Key Technology Research On Relay Protection Of Tuned Half-wavelength Transmission Lines

With the transmission distance increases,the reactive power compensation required for AC transmission will be greatly increased,while the reactive power compensation required for long-distance transmission of DC transmission is much smaller than that of AC.Therefore,UHVDC transmission is the current long-distance large-capacity power The main way of conveying.But when the transmission distance reaches half the power frequency wavelength distance,the inductive reactive power and capacitive reactive power generated by the AC transmission line will cancel each other out,and the AC transmission line will form a stable state without reactive power compensation.Considering that the ultra-UHVDC transmission system requires a large number of extremely expensive power electronic equipment,and the large number of harmonics caused by the converter valve in the high-voltage network is likely to become a hidden danger for the safe and stable operation of the main network,there is no need for half-wavelength AC with reactive power compensation.Power transmission is one of the alternatives for ultra-long-distance and large-capacity power transmission.In practical engineering applications,the transmission line distance is often difficult to reach exactly half the power frequency wavelength(3000km)under the condition of 50 Hz.Therefore,it is necessary to introduce a tuning circuit to compensate for the electrical distance of the transmission line less than 3000 km,so that it has the advantage of half-wavelength AC transmission operation.At present,there is no actual operation project of the tuned half-wavelength AC transmission line.The RTDS real-time digital simulation test platform is used to build the tuned half-wavelength transmission system model to realize the actual output of fault signals.A complete closed loop helps to verify the correctness of the protection algorithm.According to the existing UHVAC system relay protection configuration requirements,the main protection and backup protection schemes of different principles should be configured for the transmission line,and there should be a reasonable logical coordination relationship between the different protection schemes.Traditional current differential protection has a dead zone in half-wavelength AC transmission lines and cannot be directly applied.Distance protection is limited by the distribution of voltage along half-wavelength lines and cannot determine that the fault occurs in the fault area.Therefore,a method based on the direction of transient energy is proposed.Tuning the half-wavelength line longitudinal main protection scheme,the protection algorithm can realize the fault identification within 3ms,and realize the rapid export of the protection signal.And by analyzing the voltage distribution along the line after the faulty phase is removed,it is concluded that the overvoltage of the healthy phase must exceed 1.7 pu.The overvoltage value far exceeds the current safety requirements for UHV transmission.Therefore,the half-wavelength AC transmission line with tuning should be adopted Trip the three-phase trip mode.For line backup protection,it is proposed to adopt Park's transformation protection start algorithm to realize reliable start of the whole line,and realize the longitudinal backup protection based on the principle of impedance differential.At the same time,because the half-wavelength line with tuning has the characteristics of a single outlet,the busbar and the tuning circuit can be combined as the same unit for protection.A protection scheme for the busbar and tuned circuit based on the analog power direction after the test is proposed.Because the power itself is directional,it can be further protected.As a backup for line protection,the upper bus-side circuit breaker can be cut off when the circuit-side circuit breaker refuses to operate.Considering the three protection schemes,reasonable configuration is made to form a complete half-wavelength transmission line protection system with tuning.The single-ended traveling wave ranging method and the double-ended traveling wave ranging method based on the arrival time of the wave head respectively have inaccurate wave head calibration and time synchronization problems in the half-wavelength line with tuning,which greatly affects the ranging accuracy.In view of the above problems,based on the analysis of the propagation characteristics of the traveling wave after the fault of the tuned half-wavelength line,the fault traveling wave is decomposed into the forward traveling wave and the reverse traveling wave.The Bergeron equation is used to calculate the traveling wave energy superposition point along the line.According to the unique corresponding relationship between the location of the energy superimposition point and the fault location,a fault location method based on sudden energy changes along the line is proposed.