Category Analysis, Design and Implementation of a High Efficiency Multilevel Converter for Renewable Energy Systems

Resistive, capacitive and inductive load waveforms

Подпись: Ch3: 20 A Ch4: 50 A (a)
Resistive, capacitive and inductive load waveforms

The first experiments done to evaluate the prototype made use of near ideal resistive, capacitive and inductive loads. Figures 6.21 and 6.22 show a set of experiments done with resistive loads (prototype feed by a 48 V battery bank). Figure 6.21(a) presents an overall view of both transient and steady state operation of the prototype under a sequence of resistive load steps. As can be seen, despite of the large changes in the input voltage and also in the output current, the converter was capable to produce a stable output voltage.

Figure 6.21 – Waveforms for operation under resistive load:

(a) Sequence of load steps.

(b) Single load step of 3.16 kW.

A close view on the insertion transient of a load step of 3.16 kW can be observed in figure 6...

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Transformer-unbalancing control

Transformer-unbalancing control

The developed mechanism to control transformer-unbalancing makes use of the hold – on-at-zero intervals, as described in section 4.4. The two control actions involved in this process can be observed in figures 6.15(a)-6.15(d), which shows the evolution of the corrections as the converter feeds half wave loads of increasing power. In these examples, the voltage drop at the transformer primary voltage is compensated by replacing part of the hold-on-at-zero interval for additional positive intervals.

Transformer-unbalancing control

Transformer-unbalancing control

(d)

Figure 6.15 – Voltage at the transformer and output current under half wave resistive loads of:
(a) 235W (b) 480W (c) 935W d) 1340W.

Looking at figure 6...

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Switching waveforms

Analysis of switching waveforms is important to check out occurrence of stress in switches. Critical instants occur when the switches are switched off because high dv/dt and over-voltage conditions may appear.

Switching waveforms Switching waveforms

Figure 6.9 shows the voltage across all H-bridge switches (no-load). As can be seen, these waveforms have good aspect and no voltage spikes are found. Short transients presented in the falling edge of switch Sd and rising edge of switch Sc are due to the transformer-unbalancing control, as explained in section 4.4.

Figure 6.9 – (a) Voltage at H-bridge switches Sa and Sb;

(b) Voltage at H-bridge switches Sc and Sd.

Voltages across output-stage switches present also good behavior as can be seen in figure 6...

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Output voltage analysis

Подпись: Figure 6.3 - (a) No load output voltage (five cycles); (b) No load output voltage (zoom: half cycle)

As an inverter is essentially a voltage source at its output, the first parameter to be analyzed is its output voltage waveform, which is showed in figure 6.3.

As can be seen, the output waveform approximates a perfect sinusoidal shape, apart from the distortions near zero crossing. As was explained in section 4.4, these distortions correspond to a fixed time of 700 p, s where the output voltage is forced to be zero, and it is used to control transformer-unbalancing.

Because voltage steps are so small, it is difficult to identify at figure 6.3(a) each step of the multilevel waveform. In Figure 6.3(b), it is possible to observe all steps in detail...

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Experimental Results

Several experiments were done in order to evaluate the steady state and dynamic characteristics of the implemented prototype. In these experiments, measurement of power became somehow critical because of the high values of efficiency that had to be calculated; therefore high precision instrumentation was required.

Efficiency measurements were done using a high precision power analyzer instrument (NORMA D6000 of LEM Instruments [98]). This device has a specified precision of less then 0.1% for power measurements. Voltage and current waveforms were acquired using a Tektronix four channel digital oscilloscope. Figure 6.1(left) shows a picture of the experimental setup, where these equipments can be observed.

All1 experiments were done at room temperature between 23 °C and 30 °C ( Kassel – Ger...

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Output filter and protections

The output-stage is connected to the load through the circuit shown in figure 5.12. This circuit is composed by an output filter and over-current hardware protections. Although the output filter inductor was implemented with a high frequency core, for future implementations, filters built of low frequency cores may be experimented.

To Ouput Protection To Ouput Voltage Circuit Measurement Circuit

Output filter and protections

Figure 5.12 – Schematic circuit of the output filter and protections.

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Measurement circuits

Measurement circuits

The measurement circuits, shown in figure 5.11, are composed by sensors and conditioner circuits that are connected to the Analog to Digital (A/D) converter inputs of the main microcontroller. The battery voltage is conditioned basically by a simple resistive voltage divider, as shown in figure 5.11(a).

Figure 5.11 – Schematic of the measurement circuits.

The circuit used to monitor de output AC voltage is shown in figure 5.11(b), where a conventional low power transformer is used to provide isolation and also to reduce the voltage magnitude. It should be noted that the resultant voltage that is applied to the A/D converter is a rectified full-wave voltage...

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Controller

A block diagram of the controller board is shown in figure 5.10 and the complete circuits are found in appendix F. The 5 V linear regulator was incorporated to the main controller board (instead of integrated to the auxiliary SMPS) in order to minimize interference and increase reliability. The analog interface circuits are composed by the measurement circuits and also by some protections at each analog input pin of the main microcontroller. An interface composed by two transistor arrays was used to connect the microcontroller pins to the optocoupler emitters of all MOSFET drivers.

Controller

Figure 5.10 – Block diagram of the controller board.

The controller is based on one 8-bit AT90S8535 microcontroller (running at 11.0592 MHz) from ATMEL [96]...

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