90 kW PWM, Sine Wave Inverter Design: 115 VAC/200VAC, 3-PH, 400 Hz Out using a TI DSP

I recently designed a 400 Hz sine wave output inverter at 90 kW with 115 VAC (Line-to-Neutral) & 200 VAC (Line-to-Line) output using a sine PWM technique. A prototype was built and tested based on the Texas Instruments (TI) TMS320F2812 fixed-point, 32-bit DSP. This unit actually runs from 480 VAC, 60 Hz, 3-Phase and is technically a Frequency Changer. But it's called an Inverter since the power stage runs from the 500 V DC  Bus. The full load output current is 260 amps AC. The power stage uses a 3 each,1/2-Bridge sections without an Inverter neutral. Therefore, the output power is all taken line-to-line into an isolation output transformer primary. The secondary of this output transformer does have a neutral so that unbalanced line-to-neutral loads can be serviced.

The entire control section, including feedback is based on a 32-bit, TI DSP chip, the TMS320F2812 running at 150 MHz or 6.66 nSec/clock cycle. This IC has not only a very good general purpose 32-bit processor, but also 256 kByte of Flash, 36 kByte of RAM, 16 PWM channels, 16, 12-bit A/D Inputs, 56 GPIO ports, 2 UARTS, a SPI port, multiple programmable timers, and more. Our code was primarily written in C & C++ with small sections done in assembly language. I designed the hardware to connect the DSP A/D inputs to the 3-phase output voltage and current for feedback and protection. Since this is a high-power inverter with potentially lethal output voltages, a means of isolation was required between the user, the 480 VAC input, and the 200 V (L-L) AC Output. An isolation transformer with multiple secondaries was used to isolate everything from the 480 VAC input. This transformer had a 3-phase WYE output and a 3-phase DELTA output with both having the same line-to-line voltage. This provided a 30 degree phase shift between the two secondary sections such that by using 6 each, 1/2-bridge rectifiers, we obtain 12-pulse rectification. This increased the input power factor from an otherwise low figure up to about 0.93 since the multiple current pulses tend to follow the sine wave input voltage. With each phase having the input current largely following the input voltage, we approach unity power factor. Many potential customers will not buy a frequency changer (Inverter) with standard 6-pulse rectification that one would get with a single DELTA or WYE secondary winding. The input transformer also has a 3rd secondary winding to provide a single-phase 230 VAC output to run the AC fans and the 100 watt housekeeping AC/DC 24 VDC power supply.

Despite the 12-pulse rectification, some DC energy storage is still needed for momentary power outages up to 10-20 milliseconds. Since the DC Bus output voltage can get slightly above 500 VDC and electrolytic caps with that high  a voltage rating are very hard to find, I used two caps in series, each with a 350 VDC rating to get a 700 VDC total rating. Equalizing resistors were placed across each cap to ensure the DC voltage would be well balanced across the two series caps. Each cap had a values of 22,000 uF. Therefore, the total capacitance across the DC Bus was 1/2 thar or 11,000 uF and the voltage derating for safety is about 25% which is quite acceptable.    

The main inverter section uses 3 each of a single IGBT module. This module has a half-bridge or two IGBTs in series. Both series IGBTs have a fast recovery rectifier connected in antiparallel to the IGBT. Inside the module, there are actually multiple IGBTs in parallel for each IGBT in the module's circuit to obtain the proper current rating. Despite the 11,000 uF electrolytic caps, the IGBT modules still need a low-esr film cap right across the IGBT DC+ & DC- inputs to prevent dangerous voltage spikes from getting out of control. There are caps made with special terminals called IGBT caps that mount directly to the IGBT module terminals and have short, very low inductance leads. These are most often in the 1 uF to 5 uF range at 600 V to 1200 V. The inverter operates at about 20 kHz using Pulse-Width Modulation (PWM) such that the pulses on-time follows the value of a sine wave. The first channel, phase-A is the reference phase and the other two, phase-B and phase-C are time delayed by 120 degrees at 400 Hz for the proper phase shift. The IGBT output pulses have a turn-on and turn-off time in the neighborhood of 200 nanoseconds from 0 VDC to 500 VDC. The inverter next must convert these pulses back to a smoothly changing, low-distortion sine wave. All that's needed to do so is a low-pass filter to block the 20 kHz pulses and reclaim the 400 Hz sine waves. The output filter for each phase consists of an inductor and a capacitor. A starting value of 100 microHenry and 25 microFarad were used for a filter cutoff frequency of 3.2 kHz. Since the LC filter center frequency is almost 8 times higher than the 400 Hz fundamental, the Q is not very important under most loading conditions. It's critical to keep the core losses down despite the 20 kHz PWM pulses. A special core material called NanoCrystalline Amorphous was used. Of course, winding also was special in order to keep the skin effect at 20 kHz from causing high copper losses. The winding was done with 3-parallel layers of copper foil, each insulated from each other such that only the start and finish ends were paralleled, but the rest of the layers were separated. If only a single, thicker layer of copper were used, the skin depth would not penetrate to the center and the effective AC resistance and copper losses would've been too high. There is an important third major loss component in AC inductors that also had to be addressed and that is the fringing losses due to the required gaps. To obtain the relatively low final inductance of 10 0 uH, the C-Core was cut (also called a Cut Core) and insulating gap material inserted. With a single cut, the gap was too large and fringing losses were too high that resulted in a inductor getting to hot. To remedy this, we disassembled the inductors and again cut the core, this time into multiple pieces with multiple gaps. The total gap was the same, but each individual gap was much smaller. This resulted in the fringing losses being reduced greatly and the resulting inductor temperatures were down to 33% from the previous inductor version, a 66% reduction in delta-T. 

The 3-Phase inverter 1/2-bridge outputs get connected to the output transformer after the LC filter. This transformer has a 1:1 ratio. This transformer provides isolation against DC getting to the load in case of an IGBT short. The output transformer also has a 3-Wire DELTA Input and a 4-Wire WYE Output so that balanced and/or unbalanced single and 3-phase loads can be connected. The transformer uses thinner-than-normal 9-mil thick Silicon-Steel laminations to keep the core losses down as they would be much higher with the standard 12-mil or 18-mil thick steel.

A 3-Phase contactor with a 24 VDC coil can switch the load in or out based on the code in the DSP. IF the DSP senses that the load is too high or has unbalanced voltages, the contactor can open-up the load for safety. There is another contactor in the input rectification circuit, also controlled by the DSP and this is for limiting the inrush-current. There is a large resistor in series with the electrolytic caps that initially limits the inrush current to these 22,000 uF caps. After the caps have had time to charge, about 1/2 second, the contactor closes and the shorts out the series limiting resistor.

The inverter has a Human-Machine Interface (HMI), basically, a color touch screen display for user I/O. This HMI communicates using an RS-422 interface and a modified Modbus protocol to talk to the DSP. The main user controls are separate from the HMI and allow basic on/off control and status indication. These are large, industrial type switches and lamps.