Influence of Cables on Power Transmission Network Frequency Response

Harmonic resonance is an important factor to be considered in a power transmission networks during connection of remote generation units with high-voltage cables (e.g. wind or solar power plants etc.). It is known that cable characteristics differ from characteristics of conventional overhead lines. Cable capacitance is far higher than capacitance of an equivalent overhead line, what can potentially lead to the low resonant frequencies which may be triggered by various switching events, such as transformer or shunt reactor energization etc. In this paper, possible consequences of power plant connection to transmission network with long HVAC cable are analysed, with regard to harmonic resonance and frequency response of the network. The research has been conducted on developed model of the power transmission network in the software for calculation of electro-magnetic transients EMTP-RV

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Parametric and nonparametric identification of shell and tube heat exchanger mathematical model

Parametric and nonparametric models of a shell and tube heat exchanger are studied. Such models are very important because they provide information about controlling a system operation. Without the model, the control task would be difficult for tuning of controller. For many years, researchers have studied these models; however, their models are still less satisfactory since they are not in general form. This problem is caused by two key issues, namely, multiple unknown parameters and highly nonlinear structures. Energy balances have been set-up for condition of unknown parameters which involved, among others, temperature, flow rate, density and heat capacity. The identification process produces a dynamic model of the heat exchanger which is developed based on a lumped parameter system. The model developed is single input single output whereas input signal is hot water flow rate and the output is cold water temperature. The general form of the model obtained could have parametric model structures such as auto regressive with external input, average auto regressive moves with external input, output error or box-jenkins. The study in this thesis aims to solve the general form through parametric and nonparametric models which has been proposed as candidate models. Both candidate models have been implemented and tested by applying several data sets constructed in lab experiments. The first finding is the derivation of the dynamic model in the general form of the transfer function in s domain, and it has been proven that it has parametric model structure. The second finding is the first order without delay time transfer function of the nonparametric model where they have gain is 35.20C and time constant 7200s. These have proven to fulfill that the measured experimental data contains calculated error that is no than more 2%. The third finding is the parametric model obtained has proven that the measured experimental data contains calculated error level that is very satisfactory, i.e. less than 1%. This error has been determined based on the final prediction error for each model structure used. The best model has been chosen, i.e. bj31131. It has the smallest values of the loss function and final prediction error of 0.0023, and it has high values of the best fits, i.e. 96.84%. Parameter optimization has been calculated to determine minimization or maximization of functions which involved the parameter studied. It is used to find a set of design parameters that can in some way be defined as optimal. The first until the third findings are minor contribution while the parameter optimization has been a major contribution

Synchronous Machine Emulation of Vsc for Interconnection of Renewable Energy Sources through Hvdc Transmission

The majority of the energy demand over the past years has been fulfilled by centralized generating stations. However, with a continuously increasing energy demand, the integration of decentralized renewable energy sources (RES) into the power system network becomes inevitable even though these sources affect the stability of the grid due to their intermittency and use of various power converters. The transmission of power over long distances from RES is usually accomplished either by AC or DC transmission. High voltage DC transmission (HVDC) is preferred over high voltage AC transmission (HVAC) due to numerous and complex reasons, such as its lower investment cost for long transmission cables, lower losses, controllability, and limited short circuit currents. Several control methods for grid-connected voltage source converters (VSCs), such as power-angle and vector-current controls, are being adopted in RES interconnections. However, these methods face several issues when used for a weak grid interconnection. This thesis develops a control strategy for a VSC–HVDC transmission system by referring to the synchronverter concept. In the proposed method, the sending-end rectifier controls emulate a synchronous motor (SM), whereas the receiving end inverter emulates a synchronous generator (SG) to transmit power from one grid to another. The two converters connected by a DC line provide a synchronverter HVDC (SHVDC) link. Given the high demand for sustainable energy, integrating RES—which can be extended to wind-based resources—into the long-haul HVDC link becomes essential. Therefore, in this thesis, a windfarm with a type 4 permanent magnet SG is integrated into the HVDC link through a rectifier. Depending on the wind speed, the proposed control strategy automatically shares and manages the wind generator power on the DC side by using a battery energy storage system (BESS) connected to the HVDC link to stabilize the power fluctuations generated by the intermittency of the wind farm. The performance of the synchronverter-based HVDC transmission was verified by using a MATLAB Simulink model. Results show that the controller can effectively control the power flow from one grid to another and that the effect of wind fluctuation on the grid can be mitigated by introducing a BESS at the DC link. Therefore, by properly controlling the SHVDC, BESS, and RES connected to the HVDC system, the power from remote RES can be connected to a weak AC grid in a stable manner