September 24, 2013

Distributed Grid-Connected PV Integration

Executive Summary

Distributed grid-connected photovoltaics (PV) is playing an increasingly significant role as an electric supply resource and as an integral part of the electrical grid. However, PV poses some notable challenges to grid engineers, planners and operators. They include rapid output variations (ramping), daily variability of the output, effects on power quality, especially voltage and current harmonics, current “backflow” and a mismatch between PV output and end-users’ peak demand. Grid-connected or on-site electricity storage that is located near to or on-site where PV is deployed provides means to offset or manage those challenges.

Discussion

Challenges. Distributed grid-connected photovoltaics (PV) is playing an increasingly significant role as an electric supply resource and as an integral part of the electrical grid. However, PV poses some notable challenges to grid engineers, planners and operators. 

An important challenge posed by grid-connected PV is the rapid output variations (ramping) that occur as clouds pass overhead. The phenomenon is shown graphically in Figure 1. For example, PV located near coastal zones may be subject to ongoing marine layer clouds that vary throughout the day. In some cases, inland heat causes an increase of coastal cloudiness. In the southwest region of the United States, large “monsoonal” clouds occur during many hot summer afternoons just when PV output would be is most valuable. The variability can be a significant challenge for grid operators who must be adept at filling-in when PV output drops off and then reducing grid support as PV output picks up after clouds have passed. 

A somewhat related challenge is referred to as the “cloud edge effect.” Consider how the edge of a cloud with the sun behind it seems especially bright. That phenomenon acts like a lens which concentrates insolation (light and other radiation from the sun), leading to a temporarily increases the amount of insolation reaching the PV modules by as much as 25%, for a few to many seconds. Depending on how the PV system is designed, this cloud edge effect can cause short duration increases of power output that may have to be offset or managed. 

Needless to say, PV output varies as the sun rises, moves across the sky during the day and then sets at night. Although this “diurnal” or daily variation is somewhat or even very predictable, grid operators must address the variability by reducing or increasing output from some other “dispatchable” power sources, especially fossil fueled generation to accommodate the daily variation. Coincidentally, in some cases, wind generation is diminishing as solar power is increasing in the morning and wind generation is increasing as solar power output is decreasing the evening. That can either lead to offsetting variation or variation that is additive. PV and wind generation diurnal variation is shown in Figure 2. 

In some cases, especially those involving high penetration of PV in parts of the distribution system dominated by residential end-users, the amount of power generated by the PV may exceed the total demand being served by a given part of the distribution system. In those circumstances, “excess” power can have a somewhat dramatic effect on the electric service voltage. 

Another effect is known as “back-flow.” This effect entails current flow from the “low voltage side” of electrical transformers (also known as the transformers’ secondary side) to the higher voltage side (also known as the transformers’ primary side). That is important because electrical transformers are designed for one-way current flow, from higher voltage to lower voltage. This challenge tends to be more common in parts of the distribution system that are serve primarily residential end-users, because demand in those parts of the grid tend to relatively low during the day when residents are at work or school. 

Another PV integration challenge is that PV’s maximum output tends to occur before the peak or maximum end-user demand. As a result of this mismatch,energy generated in the morning and early afternoon is less valuable than if the energy was generated in the mid and late afternoons when air conditioning use is highest. Parts of the distribution system that are dominated by residential air conditioning use may be the worst case because air residential air conditioning demand tends to pick up in the late afternoon and may even last until the early evening.This challenge is less dramatic in parts of the distribution system that are dominated by commercial and industrial (C&I) energy end-use. 

There are some important localized power quality related impacts caused by distributed PVs, including:1) ramping and 2) excess output. Of particular note is the impact on voltage. Voltage-related effects on electricity using equipment can include damage and equipment may not be able to operate if the voltage is too high, low or unstable. (The distribution system is designed to operate within specific range of voltages.) Finally, operating the grid at voltage that is too high can reduce distribution equipment energy efficiency and energy end-use efficiency.

Electricity Storage for Distributed PV Integration

Grid-connected or on-site electricity storage that is located near to or on-site where PV is deployed provides means to offset or manage those distributed PV related challenges. 

First, storage can be used to “smooth out” variability that the grid must accommodate. For example, storage can be used to provide localized ramping “service” as depicted in Figure 3. 

Second, storage can be charged at night from the grid and/or in the morning using energy from the PV system so the combined output (PV plus storage) is constant during peak demand periods and so current backflow is reduced or eliminated. Third, if the storage systems’ power conditioning unit (PCU) has the necessary capabilities, the storage system can provide what is called “Volt/VAR control” to address voltage problems. Volt/VAR control entails on going management of voltage and reactive power (which affects voltage) so the voltage is kept stable and is maintained within acceptable levels. 

Conclusions and Observations

Clearly distributed photovoltaics will be a significant contributor to the electric supply mix. PV has numerous attractive features including a fair match with demand for electricity, modularity, suitability as a distributed energy resource (DER), no air emissions and no fossil fuel use and very quiet operation. However, there are operational challenges poses by distributed PV – especially in residential areas and in parts of the distribution system with high penetration of PV. However, modular distributed electricity storage is quite well-suited to offset most those of challenges due to some attractive synergies including storage’s ability to time-shift energy and to fill-in in the afternoon so PV output can be constant and highest when needed and its ability to address the voltage-related problems that PV can cause. 

In addition to addressing distributed-PV-specific challenges, distributed storage plus PV can provide several additional benefits. For example, storage plus PV could be part of the end-user’s overall bill electricity management strategy, by reducing time-of-use energy cost and/or peak demand charges. And, depending on the location, distributed storage plus PV capacity may reduce transmission congestion, allow for transmission and distribution equipment upgrade deferral. The storage-PV system could also improve local electric service reliability and/or non-PV-related power quality problems (primarily, voltage related). 

Note also some technical synergies between PV and modular storage which means that some costs and some equipment and hardware can be shared (e.g., design and construction-related costs). Especially notable are synergies that allow sharing of power conditioning equipment (with modest additional cost). The PV-storage system could also including sharing of equipment and costs related to wiring, communication and controls and interconnection.


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