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The growing importance of distribution in grid planning

By Martin Shalhoub

 

The energy industry is at the cusp of a paradigm shift in the way it generates, distributes, consumes and stores energy. Significant, multi-faceted changes in energy supply, demand and delivery technology, customer expectations and stakeholder demands are compelling electric utilities to throw out their old business model and develop a profoundly different and expanded grid capability. Changes in planning, infrastructure, and revenue models are imperative to effectively address a range of planning and investment issues emerging from this paradigm shift. Utilities must now plan for hosting rapidly growing quantities of distributed energy resources (DER) in all its varieties, including: distributed generation, demand response resources, storage and energy efficiency applications. 

With the rapid growth of DER and large scale renewable generation, and the desire by many legislators and commissions to incent their continued growth, the need to improve and better integrate the system and supply planning under a single system framework becomes crucial. 

Despite the “integration” implied in its name, utilities have often completed their transmission and distribution plans as activities largely independent and discrete from each other, and distinct from their generation focused integrated resource plan (IRP). Now, however, with the use of utility controlled generation decreasing, a truly integrated and interactive planning process is required.  Utilities must include a broader landscape of options and system components to optimize its portfolio. This requires a more integrated planning approach that encompasses all the elements of the system.

High Renewable Penetration is Not New

While we are navigating through new waters for the U.S. utilities, we are not in unchartered territory. Europe remains well ahead of any state in the U.S. in accommodating large contributions from variable renewable energy.   

Though system-level data is useful, it tends to mask the more extreme stress tests to hosting renewable generation.. Utility systems in Germany have experienced distribution feeder-level DER generation peak outputs that are over 300% of the feeder peak demands.  Even with the strong transmission level grid connections to other utility systems, German utilities are beginning to see renewable driven issues at the transmission system-level that were thought to be limited to distribution feeders or islanded microgrids.   

With increasing DER penetration levels on the distribution system, it is becoming necessary to model in detail those distribution circuits with high DER levels. This can provide an accurate assessment of the impact that fault currents may have on the operation of transmission protection and control systems and the effect of reverse power flows on voltages, electric losses and spinning reserve levels. 

Planning for the Unknown

Understanding planners’ first axiom – no matter what you predict, the future will be different – U.S. utilities need to plan for flexibility, now more than ever. Flexibility for utilities has commonly been addressed with scenario planning. Also, most utilities are not yet applying robust sub-hourly or extreme outcome planning to distribution systems. Stochastic, or probabilistic analysis, must be incorporated to better assess the impacts of key inputs on planning outcomes. Utilities must now extend and refine rigorous planning techniques that take into account individual distribution feeders and what is happening in the distribution system.

While distribution planning has its own set of challenges, the transmission system must also be equipped to address the DER impacts that flow from the distribution and sub-transmission systems.  Just as historic distribution planning has only viewed electric service customers as loads, resulting in the transmission planning approach of modeling distribution substations loads, new transmission planning approaches must be included to account for the impacts of the DER contribution to the transmission system.  Considerations must be given to modeling DER impacts on an individual feeder basis and their collective impact to the transmission grid, including feeder-level load forecasting, resource planning, steady state analysis, transient stability analysis, and protection coordination and control.

The last cog in this machine is the economic evaluation. The nodal market is more and more prevalent throughout the globe. Historically, analysis has been focused on transmission nodes, but with the strides in DER this focus will change to accommodate nodes that are likely to dip into the distribution system where the transmission node no longer makes sense. This will impact the entire industry and open the gateway to planned microgrids that are intended to operate across the distribution and transmission grids in a seamless environment. 

In most electrical systems in the U.S., distribution plans have not yet faced the internal and external scrutiny of the generation and transmission planning processes. However, the standard of integrated, granular analysis across distribution, transmission and generation networks is expected to significantly increase based on emerging regulatory, planning and operational best practices. 

Solar PV Generation Variability

Most of the anticipated DER will take the form of solar photovoltaic (PV) resources. Unfortunately, scattered clouds can create a widely variable output from solar PV. These periods of widely varying outputs create an increased burden on distribution voltage regulation and the handling of reverse power flows. 

The variability of a single solar DER installation should typically have an immaterial impact on a distribution feeder, substation or bulk generator. However, the PV output operates in unison across wide areas. With significant PV penetration on a feeder, it can have rapid and profound synchronized output changes from the entire DER based on passing cloud cover. This creates synchronized changes of power flows and voltages over very compressed time frames. 

This rapid fluctuation in PV output across a system with high PV penetration may create difficulties at the transmission and bulk generation levels, such as unacceptable voltage variations and the need for additional spinning reserves. Accommodating this synchronized variation of solar DER output through the entire system will require a flexible and responsive fleet of energy sources enabled with advanced grid controls and automation. The larger the DER penetration levels, the greater the potential for the impacts throughout the system.

Proactive implementation in Planning

The stakes are high given grid operations and societal benefits of applying integrated planning methods effectively, especially when compared to the potential costs and adverse system impacts of applying it poorly or not at all. Utilities must rethink and adapt their planning practices in order to remain successful in this rapidly transforming environment of DERs.  These industry changes require utilities to better assimilate the intelligence, resource choices and behavioral characteristics of the key stakeholders in this new planning paradigm. Ultimately, utilities must improve the internal alignment and integration of their planning activities under a consistent set of planning parameters across generation, transmission and distribution grids.

Martin Shalhoub is the head of Siemens PTI.

 

 

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