The use of grid-tie inverters (GTIs) in solar energy systems is emerging as a major application for highly repeatable ac-power sources. These power sources find use in both the design phase and production testing to confirm the inverters’ ability to withstand variations in utility line power and demonstrate conformance to applicable standards. Yole Development, a leading market research firm, predicts the photovoltaic inverter market to more than double over the next five years.
Test issues dictate the need for power-source features that make testing easier as well as more accurate with greater repeatability. Furthermore, the environmental and economic impact of wasting electrical energy demands considerable attention for the reduction of energy consumption. Both aspects define the requirements for an advanced power source.
Today’s Power Source Requirements
Since the utility line power in most industrialized nations typically has distortion levels of 3% to 5% with voltage fluctuations and dips easily exceeding 10% on an almost daily basis, an alternative power source is required for these tests. To further complicate the testing process for global products, the variations of utility voltage, ranging from 120V/60 Hz in North America to 220V/230V at 50 Hz in most of Asia, South America, and Europe, or 100V at 50/60 Hz in Japan, make programmability essential to the power source.
To produce the voltage levels, distortions, dips, and interrupts that end products normally experience while operating off the utility power line, the power source used in product testing requires either manual or computer programming capability. While these immunity tests evaluate a product’s ability to withstand common public supply disturbances, additional tests are necessary to measure emissions or the disturbance contribution that the product itself may produce. Accomplishing both requires clean ac power sources that supply power and receive power from the product under test. The latter requirement defines a regenerative system.
Regenerative Mode Operation
In the regenerative mode, an ac-power source can accept and sink (SNK) power returning from any connected equipment to the utility grid. This power return can be a short-term event or a semi-permanent condition. Turning off an electric motor or a reactive load can create a short term situation. In contrast, a solar power or wind power-based inverter supplying power back to the source is essentially a semi-permanent condition. For efficient ac line simulation, products like AMETEK’s MX Series programmable power sources use switch-mode technology instead of a linear design to avoid the excess heat and provide higher efficiency.
A solar inverter producing sufficient power can feed power continuously back to the source. When the power level cannot cover the load demand, the direction of power flow can change dynamically, even on a half-cycle by half-cycle basis. A power source with SNK capability can address the continuous, intermittent or half-cycle situations as well as short-term events in the power flow applications.
Using the MX Series with the SNK option as an example, the current limit for power sourced by the MX can be set to 40A with the regenerate control state OFF, while the maximum current that is returned by the MX to the utility could be set to 10A with the regenerate control state ON. In regenerative mode, the current limit functions exactly opposite the normal operating mode of a power source. Instead of reducing the voltage to limit the current, the MX will increase its voltage level to the user-programmed overvoltage limit.
To meet the rising demand for electricity, utilities can acquire surplus energy from photovoltaic systems, micro turbines, fuel cells, and other local generating technologies. However, the performance, operation, testing, and safety of interconnection products and services must meet the requirements of IEEE 1547. A programmable power source in this application must provide a means to interconnect an electric power system (EPS) with a distributed resource (DR) such as a solar panel’s photovoltaic inverter (see fig. 1) as well as repeatedly perform the testing required by the standard.
An incorrectly established interconnection can incur a problem situation known as islanding. As defined in IEEE 1547, 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. Since unintentional islanding of a distributed power source may cause power quality issues, interference with grid protection devices, and other problems, an anti-islanding function in equipment ensures the detection of electrical islands and proper disconnection from the electric power system. The MX operation with the regenerate state ON supports the balanced-mode anti-islanding test required by IEEE 1547 and other standards such as UL 1741and CA Rule 21.
A Solar Power Example
Figure 2 shows an integrated test set up for measuring an inverter’s performance. We sometimes refer to the phase-to-phase 240V/60Hz configuration, a 240V delta without a neutral tap, as the stinger mode in US systems. This testing provides measurements similar to those obtained from European or Asian 220/230V-50Hz single-phase systems. The similarity allows the use of general data and eliminates the need for duplicate screens for various worldwide power systems.
In this example, we measure power flow in each of the three legs. When the inverter is not powered or synchronized to the 240 Vac from the MX45-3Pi, power is supplied only from the MX to the load. With power from the DC source or a solar panel, the inverter comes on line, synchronizes and begins to supply power.
Eric Turner is the Product Marketing Manager and head of Renewable Energy Initiatives at AMETEK Programmable Power. His career includes ten years of International sales in over 45 countries and extensive experience with design and test of sophisticated missile guidance systems. Eric can be contacted via email at [email protected].