3 Ambitious development targets

All of the following targets were achieved without exception:

• extremely high efficiency even with partial load: over 93% for 25-100% of maximum power

• maximum efficiency over 94%,

• marketable price-performance ratio compared to inverters with 1 - 50 kW,

• very high reliability,

• integrated diagnosis and transmission of measured values,

• extended temperature range (ambient temperature -25 °C to +60 °C) and

• degree of protection NEMA 4X (outside mounting).

Other requirements for installation and safety (fig. 2) are based on the relevant regulations for electromagnetic compatibility and grid-tied operation etc. and also depend on specific conditions of use.

Fig. 2: Protected against all types of interference – example of a String Inverter

In addition a String Inverter improves the plant’s efficiency, especially because losses due to mismatching or local and temporary partial shadowing are reduced (MPP tracking for local groups). Furthermore it avoids mismatching based on the parallel connection of several strings (worsening when the plant ages) because only modules connected in series are coupled via a String Inverter with its own MPP tracking. This and the elimination of serial diodes improve plant efficiency by approx. 1-3%.

In close cooperation with internationally accepted manufacturers and research centres (Source: SMA Regelsysteme GmbH, ISET, IEE) detailed studies are developed that carry out the special requirements of photovoltaic energy feeding to the low-voltage grid, showing 30 different switching topologies. The optimum solution to achieve high efficiency and low cost while ensuring electric separation of the grid and PV connections was found to be a pulse-controlled inverter with toroidal-core transformer.

The DC voltage from the PV panel is converted into an AC intermediate circuit via a full bridge. A pulse-width modulated regulator working beyond the human audible frequency spectrum at 20 kHz approaches a sinusoidal 50/60 Hz AC current. The low-frequency toroidal-core transformer then steps up this current to grid voltage level. An LC low-pass filter connected in the outgoing circuit retains the operating frequency. A two-pole relay controlled by the system management couples the power unit to the grid.

4 High efficiency even with partial load

To yield maximum PV power on an annualized average the power unit has been dimensioned so as to guarantee high efficiency with partial load as well.

In order to achieve a high efficiency under partial load with only a minor decrease under full load fast power semiconductors with an extremely low forward resistance are used as switches in the converter bridge. Detailed tests have been run with semiconductors of a considerably higher current carrying capacity to increase the nominal power efficiency. The tests have shown that those semiconductors are unsuitable not only because of higher costs, but also because of their lower efficiency under partial load. While forward power losses would be decreased one would have to accept an increase in reloading losses in the gate capacities.

But the selection of suitable power transistors is not the only condition for optimizing efficiency in the converter bridge. Different modulation procedures were assessed in numerous series of tests. It was found that the combined PWM/50-60 Hz mode of the converter bridge yielded the highest efficiencies for partial loads The resulting distortions of the output current at zero crossing impose tougher conditions on current control. Combined PWM/50-60 Hz mode consists of the sinusoidal pulse width modulation (PWM) of the circuit breakers with potential to ground as well as the 50/60 Hz pulse of the circuit breakers on the plus potential in the full bridge. The branch of the bridge lying in the respective current path is monitored by two different and autonomous overcurrent detectors. In case of overcurrent the entire converter bridge blocks. This makes the inverter absolutely safe in case of a short circuit both on the DC and the AC side. The losses in the inductive components such as the toroidal-core transformer and the storage choke were minimized for partial loads as well. The resulting efficiency curve of the a String Inverter (SMA Regelsysteme GmbH, model: Sunny Boy 2500U) is shown as a function of input power in fig. 3.

Due to the extremely high efficiency and a large heat sink the occurring power loss results in only a slight temperature increase. Even with extremely high ambient temperatures the components’ temperature is still far below the admissible limits. Moreover the inverter is protected by an overall temperature monitoring with a special algorithm for power reduction.

Fig. 3/4: Operating characteristics of the String Inverter (SMA Regelsysteme, Sunny Boy 2500U)

a) With a typical PV voltage of 300 V its efficiency reaches over 90% as early as at 10% of nominal power and up to 95% at maximum.

b) The current waveform properly aligns with the grid.

5 System management and grid monitoring

System management is the central element for the operation of the String Inverter. It is used for data acquisition, closed-loop control, communication and open-loop control of the entire operation process. It uses of two microcontrollers communicating with each other: system control and current control.

The power unit is controlled by a primary digital voltage controller with a secondary analogous current controller. The voltage controller is provided its required value from the MPP controller which is digital as well. The sinusoidal grid current control is the most time-critical of all the control functions the system management has to perform. Therefore this task is executed by an external analog circuit which includes a PD controller with amplification which can be changed by the processor. The total harmonic distortion of the feed-in current is very low due to the combined analog/digital control.

This kind of String Inverter has a standard autonomous disconnection device. The unit is integrated in the inverter and continuously monitors the relevant grid values such as voltage, frequency and impedance. Reliable functioning is guaranteed by the redundant measuring setup and an automatic self-test before each connection to the grid.

Fig. 5: Structure of grid monitoring by two separate switching devices

Two autonomous different Mains Monitoring units with allocated Switching Devices (MSD, translation of the German ENS). MSD1 and MSD2 are both based on a microcontroller. Both monitoring units can interrupt the energy flow into the grid independently from each other (fig. 5).

The procedure to measure grid impedance was designed so as to make optimum use of the existing measurement equipment and guarantee sufficient measurement accuracy. It generates a current pulse around the zero crossing of the grid voltage. The evaluation is then not done on the resulting change of voltage which is hard to assess by measurements, but the shifting of the zero crossing. The two microcontrollers independently calculate the grid impedance from the duration of the period following complicated mathematical procedures. The energy required for the current pulse is first taken from the grid, intermediately stored and fed to the grid again after the impedance has been assessed. The advantage of this method is that it is completely independent of spot radiation power.

6 Diagnosis and communication

The modular PV system technology concept lets the String Inverters be spread out all over the plant. A central PC with special Windows program can check the function of each single inverter in a simple and fast way by monitoring the status of the device and the measurements taken.

Signals are transmitted between the PC or the data logger and all inverters via the existing power cable by the Powerline modem integrated in the String Inverter – no additional signal lines are required.

Communication enables the following functions:

• Continuous acquisition of operating data of all String Inverters and PV module groups connected

• Monitoring of operating states and signaling of operating failures

• Online transmission of measured data from a selected String Inverter

• Identification of faulty strings

7 Experience and future inverter topologies

By now String Inverters are the first choice on solar markets for grid-tied inverters due to their enormous technical advantages. Extensive experience has been gained during the development, production, world-wide operation and sales of String Inverters. It has been found that a further drastic reduction of specific inverter costs (DM/W) right now is only possible by increasing the inverter’s nominal power if one doesn’t want to carry out intensive and costly development of active and passive components which would lead to a higher degree of integration. With power above 3 kW, however, it is no longer possible to use pure string technology due to the resulting high string voltage. Therefore the market does not offer “pure” String Inverters for this power range (single inverter > 3 kW).

Two years ago manufacturers brought up Multi-String Inverters. The declared target was a further drastic reduction of specific inverter costs so that the advantages of string technology could fully be used with the Multi-String inverter as all strings are connected individually to one common inverter bridge of 5 kW via MPP-controlled extendable DC/DC converters. Depending on the number of strings in the PV plant the Multi-String Inverter can be equipped with a variable number of DC/DC converter modules.

Today a typical application of the Multi-String Inverter is its use in a grid-tied PV plant with more than 3 kW. The following illustrations show a few possible plant structures which offer new and interesting configurations for PV architecture among other things. Fig. 10 shows the example of a 3 kW plant where the advantages of the string-oriented concept can be fully used. You can see that strings with PV modules of different nominal data, size or technology as well as strings with different orientation (east, south, west) and inclination or strings with differently strong shadowing can be connected to one inverter and still work in their individual MPP each.

8 Advantages of a Multi-String Inverter in PV plants

a) Strings with different nominal data (size, type of solar cells)

b) Strings with different orientation (west, south, east) and inclination

c) String with differently strong shadowing

Fig. 6: Advantages of a Multi-String Inverter in PV plants with strongly varying data for different strings

Since PV plants were first connected to the grid for research and demonstration purposes a well-advanced technical and qualified standard could be realized for photovoltaic system components manufactured in serial production. Depending on the type of application, conditions on site and the basis of calculation total costs of a 1 kWp plant have been cut approximately by half from 1990 till today. Modular system technology – starting in 1995 - made a substantial contribution to this development. Since then string technology has been used more and more for plants up to several megawatts. Consistent further development, making use of all well-known advantages of string technology with a Multi-String inverter, will further lower the price and system efficiency especially of larger-scale PV plants.

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