RE Intermittency and System Flexibility

Traditional power systems handle uncertainty from generation, network, and demand using operating reserve, contingency and security analysis. RE increases existing variability and uncertainty in wide spatial and temporal scale, which necessitates faster response and increased operational frequency of system balancing resources. The property of a system to cope up with any externally imposed variabilities/ imbalances is termed as its flexibility.

Flexibility of a system resource can be understood in three dimensions: range of power output (MW), speed of power output change (MW/ min), and duration of providing energy (MWh). A single resource cannot always respond in these three different dimensions, and the operators need to maintain a diverse portfolio of flexible components for day-to-day balancing. Resources having wide range between maximum and minimum output, can respond to a large range of variation, while others having fast response time can damp any quick imbalance within a short duration, saving the system from any negative consequences.

Entities with the ability to deliver energy at longer time span can provide flexibility to address disturbances for stretched duration [49, 50].

An interconnected power system could harness flexibility from different sources, may it be from supply, demand, or transmission side. Though there is consensus on the importance of flexibility with respect to increased RE penetration, identification of appropriate technological option for a national energy system is still challenging. Techno-economic uncertainty of new

2.2 Effect of RE Intermittency on Power System Operation and Planning 17 flexible resources like storage and their higher costvis-a-visexisting options (e.g. gas fired plants), and inefficient planning methods are main reason behind this.

Sources of Power System Flexibility

Generation Flexibility: Flexibility on the generation side can be obtained from fast acting gas, oil-fueled, modern coal-fired, and hydropower plants. Modern nuclear power stations can also provide limited level of flexibility [51–53]. Load following and frequency regulation are the key flexible services that could be obtained from generation side.

Flexibility using Demand Side Management: Demand side management (DSM) actions are measures to obtain a load curve favorable to both customers and utility. Thus, DSM can potentially act as a flexible resource. Peak shaving, valley filling, load shifting, strategic load reduction and growth,etc. are some DSM mechanisms [54, 55]. DSM can either be incentive based or price based. Price based DSM refers to changes in electricity usage pattern by customers, in response to the price change. Some price based DSM mechanisms are time-of-use tariff, real-time pricing, and critical-peak-pricing, etc. Incentive based DSM programs give customers benefit additional to their retail electricity rate. Direct load control, demand bidding/ buyback programs, capacity and ancillary services market mechanisms are some incentive based DSM measures [56–58].

Flexibility using Energy Storage: Energy storage technologies, either in generation or demand side, provide a range of services which system operators could utilize to meet their flexibility need [59, 60]. Storage system could be used either in energy management, power back up, or power quality applications. Bulk storage systems such as pumped hydro, compressed air, and battery storage technologies like sodium sulfur, vanadium redox, lithium ion, and zinc bromide are suitable for energy management services (energy arbitrage, load leveling, transmission and distribution capacity deferral,etc.) due to their long discharge time [61]. Power quality, system stability, and frequency regulation applications require discharge time from seconds to minutes. Small-scale storage such as flywheels and capacitors are useful for this application. Power backup service requires storage system to follow the load with high ramping capability, with a discharge rate between minutes to hours. Lead acid, Nickel metal hydride, and nickel-cadmium batteries could provide these services. Small-scale storage,e.g.batteries and electric vehicles on demand side, could also provide DSM services [62–64]. Thus, storage systems could be useful in integrating large-scale fluctuating RE [65–67].

Flexibility using Inter-Connection: System operational flexibility can also be obtained from other regions connected via transmission lines when flexibility in one area is not sufficient or expensive [68]. Availability of transmission capacity plays a crucial role in mitigating residual load fluctuations due to increased penetration of variable RE generators [69]. A robust and interconnected network is critical to ensure large-scale RE penetration [70–73].

Renewable Energy Curtailment

Stable power system operation requires load and generation balance at every point of time.

At times of RE over-generation, inflexibility of thermal generators and network security criteria may restrict its full utilization [6, 74, 75]. This reduction of generation from variable RE generators is referred to as RE curtailment, which has operational as well as economic consequences [11]. RE over-generation occurs when residual demand is lower than firm- load served by must run base load plants. Also, output variation of base load plants (to support RE fluctuation) is limited by their ramp up/down rates. Therefore, if sufficient balancing resources, reserve capacity, and storage facility to support RE fluctuation in real time are not available, operators are forced to curtail some part of the available RE power to maintain system stability [6]. A sudden unplanned increase in RE generation creates network congestion, which often leads to RE generation curtailment [76]. Significant RE penetration can also cause fluctuation in voltage and frequency, thereby giving back-down signals to RE generators. Curtailment decreases capacity factor of renewable power plants, and thereby reduces project profitability, increases financing cost, weakens investor confidence in RE, and makes it challenging to meet carbon emission reduction targets [74, 77].

In several countries, RE curtailment has been a problem associated with large-scale RE integration. Its degree and impact largely depends on RE penetration level and system configuration. Levels of wind energy curtailment experienced in the United States differ substantially by region and utility. In Electric Reliability Council of Texas (ERCOT), 17%

wind energy curtailment was observed in 2009 which reduced to 4% in 2012 and 1.6% in 2013. Transmission inadequacy, oversupply, and inefficient market design are the primary reasons in this case [78]. In California Independent System Operator (CASIO), curtailment predominantly occurs due to oversupply, generator ramping constraints, line congestion, and must run status of hydropower plants in spring. Here, in early 2014, 19.39 GWh of wind curtailment was witnessed [74]. Bonneville Power Administration (BPA) reports around 2% wind curtailment, mainly due to shortage of reserve capacity. Wind curtailment level of 1%–4% in Midcontinent Independent System Operator (MISO) and 1%–2% in Public Service Company of Colorado are usual [78]. In China, total curtailed wind power during

2.2 Effect of RE Intermittency on Power System Operation and Planning 19 2010–2013 was around 60 TWh, with some provinces having around 30% curtailment in 2012. This was mainly due to limited transmission capacity and mismatch between generation and consumption profile [79–82]. Curtailment rates of several European countries are low, despite having significant RE penetration levels. Strongly interconnected network and well-functioning international power market are the two supporting factors here [82].