Dynamic Study on the 400 kV 60 km Kyndbyværket – Asnæsværket Line

Until recently, the use of HVAC underground cable systems has been mainly limited to densely populated area. As such, HVAC underground cable systems are limited both in length and numbers to date. This tendency has been changing over the past ten years as the service experience of HVAC, cable systems have shown to be satisfactory. The applications of HVAC cable systems are proposed more often for transmission projects, and recently proposed HVAC underground cable systems are longer compared to existing cable systems. Due to this historical background, HVAC underground cable systems have been studied and tested primarily with short cable lengths. Some knowledge from short cable lines can be directly applied to long cable lines, but there are several phenomena which are peculiar to long cable lines. The main objectives of the PhD project are to shed light on the phenomena peculiar to long cable lines. The PhD project focuses on the 400 kV 60 km Kyndbyværket – Asnæsværket line, which will feed power to Copenhagen, the Danish capital. The PhD project addresses major problems and potential countermeasures related to the installation and operation of a long cable line, through: (1) Insulation coordination study (2) Derivation of theoretical formulas of sequence currents (3) Identification of dominant frequency components contained in the overvoltage (4) Analysis on the statistical distribution of energization overvoltages (5) Protection study First, the insulation coordination study have found that it is feasible to build and operate the 400 kV 60 km Kyndbyværket – Asnæsværket line and have identified necessary considerations in the equipment specification and required countermeasures against possible problems. Especially, severe temporary overvoltages caused by parallel resonance and system islanding are observed, which requires a consideration in the selection of surge arresters. Second, the PhD project has derived theoretical formulas of sequence currents of a cross-bonded cable and a solidly-bonded cable. These formulas are obtained by solving equations which are derived from the setups for measuring sequence currents of cross-bonded and solidly-bonded cables. For a cross-bonded cable, the equations are solved utilizing the known impedance matrix reduction technique. The derived formulas consider the cable as a cable system; they can thus consider sheath bonding and sheath grounding resistance. An accuracy of proposed formulas is verified through a comparison with EMTP simulation results. The verified accuracy of the proposed formulas shows sequence impedance / current can be obtained before the installation without making measurements for a majority of cables. This gives an important advantage in setting up transient overvoltage studies as well as planning studies. Third, as the switching of EHV cables can trigger temporary overvoltages, it is important to find the dominant frequency component contained in the switching overvoltages of these cables. Since there are no theoretical formulas to find the dominant frequency, it is generally found by means of time domain simulations or frequency scans. The derivation of theoretical formulas has been desired as the formulas would be useful in verifying the results of time domain simulations or frequency scans. Additionally, the formulas could eliminate the necessity of building simulation models of some network components. The PhD project has derived the simple theoretical formulas for estimating the propagation velocity and dominant frequency from impedance and admittance calculations. The comparison between the proposed formulas and the simulation results is performed using the Kyndbyværket – Asnæsværket line. From the comparison, the derived formulas are found to be sufficiently accurate to be used for efficient analysis of resonance overvoltages. In addition, the accuracy of the formulas derived demonstrates that the propagation velocity and the dominant frequency are determined by two inter-phase modes for long cables. Forth, the statistical distribution of energization overvoltages of EHV cables is derived in the PhD project from a number of simulations. Through the comparison with the statistical distributions of energization overvoltages of overhead lines, the main characteristics of the statistical distribution for cables are identified. In particular, it has been found that line energization overvoltages of cables are lower than those of overhead lines with respect to maximum, 2 %, and mean values. The standard deviation has been found to be smaller for cables. The main characteristics of the statistical distribution are found to be caused not by random switching by accident, rather there are contributing factors and physical meanings behind the characteristics. These contributing factors and physical meanings are identified from the theoretical analysis of voltage waveforms of energization overvoltages. These findings are useful not only for the determination of insulation levels of cable systems, but also for insulation coordination studies of cable systems. Finally, through the calculation of the ground loop impedance for cable lines, it has been found that, for long EHV cable lines, the reliable operation of the ground distance relay is possible with a typical relay setting. It is known that the ground loop impedance of EHV cable lines does not have a linear relationship to the distance. There is a discontinuity in the ground loop impedance at cross-bonding points, which may have an ill effect on the reliable operation of the ground distance relay. However, the discontinuity of the ground loop reactance of the long EHV cable lines is small enough for the ground distance relay to operate satisfactory with a typical relay setting. Effects of parameters, such as substation grounding, cable layouts and transposition, are also found through the analysis.