In the field of renewable energy quality assurance, proficiency testing (PT) of conversion efficiency for photovoltaic (PV) grid-connected inverters is a critical technical activity to ensure measurement consistency among laboratories. As a practitioner involved in PT scheme design, I have focused on the development of robust sample preparation and validation procedures. This article systematically describes the design basis, key parameter selection, structural design, and verification of uniformity and stability for PT samples, with particular emphasis on understanding different types of solar inverter architectures and their influence on efficiency characterization. The work is based on the Chinese standard NB/T 32004-2018 Technical specification for PV grid-connected inverter and the guidance document CNAS-GL003:2018 Guidance on evaluating the homogeneity and stability of samples used for proficiency testing. I will illustrate the approach using a 6000 W PV grid-connected inverter PT sample as a case study.

Proficiency Testing Sample for Inverter Conversion Efficiency

1. Design Basis and Requirements

1.1 Technical Standard Requirements

The preparation of PT samples strictly follows the core requirements of NB/T 32004-2018, specifically clauses 5 to 9 covering environmental conditions, safety, basic functions, performance, and protection. Among these, conversion efficiency is the paramount performance index. According to the standard, conversion efficiency is defined as the ratio of AC output energy to DC input energy over a specified test period. For a PV inverter, the key efficiency indicators include dynamic MPPT efficiency, static MPPT efficiency, and overall conversion efficiency. Therefore, the sample must satisfy the following test adaptability requirements:

• DC input voltage range: 240 V to 440 V, with emphasis on consistency at 340 V (typical operating point) and 440 V (upper operating point).
• Coverage of all load points specified in the standard: 5%, 10%, 20%, 30%, 50%, and 100% of rated power.
• Quantitative requirements: static MPPT efficiency ≥ 99%, dynamic MPPT efficiency ≥ 95%; power factor ≥ 0.98 when active power > 50% rated power, and ≥ 0.95 when active power is 20% to 50%; total harmonic distortion (THD) ≤ 3% for rated power > 50%; three-phase current unbalance ≤ 2%; DC current component ≤ 0.5% of rated current; capability to operate at 1.1 times rated power continuously; grid voltage tolerance of 85%–110% of nominal voltage and frequency tolerance of 47.5 Hz–50.5 Hz.

1.2 Proficiency Testing Standard Requirements

The uniformity and stability of PT samples are evaluated according to CNAS-GL003:2018. For uniformity, all samples in a batch must exhibit statistically insignificant differences in core characteristic (conversion efficiency). This is achieved by using identical inverter models, same batch of power modules, and MPPT controllers. For stability, the sample must remain unchanged over the entire PT cycle, including transportation, storage, and testing phases. The assessment uses statistical tests such as ANOVA and t-test. Additionally, sample size and weight must be controlled to facilitate economical transportation and ease of handling.

2. Key Parameters and Structural Design

2.1 Selection of Power Rating and Rationale

After evaluating market trends and laboratory capabilities, I selected a 6000 W grid-connected inverter as the PT sample. This power class is widely used in distributed PV applications (e.g., residential rooftop systems and small commercial installations), offering strong representativeness. Moreover, 6000 W is convenient for test bench construction in most laboratories while still reflecting realistic efficiency behavior. The sample belongs to the transformerless types of solar inverter, which is the dominant topology in modern residential and commercial installations due to higher efficiency and smaller size.

2.2 Determination of Electrical Parameters

The following tables summarize the key input and output parameters.

2.3 Efficiency Target Values and Curve

The target conversion efficiency was set based on typical performance of transformerless inverters. The expected maximum efficiency is ≥ 96%, while efficiency at 20% load is ≥ 94%. For the PT scheme, these targets are used to define the assigned value and standard deviation for proficiency assessment. The efficiency curve provided by the manufacturer shows a typical shape for high-power-density MPPT algorithms.

2.4 Structural Design and Topology

The sample adopts a modular architecture consisting of the following stages: photovoltaic input unit, MPPT DC-DC converter, DC bus, inverter bridge (DC-AC), filter and grid-tie unit, and control/monitoring modules. The design follows the principles described in product design manuals. The inverter is transformerless, which reduces weight and cost while improving peak efficiency. This topology is one of the most common types of solar inverter for distributed generation. Protection features include PV insulation detection, residual current monitoring (RCMU), ground fault circuit interrupter (GFCI), and redundant communication interfaces (RS232/RS485, Wi-Fi/GPRS). The enclosure has an IP65 protection rating, dimensions of 410 mm × 445 mm × 210 mm, and a weight of approximately 26 kg, making it convenient for PT logistics.

An example of a practical grid-connected solar inverter system is shown below.

3. Test Requirements and Verification

3.1 Test Conditions and Setup

All efficiency measurements are performed in a controlled environment: temperature 25 ℃±2 ℃, relative humidity 45%–75%, atmospheric pressure 95 kPa–106 kPa. The sample is wall-mounted with brackets and vertical orientation, with terminals pointing downward. Clearance around the sample is at least 500 mm on top, bottom, front, and sides, with the back flush against the mounting surface to ensure proper ventilation. The test platform comprises a PV array simulator (DC source), a grid simulator (AC source), and precision power analyzers for measuring DC voltage $U_{DC}$, DC current $I_{DC}$, AC voltage $U_{AC}$, and AC current $I_{AC}$. The sample’s protective earth terminal is properly grounded before any power application.

3.2 Uniformity Testing and Evaluation

Because the sample is a single-unit design that undergoes repeated measurements, I adopted a full-inspection approach. Two units were procured: one primary sample for circulation and one backup. For uniformity evaluation, each unit was tested at multiple load points (5%, 10%, 20%, 30%, 50%, 100% of rated power) under a fixed MPPT voltage (340 V and 440 V). The data were analyzed using one-way ANOVA. The null hypothesis is that the between-sample variance and within-sample variance are equal. If the calculated F-value is less than the critical value $F_{\alpha}(f_1, f_2)$ at significance level $\alpha = 0.05$, the samples are considered homogeneous. After the samples are returned, a reverse check applies the criterion $S_s \leq 0.3\sigma$, where $S_s$ is the standard deviation of sample means and $\sigma$ is the target standard deviation for proficiency assessment. The following table presents the ANOVA results for the 30% load point at $U_{MPPT}=340$ V.

Similar ANOVA tests were conducted for all load points and both MPPT voltages. All F-values were below the critical threshold, confirming sample uniformity. The reverse check after return also satisfied the $S_s \leq 0.3\sigma$ criterion.

3.3 Stability Testing and Evaluation

Stability was assessed by simulating transportation vibration and harsh storage conditions. The sample was packed according to the PT packaging specification and subjected to random vibration on a vibration table following the acceleration spectral density curves defined in GB/T 4857.23-2021. The test was performed in vertical, lateral, and longitudinal directions for 2 hours each. The key points of the PSD curves are:

After vibration, the sample was placed in a climate chamber simulating a southern warehouse environment (40 ℃, 93% RH) for 48 hours. Efficiency was measured before and after these treatments. The data were analyzed using a two-sample t-test. If the calculated t-value is less than the critical value $t_{\alpha/2}(n_1+n_2-2)$ at $\alpha = 0.05$, no significant change is detected. For the typical load point of 50% power at 340 V, the results were:

The reverse check using the criterion $|\bar{x}_1 – \bar{x}_2| \leq 0.3\sigma$ also confirmed stability. All load points passed the t-test and the reverse check, validating that the PT sample remains stable under realistic transport and storage conditions.

4. Discussion on Types of Solar Inverter and Sample Representativeness

The 6000 W transformerless inverter used in this study represents only one types of solar inverter category. In the broader landscape of solar inverters, we also find string inverters (with or without transformers), microinverters, central inverters, and hybrid inverters. Each type exhibits different efficiency characteristics, MPPT algorithms, and electrical behavior. For PT programs aiming to assess laboratory proficiency across diverse inverter technologies, it may be necessary to design multiple sample categories or to select a representative topology that challenges both measurement accuracy and interpretation of standards. The transformerless type was chosen because it is the most common in new installations due to its high efficiency and low cost, but laboratories should be aware that other types of solar inverter may require specific test configurations (e.g., galvanic isolation, higher common-mode voltage). The sample preparation methodology described here can be adapted to other power ratings and topologies by adjusting the parameter table and validation criteria.

5. Conclusion

This article has presented a comprehensive approach to preparing and validating a proficiency testing sample for conversion efficiency of photovoltaic grid-connected inverters. By defining key electrical parameters, adopting a modular structural design, and rigorously verifying uniformity and stability through ANOVA and t-test criteria, I have demonstrated that the 6000 W transformerless inverter satisfies PT requirements. The methodology not only supports the specific case study but also provides a template for future PT schemes targeting various types of solar inverter. The inclusion of vibration and environmental stress tests ensures the sample remains stable throughout the entire PT cycle. The work contributes to the standardization of PT practices in the PV inverter testing community, thereby enhancing overall measurement quality and regulatory oversight.