The thesis’s structure is organized in the manner of 6 chapters that comprise the following categories: in-troduction, theory, approach and method, results and discussion, and conclusion.
Chapter 2 describes the theoretical framework of the thesis, which includes an overview of common grid problems that can occur in both the distribution and transmission grid and the reason for their occurrence.
This also includes the benefits and operation approaches for different types of DFRs.
Chapter 3 continues with the theory of LFM strategy and the explanation of the different LFMP and their roles and responsibilities in the flexibility market. This explanation will lead to a better understand-ing of the market strategy and coordination and the two-stage stochastic optimization, which is the main focus of the following parts. In addition, this will also lead to a better understanding of the choices made behind the implementation of the multi-period hybrid AC/DC-OPF.
Chapter 4 describes the approach and method, which encompasses the analysis and explanation for choosing the given power flow models in the distribution and transmission grid. These models are then combined to make a single hybrid AC/DC-OPF model. The last extension of the model is to implement the multi-period approach, which results in a single multi-multi-period hybrid AC/DC-OPF model. This model allowed the implementation of DFR with their respective operational possibilities and limitations.
Chapter 5 demonstrates the capabilities of the developed multi-period hybrid AC/DC-OPF model through different test cases, which involve solving different grid problems, as discussed in Chapter 2.
Lastly, Chapter 6, will conclude the LFM strategy and performance of the multi-period hybrid AC/DC-OPF model and what this thesis has achieved using this model. The combined results will also provide recommendations for further expansion of the model and better test cases.
2 Flexibility Analysis for Improving Grid Operation
With the increasing electrification of society and integration of DERs, the distribution grid is undergoing profound changes. On the positive side, electrification and DERs can lead to a more efficient, sustainable, and low-carbon society. Unfortunately, this sudden evolution also introduces particular challenges for the distribution grid’s operation. Analysis from Navigant report [4] predicts that the amount of installed DER will increase in the coming years as shown in figure 1. The main DERs predicted by the report to be increase are distributed generation, Distributed Energy Storage System (DESS), Electric Vehicle (EV) charging load, Demand Response (DR), energy efficiency, and Distributed Energy Resource Management System (DEMRS). So, preparing the distribution grid for the implementation of DERs will allow for a smooth transition to a more complicated but efficient distribution grid. This chapter will discuss how the distribution grid can benefit from DER usage in solving impending grid problems and challenges.
Figure 1: Predicted increase of installed power from different DER in the coming years, as analysed by Navigant report [5].
2.1 Flexibility Resources in Distribution Grid
When talking about flexible resources in the distribution grid, we refer to the usage of DERs. These can be defined as production and supply resources connected directly to the distribution grid for energy management [6]. Since these DERs are being used as flexibility, they will be referred to as distributed flexibility resources (DFR) from here on in this thesis. These assets include Energy Storage System (ESS), DGs, load shifting, and load shedding, which will provide flexibility for the own developed multi-period hybrid AC/DC-OPF model. This chapter will also include a theoretical introduction to these flexibility assets and explain their activation and form of operation. In addition to several other papers studied during this thesis, this explanation will mainly encompass the proceeding work conducted in [3].
2.1.1 Energy Storage System
ESS are defined as equipment that can store electric energy over time in a given state until converted back to electric energy [7]. Adapting this type of DFR can benefit the power system in several ways. The specialization project [3] describes the operation and activation of ESS, with five categories as potential ways to improve the grid efficiency. This thesis will focus on three of these categories:
• Ancillary services: Using ESS for purposes like voltage control, load following, and supply reserve.
• Grid system: Using ESS to solve disturbances in the power grid and congestion management.
• End-user/Utility customer: Using ESS to reduce overloading of the grid by utilizing load shifting.
ESS can have different attributes depending on their location in the grid, how much energy they can store, and other properties related to the operation. Based on these attributes, ESS can suit different roles. The capacity of the ESS utilized in this thesis will be in the range of medium capacity and will therefore suit the roles of:
• Medium-power applications in isolated areas, such as individual electric systems and towns that utilize ESS for electric supply and end-user/utility customers.
• Network connection application with peak leveling, which suits the role of grid system management and renewable integration.
• Power-quality control applications so solve grid system disturbances.
ESS can be viewed as a consumption or production unit from the grid’s perspective, depending on its situation. In simple terms, when the prices are high, ESS should try to sell its power, and when prices are low, it should try to purchase power. The balance between the demand and supply will also influence the operation of ESS. In order to avoid load shedding, storing and discharging of ESS can occur despite the prices not being optimal for its operation to achieve cost minimization. Based on these premises, the use of ESS as load leveling assets [8] can solve both congestion and voltage problems by discharging itself according to the grid situation. Figure 2 presents a concept of load-leveling, where power is stored during low demand and released during high demand.
Figure 2: The concept of load leveling during peak hour consumption [9].
2.1.2 Distributed Generation
There can be a wide range of DG’s with different criteria and operational conditions in a distribution grid.
A common definition for a DG is an electric power source connected directly to the distribution network or on the customer side of the meter [6]. Instead of listing the numerous DG types, they are sorted based on one of two categories: intermittent and non-intermittent DG. Intermittent DG means that a DFR is not available on-demand; there are external factors that decide production. DFR can then be anything from a small, intermittent, and renewable photovoltaic array used by end-users to large combustion turbines that are non-intermittent and non-renewable operated by a commercial producer [10].
The use of intermittent production from solar and wind will allow for an entirely new complexity level for operation planning. Therefore, only non-intermittent DGs are considered in test cases performed later in the thesis. In regards to capacity, if using [6] as a standard for DG’s rating, their capacity in this thesis will be of medium and large magnitude. DGs have many of the same properties as ESS when solving grid problems such as congestion and low voltage magnitudes. Their advantages come from the higher capacity and the ability of on-demand production without the need to store energy in advance. This advantage enables a more selective operation relative to prices. Based on this, DG’s activation has been deemed more expensive than ESS for the performed test cases.
2.1.3 Load-Shifting
Instead of regulating the production to match the consumption, load shifting tries to regulate the con-sumption to even the system’s capacity. This technique shifts a part of the concon-sumption during load peak hours to off-peak hours, preventing the occurrence of grid problems in the first place [11]. This way, power-intensive tasks which are flexible with their time of use can shift their operation to off-peak hours. The result of this action is that consumption is more evenly distributed throughout the day, while the total energy consumption remains unchanged. In regards to solving grid problems, load shifting can significantly
peak hours. Voltage problems in the distribution grid can also be solved with load shifting if the active power is the dominant factor for voltage regulation. Figure 3 illustrates the concept behind load shifting, where the solid line indicates the original load profile while the dashed line illustrates the load profile after load shifting.
Figure 3: Visualisation of how load shifting reduces the load during peak hour consumption [12].
2.1.4 Load Shedding
Load shedding is not a novel concept in the power industry. Considering load shedding as a DFR might not be entirely correct, as the action is not entirely ”flexible”. The action of load shedding is the deliberate shutdown of electric power in part, or parts of a power distribution system [13]. This shutdown is the last resort action the system operator can do in extreme grid situations, which, if not treated, could potentially lead to severe grid damage and cascading outages. Unlike ESS, DG, and load shifting in the LFM, load shedding is not supposed to be a planned action, and thus all use of this measure should be avoided.
Therefore, in the LFM, load shedding should only be utilized in handling extreme grid situations where no other measures are available.