Guideline for Using IEEE 1547 for Solar PV Interconnection Page 1 A Guide for Iowa s Municipal Electric Utilities On the How the IEEE 1547 Distributed Generation Interconnection Standard Affects Solar Photovoltaic Array Inverter Interconnections What is IEEE 1547? The Institute of Electrical and Electronics Engineers (IEEE) is an association of 425,000 electrical engineers from around the world that are dedicated to advancing technological innovation and excellence. One of their tasks is to develop standards for the design, application, and manufacturing of electrical products. One of their 1300+ standards is IEEE 1547 IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems. Responding to pressure and financial support from the Department of Energy, a broad range of stakeholders in the electric power industry started the IEEE standards-making process in 1999 to develop a standard that was greatly needed by utilities, distributed generation (DG) manufacturers, and regulators in the US. A record breaking 204 people provided input to a core group of about 75 people, including the author that attended numerous 2- to 3-day working committee meetings. After a long and difficult process involving 11 drafts, the standard was published in 2003 and has been universally adopted by all involved. There are six complementary standards that provide application and design guides and more details for specific issues. A process has recently been started to review and possibly update the main 2003 standard to address how DG should respond when the regional grid frequency is too high, or too low, because of too much or too little generation and when the grid voltage is abnormal. This review is being made because there is much more DG connected to the grid today than there was in 2003. With the advancement of technology, DG experts now believe that DG can be better used to help support the grid during abnormal conditions. What is the purpose of IEEE 1547? The purpose of IEEE 1547 is to provide requirements for the performance, operation, testing, safety considerations, and maintenance of the interconnection for the interconnection of DG with utility electric systems. Utilities require the owner of a DG to abide by the standard s requirements before it can interconnect its solar PV, wind turbines, and engine generators to the grid. The standard gives utilities confidence that the DG will respond in a predictable way to disturbances on the electric system, and that the DG will not cause electrical problems for any utility s electric system under normal operation. In short, it makes the interconnection and operation of DG on the electric system more reliable, safe, and easier for all involved. What does IEEE 1547 not do? The standard has nothing to do with electrical code design requirements or enforcement, net metering, avoided costs, or buy-back rates. Although it does not limit the amount of DG on the utility's electric system, it informs the utility and DG what standards and criteria must be met to ensure reliable operation with a high penetration of DG. Meeting these criteria for high-penetration situations typically involves extra interconnection equipment, revised operating requirements for the DG, and special operating procedures.
Guideline for Using IEEE 1547 for Solar PV Interconnection Page 2 The standard generally reflects the utility s perspective on its side of the meter while the local electrical codes affect the installation on the customer s side of the meter. However, communication between the utility and their local electrical inspectors and contractors about the solar PV interconnections would likely be beneficial. Key Requirements of IEEE 1547 There are basically eight key requirements of IEEE 1547. 1) The DG should not actively try to regulate the voltage at the interconnection point, so that the DG doesn t interfere with the utility s existing substation and line regulators which already do this task. All solar PV inverters are designed to operate at unity power factor, so that they don t absorb or generate reactive power (kvars). Therefore they meet this standard. However, if the utility wants such help regulating the voltage from large DGs, then it can request the DG to help do this. In a few cases larger DGs are requested by utilities to hold a set voltage level, in order to help stabilize the grid voltage during disturbances. 2) The DG should not cause the distribution grid voltages to be outside of the ANSI Standard C84 Range A, which is basically 114 to 126 volts, which is ±5% on a 120 volt base. Generally this is not an issue with DGs that are sized to the customer s load level, especially if they are for residential or small commercial customers. 3) The DG should not cause overvoltages in the utility s distribution system. Again, generally this is not an issue for municipal utilities with DGs that are sized to the customer s load level. However, this can be an issue for large DGs, such as a 1.5 MW wind turbine that is connected to a 12.47 kv distribution line 2 line-miles from the substation, where the voltage might be too high during light load periods when the wind turbine might be generating full power. In these cases, the DG may be required to regulate the voltage or operate at a power factor other than unity. 4) The DG should not cause the utility's breakers, reclosers and fuses to not operate correctly. The utility s substation and line reclosers trip for line and ground faults because they detect the fault current. The addition of DG should not appreciably decrease these fault currents to the point where they cannot be detected. Since solar PV inverters typically contribute 110% to 150% of their rated full load current to distribution system faults, they will not typically affect the utility s breakers, reclosers and fuses from clearing faults. This can become an issue only when the size of the PV array becomes larger, or when the cumulative amount of solar PV arrays approaches the amount of load on the feeder. 5) The DG should trip itself off line for faults on the utility s distribution feeder. All inverters have internal protections that detect under and over voltages, which indicate the presence of utility faults, or that the utility s feeder has tripped off line. This is not a problem for residential and small commercial solar PV arrays, or when the cumulative amount of DGs on a feeder is modest. 6) The DG should not reenergize itself after it trips for a fault until the utility s breaker or recloser reenergizes the feeder. Solar PV inverters meeting the IEEE 1547 standards do not cause this problem.
Guideline for Using IEEE 1547 for Solar PV Interconnection Page 3 7) The DG should respond to any overvoltage on the utility feeder by tripping off line. Again, inverters meeting the IEEE 1547 standard will trip off for utility feeder overvoltages. 8) The DG should detect and then trip off line if the utility breaker, recloser or fuse trips the feeder off line. In some cases it may take a solar PV inverter a short period of time to detect that the utility feeder is disconnected from the substation. During this short period of time, the feeder section can become an island that is being partially energized by the DGs on that section. The IEEE 1547 standard requires that these unintentional island conditions must be detected by the DGs and then tripped off within 2 seconds. Since essentially all new inverters meet the IEEE 1547 standard, this is not an issue if the cumulative amount of solar PV DG on the circuit section is less than about 75% of the daytime minimum load on that circuit section. A circuit section is any section of the utility feeder that can be tripped off line due to a breaker, recloser or fuse. Other Requirements of IEEE 1547 There are several other requirements in IEEE 1547 that guide the manufacturers and are embodied in the design of the DG equipment. These requirements include: 1) A DG larger than 30 kw should have field adjustable trip points for frequency and voltage deviations. 2) The DG should not cause voltage fluctuations on the grid that are greater than 5%. Typically this would only be an issue for a large wind turbine that starts up or trips off. 3) The DG should not cause voltage flicker that is objectionable to customers. Currently in Iowa, only large wind turbines could cause voltage flicker that might be objectionable to customers. However, very high penetrations of solar PV could potentially cause this. 4) The DG should not cause harmonic distortion that is greater than 5%. All new solar PV inverters have distortion levels less than 5%. 5) The DG should never energize a dead utility feeder. All new solar PV inverters should never do this. 6) The DG must be able to have the kw, kvar, and voltage monitored if the DG is greater than 250 kw. All new large solar PV inverters have monitoring capabilities built in. One IEEE 1547 requirement is to have a disconnect switch near the DG so that the DG can be disconnected by the utility from the electric system in emergencies. For solar PV systems this would be a disconnect switch near the PV array that is accessible to the utility, and that can be locked out by the utility. The switch must also clearly indicate whether it is open or closed. Real world experience over many years has shown that such disconnect switches are almost never used by utilities. Because of this, many utilities do not require this disconnect switch for smaller DGs. Sometimes a DG s inverters or interconnection equipment can simply fail to operate correctly due to a defect or damage. If the DG is small, this type of problem might not ever be found, simply because it is too small to have any effect on the grid. However, a larger DG that has a defect will eventually be detected, because it may be under-generating power or causing a problem on the grid.
Guideline for Using IEEE 1547 for Solar PV Interconnection Page 4 Rule-of-Thumb Interconnection Guidelines In order to simplify the utility s review of solar PV interconnection applications, a series of approval guidelines has been developed based on the author s study and experience. Use of these guidelines will streamline the process for the utility for the majority of solar PV interconnection applications. These guidelines have been incorporated into an Excel spreadsheet. The key requirement the utility should look for is that the solar PV inverter is compliant with the IEEE 1547 standard. The only other major factors are the size of the solar PV system relative to the feeder daytime minimum load, and the amount of other DG already on that same feeder. In general for the typical solar PV interconnection applications that Iowa municipal electric utilities will receive, if the cumulative solar DG on the specific feeder is comfortably less than the minimum daytime load and the solar PV is sized to the customer s needs, then there should be few reasons to perform additional studies before approving an interconnection application.
Guideline for Using IEEE 1547 for Solar PV Interconnection Page 5 Hawaiian utilities have many feeders where the amount of connected solar PV is higher than the daytime minimum loads which means that during sunny days the excess solar PV power is being fed back from the substation into the transmission grid. Figure 1 below shows the feeders on the island of Oahu with very high penetration of solar PV power. FIGURE 1 Based on the lessons learned in Hawaii, better guidelines will be developed in the future for high solar PV penetrations. Thomas A. Wind, PE Wind Utility Consulting, PC Consultant to the Iowa Association of Municipal Utilities August 23, 2014