| The amount of non-linear loads and the cables supplying them have been rapidly growing in the last decade.Therefore,it is essential to evaluate the ampacity of cable when designing the urban electrical network.Although,numerous effects have been considered in the area of the ampacity estimation,the knowledge is not adequate to solve new problems.In this thesis,EV(Electrical Vehicle)charging loads and High Speed Railway(HSR)traction loads are investigated in terms of their impacts on the MV and HV cable system respectively.In this thesis,the model to calculate the ampacity and the temperature of MV and HV cables are based on the IEC 60287 equations.Firstly,the harmonic orders of non-linear loads can be obtained by using the fast fourier transformer(FFT)on the current signal in time domain.Then,the AC resisitance of the conductor and the loss ratio of the metallic sheath and armour layer to the joule loss of the conductor can be calculated.To analyse the impacts of the transient characteristics of the HSR traction load on the transient temperature rise of HV cable,the equation in IEC 60853 used to calculated the joule loss of the phase current is modified to be suitable for the harmonic current.Then,the transient temperature rise due to the joule loss of each cycle current is formulated.Finally,the cumulative transient temperature rise in presence of harmonic current can be deduced by the integral in a specific time span.Besides,the circular current loss in sheath are formulated as a ratio of the joule loss of core conductor.In this model,the mutual inductance between the centre conductor and the sheath of HV single core cable is a key parameter which was calculated by the geometric mean method without considering the corrugated sheath.To obtain the the mutual inductance with corrugated metallic sheath,the analytical solution is deduced based on the Maxwell equations.The parametric equation of the helical and annular corrugated sheath cable are proposed as the limits of surface integral of the magnetic flux within the sheath.The mutual inductance of 110 kV and 220 kV cable with 400,800,1200 mm~2 cross-section are analysed.The results show that the mutual inductance decreases with the embossing depth increasing.The results of analytical solution and numerical solution are numerically equal.Besides,the mutual inductance calculated using the analytical method is smaller than the value calculated using the geometric mean distance 0.09~0.14%.For a long cable with multiple pitches,the mutual inductance of HV cable with helical and annular corrugated sheath are identical and smaller than a cable with smooth sheath.To meet the rapidly growing EV charging demand,it is important to determine maximum ampacity and avoid overloading of MV cables in power distribution systems.Previous works on modelling and analysis of relationship between the load demand of MV cables and EV penetration level did not consider the contribution of harmonic currents.This thesis presents a methodology for modelling and analysis of the impact of EV charging loads on cable ampacity and temperature in presence of different levels harmonic currents under two typical scenarios of EV charging modes,i.e.,uncontrolled domestic charging scenario and uncontrolled off-peak domestic charging.The proposed method determines the ampacity and temperature of three-core cable taking account for cable geometry,harmonic levels,stochastic nature of the charging power,start time of individual charging,seasonal temperature under different EV penetrations.A generic urban distribution network with residential load ratio of 0.7,which is representative of urban residential distribution network in China,is adopted as an example.Results show that the ampacity of a 400 mm~2 10 kV cross-linked polyethylene(XLPE)cable decreased by 10.79 A and 2.03%when the harmonic level is at the maximum allowed limit as given in IEC 61000-3-2.Meanwhile,the ampacity decreased by 23.34 A and 4.09%at the maximum limit given in IEC 61000-3-4.In presence of 36.15%and 41.66%total harmonic distortion current which are the most serious harmonic distortion in the example network with 30%EV penetration,cable ampacity decreased by 42.86 A,8.06%and 61.35 A,11.35%respectively.Optimizing cable circuit layout in a cable duct bank to minimise the maximum operational cable temperature is an important topic due to increasing adoption of cables and cable duct banks in urban areas.Previously most efforts focused on initial design of new cable duct banks.However,in some existing,operational cable duct banks with spare ducts or ones unoccupied with operational cables,new circuits may be laid to reallocate currents from heavily loaded circuits to lower the maximum operational temperature to the allowed safe level.This thesis aims to contribute to the optimal selection of spare ducts to minimize the maximum temperature in an operational cable duct bank.The method proposed to solve the spare ducts selection problem(SDSP)in the thesis extends the particle swarm algorithm(PSO)with customized velocity and position updating equation to matrix searching of possible solutions.The method has been implemented to a 3×8 cable duct bank in which 16ducts have been occupied with operational cables.Results show that,the maximum temperature is reduced from 92.0?to 45.3?when the current of the 4 most heavily loaded circuits are reduced by half by having 4 additional parallel circuits and that the result of the proposed method is 2.7?lower than the solution adopted by the owner utility company.To analyse the impacts of the high-speed railway(HSR)traction load on the HV cable system,an on-site field test by using three Rogowski coils to measure the conductor current of three phase conductors have been done.The results show that the current of the three phases are unbalanced.The maximum unbalance factor is up to 0.77 and the average value is 0.49.Meanwhile,the r.m.s value of the sum of the three phase vector current is up to 24.22A.Besides,the impacts of the unbalanced phase currents on the induced electromotive forces(EMFs)of the three sheath loops are simulated.The results show that the EMFs will increase when the phase currents are set as unbalanced in three layout arrangement,e.g.isosceles right-angled triangle,parallel and regular triangle.Comparing with the balance current,the EMFs induced by the unbalanced phase current increases by 37%,22%and 30%in the parallel layout.It is recommended that the parallel layout arrangement should be avoided.To analyse the impacts of the harmonic and transient characteristics of the HSR traction load on the transient temperature of HV cable,the current signal of the conductor in time domain is transformed to the frequency spectrum using the FFT.The results of the measured phase current of the HV cable show that,the maximum THD_i is up to 11.65%,the average value is below 5%,mainly composed of the low frequency content.Besides,the results of the simulation show that the ampacity of the HV cable with large cross-section decreases more than the small ones,and the 2500 mm~2 220 kV is derated by 6.85%.Moreover,the results of simulation show that the temperature rise of the conductor due to the 244.15 A phase current at 7:05 a.m.is 10.64?.However,the temperature rise of the conductor is up to 35.16?at 17:25 p.m..The transient characteristics should be taken into consideration when designing the ampacity of the HV cable because of the effect of cumulative transient temperature rise. |
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