| MMC-HVDC is a kind of power electronic equipment with fast power control characteristics.When the electrical distance between the sending converter station and the thermal power unit is close,MMC-HVDC may cause the unit to have electrical negative damping characteristics,which will cause some shafting modal divergence and the risk of sub-synchronous oscillation(SSO)of the generator and endanger the safe and stable operation of the power grid.The complex torque coefficient method is a classic method to analyze the torsional vibration phenomenon of the thermal power unit shaft system,and its corresponding negative damping criterion can effectively analyze whether the unit has the risk of subsynchronous oscillation.Based on the complex torque coefficient method,this paper focuses on the subsynchronous oscillation characteristics of the thermal power unit Connected via the MMC-HVDC system.Firstly,the dq-axis impedance model of the MMC single-ended system is derived to provide model support for obtaining the electrical damping characteristics of the system in the next step.In the modeling process,the arm average model of MMC was used,the dynamic characteristics of the equivalent capacitance of the bridge arm were considered,and the electrical quantities of the system were expressed in the form of common mode components and differential mode components("∑-Δ").The state equation of the 18-order MMC system is obtained,and the voltage-current relationship is further considered to derive its dq impedance model;the correctness of the model is verified in the time-domain simulation using the voltage injection method.Then,the expression of the electrical damping characteristic of the system is derived.Based on the basic principle of the complex torque coefficient method,the small-signal model of generator is established with two pairs of input and output relationships formed by electromagnetic torque and rotor speed,voltage and current,and the dq impedance model is used to equivalently model the external network of the system,which is seen from the voltage and current ports of generator,thereby simplify the ratio relationship between the generator’s electromagnetic torque and the rotor speed,that is,the expression of the electrical damping coefficient.Finally,based on the electrical damping characteristics,the influence of key parameters on the SSO characteristics is analyzed.Increasing the strength of the AC system,reducing the DC transmission power,and increasing the proportional coefficients of the inner and outer loops can help reduce the risk of SSO in thermal power units;while integral coefficients of the inner and outer loops,the proportional coefficients and the integral coefficients of the circulation control have almost no effect on the SSO characteristics of the thermal power unit.When series compensation is put into operation,a thermal power unit connected via series compensation and MMC-HVDC is formed.Since the electrical resonance frequency of the system is far away from the torsional vibration frequency of the shaft system,the interaction between the machine and the network is weak.At this time,the electrical damping value is still a positive value,which explains why the electrical damping curve does not appear concave due to series compensation. |
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