Power Inverter Certification According to Standards and Grid Codes

California-based EPC Power is a cutting-edge power conversion company specializing in utility-scale inverters ranging from 1 MW to 6 MW. The systems range from 250 kW to 100 MW. They suit various applications such as:

• Microgrid

• Uninterrupted Power Supply (UPS)

• Black Start - restarting after blackouts - in parallel with other generation sources.

The issue

EPC power conversion systems are used throughout the world by different governing bodies. Engineers put them to the test through the most extreme weather conditions. 

With the increasing number of renewable energy resources often interconnected to the electrical grid, these power systems, known as Variable Energy Resources (VERs), are rapidly increasing. They often interconnect to Bulk-Power System (BPS) facilities. VERs also commonly apply as Distributed Energy Resources (DERs) widely distributed by geography, unlike traditional power generation resources.

There is a dizzying array of standards for inverters and power converters around the world. Because EPC Power sells PV inverters internationally, its products must be certified to North American standards (UL 1741, IEEE 1547, and CSA 22.2) as well as Australian and European safety standards and grid codes, including IEC 62109 and VDE) and quality standards, including ISO 9001:2015. 

Keeping up with all these standards is a challenge for EPC Power engineers. They needed a new power measurement data acquisition (DAQ) system capable of handling a broad range of power testing and performing complex calculations based on the incoming voltages and currents. However, I should describe the full scale of challenges before presenting the solution. 

We depend on reliable power

Most of the world takes power for granted. We wake up this morning, turn on the lights, use our electric toothbrush, shaver, and hair dryer, and then make breakfast in the kitchen. We rarely wonder if the power is going to work. We are so conditioned to reliable power that when a storm knocks down the power lines, it is a significant disruption. There is a mad dash to the hardware store to buy generators to keep our refrigerators running. 

There is no tolerance for unreliable products in the power generation and conversion markets. Power conversion systems must be validated, which includes rigorous environmental testing in extreme weather conditions. People in Arizona assume that power conversion systems will work in the desert heat, just as those in northern Minnesota expect them to work at -40°. They simply have to work, no matter what.

Different grids around the world

In 1891, Westinghouse engineers in Pittsburgh, Pennsylvania, were the first to define a grid frequency and chose 60 Hz. General Electric and Westinghouse had been debating the optimal power frequency. Neither frequency had a compelling advantage, but they selected 60 Hz because it “felt less likely to produce annoying light flicker.”

Later that year, the German company AEG (descended from a company founded by Edison in Germany) decided to build the first German power-generating facility to run at a 50 Hz fundamental frequency because it was said to be more “metric-friendly” than 60 Hz. These early decisions are what created different grid characteristics around the world. Figure 1 illustrates these differences.

International travelers immediately notice that the power plugs and receptacles differ on different continents. Today, to accommodate the grids globally, most AC appliances and AC/DC chargers have frequency/voltage ranges between 50-60 Hz and from 100 to 240 VAC. We can take our notebook computers and electric razors to any world country and use them with a simple and inexpensive plug adapter.

A variety of norms and standards

The range of different norms around the world also challenges the power industry. This situation arose due to the historical power implementation and its regulation by diverse governing bodies.

Today, we have different physical power combinations for the fundamental grid operation, and in addition, we have hundreds of divergent regulatory bodies. Nearly every country, or country union, has its own regulatory body defining the standards applicable for selling a product in that respective country or region. Each regulatory body can reference international, regionally, or nationally based standards. 

There are hundreds, if not thousands, of such bodies at the national level. From a manufacturer’s perspective, this introduces tens of thousands of combinations of compliance validation tests for export, international, and intercontinental sales.

Much of the time spent in the power conversion product R&D, the engineers spent validating compliance with standards and working with standardization bodies to approve products on the world stage.

The following is a list of the most common international standards that pertain to the power industry:

• American Society for Testing and Materials (ASTM International)

• Institute of Electrical and Electronics Engineers (IEEE)

• International Commission on Illumination (CIE)

• International Electrotechnical Commission (IEC)

• International Organization for Standardization (ISO)

• National Electrical Manufacturers Association (NEMA)

• Society of Automotive Engineers (SAE International)

• Underwriters Laboratories (UL)

In general, these standardization organizations have separate ways of testing and evaluating pass/fail criteria for various critical conditions of the grid. Many of these conditions sum up to the following:

• Voltage Ride Through (VRT)

• Low Voltage Ride Through (LVRT) – Sometimes referred to as Under Voltage Ride Through (UVRT)

• Overvoltage Ride Through (OVRT)

• Frequency Ride Through (FRT)

• Under Frequency Ride Through

• Over Frequency Ride Through

• Rapid Voltage Changes (RVC)

• Flicker

• And more

Key power parameters to be validated

In response to these conditions, the standardization organizations have defined criteria to evaluate Voltage (V), Current (I), Active Power (P), Reactive Power (Q), Apparent Power (S), Symmetrical Components (Positive, Negative, Zero Sequence), et al, regarding the component or system under test. As defined by the regulatory bodies, the Variable Energy Resources (VERs) must be able to do some or all of the following:

• Disconnect and stay disconnected until manually reconnected.

• Disconnect temporarily from the grid, then reconnect and continue operation after the event.

• Stay connected, and do not disconnect from the grid.

• Stay connected and support the grid with reactive power known as transient reactive

The solution

EPC Power uses two Dewesoft SIRIUSi-XHS-4xHV-4xLV high-speed power analyzers to synchronously measure two 3-phase AC power systems plus their DC components. The SIRIUS XHS platform performs continuous or triggered recording up to 15 MHz. Inverter testing is rigorous, and the SIRIUS XHS is a powerful tool for capturing and evaluating high-speed transient signals. 

The electrical power software module in DewesoftX provides engineers with many options for real-time computation of power, power quality metrics, or recalculation of metrics in a post-processing environment. The combination of acting as a transient recorder, oscilloscope, power meter, and power quality meter gives the user the ultimate flexibility.

Measurement procedure and setup

EPC Power has an extensive test lab where they evaluate the product performance under various conditions. This lab contains many components, including big-scale power supplies, load banks, etc. All these can simulate realistic Voltage Ride-through states, Frequency Ride-through states, and other conditions that may occur in the field, and engineers need to evaluate in qualification.

Figure 5 shows the measurement point of the three-phase system. Behind the wall resides the state-of-the-art, proprietary testing instrumentation engineers use to simulate these field conditions. This configuration allows EPC power to quickly and effectively swap out products for evaluation.

Figure 6 shows the instrumentation cart that contains two SIRIUS XHS measuring instruments. Each has four high-voltage inputs and four low-voltage inputs. The two SIRIUS XHS instruments are synchronized and connected to the measurement PC via the ethernet connection. This connection allows you to locate the computer far from the test cell.

Flexible, user-configurable displays

The Electrical power module in DewesoftX creates an overview screen that is customizable to fit the needs of each independent test. Graphical widgets such as digital meters, recorders, scopes, vector scopes, tables, FFTs, harmonic FFTs, and many more make it easy to adjust the software to fit the unique needs of every test.

On-line and off-line mathematics

The Power Module has a simple-to-use interface for DC, single-phase, 2-phase, 3-phase (Star, Delta, etc.), or any combination thereof. Line frequencies can be specific or variable to fit any power system. Power parameters can be calculated based on an exact number of cycles, as defined by IEC and IEEE, and additionally on ½, 1, 2, or 4-period basis. In addition, the module allows defining overlaps for a smoother output.

With the check of a box, the power module can calculate power quality metrics and parameters such as phase angles, P, Q, impedance, and inter-harmonics individually at the harmonic level. 

This feature enables a critical understanding of content outside the fundamental frequency harmonic-by-harmonic evaluation. The DewesoftX Power Module has predefined calculations for harmonic distortion, symmetrical components, Rapid Voltage Changes (RVC), voltage flicker, and flicker emission.

The DewesoftX math module allows engineers to define any other math function not included in the power module by default. Such formulas, as shown in Figure 7, are very helpful in changing units to look at percentages, calculate efficiencies, measure the timing of waveform characteristics, etc. 

The DewesoftX math module works hand in hand with the power module, which allows you to do custom calculations and runs in real-time and during post-processing.

Controlling setpoints over CAN network

One pain point for many of these types of tests is controlling setpoints. Historically, engineers have written third-party programs to send commands to controllers to sweep through several setpoints to acquire the appropriate data. The SIRIUS-XHS modules have an integrated Controller Area Network (CAN) port, which allows the user to define a test script in Dewesoft and then send setpoints to any controller capable of CAN communication. 

In DewesoftX, the math channel showing the “wanted values” is easily created and simply assigned to transmit over the CAN network. DewesoftX will directly export the DBC file for the controller to decode the message, thus supporting seamless integration of the data acquisition and control of the test bench.

Summary and conclusion

As EPC Power blazes a new path forward for power conversion technology, Dewesoft’s power analysis tools are continuously advancing to keep up and support the latest trends in test and measurement. Dewesoft’s easy-to-use software interface and advanced power analysis tools make it the perfect fit to accommodate the testing requirements dictated by all the various organizations, governing bodies, and other regulations. 

Voltage or Frequency Ride Through Testing, RVC, and Flicker are just a few of the ways that thousands of out-of-the-box calculations for certification or R&D. Pairing the pre-defined calculations from the power module with Dewesoft’s advanced math module makes Dewesoft an invaluable tool for any R&D in the power market. 

Finally, by executing setpoints, load profiles, or other controls from the Dewesoft environment through CAN, MODBUS, or other protocols, the test setup is greatly simplified by removing many unnecessary process components.

Dewesoft and EPC Power look forward to a brighter, more powerful future as we continue to collaborate and innovate new technology related to power analysis.

Learn more about EPC Power Corporation.

Learn more about Dewesoft power and energy test and measurement systems.