What the experts say

Consultancies DNV GL and Sgurr Energy have both undertaken independent technology review of Huawei’s smart inverter technology. Here, the main findings of their analysis are presented.

DNV GL reviewed the inverter series manufactured by Huawei and configured for sale in the United States. It looked at the following string inverter products:

  • SUN2000-25KTL-US
  • SUN2000-30KTL-US
  • Smart Logger

The Huawei SUN2000-25KTL-US and SUN2000- 30KTL-US String Inverters were designed and configured for North American markets, and are based on the SUN2000-40KTL model. With approximately 683MW of international projects using the similar SUN2000-40KTL, the North American models should benefit from the Huawei experiences from other regions.

Huawei has recently started to deploy smart PV inverters in the US. The SUN2000-25KTL-US and SUN2000-30KTL-US String Inverters are the first inverters designed by Huawei specifically for the North American market. The compliance testing for these products to UL 1741, CSA–C22.2 and UL 1699B has been completed, making installations in the US possible.

Technical evaluation

Overview: The Huawei series of field-installed smart PV inverters presently includes 12 primary models ranging in power rating from 8kVA to 40kVA continuous AC output power. These 1,000V (DC) maximum input voltage inverters operate with a three-phase output and are designed for outdoor larger scale PV plant operation.

Topology: The 25kW model includes three separate MPPT circuits configured such that each pair of the six PV circuits is controlled by a separate MPPT algorithm. The AC side of the inverter includes an EMI filter, AC Surge Protection Device (SPD) and a relay to provide AC isolation. The design provides a DC disconnect switch, DC Surge Protection Devices (SPDs), EMI filtering and input current monitoring on the PV side of the inverter. An isolation contactor or relay is not provided as a means of automatically disconnecting the inverter from the DC energy source in the event of an inverter fault.

Environmental characteristics: The Huawei inverters are listed in their data sheets as having a temperature rating from -25° to 60°C, and 0-100% relative humidity. All models of the Huawei smart PV inverters are designed for outdoor installations. DNV GL is very positive about the use of natural convective cooling without external fans as an appropriate feature for the Huawei string inverter products. Huawei does employ internal fans to circulate the air intermittently when the internal enclosure temperature is high. Huawei reports that based on the low number of fan operating hours due to its intermittent use, and the characteristics of the fan specified, the calculated life of the internal fans is greater than 25 years.

Power quality and grid support: The Huawei smart PV inverters also implement the interface to the utility grid. This includes functionality for providing high power quality and grid protection. The SUN2000- 25KTL-US inverter will deliver rated real power from unity to +/- 0.91 power factor. DNV GL views positively that the Huawei smart PV inverters have reactive power capabilities which may become important for larger installations in North America.

CEC ratings: An important efficiency measurement is a blended average that is used in California and across North America developed by the California Energy Commission (CEC). The inverter test protocol developed by the Energy Commission to determine inverter performance data shall be used along with the UL certification for safety and reliability. Huawei lists the CEC efficiency for the two models under review as 98.0%. DNV GL verified the listings; an average CEC efficiency of 98.0% for a 25kW or 30kW string inverter places the Huawei inverters among the best regarding inverter efficiency performance.

Maximum Power Point Tracking (MPPT): The MPPT is the algorithm that is implemented in a PV inverter to capture as much energy as possible from the PV array by continuously operating at the highest power level. The data sheets for the two models under review indicate a MPPT range of 560 to 850VDC. Huawei states that the inverter can start at a DC voltage of up to 1,000 volts. The DC voltage will then be pulled-down to operate within the MPPT range.

Quality and reliability

High reliability of PV inverters is critical for the successful operation of PV power plants. Inverter reliability has historically been an issue and is an important focus in the supplier decision making process by PV inverter customers.

Design for reliability: The SUN2000-25KTL-US and the SUN2000-30KTL-US are both based on the SUN2000-40KTL international model, which has been shipped in significant volume. Compliance requirements and differences in the application of the inverters created the necessity for design changes to the North American model. DNV GL recognises that when a new model is based on a redesign or modification of an existing inverter, the design process is sometimes altered appropriately to focus on verifying and validating only the specific design changes. That being said, the field learnings and collected field and laboratory data from the legacy designs should support improved practices when new versions are delivered.

Component selection and de-rating: In addition to designing for low stress on components, the individual component selection is critical. One area that can have a significant negative impact on the reliability and lifetime of an inverter is the use of components with known wear-out mechanisms. This typically includes mechanical components such as fans. DNV GL views the use of natural convection cooling with no cooling fans positively based on the impact on product reliability.

Product testing: Huawei has an extensive set of reliability testing laboratories known as the Huawei Global Compliance Testing Center. Inside these facilities there are a host of environmental chambers for Thermal Cycling, Damp Heat, Ultraviolet, Humidity Freeze, Dust, Salt Fog and other testing needs. DNV toured these facilities as part of the site visit and witnessed a number of ongoing tests in the chambers for both inverter and other product devices. The Huawei GCTC facility is quite impressive.

Field failure analysis: The Failure Reporting and Corrective Action System (FRACAS) is Huawei’s primary tool for identifying root cause for field failures and investigation. DNV was shown an example of how Huawei uses this system for driving to root cause on a manufacturing defect which resulted in field issues. The root cause of the issue was determined and multiple changes to product handling, tooling, and manufacturing were implemented to drive the defect out of the system design. DNV supports the use of such a system for identifying root cause and driving process.

Summary

DNV GL reviewed the product development and verification data provided by Huawei and found the engineering methods generally acceptable as sound product development practices. The string inverter characteristics and specifications appear to align adequatel

SgurrEnergy

SgurrEnergy was commissioned by Huawei Technologies Deutschland GmbH to conduct a detailed independent technical review of the SUN2000 series string inverter.

Overview

The ethos of the SUN2000 string inverter series design is to minimise maintenance with high reliability and efficiency. The SUN2000 is a transformer-less grid-tied PV inverter which converts high voltage DC power into sine wave three-phase grid-frequency AC. The absence of a transformer leads to higher efficiency, reductions in weight and size and a lower cost due to the reduced number of components.

The component parts of the SUN2000 series inverter system are located in a single aluminium cabinet with integral heat sink. The cabinet is IP65 rated for outdoor use which means the electronic components are protected against dust and water ingress which reduces the likelihood of damage to components.

The MPP voltage range of the inverters is wide meaning multiple system configurations are possible.

Inverter circuit layout

The SUN2000 is a compact 40-55kg single aluminium cased unit. It comes with an integral heat sink and attachable wall mount. There is a door mounted LCD graphics capable display with additional monitoring. DC voltage, DC Current, AC Voltage, AC power, yield and temperature can be accessed and displayed in either daily or monthly values. Faults are logged in the alert section of the display. This gives a high spatial resolution of the PV system and inverter performance, allowing rapid determination of faulty areas.

The design of the inverter provides a safe and efficient means of converting DC power to AC. The inclusion of three or four dependent MPPTs increases the efficiency of the inverter in multiple operational environments.

Inverter components

The manufacture of inverters requires use of sub-components and therefore the selection, supply and control of these parts plays an important role in the overall design and assembly process. Huawei implements a supply chain incorporating five supply centres, one in China and four in other territories. The main components for the SUN2000 inverter series are sourced from USA, Hungary and China through established supply chains.

A large number of components are manufactured in China with the remainder manufactured by established manufacturers with strong supply chains. This gives confidence that supply chain issues will be minimised. A review of key components was undertaken and [Sgurr] believes that the components within the inverter are of good quality and suitable for use within this inverter.

Performance

Efficiency: Huawei claims a maximum efficiency across the range from 98.5% to 98.7%. This has been measured according to IEC 61683 standards specified within an inverter technical standard prepared by China General Certification (CGC). This efficiency range is at the upper end of comparable products. Independent testing by Photon confirmed the efficiency profile of the inverter according to the manufacturer’s claims.

MPPT: The Huawei SUN2000 series has a maximum of four maximum power point trackers (MPPT) for increased versatility when connected to different module types, outputs, quantities or arrangements. MPP tracking is utilised to maximise the power from a PV array. The tracking algorithms operate by continuously monitoring the voltage and adjusting the load impedance to establish the most efficient operating voltage for each input. The MPPT concept is acceptable and in line with industry practice.

Summary

Overall, the reliability of Huawei inverters is reasonably well proven, with a significant amount of data provided for review including operational data and overall recorded failure rates even though the inverters have not been in production for a significant period of time. Whilst inverters are typically expected to last for at least half the project lifetime of a PV plant (25 years), considering a bathtub failure rate and relatively low current failure rate for the initial period, as well as accelerated lifetime tests completed on the inverters, [Sgurr] is satisfied that the inverters have a good level of reliability.