# Introduction

Integration of large‐scale “variable” renewable energy (VRE) generation resources – wind and solar – into national grids has become an interest in many developing countries. A significant drop in investment costs of these technologies, their ability to reduce reliance on imported fuels, and the lack of emissions of either local air pollutants of carbon dioxide – all contribute to their continued uptake. A significant volume of work on renewables has been carried out over the last 20 years, especially since 2012 when large‐scale grid‐connected solar and wind began to ramp up quickly in many countries around the world. NREL’s _Renewables Future Study_ (NREL, 2012) was one the first studies in this recent body of work, and it laid the foundation for much of the analysis done subsequently. Mills and Wiser (2012) and Madrigal and Stoft (2012) focused in more specific detail on generation and transmission planning aspects, respectively.

A large number of studies that have been done over the past 20 years may be categorized as follows:

1. Simple “back‐of‐the‐envelope” approaches that rely on comparison of levelized cost of electricity (LCOE) or auction prices more recently, of VRE and non‐renewables. Ueckerdt et al (2013) discusses a number of these studies and the pitfalls of such an approach in failing to capture the variable nature of renewable resources;

2. Highly detailed production costing (e.g., Olsen et al, 2013) and load flow studies (e.g., O’Sullivan et al, 2014) that look at generation and transmission system operational aspects, respectively; and 3. Planning models of various vintages from WASP‐IV that had its beginning in the seventies (e.g., Botswana Study, 2009) to modern tools like PLEXOS (e.g., IRENA 2016 presentation in Namibia and ECA 2017 study for South African Power Pool).

Many LCOE studies have been undertaken to assess various policies and investment decisions. While the LCOE approach focuses on the economics of renewables, it ignores practically all the technical aspects that are important in power system planning. The second category of more technically focused studies, on the other hand, are completely devoid of economics and are essentially a “feasibility check” on a given power system plan, including those with a high share of VRE.<sup>2</sup> These studies do not provide insight as to how much renewables _should be_ in the mix.

Studies in the third category consider economic and technical issues jointly. Least‐cost planning studies, for example, consider VRE resources as part of the portfolio of generation resources (Blanford and Niemeyer, 2011). Key issues to address in these models are: (a) the level of detail that can realistically be modeled; and (b) coordination among a hierarchy of long‐ and short‐term models. IRENA (2017) is an excellent review of these issues and the capabilities of existing models. There has also been a limited effort to link among these models. For instance, Diakov et al (2015) includes some features of dispatch in a capacity planning model. De Sisternes et al (2016) also make an effort to approximate hourly representation of load in a capacity planning model.

> 2 Load/power flow and stability studies, for instance, check the ability of the transmission network to accommodate the generation from all sources including VRE without violating line flow and voltage limits, and the ability of the system to restore normal operation following loss of a major generator/line.

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