Protection Scheme For Transmission Lines Connected With Renewable Energy Resource Systems

With the rising energy demand and the implementation of some favorable energy policies in China,renewable energy resource systems are gradually connected to the power system.The operating modes and integration methods of renewable energy resource systems are flexible and diverse.They can be connected into the network in the form of distributed generators.On the other hand,they can be in large-scale use and be transferred over long distances.Unlike traditional synchronous generators,there are a large number of power electronic devices in the renewable energy resource system.The fault characteristics of these systems are determined by the control strategy during the fault period.It becomes difficult for system operators to analyze the fault characteristic utilizing traditional methods.With the increasing penetration of renewable sources,their complex fault characteristics bring a lot of challenges to the relay system.The existing protection schemes and setting principles may not be adaptive to the change of the power system with the connection of renewable energy resources.Relay protection is the first choice to guarantee the safety and reliability of the utility network.How to improve the performance of relays is a key problem to promote the development of renewable energy resource systems.Based on the recent grid codes,control strategies under normal operating and fault conditions for renewable energy resource systems are firstly analyzed.The renewable energy resource systems include doubly-fed induction generators(DFIGs)and inverter-interfaced generators(IIGs).The sequence impedance characteristics of these systems are unveiled.Then the thesis focuses on the behavior of the phase selectors for the transmission line emanating from IIGs.With the utilization of the adaptive protection,the current protection,the distance protection,and the current differential protection are improved respectively when the transmission line is connected with wind generators and IIGs.The rest of this thesis is as follows:1.The fault impedance characteristics of DFIGs and IIGs are summarized.All the newly released grid codes require renewable energy resource systems to have the ability of low voltage ride-through(LVRT).Meanwhile,DFIGs and IIGs should provide some ancillary services to the utility grid,such as dynamic reactive power output.Crowbar circuit is the most popular method to achieve the LVRT of wind generators,whereas the positive sequence control strategy is always adopted to control IIGs during the fault period.In this case,the fault currents and equivalent sequence impedances of these two sources are analyzed,and the difference between the sequence impedances is also described.The following proposed protection schemes are all based on the fault analysis in this chapter.2.A criterion for the phase selector on the IIG side is proposed to overcome the failure of the phase selector.This criterion is based on the low voltage and the impedance comparison.When the transmission line is connected with the positive-sequence controlled IIGs,the performance of the phase selectors on both sides of this line is analyzed.The phase selectors on the system side,i.e.,the phase selector of the sudden-change of two-phase current difference and the phase selector of the phase comparison of symmetrical components,perform well.However,the phase selectors on the IIG side fail to select the fault phase.To solve this problem,the potential criterion for the phase selector on the transmission line connected with IIGs is proposed.This criterion detects the low voltage and compares the impedance measured by the relay with the predefined value.3.With the introduction of the adaptive correction coefficients,an adaptive current protection for the instantaneous overcurrent protection and the time delay instantaneous overcurrent protection is proposed.After the crowbar circuit is activated,the equivalent positive-sequence impedance of DFIGs is not equal to the negative-sequence impedance any more.If the setting principle for the adaptive current protection just considers fault types and prevailing system parameters,those current relays are very likely to mal-operate or fail to operate.With the help of the adaptive correction coefficients,an adaptive current protection is proposed to ensure the performance of relays.4.An adaptive distance protection for the zone 2 protection is proposed when DFIGs are integrated into the grid.Due to the exclusive sequence impedance characteristic of DFIGs,the impedance seen by the zone 2 protection on the upstream line of the connection point of wind generators is greater than the predefined setting,when an unbalanced fault occurs on the downstream line.There exists a high probability that the conventional adaptive zone 2 protection of distance relays refuses to operate.Based on the positive-,negative-,and zero-sequence branch coefficients,an adaptive branch coefficient is proposed to calculate the zone 2 setting of distance relays.With the utilization of the adaptive protection,the proposed scheme can realize the online real-time calculation of setting of zone 2.Without losing the selectivity of relays,a better coverage of zone 2 can be achieved,and the sensitivity of the protection is enhanced.5.A sequence-component based current differential protection is proposed for the transmission line emanating from IIGs.Due to the weak-infeed characteristic of IIGs,the conventional current differential protection is not sensitive to the internal faults in some extreme situations.When the positive sequence control strategy is adopted to control IIGs during the fault period,there is a remarkable difference between the negative-sequence components of fault currents on both sides of the line under an internal unbalanced fault.Hence,the proposed scheme reduces the contribution of the positive-and zero-sequence components of fault currents and then defines the calculation currents on both sides of the line.With the introduction of a differential coefficient,the protection scheme discriminates the internal faults accurately from the external faults and the normal operating conditions by means of the magnitude of this coefficient.Simulation results validate the effectiveness of the proposed scheme.Finally,the conclusion of the thesis is conducted,and the prospective researches concerning the protection for transmission lines connected with renewable energy resource systems are proposed.