SG125HV Technical White Paper

Time:2017-07-10 Page view:2439

SG125HV Technical White Paper

1.      Overview

Until recently, utility scale solar power systems have been designed with the largest capacity central inverters available.  With the introduction of string inverters, systems with capacities in the range of 1MW to 10MW have been successfully designed and installed using three-phase, 1000VDC string inverters with capacities in the 30kW to 60kW range.  Sungrow’s introduction of the SG125HV, a 1500VDC string inverter with a 125kW capacity has made even the largest utility scale solar power systems within the range of the string inverter system architects.

2.      Features


Fig-1: SG125HV Appearance

The SG125HV is the world’s highest power inverter in the string category.  Weighing in at 159 pounds, so it stays in the two-man installation category.  The key specifications for the SG125HV are listed below.  The following paragraphs will describe the system design parameters that utilize these specs to connect them with utility scale system design and ultimately, the Virtual Central Solution.

3.      Technical Specifications

Max DC input voltage


Max output power MPPT range

860VDC to 1250VDC

Max DC input short circuit current




DC inputs


Max AC output power (at 50C)

125kW / 125kVA

AC output voltage (nominal)


Max AC output current


Max efficiency


CEC efficiency


Max DC input cable size


Max AC output cable size


AC output configuration

Three phases and ground

Inverter total weight

159 lbs

Inverter dimensions


Display/ communication

LED/RS485 (dual inputs), Bluetooth+APP

Operating temperature Range

-25°C to +60°C(Derated above +50°C)

3.1      High Efficiency

Sungrow’s SG125HV was designed to have a power converter platform that was more efficient than previous generations of products. Utilizing 5-level converter bridge, the SG125HV provides extraordinary best-in-class efficiency, further increasing project ROI by maximizing input vs output power conversion efficiency and minimizing loss. The maximum efficiency of SG125HV can reach 98.9%, and the CEC efficiency is 98.5%. Greater efficiency equals togreater AC power yield.

3.2      Wide Operating temperature range

Sungrow’s SG125HV has one of the widest operating temperature ranges available in the industry, with full power available from -25°C to 50°C and operation in power de-rate mode available up to 60°C.

The SG125HV is rated for 125,000W at ambient temperatures up to 50°C. At temperatures above 50°C the inverter will ramp down power or ‘de-rate’ in order to maintain control of internal temperature and losses.  The de-rated operation will continue until the ambient temperature exceeds 60°C.

3.3      Less cable cost, save Capex

Sungrow’s in-depth project design analysis and Capex economics reviews have determined that in most cases it is overall more cost-effective to have the 1500V DC SG125HV inverter located nearer the AC POC than the PV array field. As previously explained, utility-scale project cabling costs are dominated by voltage drop concerns and longer runs of two cables (DC+/DC-) at 1500V are simply less expensive than running three cables (A, B and C phases) at 600V AC. In order to facilitate the recommended design scheme that uses external (i.e. mounted within or immediately peripheral to the PV array) combiner boxes with longer DC+/DC- ‘homeruns’ back to the inverter located near the POC, the Sungrow SG125HV inverter utilizes a single DC input architecture that allows larger homerun cables or even trunk bus cables to be connected directly to the inverter without the need for fusing or other maintenance-intensive components within the inverter.


Fig-2: DC input cable vs. AC output cable

3.4      AC output configuration

In an effort to further reduce project Capex and push the inverter’s efficiency to the highest levels, a voltage level higher than 480V AC was required. The SG125HV outputs a nominal 600V AC output voltage which reduces AC output current by 25% when compared to the same amount of power at standard 480V AC. Lower current yields smaller cable costs by the same percentage and greatly benefits project Capex.

In addition, the inverter does not require a neutral connection from the primary transformer, so only 3 phases plus a PE ground are connected to each inverter, further reducing cabling costs vs competitors which require a neutral conductor in addition to the three phases cables and PE ground.

3.5      Flexible wire space adapt to more cablesizes

The maximum AC output current for the SG125HV is 120A but the terminal block can accommodate up to a 350MCM wire.  The ampacity of a 350MCM wire is 350A in conduit so there is ample allowance for wire oversizing to reduce voltage drop with longer AC runs.

On the DC side, the maximum DC current input is 148A. This current value does not represent a limit on the amount of DC current the array connected to the inverter can be sized to; The max. DC short-circuit current is 240A,which allows the DC/AC ratio up to 1.5:1; the high DC/AC ratio is becoming increasing common with modern PV plant ROI economics.

The maximum wire size that the SG125HV DC terminals can accommodate is 350MCM which is good to more than 350A continuous@ 90°C and allows for very large DC:AC ratios approaching 1.5:1 and beyond for maximum DC system design flexibility. 

3.6      Small and light to bring thesimpleinstallation

The SG125HV is comparably sized to inverters with half the power rating and was designed to be simple to install. A metal backplate is provide, along with fastening hardware, which gives installers flexibility with mounting options. The inverter can be hung on appropriately-rated strut supports, on concrete walls, or virtually anywhere the mounting bracket can be installed.

A comparison of the cost to install central inverters with the costs of installing string inverters brings up some significant differences. Aside from the MV transformer, installing the string inverters and related BOS does not involve anything more than just manpower.  There is no need for wide access paths or bringing heavy lifting equipment to the site.

For string inverters, especially the SG125HV at 159 pounds, there is no need for massive concrete structures or skid platforms.  The inverters can be mounted to existing solar array structures or a relatively simple rack made of Uni-strut.  Some shading should be considered as good engineering practice to reduce the thermal load and to reduce the over-all weather impact on the units.

To create the Virtual Central Solution, it is not absolutely necessary for all the SG125HV inverters to be mounted right next to each other although it can save in integration costs in the long run.  The inverters can be spread out individually or located in groups.  This approach can reduce the DC wiring lengths without creating unreasonably long AC wire runs. However, the closer the groups are to each other, the easier it will be to integrate the communication and control system.

It is best to parallel the several SG125HV inverters via one or more AC breaker panels.  Fused disconnect switches can be used as well.  This helps in both circuit protection as well as in providing electrical isolation on the AC side which will facilitate the removal and replacement of individual inverters.

3.7      Stable display and communications

For user convenience, the SG125HV includes an LED HMI display panel to indicate inverter operating status and other system parameters such as Bluetooth connectivity, serial communications status, and fault and ground impedance status.

Customer communications to the SG125HV inverter are done through the serial RS485 . Additionally, the inverter allows Bluetooth connection to smart devices enabling the Sun Access App to communicate with the inverter wirelessly to set parameters, check fault codes, etc.

4.      Virtual Central Solution

The AC output capacity of the SG125HV is 125kW which is large enough to be a practical “building block” or “power module” for what we will call a Virtual Central Solution.  Since there are string combiner boxes required in the system architecture for the SG125HV, it can be thought of as a “mini central inverter” or as an integrated power module of a Virtual Central Solution.


Fig-3: Virtual Central Solution

The Virtual Central Solution can be customized to suit the needs of the site more effectively than with a central inverter that has a fixed output capacity.  Since the SG125HV can be paralleled in almost any quantity and connected to a single, LV winding of an MV transformer, the capacity of the resulting power station, or Virtual Central Solution, can be set at any multiple of 125kW. 

For example, ten of the SG125HV inverters can be integrated together to make a 1250kW inverter and 20 can be paralleled to make up a 2500kW inverter.  The integration of the ten, or fifteen, or twenty inverters can be done at the site or pre-integrated off-site and brought to the site as a subsystem.  Due to the low weight of the inverter and the small size, it can be mounted to the array structure itself or to a simple, Uni-Strut rack.

The SG125HV was designed with a single DC input and a single MPPT.  There are several reasons for this, but the over-all impact is one of a lower installed cost.

With the 1500VDC SG125HV inverters, the best system architecture is more like a central inverter architecture with the inverters located closer to the point of connection (POC).  As shown in Figure 2, since the nominal DC voltage is closer to 1050V, the DC current is closer to the same as the max AC current of 120A at 600VAC nominal output.  The savings in wire cost comes by way of the two wires on the DC input side vs. the three wires on the AC output side.

A similar case can be made for using a wiring harness/trunk buss instead of a conventional combiner box.

Another reason for not having an integrated combiner box is the size and weight that would have to be added to the inverter.  This would make it too large and heavy for two installers so it would have to be designed in two separate pieces, adding significant cost to the inverter as well as the required mounting structure, and increasing the time to install.  It would force the systems to be a distributed architecture with the AC side being the distribution voltage.

In the bigger picture, the reduction of the number of string combiner boxes needed for a 1500VDC system compared to 1000VDC systems, the savings realized in using the DC wiring for the longer runs, and the ability to centralize the location of the inverters, has the combiner impact of reducing the total installed costs.  The actual savings will vary depending on several variables, but the net effect will be positive in most cases.

Virtual Central SolutionFor the fastest command-response times, the parallel RS485 input normally used for tracker control can be used.  If dedicated to the command and control, this parallel input can achieve response times in the area of 200ms or less.  This type of rapid response time will be required by utilities going forward.

Having one SCADA interface for a large power block, with response times in the 100ms to 200ms range is as good as a central inverter.  A list of some of the other main advantages is noted below.

●    Increased up-time and lower O&M costs mean a higher ROI.

●    Instead of one or two MPPTs for a central inverter power station, the string inverters offer one MPPT per inverter so 20 MPPTs per 2.5MW power station.

●    The status and input/output information for every 125kW inverter can be monitored for a more detailed record of the whole power station performance characteristics

●    Grouping the inverters makes it easier for maintenance and service compared to a distributed architecture where the service personnel have to move from inverter to inverter throughout a large solar array field.

5.      Summary

In summary, the features, capabilities, and the range of system architectures available with the Sungrow SG125HV, it has become one of the most asked for product since it was first introduced at the 2016 SPI show.  The inverter itself as well as the Virtual Central Solution applications will change the paradigm for the string vs. central inverter decision making process from here on.

Sungrow is considering offering a pre-integrated assembly of ten, SG125HV inverters, including all wiring and AC power panel.  This would reduce the field integration time and help ensure a higher quality installation.