J. Verboomen

Phase shifting transformers: principles and applications

The purpose of this paper is to give a short overview of existing technologies regarding phase shifting transformers (PSTs). A classification is made based on the symmetrical or asymmetrical and on the direct or indirect character of the PST. As a case-study, the PSTs in Meeden, The Netherlands are studied more profoundly. Furthermore, a model is developed on a real-time digital simulator (RTDS) in order to demonstrate the capabilities of the PST

Analytical Approach to Grid Operation With Phase Shifting Transformers

Analytical expressions are derived to gain insight in the operating principles of phase shifting transformers (PSTs) in a highly meshed grid. To this extent, the dc load flow algorithm is adapted to account for such devices. This leads to a linear expression for the relation between PST settings and the active power flow in the lines. Based on these equations, the total transfer capacity (TTC) can be described mathematically, which allows for optimization. Furthermore, the linear least squares method is used to distribute a cross-border transport evenly over the interconnectors.

Power flow controlling devices: an overview of their working principles and their application range

The ongoing liberalization process causes an increase in international power flows, putting substantial strain on the transmission grid. This is especially a problem when unidentified power flows or loop flows are involved. At this moment, several congestion problems arise in the European grid. Controlling power flows is one way to extend the utilization of the current grid, without investments in new transmission lines, being problematic due to political, environmental and social considerations. In this respect, power flow controlling devices are rapidly gaining interest of utilities and transmission system operators (TSO). New devices are installed, and it is expected that their number will rise significantly in the near future. This paper provides an overview of the current available power flow controlling devices and their applications. Both electromagnetic transformer based solutions as the fairly recent power electronics developments are covered. Special attention is given to HVDC, both line commutated and voltage source converter based, as they are able to fully control the power flow. In a final part, the use of multiple controllable devices in a meshed grid is treated with special emphasis on the need for global control. As a conclusion different technologies are compared

Border-Flow Control by means of Phase Shifting Transformers

In this paper, a control scheme is demonstrated that regulates multiple phase shifting transformers (PSTs) to equally load the interconnectors of a border. A crucial step in the development of the control scheme is the derivation of phase shifter distribution factors (PSDFs), which indicate the influence of a PST on the active power flow on a certain line. Based on these PSDFs, the Linear Least Squares (LLS) method is used to calculate the optimal PST settings. The degree to which an even repartition can be obtained, depends on the number of PSTs in relation to the number of interconnectors. As a case- study, simulations are performed involving the Dutch and Belgian interconnectors.

Impact of Stochastic Generation in Power Systems Contingency Analysis

A new methodology is proposed for the contingency analysis of power systems with a high penetration of stochastic generation. The essence of the proposed technique is the use of a probabilistic risk measure for the security assessment of the N subsystems deriving from the application of the N-1 criterion. This measure, namely the stochastic stress of the system, corresponds to the concurrent behavior of the stochastic system inputs (loads and stochastic generators) that are situated in the lower voltage levels of the system. In the context of Monte Carlo simulation, this problem is equivalent to the sampling of a large number of non-trivial dependent random variables (stochastic power injections). The modeling procedure is split in two independent tasks: modeling the marginal distributions and modeling the stochastic dependence structure. The second part is the most cumbersome modeling problem. For this, a two-step method is used. First, clusters of positively correlated variables are defined and are modeled based on the concepts of perfect correlation (comonotonicity). Then, the exact correlations between these clusters are modeled based on the joint normal transform methodology. This powerful computational method can be easily applied to large systems with a high number of stochastic generators. The application of the method for the contingency analysis of the 5-bus/7-branch test system (Hale network) with a high penetration of wind generation is presented in the paper

A model of the static synchronous series compensator for the real time digital simulator

This paper presents a detailed simulation model of the static synchronous series compensator for the real time digital simulator. The RTDS is a fully digital electromagnetic transients power system simulator that operates in real time and replaces the previous generation of network analyzers which were based on analog technology. It has been created specifically for real time power system simulation, utilizing advanced custom hardware and software technologies. The SSSC is a FACTS device which is based on the voltage source converter technology employing gate turn-off thyristors in the 2 level inverter which is coupled to a DC capacitor on one end and to the transmission line through a series transformer on the other. The model is verified for standard operational characteristics of the device in a simple 2 bus network and is prepared for further implementation in bigger and more complex networks

Indication of safe transition paths of phase shifter settings by greedy algorithms

In a liberalised market environment, the use of phase shifting transformers (PSTs) or other power flow controlling devices allows the transmission system operator (TSO) to utilise the available grid infrastructure in a more optimal way. In previous work, research has been performed on how to coordinate multiple devices in order to maximise the total transfer capacity. Once the optimal phase shifter settings are determined, the question is how to go from the current setting to this optimal point. In this paper, algorithms are developed to calculate a safe transition between two sets of PST settings. The problem is modelled as a graph in which each combination of PST settings is represented by a vertex (node). Classical shortest path determination algorithms have an unacceptable calculation time for this problem, and an alternative solution must be found. The requirement of the shortest path can be relaxed to a requirement for a good path. This enables the use of a greedy algorithm, which is developed and tested in this paper. Also, an adapted form of the greedy algorithm is proposed, in order to avoid excessive switching between multiple PSTs.

Estimation of Power System Variability due to Wind Power

The incorporation of wind power generation to the power system leads to an increase in the variability of the system power flows. The assessment of this variability is necessary for the planning of the necessary system reinforcements. For the assessment of this variability, the uncertainty in the system inputs should be modeled, comprising of the time-dependent stochasticity of the system loads and the correlated wind resources. In this contribution, a unified Monte-Carlo simulation methodology is presented that addresses both issues. The application of the method for the analysis of the wind power integration in the New England test system is presented.

Coordination of Phase Shifters by Means of Multi-Objective Optimisation

In a liberalised market environment, the use of phase shifting transformers or other power flow controlling devices allows the transmission system operator (TSO) to utilise the available grid infrastructure in a more optimal way. However, each phase shifter adds a degrees of freedom to the control problem, making optimisation more difficult. Furthermore, there are multiple objectives when optimising power system control. In this article, particle swarm optimisation is used to determine the optimal (coordinated) setting of the phase shifting transformers in the Netherlands and Belgium. In addition, the effect of this optimisation on the active power losses is considered, leading to a multiple-objective optimisation problem. The mathematical principles which are needed to deal with such a problem are described, such as the Pareto front. This is an optimal curve, which allows the user to make a trade-off between the importance of the objectives. If the front is convex, it can be found by a technique called conventional weighted aggregation. In this paper, the Pareto front of the losses versus the import capacity is constructed by using this method