Nicklas Johansson

Interarea Oscillation Damping Using Active-Power Modulation of Multiterminal HVDC Transmissions

Analytic investigations of interarea oscillation damping of an idealized two-machine system using active-power modulation of three- and four-terminal high-voltage dc (HVDC) transmissions are presented. It is shown that, generally, the pairing of two terminals gives the highest damping. Based on the analytical results, recommendations for how the pairing should be made are developed. Simulations, both on a two-machine system and on a more realistic 23-machine system, verify the analytical results.

A Comparison of Different Frequency Scanning Methods for Study of Subsynchronous Resonance

This paper compares four different methods for determining the electrical damping of a power system seen from one generator as a function of frequency. This information is useful when the risk for subsynchronous resonance (SSR) in the system is evaluated. The study compares one frequency scanning method which is implemented in a time-domain digital simulation program with three methods of different complexity based on analytical calculations. The time-domain simulation method is easily implemented with a detailed model of the power system including complex load and generator models, whereas the analytical methods are based on simpler models of the power system. The computational effort is much larger for the time-domain method than for the analytical methods. In the study, all methods were used to determine the damping characteristics of a four-machine power system in different configurations. The study shows that fast analytical methods may provide results which closely agree with the detailed method of time-domain simulation. However, the study also shows that the level of accuracy in the analytical model is very important.

Adaptive Control of Controlled Series Compensators for Power System Stability Improvement

This paper describes the design and verification of a time-discrete adaptive controller for damping of inter-area power oscillations, power flow control, and transient stability improvement. Only locally measured signals are used as inputs to the controller. The controller may be used with any FACTS device which operates as a variable series reactance in the power grid, such as for example the TCSC. The controller is based on a reduced system model which relies on the assumption of one dominating inter-area oscillation mode in the power system where the FACTS device is placed. Verification of the controller is performed by means of digital simulations of a four-machine system commonly used to study inter-area oscillations.

An Adaptive Model Predictive Approach to Power Oscillation Damping Utilizing Variable Series Reactance Facts Devices

This paper describes an adaptive method of controlling FACTS devices for power oscillation damping. The method is based on step-wise series reactance modulation. Here, a reduced model of the power system with only two rotating masses is used as a basis for the control design. The model parameters are updated using local measurements of the active power on the controlled line. An adaptive closed loop controller is developed based on the principle that it is possible to stabilize an oscillation in a power system which is characterized by one major mode of oscillation by switching a reactance in series with one transmission line in a small number of steps. The reduced model parameters are recomputed when new information of the system response is known making the control scheme an adaptive one. The paper also includes the derivation of a damping controller with a power flow control feature and a verification of the controllers using digital simulations of power system models of different complexities

Impact on Interarea Modes of Fast HVDC Primary Frequency Control

Primary frequency control is often added to the control system of a high-voltage DC (HVDC) transmission that interconnects two nonsynchronous ac grids. As the active-power actuation lag is typically in the range of hundreds of milliseconds, the HVDC primary frequency control can be made fast enough to permit inertia sharing among the interconnected ac grids. This reduces the maximum frequency fall after a loss-of-generation contingency (the nadir). A fast HVDC primary frequency control may affect the interarea modes of the interconnected grids. To this end, it is shown that, at least for the cases studied, the modal damping never decreases through the introduction of the mentioned control.

Discrete Open Loop Control for Power Oscillation Damping Utilizing Variable Series Reactance Facts Devices

This paper describes an open loop method of controlling FACTS devices for power oscillation damping. The method is based on step-wise series reactance modulation. The principle of the method is to stabilize an oscillation in a power system which is characterized by one major mode of oscillation by switching a reactance in series with one transmission line, thereby changing the total reactance between the areas participating in the oscillation. In order to stabilize the system during an oscillation, the stationary voltage angle difference between the areas is changed to coincide with the present angle at a point where the speed of the lumped machine representations of the areas is nominal. This is the case at the peaks of the oscillation. To determine the required size of the reactance step, a reduced model of the power system is used. The model parameters are continuously updated using local measurements of the active power on the reactance controlled line. Several approaches for damping with different numbers of steps are presented and verified using digital simulations of power system models

Estimation of grid parameters for the control of variable series reactance FACTS devices

For high performance control of flexible AC transmission system (FACTS) devices with controllable reactances, a representation of the surrounding grid is essential. Using such a model, an adaptive control strategy can be developed which optimizes the control in real time as the grid parameters change. This paper proposes such a generic grid model and derives the theory of how to estimate the main parameters using measurements of the line active power response from small step reactance changes. The estimation methods are verified using simple grid models in PSCAD simulations and more advanced grid models using SIMPOW simulations of a modified version of the CIGRE Nordic 32 grid. This work should be thought of as a foundation for developing control systems for variable series reactance FACTS devices

Preliminary Design of Power Controller Devices Using the Power-Flow Control and the Ideal Phase-Shifter Methods

This paper introduces a new method for the preliminary design of power controllers (PCs) in the electric power grid. The method, which is denoted the ideal phase-shifter (IPS) method, utilizes the concept of the power controller plane where the active power of the PC line is plotted versus the difference in voltage angle between the PC terminals. The power controller plane makes it possible to graphically visualize the working area of a PC in a power grid and thus determine the grid situations which are dimensioning for the PC. The IPS method offers the possibility of plotting the grid characteristics in the power controller plane which are unbiased with respect to the reactive properties of the PC. This makes the method suitable for comparison and preliminary design of PCs of different types and with different characteristics by simple geometrical considerations. In this process, the IPS method uses the power-flow control method for deriving the PC characteristics. This paper includes an application example of the method where it is used for dimensioning of two different PCs in a 26-bus test system.

Adaptive control for power oscillation damping by means of a Thyristor Controlled Series Capacitor (TCSC)

This paper describes an adaptive method for control of a TCSC device for power oscillation damping. The aim is to mitigate power oscillations in power systems with one dominating oscillation mode. To design the controller, a simple generic model of the power system with only a few variables is used. Initially, a fixed-parameter controller which is based on a pole-placement technique is designed. The controller is then implemented in a digital simulator and tested in a four-machine power system. The fixed-parameter controller performs well in most cases but in some contingencies, a performance decay is seen. To improve the controller performance, an adaptive control design has been investigated. The investigated controller is a self-tuning regulator and a recursive least-squares method is used to estimate the system model parameters. An open-loop controller for transient stability improvement and a closed-loop PI-controller for power flow control are also presented. The adaptive controller has been validated with good results by means of simulations of a number of contingencies in different operating conditions of the four-machine system.

Frequency response improvement with synchronous condenser and power electronics converters

This paper gives an overall review of the working principle of the inertial response from the synchronous condenser for frequency support in the grid. In addition to this, four proposed frequency control strategies for improvement of network frequency response by static energy storage devices, such as the Static Synchronous Compensator (STATCOM) with suitable energy storage elements or the battery energy storage system (BESS), are presented. The proposed frequency control strategies are evaluated and benchmarked with the synchronous condenser in the same network. It is shown that a power electronic converter based system with energy storage can be designed with lower rating, still providing the same or better frequency support than the synchronous condenser.