**3. Weather-dependent variability of renewable resources**

The wind energy and PV are expected to have a lion's share in the prospect of the power utility. So, the future energy source is pivoted on the in-depth realization of their variability. Resource variability is a multi-faceted notion expressed by a range of distinctive characteristics. Simultaneously, research to date tells that there is restricted knowledge about the variability of the future power system. The variable attribute of climatic fluctuations is the reason of inconsistency of the RES and creates uncertainty in the energy production on the range of seconds, hours and days even. It is estimated that clouds limit up to 70% of daylight hour solar energy potential. Grid sometimes deals with aggregation over massive areas and this mitigates the variability of every single RES.

Presently a large variation is tackled via switching in fast-acting conventional sources depending on the climate forecasts on a minute-by-minute and hourly basis. Such variability can additionally be taken care by setting up large scale storage on the grid or; by the long-distance transmission of RE linking to larger pools of such generations in order to equalize regional surplus or shortfall nearby in future. Graabak et al. [9] have addressed the variability characteristics such as: (i) Distribution long term, (ii) distribution short term, (iii) step changes, (iv) autocorrelation, (v) spatialcorrelation, (vi) cross-correlation and (vi) predictable pattern.

*Wind Solar Hybrid Renewable Energy System*

last 10 years with some vision of future trends.

smooth and reliable coordinated control.

(3) reactive power support.

**2. Regulatory, project finance and technical perspective**

Energy storage for grid applications lacks a sufficient regulatory history.

The renewable electricity demand is predicted to add up 20% more within the next 5 years. They can have the quickest development within the power sector, providing nearly 1/3rd of the requirement in 2023 [5]. Further, there is forecast to exceed 70% of world electricity produced, primarily by PV and followed by wind, hydropower, and bio-energy. Hydropower remains the biggest such supply, meeting 16% of the world electricity demand by 2023, followed by wind (6%), PV (4%), and bio-energy (3%).

Whereas active regulation of voltage was not permitted and the DERs had to trip on abnormal voltage or frequency, participation in voltage and frequency control was desirable due to a gradual increase of the percentage of DER in power system. This was resolved in 2003. The first amendment to this came after a decade (11 years) but the second one came just after 4 years of first [6]. This comes in line with the steeper increase of DER penetration than the previous decade. As the DER are geographically dispersed, the communication interface between DER and the main grid and in between the DERs has been an additional demand of the hour for

Some of the distribution grid safety demands are (1) short trip times, (2) ridethrough with momentary cessation (3) voltage rise concerns (4) protection coordination (5) islanding concerns for the safety of workers. Bulk system reliability demands (1) long trip times (2) ride-through without momentary cessation

if operated in a manner coincident with grid needs that respect storage limitations. These DGs have made the grid more resilient, efficient, environment-friendly, flexible, less vulnerable, easier to control, immune to issues at some other location, slow gradual capital investment, integrating to grid with minimal disturbance to existing loads during commencing. Participation of DERs in operation is profitable in respect of load shifting without grid up-gradation curtails peak demand, grid support by storage responding to demand thereby improving frequency response reducing spinning reserve. EVs and MGs can provide ancillary services. Under normal operation of the grid, varying capabilities of the DERs support voltage and reactive power whereas under fault voltage and frequency ride through capability is expected. Under such fault, the inverter must respond as per requirement. With the coordination of inverter-based resources in a group, it is possible that the DERs counteract to grid contingencies such as voltage and frequency deviations, and assist in fast recovery. So they are termed virtual inertia. But, at the same time, some issues are of concern and have drawn the attention of researchers. They are mainly due to stochastic nature such as load following, power vs. energy profile in storage, stability, reliability, cost, control architecture, autonomous control, power quality issues and grid interconnection. Considering these issues, in [2] the feasibility study, the unit commitment for reliable power supply and modeling of energy systems of PV, wind and diesel generator are focused. In the past decade, more significant development has taken place with various combinations of sources and storage. Optimization in all respect of wind energy for grid integration has been thoroughly reviewed [3] and observed to have good success. The control topology and the objectives have also changed in recent years. In addition to other reviews, control aspects and reliability issues with such sources are discussed [4]. The application of evolutionary technique and game theory in hybrid renewable energy is also presented. The chapter revisits the changing requirements due to DG, summarizing the research works done in the

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Distribution can be short-term (minutes, less than 1 h) or long-term (1 h or more). These terminologies carry their own implication.

Many such related papers refer to "step changes" as a variability characteristic. These are the alteration in the available resource that takes place in small time steps of minutes to some hours. Another variable characteristic is autocorrelation [10] which figures out the statistical relation among values of the same parameter in a series. The relationship of wind speed information between different locations and the corresponding relationship of solar irradiance for different locations are under study by several projects. This spatial correlation is perceived as one of the instrument to gauge variability characteristics. Wind and solar sources may also show one kind of diurnal and seasonal trends.

Power from sun, wind, and ocean additionally exhibit predictable seasonal patterns recognized as a distinguishing variability characteristic. Pattern forecast for this trend of wind and sun is complicated, and it is a subject matter taken up in many papers. In a precise study, Tande et al. [11] have viewed reanalysis data set for illustrating of wind variability characteristics. With information of a temporal resolution of 6 h and a spatial resolution of 2.5° in each latitude and longitude, a two-dimensional linear interpolation of neighboring locations is utilized to get wind speeds at the chosen sites. Both offshore and onshore information can be dealt with in this way for explaining the variability. It is apparent that entry of offshore wind generation and its variability will noticeably affect the grid.

In the study performed by Wiemken et al. [12] record from 1995 extracted from 100 monitored PV systems (rooftop plants 1–5 kW) with a 5 min time resolution ensembled for 243 kW (grid connected) is used. A model is developed taking onshore wind and PV energy generation for the period 2001–2011 across 27 nations in Europe. The data is taken from NASA for hourly values of wind speed and solar irradiance documented at a spatial resolution of 0.5° E/W and 0.66° N/S. The generation from wind and PV translated from the climatic record were later on combined to structure regional or nation-specific datasets. The model first considered PV and wind sources to contribute half of the energy supply of total requirement. Further PV share in the wind/PV proportions of 0, 20, 40 and 60% are investigated.
