HIGH FREQUENCY TRANSFORMER WITH TRANSFORMER SWITCHOVER

Transcription

1 OPTIMUM EFFICIENCY AND FLEXIBLE USE HIGH FREQUENCY TRANSFORMER WITH TRANSFORMER SWITCHOVER One of the many requirements of the modern inverter is a broad, coordinated input and MPP voltage range with a consistently high degree of efficiency across the entire operating range of the inverter. To satisfy this requirement, Fronius is implementing a high frequency transformer (HF transformer) in most of its current inverters. This HF transformer has a transformer switchover that ensures a consistently high degree of efficiency right across the input voltage range. It is often incorrectly assumed that the maximum degree of efficiency at a particular voltage is one of the factors responsible for producing a good annual yield, when it is in fact the more or less constant degree of efficiency over the entire MPP voltage range. Thanks to the HF transformer switchover feature, the Fronius IG Plus and the Fronius CL offer maximum efficiency for almost any permissible string length. Basic inverter concept There are basically three different inverter technologies: / an inverter with a 50 Hz transformer. / an inverter without a transformer. / an inverter with a high frequency (HF) transformer. 50 Hz technology The applied DC voltage is converted to a 50 Hz AC voltage via a full bridge (S1...S4). This is then transmitted via a 50 Hz transformer and subsequently fed into the public grid. Benefits: / High degree of reliability due to fewer components. / Safety through galvanic isolation of the DC and AC sides. Disadvantages / Low degree of efficiency resulting from high transformer losses. / Heavy weight and volume (e.g. due to 50 Hz transformer). 04/2012 1/7

2 Transformerless inverter technology The existing DC voltage is converted to a square 50 Hz AC voltage via a full bridge (S1...S4), then smoothed to a sinusoidal 50 Hz AC voltage via the chokes (L1+L2) and fed into the public grid. Benefits: / Compact and light due to lack of transformer. / Very high degree of efficiency (e.g. no transformer losses). Disadvantages: / Additional safety measures (residual current circuit breaker) required. In some countries, a lack of galvanic isolation between the DC and AC sides is not permitted. / Complicated lightning protection. / Not compatible with modules that must be earthed (e.g. some thin film technologies or rear-contacted cells). HF technology This technology combines the advantages of the previous technologies. The full bridge (S1...S4) generates a high-frequency square-wave signal with khz, which is transmitted via the HF transformer (Tr1). The bridge rectifiers (D1...D4) convert the square-wave signal back to DC voltage and store it in the intermediate circuit (L1+C2). A second full bridge (S5...S8) then generates a 50 Hz AC voltage, which is smoothed to a sinusoidal 50 Hz AC voltage via the chokes (L2+L3) before being fed into the public grid. Benefits: / Compact and light, as the HF transformer is very small and light. / High degree of efficiency through reduction of transformer losses. / Safety through galvanic isolation between the DC and AC sides. / Suitable for all module technologies, as module earthing (positive and negative) is possible. Transformer switchover Depending on the input voltage, the various technologies produce the following efficiency curves: 04/2012 2/7

3 When using an inverter with a 50 Hz transformer, there is always a fixed transformer transmission ratio between the primary and secondary side (DC and AC). The higher the input voltage, the lower the efficiency. This is connected, among other things, to the utilisation rate of the transformer, which drops at high voltages, causing the losses to increase. In the case of transformerless inverters, the solar generator voltage must be greater than the amplitude of the mains voltage, which means that efficiency, taking into account tolerances, is at its optimum at an input voltage of approx. 350 V. This results from the mains voltage at 230 V, where the peak voltage is 325 V and additional semiconductor losses amount to approx. 10 V. Outside this voltage range a voltage step-up or step-down converter is active that increases or decreases the input voltage to the required value and reduces efficiency. The HF transformer concept with a fixed transformer transmission ratio reduces efficiency even at higher input voltages. However, by changing the transformer transmission ratio (transformer switchover), several efficiency peaks can be achieved through optimised transformer utilisation, thus ensuring a consistently high degree of efficiency right across the input range. 04/2012 3/7

4 Transformer switchover basics In practice, it is rarely possible when designing the system to select the best input voltage range for the inverter, i.e. the range with the best conversion efficiency according to the datasheet. For optimum yields with any permissible connection, it is therefore essential that the efficiency is consistently high across the entire operating range. Mode of operation The HF transformer has three windings (V1, V2 and V3) on the primary side. Another winding is used for transmission in accordance with the input voltage, and this changes the transmission ratio. E.g.: V = U1, V = U2, V = U3 To guarantee the best possible transmission, the switching limits change with the output voltage (e.g. for USA mains). This ensures that there is always a constant voltage (U) on the secondary side, transmission losses are minimised and the degree of efficiency over the entire voltage curve is consistently high. For each switching operation between two windings, the power is briefly switched to zero before the next winding is connected and the power switched on again. This practically eliminates all switching losses. Advantages of transformer switchover The transformer switchover results in a consistently high degree of efficiency across the entire input voltage range. It is not the maximum degree of efficiency at a particular voltage but the consistently high degree of efficiency right across the MPP voltage range that is partly responsible for a good annual yield. Thanks to the HF transformer switchover, the Fronius IG Plus and the Fronius CL offer maximum efficiency for almost any permissible string length. At the planning stage there is therefore no need to consider whether the system is designed for high or low voltages. The voltage variation due to different temperature conditions during normal operation is also compensated for by the transformer switchover. Disadvantages of transformer switchover Due to the relay switching time, a short pause (200 ms) is necessary when switching to the next transformer winding. During this short period, the input voltage increases towards the open circuit voltage, meaning that the secondary components must temporarily be able to withstand a higher voltage. 04/2012 4/7

5 Transformer switchover details Each power module of the Fronius IG Plus and Fronius CL inverter range is fitted with an HF transformer, which has various active transmission ratios depending on the input voltage. There are three different areas with three different voltage ratios, so Fronius inverters with HF transformers also exhibit three efficiency peaks. This enables a consistently high degree of efficiency to be achieved across the entire input voltage range. This is important at the planning stage and also when the temperature changes during system operation, as the voltage of the module varies according to temperature. Thin film module example Due to the special design of the thin film module, these generally have lower currents and higher module voltages than crystalline modules. To enable reasonable string lengths and module combinations in combination with the smaller fill factor, the inverter needs a broad and coordinated input and MPP voltage range with a consistently high conversion efficiency across the entire operating range. Due to the described module properties, there are very few different string lengths within the operating range of the inverter compared with crystalline modules. The following figure illustrates that three different string lengths are possible in the case of a thin film module with open-circuit/mpp voltages of 95 V and 65 V respectively and a Fronius IG Plus with an operating range of 230 V to 500 V. The temperature coefficients of the modules and the temperature range in question of -10 C to +60 C result in the following MPP ranges according to string length: It is apparent that a configuration covering the entire MPP voltage range and all different string lengths is feasible due to the consistently high conversion efficiency. 04/2012 5/7

6 Comparing this to the other inverter concepts demonstrates that it is usually only possible to have one string length at optimum efficiency, while other string lengths lead to significant declines in efficiency and hence reduced yields. In the case of an inverter with a transformer but no switchover, as shown in the following figure, the efficiency figure falls across the entire MPP voltage range of the inverter as the DC voltage increases. Only in the lower MPP voltage range (red range) is the highest maximum efficiency achieved. An inverter without a transformer achieves its optimum efficiency at one particular DC voltage. At all DC voltages above or below this level, it requires a step-down or step-up converter, which also results in efficiency losses (see following figure). 04/2012 6/7

7 The ideal string length lies in the middle MPP voltage range (blue range). Using the lower or upper MPP voltage ranges leads to reduced yields. Summary In addition to the above mentioned advantages of galvanic isolation and lower transformer losses, Fronius inverters that employ HF transformer technology exhibit a consistently high degree of efficiency right across the input voltage range, as the transformer switchover takes place on the input voltage side. They are thus well able to cater for the requirements of thin film modules, as well as crystalline ones, and can therefore be used with all module types. Characters: 8,474 (without spaces) Words: 1,586 Enquiries: Editor: DI Jürgen Wolfahrt, +43 (0) , wolfahrt.juergen@fronius.com, Froniusplatz 1, 4600 Wels, Austria. Trade press: Andrea Schartner, +43 (664) , schartner.andrea@fronius.com, Froniusplatz 1, 4600 Wels, Austria. 04/2012 7/7