is no optional Braking Chopper Unit installed, the energy stored in all DC link capacitors will be discharged slowly via those high ohmic balancing resistors when the ACS 5000 switches off the main circuit breaker. The DC link discharging takes several minutes. When the DC link voltage reaches 1000 V, the drive performs a firing through to discharge quicker.

NP Filters:

The Neutral Point of the DC link is grounded to function earth via the Neutral Point filter. The filter smooths the NP voltage ripple, which is good for control and makes the system more robust. The NP filter consists of two resistors and one capacitor.

Grounding Switch:

The safety grounding switch guarantees personnel safety when drive maintenance is performed. The safety grounding switch connects the plus, minus and neutral DC busses to ground. That ensures that the power has been removed and all energy stored in the DC link capacitors has been discharged before gaining access to the power parts. A yellow light “GROUND SWITCH UNLOCKED” located on the door indicates when the DC busses are discharged to a safe level. This indicates that the electromechanical lock of the grounding switches is released and it is allowed to operate them. Never try to move the grounding isolator without the yellow light being on! Because ACS 5000 has three physical grounding switches, there are in total three feedback signals connected in series, both for the open and closed status. Drive status “grounding switch closed” is reached only if all three grounding switches are closed. This releases the electromagnetic locking mechanism and it’s then possible to open the doors. Drive status “grounding switch open” is reached only if all three grounding switches are open. Otherwise, “DC Ground unknown” status will show up on the control panel.

Integrated Gate Commutated Thyristors:

The five level inverter cells re-create a 3-phase system at the motor terminals with variable frequency and variable voltage and allows four-quadrant operation. The inverter switches used are IGCT’s. This semiconductor type combines the strength of the latest GTO and IGBT technology. Two types of reverse conducting IGCT’s, so-called RC-IGCT‘s, are used in the air-cooled ACS 5000: 51 mm RC-IGCT’s are used in 3.5 MVA frame size and 68 mm RC-IGCT’s are used in 7 MVA frame size. The freewheeling diodes are integrated together with the IGCT on the same wafer. That results in a very compact design.

Neutral Point diodes and Balancing resistors:

Neutral point diodes are characteristic components of a three-level inverter. They provide one more output voltage level, and the peak output voltage is reduced by half. This configuration increases the freedom of the output vector control. With the same output switching frequency, the quality of the output waveform is greatly improved. In addition, the neutral point diodes will divide the DC voltage equally across IGCT’s and protect the components when the IGCT’s of the other leg are turned off. Balancing resistors provide voltage sharing when all IGCT’s are turned off and prevent unbalance between half DC link voltages.

Clamping circuit:

In order to decrease the rate of short circuit current rise and to control the IGCT’s freewheeling diode reverse recovery in a safe range during modulation, a clamping circuit is used in the inverter. The clamping circuit consists of current limiting chokes, clamping resistors, clamping capacitors and clamping diodes. The current limiting chokes are used to limit the di/dt through the IGCT’s and anti-parallel freewheeling diodes during modulation. High di/dt could damage the semiconductors. The clamping resistors are used to consume the energy saved in the current limiting chokes when the current flow is cut off. The clamping capacitors are used to limit the du/dt when the IGCT’s, which are connected to the DC bus bar, are switched off.

Output Filter:

Because of the high voltage level of a medium voltage inverter, the switching frequency is relatively low, so the output voltage harmonics will cause motor harmonic heating and torque ripple. Furthermore, a high du/dt will be disruptive for the motor insulation. On the ACS 5000 inverter output, a three-phase output filter is installed to limit the harmonics and the du/dt. This makes the output waveform closer to sinusoidal and allows to drive a standard motor. The AC reactors and resistors are mounted at the rear side of the power cell. The star point of this filter is connected to ground via a capacitor in the Control Unit. The voltage across this capacitor is monitored by an EAF board. The EAF board initiates an earth fault signal in case the voltage exceeds a certain value which is pre-set on the board by DIP switches. The default setting is 500 V. This method detects only ground faults between converter output and the motor windings.