Renewable Distributed Energy Resources - Technical Benefits and Challenges

By Babak Enayati

Various policies, regulations, and legislation seek to deliver environmentally beneficiary objectives for the power generation sector. The North American electric power system is becoming more reliant on wind, solar, natural gas, and demand response. In addition to the changing energy landscape in the wholesale market, most of the states in the U.S. have ambitious goals to deploy Renewable Distributed Energy Resources (DERs) in the retail market.

Many countries around the world have implemented portfolio standards for renewable energy, in order to accelerate the deployment of these sources, which are usually dispersed across the distribution power system. As the penetration of renewable power generation increases, electricity grids are beginning to experience challenges, often caused by inherent characteristics of the former. To name a few of these characteristics, the intermittent nature of some of the most common renewable generation types, sudden changes in their output due to grid disturbances, low short-circuit duty of the inverter-based generators, and impact on the transmission and distribution system protection, are the most distinctive. This article outlines major high level technical benefits and challenges associated with high penetration of DERs from distribution/transmission planning, operation and protection perspective.

Technical benefits:

1. Investment deferral: In areas of the electric grid where load growth may require system modifications (e.g. transformer and distribution feeder upgrades), DERs like energy storage have shown benefits in deferring the investment as the required load growth energy can be handled accordingly.
2. Resiliency: Energy produced by DERs during power system outages improves the system resiliency. Realizations of such systems are the microgrids and/or the islanded systems.
3. Reduced Transmission and Distribution losses: Since load on the distribution system can be fed locally through DER installed in close (electrical) proximity, the current flow exchanges between the transmission and distribution systems are reduced. Hence, the overall system losses are reduced.

There are many non-technical electric benefits associated with renewable DERs, as well, such as those of the social and policy types. Nevertheless, there have only very recently been analyses that quantified those benefits.

Technical challenges:

Power Quality: Increase in DER penetration, exposes the electric grid to some major power quality issues such as the following:

• Harmonics: According to the IEEE 1547 “Standard for Interconnecting Distributed Resources with Electric Power System (EPS)”, each DER must meet certain total harmonic distortion requirements. However, the sum of all harmonics that can be injected into the system is a concern for many utilities.
• Rapid voltage change: DERs that do not have a ramp rate or soft start capability may cause rapid voltage changes on the EPS, which may violate the utility distribution system design standards.
• Temporary and transient over voltage: This is becoming a concern for distribution and transmission utilities as the DERs may not be able to follow certain power system grounding requirements. Transmission and distribution systems may experience overvoltage for certain system faults if the DERs do not provide the proper grounding resources.

1) Impact on the bulk power system reliability: Traditionally, DERs used to be considered as a passive load resource on the Bulk Power System. In areas where the penetration of DERs is increasing, this assumption will no longer be valid. As deployment of DERs on the distribution system increases, the impacts on the Bulk Power System reliability need to be addressed promptly. In order to avoid large scale power system blackouts, once the bulk power system experiences frequency or voltage disturbances, the DER must remain online and support the grid until the latter has fully recovered (i.e. DERs with ride through capability). Some distribution utilities are concerned that the ride through requirement may have adverse impact on the distribution system protection, related to the prolonged fault and islanding detection. As a potential solution, the electric power industry is moving towards deploying communication based anti-islanding protection schemes on the distribution system i. e. Phasor Measurement Units, Power Line Carrier, etc.

2) Impact on transmission and distribution system protection: The adverse impacts of DERs on the system protection can be categorized into:

• Transmission system protection. Typically, distance protection is utilized in the transmission system to detect and isolate the transmission system faults. If the distribution system connected DER on the faulted transmission line feeds fault current into the transmission system fault, it may cause overreach, hence, misoperation on the distance relaying protection zones (infeed impact).
• Distribution system protection: DER provides a parallel current source to a distribution system fault, which lowers the fault current contribution from the substation. This may impact/delay the operation of the upstream relaying on the distribution feeder (relay desensitization)

2) Visibility and control: Due to above mentioned challenges, as the penetration of DERs on the EPS increases, utilities are becoming concerned about reduced visibility and control over DERs. This issue needs to be addressed promptly by deploying secure communication infrastructure and protocols between the DERs and the utility distribution management system, etc.

3) Modeling: Currently, the tools that are widely used by the distribution utilities for the DER interconnection studies (planning, protection, etc) are not capable of modeling accurately DERs with grid support functionalities (i.e. active DERs). To address this concern, some open-source modeling tools have been proposed by research consortia. Some of the active DER characteristics are considered as proprietary information by the respective manufacturers and cannot be modeled by the proposed tools in a standard format. Lacking standardized models for the various DER devices, assumptions have to be made by utilities. Furthermore, the commercially available tools provide limited results for the wide area protection and load flow analyses.

In light of the major planning, protection, and power quality challenges associated with high penetration of DERs on the EPS listed above, utilities, government officials, and the DER manufacturers have been and will be collaborating closely to resolve all concerns in a cost efficient and timely manner, so as to avoid missing on the overwhelming opportunities of these technologies.

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