Photovoltaic Grid-connected Inverter Testing

Photovoltaic grid-connected inverter inspection mainly conducts scientific testing and analysis on the power quality testing, protection function testing, efficiency and power characteristic testing, electromagnetic compatibility testing, environmental adaptability testing and safety regulation testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Power quality testing: Mainly evaluates whether the output power of the inverter meets the grid connection requirements. The core items include the voltage and frequency adaptation ranges, total harmonic distortion rate and the contents of the various harmonic orders, DC component injection limits, power factor control ability, as well as the voltage fluctuations and flicker. These indicators are directly related to the stability of the power grid and the safe operation of the other electrical equipment.

Protection function testing: Verifies the protection and response abilities of the inverter under the grid abnormalities or faults. The key testing items cover the over/under-voltage protection, over/under-frequency protection, anti-islanding protection, low voltage ride through, over-current protection and DC side reverse polarity protection. These functions are the key barriers guaranteeing the safety of the power grid and the photovoltaic system itself.

Efficiency and power characteristic testing: Measures the energy conversion efficiency of the inverter converting the DC power into the AC power. Including the maximum conversion efficiency, European efficiency, China efficiency, MPPT efficiency, power-voltage characteristic curves, rated output power and overload capacity. The efficiency indicators directly affect the power generation and economic benefits of the photovoltaic power station.

Electromagnetic compatibility testing: Evaluates the operation stability of the inverter in the electromagnetic environments and its electromagnetic interference levels to the power grid. Divided into the electromagnetic emission testing (conducted disturbance and radiated disturbance) and immunity testing (electrostatic discharge, surge immunity and electrical fast transient burst immunity, etc.), ensuring that the equipment works reliably in the complex electromagnetic environments and does not interfere with the other equipment.

Environmental adaptability testing: Inspects the long-term operation reliability of the inverter under the different environmental conditions. Mainly including the high and low temperature operation and storage tests, damp heat testing, salt spray corrosion testing, vibration and shock testing, and protection grade verification. These tests simulate the harsh challenges that the inverter may face in the actual installation environments (such as the rooftops and outdoors).

Safety regulation testing: Ensures that the inverter meets the basic personnel and equipment safety requirements. The main items include the clearances and creepage distances measurement, insulation resistance testing, power frequency withstand voltage testing, leakage current testing, grounding continuity testing, as well as the enclosure protection and mechanical strength inspection. It is the mandatory inspection that must be passed before the product launch.

Testing Scope

Classification by power grades: The testing covers the full series of products from the micro inverters (hundreds of watts to several kilowatts), string inverters (several kilowatts to hundreds of kilowatts) to the central inverters (hundreds of kilowatts to the megawatt level). The inverters of the different power grades have the different testing emphases and standard requirements. For example, the large-power models focus more on the grid support ability.

Classification by grid connection types: Including the testing of the single-phase grid-connected inverters and three-phase grid-connected inverters. The single-phase models are mainly used for the residential and small commercial systems, with the testing focusing on the harmonics and anti-islanding protection; the three-phase models are used for the industrial, commercial and ground power stations, and in addition to the basic items, the three-phase unbalance degree, etc. also need to be tested.

Classification by topologies: The testing objects cover the inverters adopting the different technical routes, such as the types with the power frequency transformers, high-frequency transformers and without the transformers. The transformerless types are widely used due to their high efficiencies, but their DC component suppression and insulation requirements are the emphases and difficulties of the testing.

Key components and software evaluation: The testing scope is not limited to the whole machine, but also extends to the core components, such as the IGBT/MOSFET power modules, DC bus capacitors, filter inductors and MPPT control circuits. At the same time, the control algorithms, communication protocols (such as Modbus and SunSpec) and firmware functions of the inverter are also verified.

Full life cycle coverage: The testing covers the type tests in the product research and development stage, routine inspections before the ex-factory, on-site acceptance tests after the installation and the periodic performance monitoring during the operation. The testing purposes and item depths of the different stages are different, jointly guaranteeing the quality of the inverter in the whole process from the "birth" to the "retirement".

Specific application scenarios: For the special environments or applications, the testing scope is extended in a targeted manner. For example, the inverters used for the offshore photovoltaic platforms need to strengthen the salt spray corrosion and vibration testing; the hybrid inverters supporting the energy storage systems need to add the special tests such as the charging and discharging conversion and off-grid operation modes.

Testing Methods

Steady-state performance testing method: Under the simulated grid standard conditions in the laboratories, the programmable AC power supplies, DC power supplies and precision power analysers are used to build the test platforms. The inverter is made to operate stably at the rated power and the different load points (such as 5%, 10%, 25%, 50%, 75% and 100%), the data are acquired for a long time, and the steady-state parameters such as the efficiency, harmonics and power factors are calculated.

Dynamic response testing method: Evaluates the response speeds and characteristics of the inverter to the grid disturbances or its own command changes. The transient events such as the voltage swells/dips and frequency changes are created through the grid simulators, and the output voltage and current waveforms of the inverter are recorded with the high-speed oscilloscopes or recorders, analysing its dynamic adjustment times, overshoots and recovery stability.

Anti-islanding testing method: When the inverter operates in the grid-connected state, the grid simulator is suddenly disconnected to create an "island" state. Through the combination of the passive methods (monitoring the voltage/frequency offsets) and active methods (injecting the micro disturbances such as the frequency offsets and current harmonics), it is verified whether the inverter can accurately detect and quickly disconnect the connection with the grid within the specified time (usually <2 seconds).

Low voltage ride through testing method: Simulates the scenario of the voltage dips caused by the short-circuit faults in the grid. The grid simulators are used to generate the voltage dip waveforms conforming to the standard curves (such as the requirements of the Chinese NB/T 32004), testing whether the inverter can maintain the grid connection during the voltage dips and provide certain reactive current support, helping the grid recovery.

Electromagnetic compatibility testing method: Carried out in the semi-anechoic chambers or shielded rooms. The emission testing uses the receiving antennas and spectrum analysers to measure the radiated and conducted noise during the inverter operation. The immunity testing uses the electrostatic discharge guns, surge generators and other equipment to apply the disturbances to the inverter, observing whether its functions or performances are degraded, judging its anti-interference ability.

Environmental stress testing method: The inverter is placed in the programmable high and low temperature damp heat test chambers, and the cyclic tests are carried out according to the temperature curves (such as -25 ℃ to +60 ℃) and humidity conditions specified in the standards. At the same time, the mechanical stresses during the transportation and operation are simulated on the vibration tables. After the testing, the electrical property re-tests are immediately carried out, evaluating its environmental tolerance.

Testing Standards

Domestic Standards:

1. GB/T 37408, Technical requirements for photovoltaic grid-connected inverter

2. NB/T 32004, Technical specification of photovoltaic grid-connected inverter

3. GB/T 30427, Inverters for use in grid-connected photovoltaic generating system — Technical specification and test methods

4. GB/T 19964, Technical requirements for connecting photovoltaic power station to power system

International Standards:

1. IEC 62109-1, Safety of power converters for use in photovoltaic power systems — Part 1: General requirements

2. IEC 62116, Utility-interconnected photovoltaic inverters — Test procedure of islanding prevention measures

3. IEC 61727, Photovoltaic (PV) systems — Characteristics of the utility interface

4. IEEE 1547, IEEE Standard for interconnection and interoperability of distributed energy resources with associated electric power systems interfaces

5. EN 50530, Overall efficiency of grid connected photovoltaic inverters

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Grid simulator and AC load: The core equipment, used to simulate the various normal and abnormal grid working conditions and provide the adjustable AC loads for the inverter. It is required to have the high power output capacity, fast programming response and low harmonic distortion, and can precisely reproduce the complex waveforms such as the voltage dips and frequency changes.

Photovoltaic array simulator: Replaces the real photovoltaic modules to provide the DC inputs for the inverter. It can simulate the I-V curves under the different irradiances and temperatures, and can quickly simulate the I-V curve changes caused by the cloud shading, etc., used to test the MPPT dynamic performance and efficiency of the inverter.

Power analyser: The key instrument of measuring the electrical parameters. It needs to have the multi-channel synchronous measurement, high bandwidth, high precision (usually required above 0.1%) and harmonic analysis functions, used to precisely measure the input/output powers, voltages, currents, power factors, frequencies and the harmonic contents up to the 50th order of the inverter.

Oscilloscope and recorder: Used to capture and analyse the transient and dynamic event waveforms. It needs the high sampling rates, deep storage depths and multi-channel isolated probes, so that when testing the low voltage ride through, switching transients and protection actions, the voltage and current details at the millisecond or even microsecond levels can be accurately recorded.

Electromagnetic compatibility testing system: Including the receiving antennas, spectrum analysers and EMI test receivers for the radiated emission testing; line impedance stabilization networks for the conducted emission testing; as well as the complete sets of equipment such as the electrostatic discharge generators, lightning surge generators and electrical fast transient burst generators for the immunity testing.

Environmental testing equipment: Mainly including the high and low temperature damp heat test chambers (temperature ranges often needing -40 ℃ to +85 ℃), salt spray test chambers, vibration test tables and shock test tables. These equipment are used to assess the adaptability of the inverter to the environmental stresses such as the temperature, humidity, corrosive atmospheres and mechanical vibrations.

Safety regulation tester: Integrating the functions such as the withstand voltage testers (output 0-5 kV AC/DC), insulation resistance testers (usually 500 V/1000 V) and grounding resistance testers. Used to efficiently and safely complete the various mandatory testing items required by the electrical safety specifications.