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

Inverter improvement

Further inverter no-load reduction can be mainly achieved by modifying the multiple­winding transformer design. The simplest way to reduce the transformer no-load consumption is to increase the number of turns in its primary. In this case, if a same core is used, then coil wire area must be decreased in order to fit the same window area. In consequence, coil resistance of both primary and secondary are increased by the square of the voltage change factor, and the same occurs with the losses due to winding resistances.

The relation between no-load consumption and coil resistances can be estimated through the transformer no-load power versus input voltage characteristic, and considering that the transformer wiring losses can be modeled by a single equivalent primary resistance (Req), then i...

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Economical viability

Table 7.2 shows the average price of some commercial inverters that were obtained thought internet from diverse worldwide sellers.

Table 7.2 – Prices of selected commercial inverters.

Inverter

Power

Mean Price [€]*

€/W

Phoenix 48/3000/35

3000

2.800

0.93

Studer C3548

3500

1.800

0.51

Xantrex SW2548

2500

1.800

0.72

SMA Sunny Island 3324

3300

2.300

0.70

Average ==>

0.72

* prices from internet, search date: 25/03/2006.

The achieved average value of 0.72 €/W is in accordance with the value announced by the website "solarbuzz. com", that is equal to 0.69 €/W in march of 2006 [99]. Thus, price of an inverter of 3000 VA is expected to be around € 2100.00.

In order to have an idea of the expected economical viability of the p...

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Final Analysis

7.1 Comparison with other inverters

A summary of the main characteristics of the implemented prototype and some commercial inverters is shown in table 7.1.

Table 7.1 – Specification data of some commercial converters and implemented prototype.

No

Inverter

Vin

[Vdc]

Vout

[VRMS]

Power

[VA]

npk

[%]

No-Load

[W]

Surge

Power

Surge

Current

Weight

[kg]

1

Implemented prototype

48

230

3000

96.0

18.6

1.5x

1.5x

2

Phoenix 48/3000/35

48

230

3000

95.0

10.0

2x

18

3

Studer C3548

48

230

3500

95.0

12.0

3x

30

4

Dakar 48/3000/50

48

230

3000

90.0

4.8

2x

36

5

SMA Sunny Island 3324

24

230

3300

94.5

22

1.5x

3.5x

39

6

SMA Sunny Island

60

230

3300

92.0

<10

2x

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Non-conventional experiments

The maximum allowed THD of 5% has imposed a limit to the hold-on-at-zero interval so all presented experiments were done with a fixed value of 0.7 ms. However, if the hold-on-at-zero interval is increased, then it is possible to reduce de no-load consumption and consequently increase efficiency at light loads. On the other hand, the output waveform can become strongly distorted and the THD may be higher then 5%.

Non-conventional experiments

Figure 6.45(a) shows both variations of no-load consumption and output voltage THD as a function of the hold-on-at-zero interval (input voltage of 48 V). As can be seen, if the hold-on-at-zero interval is increased from 0.7 ms to 3 ms then the no-load consumption is reduced from 18.6W to 14.1W while the THD is increased from 3% to 26%...

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Efficiency characteristic

In this section the efficiency characteristic of the proposed converter is investigated according to the variation of input voltage, output voltage, temperature and type of load.

400 1 20

Подпись: + <~ From Battery Terminals Efficiency characteristic Efficiency characteristic Подпись: To Full Bridge Input

Because the operation frequency is low, losses are mainly attributed to conduction losses in cables, protection devices, connections, switches and transformer. Figure 6.37 shows the conduction resistance of all devices between the input terminals and H-bridge input. Total resistance in this path is 2.89 mQ what corresponds to approximately 40 % of the transformer primary resistance, showing that the design of wiring and protection have

The complete efficiency versus output power characteristic curves of the prototype is shown in figure 6.38, while figure 6...

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Characteristic curves

6.7.1 No-load consumption

Figure 6.34(a) shows the no-load consumption as a function of the input voltage. As can be seen, the input voltage has great influence in the no-load consumption and its characteristic looks like an exponential curve. This fast increase can be associated with the increase of the transformer no-load losses as its primary voltage is increased.

Подпись: No load Vb [V] (a) Подпись: P [W]

The standby characteristic of the prototype is showed in figure 6.34(b). At this operation mode, only the auxiliary power supply and controller are operating and no output voltage is produced. As can be seen, this characteristic curve is approximately linear, although it does not change significantly over all input voltage range.

Figure 6.34 – (a) No-load consumption versus input voltage;

(b) Stand-by consumption ver...

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Battery-charge-mode using Sunny Boys

The proposed converter is bi-directional and this feature can be used to charge the battery bank if its output is connected to a power source with current source characteristic. This type of operation was realized by using solar panels and grid inverters, using the configuration shown in figure 6.31. The prototype was connected to 2 Sunny Boys grid inverters fabricated by SMA. Each Sunny Boy was feed by a 900 Wpk solar panels arrays (total of twenty four 75 Wpk PV modules, organized in two arrays of 12 modules). The DC side of the inverter was connected to a 48 V battery bank.

Battery-charge-mode using Sunny Boys

Figure 6.31 – Block diagram of the battery-charge-mode experiment.

Figure 6.32(left) shows the panel array modules while figure 6.32(right) shows the prototype and Sunny Boys (at the back).

Battery-charge-mode using Sunny Boys

Figure 6...

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Startup of loads

In practice, most loads do not act like pure resistance, inductance or capacitance. Non linear characteristic and high startup currents are usually presented by real loads, such as refrigerators, incandescent and fluorescent lights, single phase motors, computers, and general electronic apparatus. The capability of the proposed inverter to operate such loads is demonstrated in this section.

Startup of loads

The refrigerator is commonly desired in residential applications and it is known to be a problem in many small stand-alone systems due to its high startup current. Figure 6.25 shows the waveforms acquired at the startup of a small refrigerator when it is connected to the implemented prototype.

At steady state operation, the measured current was 1.0 A (RMS), while according to figure 6...

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