Herwig Renner

Flicker propagation in meshed high voltage networks

Flicker emission measurements are mostly made at the point of common coupling of fluctuating loads. However, in the case of several disturbers in a network, the interaction of these loads must be taken into account. Therefore it is important to calculate the flicker propagation in networks. This knowledge can be used for evaluation of measures in networks for flicker reduction. This paper compares different calculation methods with actual measurements. All methods are based on the use of conventional tools for power system analysis and should therefore be applicable for every technical engineer.

Measurement and analysis of neutral point currents in a 400-kV-network

Neutral point treatment is a very important topic for protection and network operation. For different reasons 400-kV AC-power networks are operated with direct grounding of the neutral point. One major part of electrical networks are transformers. In transmission systems, usually at least one transformer per substation is operated with direct grounded neutral point, thus allowing earth currents to enter the transformer. New transformers are designed for very low noise emissions. During commissioning of a new transformer, unexpected noise was noticed and saturation due to DC currents respectively very low frequency currents (below 1 Hz) was assumed to be the reason. In a current research project of Austrian Power Grid and Graz University of Technology measurements of the neutral point current were performed. Analyses of these measurements revealed several frequency components in the spectrum, including DC, slow variations below 1 Hz, 16.7 Hz from railway system, 50 Hz mains, harmonics and interharmonics. This paper gives a description of the measurement setup, highlighting the problem of recording small DC currents in the presence of high AC currents. Furthermore results from a neutral point current measurement in the Austrian 400-kV-grid are presented and discussed.

Fast minimal cutset evaluation in cyclic undirected graphs for power transmission systems

The evaluation of network reliability is an important topic in the planning, design, and operation of power systems. The aim of this publication is to investigate and tune up the speed of calculating 2-terminal minimal cutsets (MC). Two fast and well known algorithms are compared with a new developed MC algorithm concerning the speed of computation. The new designed algorithm is based on a novel graph reduction method, and on an adapted recursive merge method. Eleven benchmark-networks are used to analyze all three MC algorithms. Experimental results show that the new developed MC algorithm has a linear dependency between the computation time and the graph density of a network for a fixed number of nodes. Furthermore it is shown that the proposed algorithm is faster than the minimal path based algorithms and the currently best available MC algorithm for 2-terminal reliability in complex power transmission networks. A representative 57 node power transmission network demonstrates that the new proposed algorithm reduces the computation time for all relevant MC by 96.2 %.

LVRT-retardation-device for decentralized power plants

During fault events in the electrical power grid nearby a synchronous generator, the rotor speed of the machine is increased, because dissipated power is below generated power. This may cause high rotor angle excursions and hence the machine to lose synchronism with the power grid. This paper presents a retardation device, which is able to curtail the acceleration of rotor speed during fault events. Besides, the backswing phenomenon, which decelerates the rotor speed in the first few milliseconds of the fault event. Different simulation methods are addressed within the scope of work. All simulations were performed with the power system analysis software DIgSILENT PowerFactory.

Damping low frequency oscillations with hydro governors

This paper presents a novel approach into damping inter area oscillations during poor grid conditions and low oscillation frequency ranges at the generator side. The method is based upon the application of a single input Power System Stabilizer to the actuators of hydro governor systems (PSS-G). One constraint thereby is the decoupling of the damping application from the standard operational functionalities such as primary control. The investigation is based upon a two-step approach: A principal feasibility investigation utilizing a Single Machine Infinite Bus-SMIB Model and a second step using a four generator two area model focusing on the interaction of the governor control path and the voltage control path. Local signals and signals derived from Wide-Area-Measurement System (WAMS) like speed deviation or accelerating power respectively voltage angle deviation have variously been used as input signals and compared by their impact on system damping. This article is part of a special session at PowerTech 2013 which is being proposed by the EU FP7 Real-Smart Consortium, a Marie Curie Industry-Academic Pathways and Partnerships project1. Most of the work presented in this paper has been developed during a secondment at Statnett in Oslo, Norway.

Automated power system event detector and classifier

In this paper a methodology for the on-line detection and classification of AC power system events measured at the interconnecting point between two parts of a power network is presented. These events can be isolation faults, broken lines, switching capacitors, starting motors, energizing transformers, and swinging turbines. The method uses the three phase-to-ground voltages and the three phase currents. Information about the net configuration as well as the neutral point treatment of the power system is not needed. The developed method comprises a signal pattern recognition unit embedded into a fuzzy expert system. The final result of the event evaluation is a list of all likely events including information about their direction, ranked according to their probability of recognition.

Enumeration based reliability assessment algorithm considering nodal uncertainties

A novel fast enumeration based reliability assessment algorithm based on a probabilistic load flow approach considering nodal uncertainties is proposed in this paper. The frequency and duration indices for each load point and the expected energy not supplied as well as the expected interruption costs are derived with the proposed methodology. By reducing the number of necessary system states a huge decrease of computation time is achieved while maintaining an adequate degree of accuracy. In order to verify the accuracy and the computational expensiveness of the proposed algorithm a commercial available program is used to compare the calculation results of a utility driven sub transmission power system. It is demonstrated that a fast calculation of the load indices, including station originated outages, can be performed, based on uncertain nodal powers, for instance nodal injections from wind farms or other stochastic power system loads, with a high degree of accuracy.

Systematical determination of load flow cases for power system planning

This paper presents a novel method to determine a minimum number of representative load flow cases with load and generation data in a meshed transmission grid, which cover all critical power line loading situations in a given time frame. Measured active and reactive line loading data obtained from SCADA are the basic data input for this method. In order to find the optimum number of load flow cases a heuristic search algorithm in combination with a genetic optimization algorithm is used. The comparison of the required load flow cases with conventional load situations e.g. peak load or peak generation show, that conventional load flow cases do not cover all critical power line loading situations. Load flow cases determined with the presented method give an objective and representative picture of the expected situations and can be used for power system planning in the view of increasing demand and new generation capacity installations.

Voltage stability monitoring using a modified thevenin impedance

This paper presents a method for voltage stability monitoring based on the maximum power transfer to the load. The only required information is the system topology, the data from PMUs and the operational status of synchronous generators. With this information, the Thevenin impedance seen from a load bus can be estimated and by using the Thevenin theorem for maximum power transfer an impedance based stability index can be established. Since the Thevenin impedance is influenced by the operational conditions of the synchronous generators, it is consequently not a constant value and differs from the short circuit impedance of the bus. It is emphasized that the method requires only the information of the considered subsystem. Therefore, since the computation requirement is insignificant, the algorithm can be used for online monitoring. The validation of the approach is achieved by simulating a simple transmission system.

Validation of aggregated harmonic current source models based on different customer type configurations

Harmonic voltages are important voltage quality parameters defined in EN 50160. For harmonic voltage studies in electricity networks, harmonic emission of loads is often modelled as harmonic current source. In this paper harmonic current sources were parameterised on the basis of measure-ments of total harmonic current emission of several different low voltage networks. Measurements from low voltage net-works with different typical customer configurations were used: residential, offices and PV. A real medium voltage electricity distribution network of an Austrian distribution system operator, with significant consumption of residential and office customers, was chosen for this analysis. In order to automate the harmonic load flow calculations in DIgSILENT PowerFactory for every 10-minute interval of one week, a script in DIgSILENT Programming Language - DPL was developed. Harmonic voltage results from the harmonic load flow simulation are compared with the results of harmonic voltage measurements from power quality monitoring system installed in this network. The goal of this paper is to assess the suitability of the approach, where only background harmonics and key harmonic current sources are modelled. Since the approach provided good results, it can be used in future work as a basis for optimising the number and locations of power quality monitors in electricity distribution networks.