High-Penetration Photovoltaic Integration Handbook for Distribution Engineers
The NREL handbook, High-Penetration Photovoltaic Integration Handbook for Distribution Engineers, analyzes the impacts of high-penetration levels of photovoltaic (PV) systems interconnected onto the Southern California Edison distribution system.
High-Penetration Photovoltaic Integration Handbook for Distribution Engineers (Mather et al. 2016)
This handbook was developed by NREL as part of a five-year research project. Regulators may gain significant insight into utility practices related to interconnection, their technical reticence to higher-penetration PV generation, and the basis for interconnection procedure supplementary technical screens.
With certain engineering techniques described in this handbook and forecasts of distributed energy resource (DER) deployment, utilities can model the future states of the distribution system for impact assessment. This can result in better system planning, better use of DER systems to defer upgrades and overall, a more efficient statewide interconnection standard.
Apply Time-Varying Analysis
As a first step, it is crucial to analyze the various impacts on the distribution level due to high penetration of PV. The report recommends investigating the impact of high PV penetration using time-varying analysis as opposed to single time point analysis, because it captures interactions among load, generation and control equipment that are difficult to predict using the latter. The identified impacts are broadly categorized as overload, voltage, reverse power flow and system protection impacts. PV systems should be evaluated for its normal and abnormal configurations throughout the entire load spectrum of circuit. In addition, the impact of PV should also be analyzed during contingency conditions.
Perform a Model-Based Assessment
The next step is to perform a model-based assessment study of PV impacts. The prescribed methodology requires development of system models and data necessary for time series analysis and impact of adding PV to the electric grid. A base case model of the distribution system should be developed which consists of as many components as necessary to accurately represent it. Additional data would be required to model future states of the distribution network such as load forecast, changes in network topology and so on. Measurement and validation are the most important factors to ensure the accuracy of the distribution circuit model. Once the base-case model is evaluated, it is possible to identify minimum and maximum daytime load points, and thus the critical time points with the addition of PV measurements.
Analyze Power Flow and Fault Protection
The next step of the process is the PV impact assessment, which constitutes power flow and network fault assessment. Ideally, for power flow analysis, it is desirable to analyze all days of the year. However, since these computations demand considerable time and resources, critical days can be identified from the previous step for further analysis. By simulated loss and restoration of user-selected PV generation, the percentage of PV output that causes voltage or thermal loading criteria violation can be identified. The analysis can also provide useful information for areas where large deployment of single-phase PV may be present. Results having voltage regulation issues entail a detailed dynamic study. Fault analysis evaluates the effects on fault current that results from adding the PV generation. The screening criterion for fault current is the 10% rule—if the PV fault current is less than 10% of the system fault current at the POI, then the protection may not be an issue. A marginal interrupting rating, fault sensing and selectivity may call for a detailed study. Any changes in the methodology of this analysis should be pre-approved. A comprehensive description of the detailed study parameters is described in the report along with two case studies.
Consider Inverter-Based Mitigation Strategies
The final part of the report discusses the mitigation strategies in case of high penetration PV impacts. Perhaps the most inexpensive technique to eliminate voltage variation on a distribution feeder is an advanced PV inverter. This can help mitigate load induced voltage variations and its own variable real power output on grid voltages. Mitigation methods may require actions only at PV systems or only by utility or a combination of both. Advanced PV inverters offer mitigation strategies such as Constant PF (power factor) operation and other advanced controls such as PF scheduling, reactive power compensation, Volt/VAR control, etc. Likewise, mitigation techniques to alleviate the impact of PV are available. The report provides a detailed explanation of these techniques. Regulators and utilities need to understand that modifying a PV power plant or its operation is relatively simpler than modifying utility equipment in terms of time and cost. This needs to be factored in while choosing the mitigation strategies.