As the amount of wind power in power systems has increased it has become necessary to curtail wind power in some high penetration situations. In order to assess the need for curtailment arising from voltage stability considerations we develop a security constrained optimal power flow for minimizing the expected curtailment. We find that with a very high wind penetration and wind farms operating at unity power factor curtailment becomes necessary to satisfy voltage limits. In this case the optimal solution in the studied system is to curtail at a single bus rather than curtailing by a smaller amount at several buses. However, allowing for reactive power production from wind farms reduces the need for curtailments.

Although regular curtailment of wind power has not been necessary in the Nordic power system so far, rapidly increasing wind power capacity means that it may be needed in the future. To estimate the amount of curtailment in the future Nordic power system we develop an hourly dispatch model based on open data. The model is validated against historical data and used to perform a case study for the Nordic power system in 2025 to estimate the amount of wind power curtailment under different assumptions. Curtailment is found to be below 0.3% of available generation for a 26 GW wind scenario and below 1.7% for a 33 GW wind scenario, when considering trade with neighbouring systems. The most important measures for decreasing curtailment are found to be increased transmission capacity, particularly between the areas in Sweden and those in Norway and Denmark, as well as flexibility of nuclear generation. Inertia requirements are found to have a limited impact on curtailments.

To lower CO2 emissions, policy makers want to integrate as much variable renewable energy (VRE) as possible into power systems. This has been translated into targets for VRE as a share of total electricity generation and policies that aim to maximize the use of electricity available from VRE sources. However, in this paper we demonstrate that it is a misconception that maximizing VRE production always lowers CO2 emissions. In fact there are many constraints in power system operation that can lead to situations when curtailing VRE reduces both costs and CO2 emissions. In this paper we identify these situations and constraints, and illustrate them with several examples. The examples show how different constraints from optimal power system operation, using economic dispatch (ED) and unit commitment (UC), can combine to create the seemingly paradoxical result that curtailing VRE reduces both costs and CO2 emissions. Broadly defined these situations can occur 1) due to network constraints which create the need for inefficient redispatch actions if VRE is not curtailed, 2) due to increased need for ramp capability and cycling from other units, and 3) due to reserve/security requirements which can be satisfied more efficiently by allowing VRE curtailment. To achieve the most economical and efficient operation of power systems, instead of VRE curtailment being seen as a measure of last resort to preserve system security, VRE should always be optimally dispatched through markets based on its true cost, thus maximizing the value of VRE to the system rather than its output. 

This paper presents a security constrained unit commitment (SCUC) with continuous intra-hour time-varying reserves. The hourly formulation extends the power-based UC formulation and has reserves which vary continuously within the hour, as opposed to the traditional hourly energy-based SCUC that uses constant reserves within the hour. We show that the traditional hourly energy-based formulation cannot ensure N-1 security at all times, since this formulation is not able to take the power trajectories of units within the hour into account. This is remedied by an hourly power-based version which allows the formulation of contingency constraints to guarantee N-1 security at all times within the hour. The proposed formulation uses continuous time-varying reserves which lowers the cost for providing reserves and makes better use of units’ flexibility while still ensuring N-1 security. The energy-based and power-based formulations are evaluated using different versions of a 5-min security-constrained economic dispatch (SCED) based on real load data, thus simulating the real time operation of the system under different assumptions for reserve procurement. The results show that the power-based formulation increases security compared to the energy-based formulation, both if reserves are fixed to the values from the SCUC or co-optimized in real time by the SCED.Previous article in issue.

We propose a power-based unit commitment formulation with capacity and intra-hour ramp reserves for dealing with intra-hour wind power variability and uncertainty. Although the formulation has an hourly resolution, the intra-hour ramp requirements capture wind power ramp excursions with a time duration below one hour, and thus allows the formulation to consider intra-hourly wind variability and uncertainty. This increases the security of the formulation compared to using hourly ramp reserves and allows more efficient scheduling of units with high ramp rates. We test the formulation with different durations for the intra-hour reserves, and using both hourly and intra-hourly ramp reserves, to find the best reserve formulation. The formulations are evaluated using a 5-min economic dispatch, which simulates the real-time operation of the system, for hundreds of out-of-sample realizations of wind power production. The proposed formulations are then compared to two hourly stochastic formulations and a stochastic formulation with 5-min time resolution. The proposed formulations outperform the stochastic formulations in terms of security, showing that they provide a scheduling which is more robust against intra-hour wind power variations. The proposed formulations also outperform the hourly stochastic formulations in terms of total costs, giving a better trade-off between scheduling costs and security.

The planning of future power systems with high shares of renewable generation requires modelling large and complex systems over long time periods, resulting in models which are computationally heavy to solve. For this reason methods that can be used to decrease the size of power system dispatch models are needed. A common method in large scale planning models is to decrease the model size by increasing the size of the time steps. However, using larger time steps makes the representation of variability of renewable generation and load less accurate, which can affect the results from the model. In this paper, we investigate the possibility to use a power-based version of an economic dispatch model to decrease the model time resolution while getting results which are close to the original high-resolution model. We implement both power-based and the conventional, energy-based, versions of a dispatch model with different time resolutions, and show that the power-based model has better agreement with the high-resolution model, especially as the model time step increases. For example, using the power-based model gives more accurate results for wind power curtailment in a high-renewable scenario.

This paper presents ODIN, an open dispatch model for the Nordic power system. We begin by performing a survey of existing models, and conclude that no model which simulates the different price areas in the Nordic power system and is truly open exists. Existing models either lack data to do an out-of-the-box simulation which accurately represents the Nordic power system, rely on proprietary software, or have a more coarse representation of the Nordic region. In contrast, ODIN has the following properties: (1) It is developed completely in Python and can be solved with open-source optimization tools, thus using no proprietary software. (2) It includes all data needed to do yearly simulations for the Nordic power system, with 35 years of data for weather-dependent production such as hydro power and wind power. (3) It represents all Nordic price areas and has been validated to reproduce historical production and transfer patterns. This paper describes the methodology and structure of the model and illustrates its features with examples and a case study.

In order to accommodate the increasing wind power penetration in power systems grid codes for wind power plants are being continuously updated by TSOs. In this paper we review several European grid codes for wind power released from 2015-2018. Specifically, we focus on the comparison of the new ENTSO-E grid code Requirements for Generators, released in 2016, with national grid codes to see to what extent these are in agreement and how this contributes towards harmonization of grid codes within Europe. Also, we discuss the implications of the grid codes for performing curtailments of wind power plants.

Efficient integration of variable renewable energy (VRE) such as wind power into power systems requires methods for power system operation planning that account for VRE uncertainty and variability. This has motivated extensive research into unit commitment (UC) and optimal power flow (OPF) formulations with VRE uncertainty. However, these formulations are often tested using significantly simplified representations of VRE production. We seek to address this issue by providing a model for generating realistic wind power scenarios using real production and forecast data. The scenarios are generated using 5-min production and 30-min forecast data for real wind farms from Australia. The model captures the empirical distribution of the forecast errors and the covariance between different wind farms. The high time resolution of the production data also allows the recreation of the high-frequency (5-min) component of wind power production. The resulting model is openly available, and can be used to generate wind power scenarios for use in formulations for operation planning of power systems (UC/OPF) considering wind uncertainty and intra-hour variability. The scenarios can be tailored according to preferences for, e.g., the number of wind farms and their geographical dispersion. 

This paper presents power-based unit commitment (UC) formulations with N-1 security constraints. Two different formulations are proposed, one with constant reserves within the hour and one with time-varying reserves. The formulations are compared to a conventional energy-based UC formulation using different examples. We show that the energy-based formulation does not ensure N-1 security at all times within the hour, since it does not account for the power profile of units. In contrast, the proposed power-based formulations guarantee N-1 security within the whole hour for a piecewise linear demand profile. The formulations are also evaluated using a 5-min economic dispatch based on a real load profile, simulating the real-time operation of the power system, showing that the power-based formulations also provide increased security in this case. Compared to using a power-based formulation with constant reserves, using time-varying reserves decreases the reserve cost while ensuring a similar level of security.