Olof Samuelsson

On speed stability

Distributed energy resources, such as wind turbine generators, often employ induction generators. When such a generator is subjected to a nearby fault, its rotor may accelerate and reach high steady-state speed far from that corresponding to the frequency of the system. This is the generator counterpart to induction motor stalling, which is classified as voltage stability. The phenomenon is a matter of stability, but it is not covered by current definitions of power system stability. We denote this type of stability as speed stability and propose a definition, which also includes induction motor stalling.

Monitoring of power system events at transmission and distribution level

Monitoring of events in the power system provides a great deal of insight into the behavior of the system. Events with impact on the entire power system typically occur in or near the transmission network and are therefore best monitored at transmission system substations. This puts great demands on the equipment and in general restricts the use of the data since it becomes property of the transmission system operator. An alternative is to measure at lower voltage levels. This paper documents the close agreement between voltage and frequency at low and high voltage levels during a remote and a nearby fault in the transmission network. The very high quality of the data suggests that valuable information about the transmission system can be obtained at lower voltages.

On DC injection to AC grids from distributed generation

Distributed generation, DG, introduces multi-generator grids and new modes of operation. DG will likely introduce power electronic, PE, converters on a large scale at the low- and medium-voltage levels. DC injection into the AC grid is a threat from grid-connected PE converters that are badly structured. Low DC currents in AC current are difficult to measure at those low levels decided in IEEEtrade 1547 and discussed in other international working groups. Transformers and PE can be co-designed to eliminate DC injection. Modern adjustable-speed drives, ASDs, especially, those with active rectifiers, form a basis to discuss and solve issues of DC injection to AC grids from DG also named distributed energy resources, DER or even DR. DC components from power electronics, embedded in solar power, wind power, energy storage, smart houses, smart-office buildings, own generators, etc. give rise to grid-aspects like AC components give frequencies around the fundamental one. FMEA, failure mode and effects analysis, is judged to be a tool for further synthesis

Active distribution network — Demonstration project ADINE

Since Distributed Generation (DG) often involves renewable energy, it is important to facilitate integration of DG into existing networks. This is the aim of the EU FP6 demonstration project ADINE. It is based on the Active Network Management (ANM) concept, where automation, ICT and power electronics are used to integrate more DG by exploiting active resources instead of just reinforcing the network. The resources are mobilized through ancillary services or requirements. Five enabling solutions within ANM are pushed forward in the project: Protection relay and fault location applications, coordinated protection planning, voltage control with microturbine, centralized voltage control with SCADA/DMS and a new STATCOM. All these are demonstrated in real-life. Finally the performance of each technical solution during conditions with large amounts of DG are analyzed using a real-time simulation environment with RTDS/dSPACE. Introducing appropriate parts of the ANM concept will permit connection of more DG in distribution networks.

Performance indicators for microgrids during grid-connected and island operation

This paper discusses methods for quantifying the performance of microgrids during island operation. Such methods are essential for comparing different designs and different control algorithms. Performance indicators and objectives are proposed for voltage-quality variations, for individual dips and interruptions, and for the reliability during island operation.

Influence of normalization in dynamic reactive load models

As an extension of previous studies on modeling and identification of nonlinear dynamic load models, this letter describes the influence of the normalization in dynamic reactive load models. Using measured data from normal operation, it is shown that the reactive power level, which has previously used as normalization factor is inappropriate. If instead apparent power level is used, the variability in the parameters that describe the reactive load response is drastically reduced.

Damping of electro-mechanical oscillations in a multimachine system by direct load control

Electric utility-controlled customer loads as actuators present new possibilities for power system control. The use of active loads controlled by local bus frequency is proposed for the damping of power system electromechanical oscillations. The viability of the idea is studied for one load in a three-machine power system with a meshed network. Active power mode controllability and phase angle mode observability are determined from the eigenvectors of a differential algebraic description of the uncontrolled system. The geographical variations in the entire network of controllability and observability are shown to be identical. It is presented graphically on a 3-D view of the network topology and is used as a generalization of the term mass scaled electrical distance. System zeros limit the maximum damping. An electromechanical mode pendulum analog is introduced that explains this. Time simulations verify the final controller design.

Automatic determination of parameters in dynamic load models from normal operation data

Economic and environmental concerns will slow down the expansion of the transmission systems in many countries. The de-regulation of the power supply will introduce new power flow patterns on the bulk transmission systems. The net result is that the power systems will operate much closer to their transfer limits and operate there much longer time than has been necessary. The risk for voltage collapse determines the transfer limits in many bulk transmission systems. The recording of the load behavior directly provides information about operating conditions. The applicability of this information might be used for the dimensioning of the transfer limits, the maximum transfer capacity and the calculation of security limits. The main challenge is how to manage an on-line utility able to identify the time-varying parameters of the load, and how to deal with this information to make it useful in an on-line security assessment packages are commercially available today, but its accuracy is still limited by load model uncertainty.

Islanding detection and connection requirements

This paper reflects how anti-islanding protection has developed with increasing DG penetration. Starting from a local issue it has developed to possibly affect system security. The focus here is on how islanding detection methods and anti-islanding protection settings relate to distribution level and TSO requirements. It is confirmed using field measurements and simulations that the most common passive islanding detection methods cannot identify balanced island operation. Active methods can detect a balanced island, but is on the other hand not applicable to all DG units. Altogether it is motivated to reconsider anti-islanding protection taking into account the interests of all involved parties. Doing this reveals that anti-islanding protection based on power line carrier involving the distribution company may be the solution of the future.

Estimation of Eastern Denmark's electromechanical modes from ambient phasor measurement data

In this paper we report on the preliminary results of a collaborative investigation effort between researchers in North America and Europe aiming to baseline the electromechanical modes and mode shapes of the Nordic system from synchronized phasor measurement data. We provide an overview on the Danish power grid and its interconnections, and describe the features of an experimental phasor measurement unit designed by DTU which is used to provide the measurements used in this investigation. Parametric and non-parametric block processing techniques are applied to 48 hours of ambient data recorded at the Radsted and Hovegård substations in Eastern Denmark. The estimated modes are in agreement with those shown in eigenanalysis studies, and other measurement-based investigations. More importantly, the emergence of a new 0.8 Hz mode in Sjælland is reported.