A wave of new solar photovoltaic (“PV”) installations for power generation is hitting many distribution circuits around the country. These installations are typically in the range of 10-2000 kW and comprise of a set of solar PV arrays or trays and inverter modules. The inverters are needed to change the direct-current produced by the arrays to the alternating current standard used by the distribution circuits. The smaller installations connect single-phase, while the larger sizes are three-phase. Interconnection voltage at the point of common coupling between the PV installation and the distribution circuit varies from 120 volt up to 34.5 kilovolt (“kV”).
The concept of integrating these new PV installations with existing distribution circuits is similar to that of interconnecting larger generators in the transmission grid; i.e., the new installation should “do no harm” to the existing system. There are three aspects to this concept as follows. (1) If the existing circuit meets specified standards or criteria of performance, the circuit should still meet the same standard or criteria when the new PV is installed. (2) If the new PV introduces a violation of standard or criteria, mitigation measures need to be included as part of the the new PV’s installation to resolve the violation. (3) If the existing circuit already violates a standard or criteria, the new PV either should not make the violation worse, or limit its impact such that the violation is not worse or even reduced or eliminated.
However, the standards or criteria by which a new PV installation may impact a distribution circuit are significantly different from those of new generation interconnecting to transmission grids. Distribution standards relate more to power quality type issues. In addition, power supply type issues such as thermal loading, voltage control, short circuit and stability also need to be addressed. This Blog discusses the various technical issues that may result in an impact by new PV, and options for mitigation of the impact, based on Pterra’s experience in conducting numerous integration studies of new PV. (A continuation of this Blog, Part 2, and eventually Part 3, will address each technical issue in more detail, covering the analytical approach, typical impacts and potential mitigation options.)
To begin, we list and discuss the technical topics with a general overview of the reason for considering each topic in the impact assessment of new PV. The following set of issues relate to power supply requirements:
- Thermal loading on distribution circuits under peak and day minimum load conditions, under various contingency and emergency operating states. (Day minimum is used because the solar PV is only active during the daytime hours.) This may include measurement of incremental changes in thermal losses, and identification of conditions when flow reversal occurs.
- Steady-state voltage control. This topic relates to the impact of the PV installation on the ability of the distribution circuit to maintain voltage within an acceptable range under various operating states in the timeframe from one to several minutes. As with thermal loading, typically peak and day minimum load conditions are considered. In addition, the operating duty of transformer tap-changers, voltage regulators, capacitor banks and other power conditioning equipment is assessed for possible degradation due to the new PV. PV, in particular, are susceptible to transitory outages when cloud cover shuts down then unveils, at full output, irradiation of the PV arrays.
- Short circuit levels and protection coordination. The inverters used for the PV installation produces a short circuit currents that contribute to the amount of fault current on the distribution circuit. Based on the inverter contribution, the short circuit ratings of circuit breakers and fuses are tested. Settings of protective relays, such as ground and phase over-current relays, that are dependent on the short circuit characteristic are reviewed for any loss of sensitivity or coordination.
- Frequency and voltage stability. As a non-rotating generation resource, solar PV installations introduce a unique impact to the synchronism of the interconnected system and to the ability of power control systems to maintain voltage and frequency within acceptable ranges. The impact is accentuated during islanded conditions. However, large penetrations of solar PV or a small interconnected system such as an island or remote grid may also result in impacts on stability.
- Operating reserves. As a variable resource, solar PV impacts the requirements for operating reserves for the interconnected system.
The following are the typical power quality issues associated with new PV installations:
- Voltage flicker. As a variable resource, PV introduces small and fast voltage changes on the distribution circuit that may be seen by customers sharing the same circuit as observable or irritating flicker. Industry standards define the limits on the amount of flicker that is allowed to reach customers.
- Harmonics. Inverter-based PV installations produce AC power that is replete with high frequency distortions known as harmonics. The harmonics, if large enough, can lead to overheating, representing a fire hazard and reduction of equipment life, and false tripping of branch circuit breakers. Industry standards define allowable limits for harmonic distortion per multiple of the fundamental AC frequency (60 hertz in the United States).
- Grounding. The distribution circuit is grounded to limit overvoltage during single-phase fault events. The PV installation design is reviewed to determine if effective grounding is maintained in accordance with industry standards.
- Islanding. When a distribution circuit is disconnected from the utility source, the result is an islanded circuit. If there is no generating resource such as solar PV on the island, it is normally de-energized and the customers connected to the circuit lose power. However, if there is a resource such a solar PV installation, the island may remain energized and pose several technical issues that relate to reliability and safety of the electric users and operators. For one, operating personnel may assume that the island is de-energized and initiate recovery procedures such as reclosing the utility supply that poses serious danger to personnel.
As we can see from the preceding list, the spectrum of potential impacts is broad for solar PV interconnecting to distribution circuits. Some of the impacts are potentially present for the small size solar PV installations, while some are only present for the larger sizes or larger cumulative levels of solar PV penetration.
In Part 2 of this Blog, we will take a closer look at each of the technical issues, identifying those which are commonly encountered as posing potential impacts, and how such impacts are typically addressed.