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PV effects on distribution systems

Distributed generation (DG) is being implemented across the nation’s electric grid in ever-expanding ways. During the last few years alone, we have seen major increases in grid-connected photovoltaic (PV) DG. Nearly 1 GW of installed PV capacity is now distributed across the country’s electric grid. The Solar Energy Industries Association reported that grid-installed PV capacity doubled in 2010 to 878 MW, and it had the largest single-quarter growth in the third quarter of 2011.

Integration of distributed PV with the electric utility system can still pose major challenges, however.

Electric system concerns
Distributed PV generation on the electric system can cause a wide range of concerns for the host utility system. IEEE Standard 1547, “Standard for Interconnecting Distributed Resources with Electric Power Systems,” addresses these concerns. While this standard does not specifically address distributed PV systems, it does provide the requirements for interconnecting DG facilities with the electric system. It also provides details for issues such as allowable voltage drop, short-circuit currents, and voltage flicker. Host utility owners have many other concerns such as asset impacts (e.g., loss of asset life due to increased regulator and capacitor bank switching) and feeder control (voltage and reverse power flows), which interconnection studies need to address.

Integration studies yield insight into circuit performance
Recent PV integration studies revealed circuit performance depends on several critical factors, including feeder characteristics and voltage class, PV site distance from the substation, feeder load characteristics, penetration level, and system control settings.

Feeder characteristics play an important role in how circuits perform when distributed PV is added. Circuits with higher nominal voltage will generally perform better than lower voltage circuits. PV output curves tend to match up better with commercial circuit load profiles than with residential circuits, which reduces back-feed concerns (see figure below).

Residential circuit load profile with and without PV (source: KEMA)

Both the PV station distance from the substation and PV circuit penetration play critical roles in feeder performance (see figure below).

Voltage drop of PV site for loss of output (source: KEMA)

Time-sequence power flow study approach
Utilities can undertake a time-sequence power flow study to help assess and address these PV integration issues. This study can help utilities better understand feeder performance if distributed PV is added to a given circuit or circuits. The approach includes 24- and 8,760-hour simulations using hourly load and PV output data. It also includes one-minute simulations to evaluate power quality and system control issues. Together, these simulations identify potential back-feed, voltage drop, or voltage flicker issues associated with operation of the proposed distributed PV generation.

Power flow studies also help utilities identify key asset management issues such as increases in regulator or capacitor switching operations, which could result from adding distributed PV.

Distributed PV can offer utility benefits
Despite the challenges of integration, distributed PV can offer benefits to the utility that include reduced conductor loading and system losses, delayed capital investments for capacity improvements, and improved voltage control.

Modern PV inverters have the ability to control voltage and/or vars at the point of common coupling with the electric utility. Due to the VOLT/VAR control of the inverter, the PV station can actually improve voltage control on the feeder. In some cases, it might actually help to avoid voltage collapse in the event of severe system disturbances.

The addition of distributed PV generation to the electric system is rapidly picking up pace, with ever-greater penetration across the electric power system. In fact, generator interconnection request queues are backed-up across the country. The time-sequence power flow study offers the electric power industry a fast, economical, and reliable method to evaluate distributed PV interconnection. KEMA’s Electric Transmission and Distribution group has performed many power-flow studies, and even completed some high-priority projects in less than a week.

For additional insight into PV integration and KEMA’s approach to PV interconnection studies, contact our expert: Dennis Flinn, KEMA Senior Consultant.

KEMA consultant Neil Crandell also contributed to this article, featured for TECH Notes, a monthly publication that provides business and technical insights for secure transmission and distribution systems. Sign up to receive advance notifcation.

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