As increasing amounts of solar PV connect to electric distribution systems, in the forms of utility-scale solar PV and behind-the-meter solar PV, a critical question that many in the industry have is “How Much Solar PV Can a Feeder Host?” The maximum amount of Solar PV that a given distribution feeder can host (or that can be connected to that feeder) is commonly termed in the utility industry as “hosting capacity”. Hosting capacity is dependent on multiple factors including:
· Size and location of solar PV installations
· Feeder nominal voltage level
· Thermal capacity of feeder equipment
· Steady-state voltage constraints
· Voltage deviations
· Reverse power flow
· Unintentional islanding
Size and location of solar PV installations
The size, or rated output, of solar PV and its location on the feeder greatly impacts the potential electrical issues that it may create on the feeder. This is due to the electrical impedance between the solar PV and the utility distribution substation. In general, solar PV that is connected closer to utility distribution substations have the potential for less electrical impacts such as voltage fluctuations. As the distance from the substation to the points of connection of solar PV on the feeder increases, the impedance increases and the potential for greater electrical impacts increases. With some caveats, higher amounts of solar PV can be connected close to a substation than if that same amount is connected at the end of the feeder. Solar PV connected to ends of feeders will have larger impacts, such as increased risk of unacceptable voltage fluctuations.
Feeder nominal voltage level
In general, feeders that are designed at higher nominal voltage levels (such as 34 kV) are able to host more solar PV than feeders at lower nominal voltage levels (such as 4 kV). With higher voltage levels, the amount of power that can be transmitted for the same current is increased, and the relative per cent voltage drop or rise created by a given amount of current is less. This translates into an ability of higher-voltage feeders to host more solar PV at greater distances from the substation than to a comparable lower-voltage feeders.
Thermal capacity of feeder equipment
The thermal capacity, or rating, of overhead lines, underground cables, and other feeder equipment is the amount of current that the equipment can carry without overheating. In determining equipment ratings, there are multiple factors that utilities consider, including time (duration) that the equipment must conduct the current, ambient temperature, and system contingency (reserve) planning. Depending on the equipment, there are other factors – for example, the rating of overhead conductors is impacted by the wind speed and direction, as well as solar radiation. The thermal capacity of feeder equipment should be considered for both charging and discharging for energy storage applications such as batteries.
Steady-state voltage constraints
In general, utilities have an obligation to maintain voltage at customers’ service points (typically the revenue meter) within a certain tolerance, or range. That range is usually defined through reference to a given standard (e.g., ANSI C84.1). The required voltage range is dependent upon the nominal system voltage at the service point. Different voltage ranges can be specified for system operating conditions frequently encountered and for operating conditions infrequently encountered. ANSI C84.1 defines Range A service voltage as + 5% for nominal system voltages between 120V – 600V. Since the size, location, and power factor of solar PV impacts voltage, these steady-state voltage ranges can impact solar PV hosting capacity.
The variable output of solar PV can also create changing voltage conditions over time, resulting in additional, and perhaps excessive, number of operations of the mechanical voltage regulators and load tap changers on the system. The increase in the number of operations of this equipment can result in increased maintenance costs, reduced reliability, and poor power quality.
Reverse power flow
If the amount of Solar PV generation exceeds the load on a given feeder or feeder section, reverse power flow can occur. Reverse power flow is the occurrence of power flow in the direction opposite of the power flow direction with no Solar PV connected to the feeder. High voltages on the feeder can occur, particularly during light load periods with maximum solar PV output, like a sunny weekend day on a feeder with low load. Reverse power flow also has the potential to create operational issues with voltage regulation and overcurrent protection equipment. Additionally, power flows from the distribution system to the sub-transmission/transmission system, may create operational challenges on the sub-transmission/transmission system.
When the amount of Solar PV on a feeder section is approximately equal to the load on that feeder section, the potential for unintentional islanding can occur. Consistent with the application of IEEE 1547-2018, all DER (Distributed Energy Resources) must detect the island, cease-to-energize, and trip within two seconds of the formation of the island. Anti-islanding protection is available in solar PV inverters; however, distribution organizations are concerned that the interaction of multiple inverters in an island may continue to support the island. They are also concerned that anti-islanding protection may not operate for certain operating conditions. The potential of unintended islanding is a screening factor that utilities use to evaluate solar PV for connection to feeders.
All of the above factors mean the amount of solar PV a feeder can host is, unfortunately, answered with “It depends”. Utilities have screening and study processes to address solar PV project applications. In some cases, it is obvious that given projects will not create operating problems, and those projects can go through expedited screening. In addition, more utilities have developed the capability to perform “hosting capacity analysis” and publish the results of that analysis to the public in portals, in the form of hosting capacity maps and tables.
Tim Taylor is the founder of Electric Distribution Academy, and all his courses are hosted exclusively on HeatSpring. Tim is the instructor for “Interconnection of Utility-Scale Solar PV to Distribution” and the new “Understanding IEEE 1547-2018 – Interconnection Standard for DER on Distribution”