Successful debut. science & technology. The CPS SC20KTL-O was the very first device

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1 science & technology Successful debut Chint Power s CPS SC20KTL-O achieved impressive test results despite smaller weaknesses Highlights Text: Heinz Neuenstein, Jochen Siemer Chint Power s CPS SC20KTL-O is a transformerless string inverter with a DC nominal power of 20.8 kw The MPP range stretches from 500 to 800 V; the upper limit is too close to the device s maximum DC voltage The conversion efficiency reaches a maximum of 98.1 percent, while the European efficiency came in at 97.5 percent and the device s Californian efficiency is 97.8 percent The PHOTON efficiency at high irradiation is 97.4 percent, while the device achieved a PHOTON efficiency of 97.1 percent at medium irradiation The device is light, its layout is clearly arranged and it complies with protection type IP 65 The CPS SC20KTL-O was the very first device from Chint Power Systems Co. Ltd. to find its way to PHOTON Laboratory s test bench. And while the company s name is not exactly well-known in the solar branch, Chint Power should not be mistaken as being a newcomer. The Chint Group, which has its headquarters in Shanghai, China, apparently has 16,000 employees, and is involved in low, medium and high-voltage technology, as well as measurement and automation tools, all over the world. The Chint Power Systems branch was founded in 2009 to cater to the renewable energy sector. When a company such as this manufactures PV inverters, the expectations are justifiably high and the CPS SC20KTL-O doesn t disappoint. The device does not boast any elaborate design features; the setup is simple and some parts of the interior generate a great deal of heat. Despite this, the developers really have shown extensive expertise: the device operates extraordinarily well and exhibits impressive efficiency curves. Construction The inverter tested is part of the CPS range, which includes both single-phase and threephase string inverters. The single-phase inverters have an AC nominal power range of between 1,500 and 4,600 W, while the threephase units range from 10,000 to 20,000 W. All of the models in the range are transformerless. The internal design is very compact and clearly laid out. A great deal of silicone adhesive has been used to affix large components. Virtually all circuit boards are installed on one level; only the control circuit board has been placed on a second level. The power element is attached to two large circuit boards. The lower one houses the DC input and AC output filter components, the output relay, and the auxiliary power supply. All of the boost converter components and the output bridges are on the upper level. The power transistors and diodes are stored in integrated cases and placed on the soldered side of the circuit board. They are screwed on to a cooling element on the rear side of the unit, which ensures that any heat dissipates. The boost converter s storage chokes and sinusoidal filter s chokes are found in an installation space behind and next to the cooling element at the rear of the device. A fan is placed between the two power element circuit boards and prevents any heat building up or heat pockets forming. The AC-side residual current is measured on a small circuit board situated below the large one. The display and its buttons are installed on the back of the unit s cover and project through a window making them visible from the outside. The housing is comprised of three parts: a mounting plate made of sheet steel, which supports the extruded cooling element at the bottom; a welded base section, also made of sheet steel, which functions as a basin that contains the mounting plate with the chokes; and a cover for the device, made of cast aluminum. The entire unit complies with protection type IP 65 and is therefore suitable for installation both inside and outdoors. At the bottom of the housing, there are three DC fans for the cooling element. If they fail, a certain degree 134 International November 2011

2 Romana Brentgens / photon-pictures.com (2) With its clean, modern design and streamlined interior, the CPS SC20KTL-O is finding friends fast. However, the thermographic image is cause for concern as the temperatures reached are not exactly what is expected from a modern inverter. of effort is required to remove or replace them. Moreover, exposure to ambient conditions can cause the fans to become clogged; therefore, the inverter should not be operated in excessively dirty conditions. An automatic disconnect unit, which monitors the grid for correct voltage and frequency, ensures safety is maintained. An insulation test is also performed on the solar generator. The electrolytic capacitors used in both the power element and the control electronics have a temperature class of 105 C. Therefore, they are capable of handling ambient temperatures. The solar generator is hooked up using five Helios 4 PV connectors manufactured by Amphenol-Tuchel Electronics GmbH, which are difficult to disconnect once in place due to space constraints. The grid is connected using a five-pin plug, also made by Amphenol. The inverter is not equipped with an internal DC disconnect. The device s status is indicated by the display and four LEDs. Bus communication with the inverter can be established using two RS485 interfaces. Direct communication with a PC is possible using the second RS485 interface, which can be used as an RS232 in this case. The company s own Interlink datalogger can also be connected using the RS485 interface. Operation The device arrives at the customer s home well packaged and protected by a wooden crate. The inverter is mounted on a wall using a bracket. At 50 kg, the CPS SC20KTL-O is rather light considering its nominal power. Once the solar generator is properly configured and an external DC disconnect is hooked up, the inverter is ready to start work. The unit requires around 58 seconds to complete diverse tests. It then connects to the grid and starts operation. The graphic LCD display is flush with the front cover and shows a PV system with a list of the most important measured values: DC voltage and current, AC voltage and current, AC power, the amount of energy fed in, the date, and the time. The different levels within the menu can be accessed using six buttons, which do not work as precisely as hoped. They can be used to display numerous other values being monitored. The characters shown on the display lack contour and the backlighting is not very bright. Nevertheless, it is still quite easy to read. The operating languages available for selection are Chinese, English, German, Italian and Spanish. Instruction manual An installation and operating manual is included with the device. It is available in Chinese, English, German and Italian, and the datasheet also comes in Dutch, French and Spanish. Along with general explanations, the instruction manual includes information about assembly and connection, and descriptions of operating characteristics, the display and error messages, as well as a number of block diagrams covering the inverter and system configurations. The document can also be downloaded from the manufacturer s web- The Solar Power Magazine 97.1 % at medium irradiation The Solar Power Magazine 97.4 % at high irradiation International Chint Power s CPS SC20KTL-O A 11/2011 International Chint Power s CPS SC20KTL-O A 11/2011 International November

3 Conversion efficiency The CPS SC20KTL-O only reaches its peak value of 98.1 percent in a small area. However, the area with conversion efficiencies just below that is extensive. MPPT adjustment efficiency Almost unparalleled: Very few inverters operate at such a constant level of 99 percent or more. = Overall efficiency The overall efficiency diagram looks virtually identical to that of conversion efficiency; only the area with values above 98 percent is missing. 136 International November 2011

4 site and should soon be available in all of the aforementioned languages. Circuit design The CPS SC20KTL-O is a transformerless inverter with a three-phase feed-in and a twostage design. The inverter has one MPP tracker in the input. Firstly, energy from the solar generator reaches the power stage via an EMI filter. The MPP tracker is formed by a boost converter with a storage choke and free-wheeling diode. If the input voltage is high, the boost converter transistor is shut off and a bypass circuit is activated. This reduces losses at higher voltages. The tracker feeds into a half-bridge capacitor, the midpoint of which is connected to the free-wheeling diodes in the three-point halfbridges. The pulse-width modulated output voltage is formed using three three-point halfbridges, the midpoint of which corresponds to one phase. A subsequent filter smoothes the modulated voltages blocks into sinusoidal voltage with a grid frequency of 50 Hz. An automatic disconnect separates the inverter from the grid in the event that grid voltage or grid frequency deviate from preset limits, as well as if residual current is detected on the AC side or insufficient insulation resistance is measured on the DC side. An output filter, installed immediately in front of the grid clamp, eliminates any radio interference. Measurements All of the following measurements are based on a grid voltage of 230 V. The CPS SC20KTL- O s maximum DC voltage is 850 V and the DC nominal power totals 20,800 W. A maximum PV output of 22,000 W can be connected. For MPP voltages above 680 V, the open-circuit voltage of the simulator had to be limited, as it already exceeded the inverter s maximum DC voltage based on a fill factor of 75 percent. Locating the MPP: At a predetermined IV curve with nominal power and an MPP voltage of 642 V, allowing the 58 seconds it requires to connect to the grid to elapse, the inverter requires another 106 seconds to reach the MPP. It takes around 11 seconds when changing from 642 V to 626 V; the next-highest MPP range of 658 V is reached after 12 seconds. MPP range: The MPP range spans from 500 to 800 V, which is a normal range. Based on current fill factors, the maximum MPP voltage of 800 V is too close to the maximum DC voltage of 850 V. Conversion efficiency: The CPS SC20KTL- O can operate at 100 percent of its nominal power within an MPP voltage range of 500 to 800 V. The hatched area stretching from the bottom left to the top right of the diagram indicates limitations when crystalline modules are being used, resulting from the insufficient distance between the maximum MPP and DC voltage; the area is quite large and starts at around 680 V. Another area of hatching, which indicates limitations when thin-film modules are used, starts at around 630 V. Conversion efficiencies of 98 percent and above only make up a small area in the diagram, which is surrounded by a large plateau with values just below 98 percent. The maximum conversion efficiency of 98.1 percent is reached at a nominal power of 40 percent and an MPP voltage of 611 V. This exceeded the manufacturer s specs of 98 percent. The maximum conversion efficiency drops by around 0.5 percentage points at high MPP voltages and by about 0.7 percentage points at low MPP voltages. Below 15 percent of nominal power, efficiency falls by around 5 to 10 percentage points. At nominal power, the power factor cos j was about one. MPPT adjustment efficiency: The MPPT adjustment efficiency is consistent over the entire operating range and, at 99 percent and above, is very high. There are small areas with irregularities at low powers of around 5 percent of nominal power. International November

5 Weighted conversion efficiency Straight as an arrow, almost: The inverter displays virtually horizontal progression for European and Californian efficiency. Overall efficiency at different V MPP voltages The efficiency curves at different MPP voltages confirm that the CPS SC20KTL-O gets going quickly and maintains a consistently high level. Accuracy of inverter display The measured and displayed output power deviates markedly from reality, especially in the lower power range. 138 International November 2011

6 Manufacturer s response We are very pleased that our inverter is ranked with two A grades. The test data is consistent with the results from Chint Power s lab, taking measurement tolerances into account. Overall efficiency: The maximum overall efficiency reaches 98 percent at an MPP voltage of 611 V and a nominal power of 40 percent. The progression of overall efficiency is uniformly high, similar to that of conversion efficiency. Weighted conversion efficiency: The European efficiency of 97.5 percent is achieved at 626 V and matches the manufacturer s specs exactly. The difference between the maximum conversion efficiency and maximum European efficiency is just 0.6 percentage points. At 97.8 percent, the Californian efficiency is around 0.3 percentage points higher. That figure was also achieved at 626 V. Course of overall efficiencies, average overall efficiency and PHOTON efficiency: The PHOTON efficiency at medium irradiation is 97.1 percent, while the PHOTON efficiency for high irradiation is 97.4 percent. Feed-in at nominal power: The inverter feeds in 100 percent of its nominal power over the input voltage range of 500 to 800 V at an ambient temperature of 25 C. Displayed output power: When the inverter is fed with powers varying from 5 to 100 percent of its nominal power at a constant MPP voltage of 658 V, which is in the medium range, the output power measured and displayed by the inverter deviates by up to -12 percent from the values measured by a power analyzer at low powers. At 20 percent of nominal power and above, the error rate comes in at around -3.4 percent, falling to a value of -1.3 percent at nominal power. Therefore, the display s precision is not exactly the CPS SC20KTL-O s best feature, but it is equivalent to a class C meter. Operation at high temperatures: Thanks to the broad temperature range of -20 to 65 C and the inverter s IP 65 protection type, the unit can be installed in a warm location such as below a roof, or outdoors. However, power reductions have to be taken into account at ambient temperatures above 60.8 C. Below that temperature, the inverter feeds 100 percent of its nominal output into the grid; after that point, it reduces its output. The efficiency drops by around 0.04 percentage points. The operating point chosen was 20,800 W and 642 V. Overload behavior: If the CPS SC20KTL-O is presented with an overload of 1.3 times its nominal input power, or 27,040 W, at an MPP voltage of 642 V and an ambient temperature of 24 C, the device limits power to around 21,755 W. This is equivalent to an overload of percent of the DC nominal power of 20,800 W. The device therefore features a small overload range. When power limits are applied, the inverter shifts the operating point on the IV curve toward higher input voltages. The DC voltage adjusts itself to a value of around 714 V. Own consumption and night consumption: The device s own consumption in the tested construction is around 20 W on the AC side and 6.8 W on the DC side; the manufacturer doesn t provide any information on this. At night, the inverter draws around 16 W of real power from the grid. The manufacturer specifies»less than 16 W.«Thermography: Thermographic imaging shows the inverter from above while it is operating at nominal power at an ambient temperature of 28 C. Component temperatures of up to 98.7 C were observed on the circuit boards. As the control circuit board covers parts of the power element, not all areas could be measured. The 98.7 C a rather high value was recorded around the grid isolation relay. The grid fuses also reached a considerable 92.1 C. The electrolytic capacitors in the power element were at a temperature of around 44 C. Summary The inverter s construction is very compact and clearly arranged. Virtually all of the circuit boards are on the same level. The maximum conversion efficiency of 98.1 percent is achieved in the medium MPP voltage range, although the difference of around 0.5 percentage points at higher voltages is actually very low. Due to the very good MPPT adjustment efficiency, the overall efficiency curve is virtually identical to that of conversion efficiency. The distance between the maximum MPP voltage and the maximum DC voltage is too limited, meaning the MPP range cannot be exploited in full with either crystalline or thinfilm modules. Although the inverter is not designed for use with thin-film modules as it is transformerless the manufacturer states that it has been employed in a number of systems using this type of module. At 97.1 percent, the PHOTON efficiency at medium irradiation was very high; the PHO- TON efficiency for high irradiation is slightly higher at 97.4 percent. The small difference of just 1 percentage point between the PHO- TON efficiency at medium irradiation and the maximum conversion efficiency indicates the low voltage and power dependencies. When designing a PV system, the middle third of the MPP voltage range should be selected. The inverter only features a small overload range of 4.6 percent and the output power displayed by the inverter is far from precise. The inverter s temperature range is very broad, but power limitations go into effect at an ambient temperature of 60.8 C. At just 0.04 percentage points, the conversion efficiency is hardly dependent on the temperature at all. Chint Power s CPS SC20KTL-O achieved two A grades in PHOTON Lab s test, making the device one of the top 10 inverters tested so far. Further information Contacts page International November 2011