As we have discussed, most PV systems contain power conditioning units or inverters. In addition, in order for any PV system to be connected to the utility grid, there has to be a set of test standards and codes to govern the interconnection process for a safe and reliable power delivery. In this section, we will discuss main interconnection standards that relate to PV systems such as IEEE, UL, and NEC standards. Solar professionals and designers should always look for the most up-to-date standards in this regard and consult with the local AHJ for any additional legislation.
Main Interconnection Standards
IEEE 1547 is a standard for interconnecting distributed resources with electric power systems. IEEE 1547 contains a family of standards, guides, and recommended practices. Solar professionals and designers should consult with all series of IEEE 1547 standards.
UL 1741 is the testing standard related to DG equipment such as inverters and charge controllers. It is considered a supplemental standard to IEEE 1547. UL 1741 is important because it is listed in NEC article 690.
NFPA 70 (NEC Article 690)
In addition to the sizing requirements we discussed in Lesson 8, NEC Article 690 requires that all inverters be listed and identified for interactive operation. Most requirements are based on equipment testing under UL 1741. Inverters must meet anti-islanding and disconnect from the grid when voltage is lost and must remain disconnected until grid voltage is restored to the accepted measure.
Interconnection Technical issues
All DG, including PV systems, introduce additional power at the customer location that has not been planned to exist when it was first designed. With this new addition, some technical issues and difficulties face the utility companies at the interconnection side. Some of these technical issues can be overcome by early adoption of standards and some are enforced after the systems are installed.
Islanding is the undesired condition when the DG source, such as a PV system, continues to supply power to the grid during a utility outage. This may cause a serious safety hazard to utility workers who are exposed to unexpected energized power lines. To prevent damage to personnel and equipment, all grid-bound inverters must be able to detect outages and block power transfer to meet UL 1741 equipment testing standard. Inverters with such capability are referred to as anti-islanding inverters. However, Bimodal inverters may function in stand-alone mode of operation while being disconnected from the utility grid line during outages.
Power quality is a topic that discusses several electrical performance parameters, such as voltage, frequency, and harmonic distortion. Power quality of a grid can be affected by loads and equipment connected to the grid such as power electronics equipment that operates on discrete modes and causes quality issues, which may damage sensitive equipment or create hotspots in transformers. Since DG sources including PV inverters contain switching devices, most utilities are concerned about the interconnection of PV systems and power quality issues associated with it. For that reason, utilities mandate that all DG interconnected equipment must meet certain power quality limits such as current harmonics, voltage flickers, and other parameters and utilities must continuously monitor these parameters to insure a reliable grid operation.
A particular over current problem arises when one stand-alone inverter with a 120 V output supplies a 120/240 V distribution panel. A similar problem can occur with interactive systems of single-phase 3-wire or 3-phase, 4-wire wye configuration when loads are concentrated on one phase more than the other. The single grounded (neutral) conductor can become dangerously overloaded. Therefore, the grounded conductor may carry twice its rated circuit current, and this is a serious concern discussed on NEC 705.95 that requires the sum of the maximum load between the neutral and ungrounded conductor and the inverter’s output rating not to exceed the ampacity of the neutral conductor.
In AC electricity, there are two main configurations for the 3-phase systems "WYE," or "Star," and "Delta." Please look over this article regarding Three-phase electric power for more information.
Phase voltage imbalance can occur if a single-phase inverter is connected to a three-phase power system. NEC 690.63 (705.10) doesn’t allow this type of connection unless the voltage imbalance between phases is minimized not to exceed 3 percent. Another solution is to use three similar single-phase inverters (one for each phase) that are equally loaded.