Comprehensive test techniques for grid-tied solar inverters

Eric Turner,  Product Marketing Manager, AMETEK Programmable Power, Renewable Energy Initiatives, San Diego, Calif.

The ever-increasing deployment of residential and commercial photovoltaic arrays has created a high demand for small solar inverters, but they must be performance-tested to ensure they are ready for the long haul.

The AMETEK MX is an example of a bi-directional power source that simulates the interconnection of the inverter to the grid under a range of operating conditions.

Taking solar power generating systems from the laboratory to residential and commercial installations requires testing at several points including product validation, production, and installation. As new approaches such as microphotovoltaic (µPV) inverters are introduced, the requirements for testing increase. The first illustration shows several facets of implementing solar power for a residential application. In commercial applications, multiple micro-inverters can be used. This technique improves efficiency and reliability as well as reduces safety hazards.

Inadequate testing can seriously affect any product’s success. Difficult to implement test equipment, testers with inadequate accuracy and repeatability, and equipment that does not easily interface with other parts of the PV system can significantly delay a new product introduction. Three areas of photovoltaic solar technologies that need to be explored further include solar array simulation, house-load simulation, and utility simulation. In addition, continually evolving and newly emerging standards add to the complexity of these areas.

Solar Array Simulation
Solar arrays operate in an uncontrolled environment. That is, the output depends on a range of arbitrary conditions including the intensity of the sunlight (full sun vs. cloudy conditions), ambient temperature, external shading effects (from tree branches or chimneys), dust, bird droppings, and other factors. All of these elements affect the capacity of the solar array to produce power.

The inverter is an integral part of this system and must be designed to accept maximum power transfer from the solar array. This maximum power point (MPP) is most commonly determined on a continuous basis. Most PV inverters are designed to harvest the maximum amount of energy available from the PV array at any time. To do this, they typically use an MPP tracking control algorithm to present the optimum load to the PV array for maximum power transfer.

Inverter testing for this application (during development and in production) requires a power source — a Solar Array Simulator (SAS) — that can reliably simulate actual performance. With hundreds of solar-array panels available in the marketplace, this can be a daunting requirement.

Fortunately, the National Renewable Energy Laboratory (NREL) maintains a Solar Advisor Model (SAM) database that catalogs key parameters such as Voc, Isc, Vmpp at 24° C, and standard 1000-W/m2 isolation for hundreds of commercially available PV products. The SAM provides powerful tools to help designers predict system performance for almost any fill factor or solar material. A solar array simulator with the ability to access this data and incorporate it into a realistic, dynamic, interactive test of the inverter can pay big productivity dividends.

Many solar inverters generate ac ripple on their dc input that is connected to the photovoltaic array. For single-phase inverters, the frequency of this ripple is twice the line frequency (120 Hz for US models). The simulator’s power supplies must not suppress this ripple as a function of their regulation loop.

Grid-tied photovoltaic system

A typical grid-tied photovoltaic system has several different components and voltage levels; each one requires special testing.

An increasing number of inverters (and virtually all micro-inverters) accurately measure amplitude and phase of the ripple voltage and current to track the MPP of the array. This approach allows tracking the MPP at a much higher speed when compared to conventional dithering techniques (also called perturb and observe).

Complete test system

A complete test solution verifies the product’s capabilities and ability to meet the numerous industry standards as well as the product’s datasheet performance requirements at every point in the solar array system.

Faster tracking of the MPP results in a much higher overall efficiency in cloudy conditions where the irradiance is constantly changing. It is likely that all solar inverters will use this approach in the near future, since end users are very sensitive to the overall efficiency of their solar energy installations. To satisfy this requirement, the PV simulator must be capable of reproducing the voltage and current behavior of a solar array even in the presence of this ripple. Another requirement of this process is the ability to simulate the MPP for multiple strings of solar panels because most installations use a large number of panels.

House Load Simulation
Simulating residential loads is an area that requires special test considerations. For example, if the grid-tied inverter is delivering 5 kW and the home is consuming only 3 kW, how does the inverter respond to high crest factor (HCF) loads? (HCF is the ratio of the peak value to the root-mean-square (rms) value of a waveform.) This can occur with the switching power supplies in TVs, computers, microwaves, and even during the time a refrigerator turns off and on. Products with switching power supplies can also be a source of harmonic distortion. How do all these different types of loads affect the PV grid-tie inverter? A load simulator provides the answer for testing.

The AMETEK TerraSAS is shown simulating the photovoltaic (PV) dynamic solar irradiance and temperature characteristics for multiple panel strings over a wide range of conditions.

Utility Simulation
Utility simulation is among the newest testing requirements. There are very few established standards but several areas of concern to utility companies include:

• Anti-islanding
• Dc injection
• Utility anomalies, including phase loss, voltage dips and interruptions, and frequency disturbances
• Harmonically enriched waveforms test inverter tracking capability
• Testing for interharmonic susceptibility

One of the problems that can crop up if the connection to the utility grid is not properly controlled is a situation called islanding. As defined in the IEEE 1547 standard, islanding is “a condition in which a portion of an Area Electric Power System (EPS) is energized solely by one or more Local EPSs through the associated point of common coupling (PCC) while that portion of the Area EPS is electrically separated from the rest of the Area EPS.” Because unintentional islanding of a distributed power source may cause power quality problems, interference with grid protection devices and other problems, an anti-islanding function in equipment ensures that the electrical islands are detected and properly disconnected from the electric power system.

The standards state that when the grid supply is lost, the inverter must turn off within a specified amount of time and the voltage rise must be limited. Tests to verify these capabilities are just being developed. Custom software as well as hardware is required to perform specific anti-islanding tests.

Utility companies actually have a small, 300 to 500 mV dc component on their ac power, so testing for dc injection is one of the required tests. Simulating the utility mains requires adding the dc component. This testing determines how an inverter reacts to the dc offset. Without proper design, the inverter’s power output could drop to half. The IEC committee realizes that these conditions can occur and is working feverishly to develop test standards to cope with various situations.

To produce the voltage levels, distortions, dips, interrupts and other anomalies that end products normally experience while operating off the utility line power, the power source used in product testing requires either manual or computer programming capability. These immunity tests evaluate a product’s ability to withstand common public supply disturbances. Additional tests are required to measure emissions or the disturbance contribution that the product itself may produce. Accomplishing both requires a clean ac power source that supplies and receives power from the product being tested. The latter requirement defines a regenerative system.

The importance of odd and even harmonics and their affect on the power grid is well known, however, interharmonic susceptibility and distortion is a rather recent development. Values between the integer harmonics, such as the 2.6 or 3.5 harmonic, can cause problems in some common products such as microwave ovens and washing machines. For example, the safety switches in these products can be affected by certain interharmonic values and not function properly.

All of these aspects must be addressed in a solar inverter. Testing the inverter to verify its capability and establish its performance levels requires programmable power. Surrounding the inverter with programmable devices to simulate the output of solar arrays, applying loads to the output of the inverter, and interfacing with the grid provides a comprehensive and energy efficient means of testing these devices. The second figure is an example of a comprehensive testing system.

In many cases, products are being developed concurrently with the standards. As a result, it is necessary to bridge the gap between standards and the testing that must be performed to meet them. This can be accomplished by working with a supplier that is closely tied into the standards development agencies and their subcontractors.

AMETEK Programmable Power