Distributed Grid-Connected PV Integration

Executive Summary

Distributed grid-connected photovoltaics (PV) is playing an increasingly significant role as an electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. supply resource and as an integral part of the electrical1. An adjective meaning “pertaining to electricity”. Electrical Engineer. 2. Related to, pertaining to or associated with electricity but not having its properties or characteristics. 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 powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. quality, especially voltage and current harmonics, current “backflow” and a mismatch between PV output and end-users’ peak demand1. The rate at which electric energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by.... 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 supplyA source of electric energy and/or capacity, possibly including generationThe manner in which electricity is generated. The electricity that flows through California facilities and purchases. 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 distributionThe practice of and infrastructure for distribution of electricity to end-users by utilities. Typical voltages range from 12 to 138 kiloVolts (kV) 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 qualityA measure of the level of voltage and/or frequency disturbances. 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 demandThe maximum power draw from end-user loads during specified times. For example, most utilities experience peak demand during hot summer afternoons. 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 controlIn electric power transmission and distribution, volt-ampere reactive (var) is a unit used to measure reactive power in an AC electric power system. VAR control manages the reactive power, usually attempting to get a power factor near unity (1). entails on going management of voltage and reactive powerThe portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment. Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers. It also must supply the... (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 resourceRelatively small and modular electrotechnologies that are deployed at the subtransmissionPart of an electricity transmissionAn interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. and distribution system whose voltage is lower than that of the transmission system and higher than that of the distribution system. Subtransmission circuits are usually arranged in loops so that a single line... or distribution level. (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 chargesThe price paid by a retail electricity user for each unit of power draw on the electric gridA common term used to refer to the electric utility grid.. (That power draw drives the amount of electricity generation and T&D infrastructure needed by the utility to serve all loadAn end-use device or an end-use customer receiving electric power and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or....) Typically demand charges are.... And, depending on the location, distributed storage plus PV capacityThe rate at which equipment can either generate, convert or transfer energy. may reduce transmission congestion, allow for transmission and distribution equipment upgrade deferralDelay the need to replace or enhance equipment within the grid, usually by using a power source or load management to reduce the peak load served by the equipment to below the equipment’s rated power. See also life extension and transmission and.... The storage-PV system could also improve local electric service reliabilityThe degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. May be measured by the frequency, duration and magnitude of adverse... 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 interconnectionThe physical and electrical connection between an electricity source and an external power system (i.e. the electric power grid)..