High Voltage AC Transmission System Research Articles

ANN-Based Relaying Algorithm for Protection of SVC- Compensated AC Transmission Line and Criticality Analysis of a Digital Relay

Background:The Static VAR Compensator (SVC) has the capability of improving reliability, operation and control of the transmission system thereby improving the dynamic performance of power system. SVC is a widely used shunt FACTS device, which is an important tool for the reactive power compensation in high voltage AC transmission systems. The transmission lines compensated with the SVC may experience faults and hence need a protection system against the damage caused by these faults as well as provide the uninterrupted supply of power.Methods:The research work reported in the paper is a successful attempt to reduce the time to detect faults on a SVC-compensated transmission line to less than quarter of a cycle. The relay algorithm involves two ANNs, one for detection and the other for classification of faults, including the identification of the faulted phase/phases. RMS (Root Mean Square) values of line voltages and ratios of sequence components of line currents are used as inputs to the ANNs. Extensive training and testing of the two ANNs have been carried out using the data generated by simulating an SVC-compensated transmission line in PSCAD at a signal sampling frequency of 1 kHz. Back-propagation method has been used for the training and testing. Also the criticality analysis of the existing relay and the modified relay has been done using three fault tree importance measures i.e., Fussell-Vesely (FV) Importance, Risk Achievement Worth (RAW) and Risk Reduction Worth (RRW).Results:It is found that the relay detects any type of fault occurring anywhere on the line with 100% accuracy within a short time of 4 ms. It also classifies the type of the fault and indicates the faulted phase or phases, as the case may be, with 100% accuracy within 15 ms, that is well before a circuit breaker can clear the fault. As demonstrated, fault detection and classification by the use of ANNs is reliable and accurate when a large data set is available for training. The results from the criticality analysis show that the criticality ranking varies in both the designs (existing relay and the existing modified relay) and the ranking of the improved measurement system in the modified relay changes from 2 to 4.Conclusion:A relaying algorithm is proposed for the protection of transmission line compensated with Static Var Compensator (SVC) and criticality ranking of different failure modes of a digital relay is carried out. The proposed scheme has significant advantages over more traditional relaying algorithms. It is suitable for high resistance faults and is not affected by the inception angle nor by the location of fault.

A Detailed Comparison of the Outer-loop Control Performance at the Receiving End Station of VSC Based HVDC Transmission System

The application of high voltage (HVDC) transmission for integrating large scale and/or off-shore wind generation systems with the electric grid is attractive in comparison to extra high voltage AC transmission systems due to a variety of reasons. A suitable control system is required for a VSC-HVDC system that provides good performance across a range of operating conditions. Two strategies are studied for their potential to enhance system robustness. The d-axis current control and DC power control are implemented in the outer-loop control at the receiving end of VSC-HVDC system. Small-signal analytical model is used to perform eigenvalues analysis and to design controllers. Both control techniques are investigated and the results are compared. An additional DC voltage droop control is added for both schemes. Its advantages were investigated. The droop gain and the cut off frequency of the DC voltage feedback filter are selected by analyzing the root locus of the analytical model to select the optimum values. It is established that d-axis current control with DC voltage droop control shows better performance practically for very weak AC system at both ends as well as with very long DC cable. The simulation results performed on PSCAD/EMTDC verify the feasibility of the control strategies effectively under different scenarios in order to confirm the obtained conclusions from analytical investigations.

Advance Technology in Application of Four Leg Inverters to UPQC

This article presents a novel application of four leg inverter with conventional Sinusoidal Pulse Width Modulation (SPWM) Scheme to Unified Power Quality Conditioner (UPQC). The Power Quality problem became burning issues since the starting of high voltage AC transmission system. Hence, in this article it has been discussed to mitigate the PQ issues in high voltage AC systems through a three phase Unified Power Quality Conditioner (UPQC) under various conditions, such as harmonic mitigation scheme, non linear loads, sag and swell conditions as well. Also, it proposes to control harmoincs with various artificial intelligent techniques. Thus application of these control technique such as Artifical Neural Networks, Fuzzy Logic makes the system performance in par with the standards and also compared with existing system. The simulation results based on MATLAB/SIMULINK<sup>TM </sup>are discussed in detail to support the concept developed in the paper.

Economic Assessments of LFAC and HVDC Transmissions for Large Offshore Wind Farms

Abstract Offshore wind farms extend a distance from an onshore grid to increase their generating power, but long distance and high power transmissions raise a lot of cost challenges. LFAC (Low Frequency AC) transmission is a new promising technology in high power and low cost power transmission fields against HVDC (High Voltage DC) and HVAC (High Voltage AC) transmissions. This paper presents an economic comparison of LFAC and HVDC transmissions for large offshore wind farms. The economic assessments of two different transmission technologies are analyzed and compared in terms of wind farm capacities (600 MW and 900 MW) and distances (from 25 km to 100 km) from the onshore grid. Based on this comparison, the economic feasibility of LFAC is verified as a most economical solution for remote offshore wind farms. Keywords: LFAC, HVDC, Offshore, wind farm I. INTRODUCTION In recent years, energy systems based on wind power have rapidly enlarged their application areas, especially towards large offshore wind farms (over 100 MW) and micro grid systems. The conventional onshore wind farms have small power generation and short distance power transmission to a power grid. However, for a large remote wind farms, a new power transmission system is required to provide high energy density and low loss power transmission characteristics with low investments. So, how to connect large remote wind farms to the onshore micro grid with low power losses and economic benefits is the prime concerns of researchers, and its economic power system and wind farm layouts for transmitting high power and long distance has gained more attentions. The conventional HVAC (High Voltage AC) system consists of wind generators, transformers, transmission cables and reactive power compensators, and the generated power is converted to a very high voltage (154 kV or 345 kV) by transformers. The HVAC power system transmits the power through 3 cores XPLE cables through underwater, but the transmission distance of the HVAC power system is the most critical factor against power transmission capability because reactive power losses are proportional to the distance. Therefore, HVAC transmission system is not adequate to long distance large offshore wind farms. Recently, new technologies for power systems have been reported [1]-[15] to provide alternative ways to maximize the power transmission capability. The most outstanding technology is HVDC power system, which has high economic benefits for long distant applications because HVDC power system has no limitation of the transmission capability. HVDC Transmission does not suffer from the reactive losses found in the transmission of HVAC system. However, in order to transmit DC power from a remote wind farms the generated AC power must be converted to the DC power and must be converted back to the AC power for a grid connection. Converting the AC power into the DC power requires an expensive AC to DC converter station to be installed at the remote wind farm area as well as a DC to AC power converter station at a receiving end, prior to the grid. An alternative technology is LFAC (Low Frequency AC) transmission system. LFAC transmission system uses lower frequency (50/3 Hz or 60/3 Hz) than a grid frequency (50 or 60 Hz) and requires no offshore power converter stations but an onshore frequency converter station. LFAC transmission system has an ability to extend a transmission distance and capability rather than the conventional HVAC transmission system. However, LFAC system can generate some audible noises and have transformer saturation and size problems. To adopt a best power system topology for large remote offshore wind farms, an economic analysis about HVDC and LFAC system should be performed. However, economic investments are directly dependent on a power system configuration, a distance and transmission capability. Therefore, this paper proposes HVDC and LFAC power system configurations and presents an economic assessments and comparison of LFAC and HVDC transmissions for large offshore wind farms. The economic assessments of two different transmission technologies are analyzed based on the proposed power configurations and compared in terms of wind farm’s capacities (600 MW and 900 MW) and distances (from 25 km to 100 km) from the onshore grid. From the comparison, the economic feasibility of LFAC is verified as a most economical solution for the large offshore wind farms.

Optimisation of reactive power compensation of HVAC cable in off‐shore wind power plant

High voltage AC (HVAC) transmission system is preferred to be used in off-shore wind power plant which is located within 40km from the seashore. Capacitance of HVAC cables is naturally higher than that of overhead lines, and it should be properly compensated. P model of three-core XLPE cable which is widely used in off-shore wind power plant was applied in the paper. Based on this model, impacts of reactive power compensation on HVAC transmission system, including capacity and location of compensation equipment, were analyzed and simulated then. Research showed that inductive compensation at off-shore side of the cable is more effective to decrease power loss along the cable and increase its transmission capacity than that at onshore side, while the latter is more appropriate to improve the power factor at onshore access point of the wind power plant to grid. Compensation capacity was optimized to realize the best economy with designated certain parameters of HVAC system by Sequential Quadratic Programming (SQP) method. Finally by SQP costs of 35kV and 220kV HVAC transmission system were compared, critical distance and transmission capacity of these two systems adoption were found. These results can be a reference of HVAC transmission system design of off-shore wind power plant.

Power losses from wind generated electricity in high voltage AC transmission: an analysis through simulation

In this paper, we have investigated different types of losses from wind generated electricity in high voltage AC (HVAC) transmission. Wind power generation is expensive in nature and it is thus necessary to know how much power is lost in the transmission. We have simulated an HVAC transmission system with a wind power generator in Simulink. We varied different components of the system namely wind power, transformer rating, and cable length to investigate their influence on the different types of power losses. We have analysed two different power losses in two points: transformer and cable. The experimental findings are presented in this paper.