Hybrid Simulation of ±500 kV HVDC Power Transmission Project Based on Advanced Digital Power System Simulator

Lei Chen,Kan-Jun Zhang,Yong-Jun Xia,and Gang Hu

1.Introduction

High voltage direct current(HVDC)technology has been considered as a viable alternative to AC for long distance power transmission and interconnection of power systems.HVDC technology allows power transmission between AC networks with different frequencies or networks which may not be synchronized.Since there is no skin effect on DC transmission line,inductive and capacitive parameters do not restrict the transmission capacity of HVDC systems.Besides,according to fast DC power modulation configured in a HVDC project’s control system,the power oscillation in its related AC power grids can be restrained timely,and that is helpful to enhance the transient stability of power system[1].

In China,a number of HVDC projects have been put into service,and meantime some novel HVDC technologies,such as the light HVDC based on voltage source converters(VSCs)and IGBT(insulated gate bipolar transistor)power semiconductors,have also been developed to promote the application of power electronics.As a matter of fact,there are five ±500 kV HVDC power transmission projects whose rectifier stations are located in Hubei Power Grid,and the dynamic response characteristics of these HVDC projects will undoubtedly play an important role in the secure and stable operation of Hubei power system.Therefore,it has the practical significance to study on modeling and simulation of the five HVDC projects.

By reviewing the research status in HVDC simulation field,it can be found that most of HVDC models are constructed using non-real-time electromagnetic transient simulation tools,such as PSCAD/EMTDC and MATLAB/SIMULINK[2]-[4].Admittedly,the models built by these simulation tools can reflect the operation characteristics of HVDC to a certain extent.However,since the scale of an electromagnetic transient simulation model is limited by its software algorithm,an equivalent treatment should be applied into the AC power grids connected to the HVDC model,and the interaction characteristics of AC-DC power systems may not be effectively simulated.Advanced digital power system simulator(ADPSS)independently developed by China Electric Power Research Institute(CEPRI)is a real-time hybrid simulation software with great working performance[5],[6].This software can execute the electromechanical-electromagnetic hybrid simulation,and a HVDC model can be divided into an electromagnetic transient sub-grid including HVDC primary equipment,a control system,and an electromechanical sub-grid consisting of the rest AC power systems.Consequently,not only the scale of simulation study can be guaranteed,but also its calculation accuracy may reach a higher level.

In this paper,taking the ±500 kV HVDC power transmission project from Jiangling to Echeng for modeling object,its electromechanical-electromagnetic hybrid simulation model was built according to the HVDC primary equipment’s actual parameters and the classical parameters of the control system based CIGRE(International Council on Large Electric Systems)HVDC benchmark.In the simulation analysis,the built hybrid model’s dynamic response characteristics under three different fault conditions were considered,and by comparing with the fault recording waveforms,the validities of the simulation waveforms were estimated qualitatively.

2.Simulation Principle of ADPSS

ADPSS is chiefly composed of terminal workstation,manage console,calculation nodes,and communication system[7].The terminal workstation is used for building detailed simulation model,and the manage console can carry out the comprehensive management for data store and routine calculation.Due to the use of the communication network,the data exchange between different calculation nodes can be realized.

After a simulation model’s electromechanical sub-grid and electromagnetic sub-grid are divided clearly,the two sub-grids’ simulation data will be uploaded to the manage console,and different calculation CPUs will be assigned for them.The two sub-grids will keep synchronization simulation speed,and based on the hybrid simulation interface,the two sub-grids can execute the data exchange conveniently.In addition,the electromechanical sub-grid can also obtain some other real-time simulation information from the data interface.Fig.1 shows the schematic diagram of electromechanical-electromagnetic hybrid simulation.

3.Modeling Method of the Real±500 kV HVDC Project

The ±500 kV HVDC project from Jiangling to Echeng was built for the energy transmission from Three Gorges Hydropower Station to Guangdong Load Center,and the nonsynchronous interconnection of Huazhong Grid and Southern Power Grid.This HVDC project’s rated bipolar transmission power is about 3000 MW,and its rated direct voltage/current is 500 kV/3kA.In addition,the project’s rectifier and inverter sides are respectively located in 500 kV Jiangling and Echeng stations,and its configured control system is manufactured by ABB Company.In the following text,in consideration of the HVDC project’s primary equipment and control system,their modeling methods are respectively presented.

3.1 Primary Equipment Model Building

Fig.2 shows the simulation schematic diagram of the±500 kV HVDC project model.This simulation model is composed of electromechanical and electromagnetic transient sub-models.Due to the absence of structural and operational parameters of Southern Power Grid,the AC system connected to the project’s inverter side is equivalent to an infinite power source.This infinite power source with the HVDC project’s primary equipment and control system is all built in the electromagnetic sub-grid,and the rest AC power systems connected to the HVDC project’s rectifier side will be arranged in the electromechanical sub-grid.

Fig.1.Schematic diagram of electromechanical-electromagnetic hybrid simulation.

Fig.2.Simulation diagram of the ±500 kV HVDC project model.

Table 1:Some primary parameters of the HVDC project

Fig.3.Logic diagram of the HVDC control system model.

Fig.4.V-I characteristic of the HVDC control system model.

The actual HVDC project consists of the following primary components:converter transformers,smoothing reactors,AC side filters,DC side filters,DC transmission lines,rectifiers,and inverters.There are actually 12 single-phase-duplex-winding converter transformers installed at the rectifier/inverter side,and in order to reduce the calculation node and improve the simulation velocity,4 three-phase-duplex-winding transformer models are adopted in the simulation.The smoothing reactor is imitated by a linear inductor model,and the models of the AC and DC side filters are constructed based on their practical structures and tuning parameters.As the frequency analysis of DC transmission line is not the emphasis of this simulation,the DC line whose length is 941 km can be simulated by the series of a few lumped-parameter line models.For the rectifier/inverter equipped with a large number of common thyristors,the internal fault occurring at power electronic equipment is not considered here,and the single converter valve is simulated by an ideal switching element with the parallel buffer circuit.

Table 1 indicates some primary parameters of the studied ±500 kV HVDC project,and the detailed structure characteristics of the AC and DC filters can refer to [8].

3.2 HVDC Control System Modeling

Fig.3 denotes the logic diagram of the HVDC control system model.This control model is according to the CIGRE HVDC benchmark system,and its configuration characteristics can be described as follows.

The constant-current controller is arranged at the rectifier side,and the constant-current and constant-voltage as well as constant-gamma controllers are configured at the inverter side,and the inverter’s firing angle will depend on the minimum among the outputs of the three controllers.Besides,the rectifier and inverter will both configure the voltage dependent current order limiter(VDCOL)which can reduce the reference value of direct current(Idref)in case of the large decline in direct voltage,so as to suppress the overcurrent and maintain the system voltage.

In normal state,there is a small margin(Idmarg)between the direct current references of the two constant-current controllers.Since Idref-inverterwill be smaller than Idref-rectifier,the output of the constant-current controller configured in the inverter side will be regulated to its maximum,and accordingly this controller will not be selected among the three controllers.Then,the inverter’s firing angle will be dominated by the constant-voltage controller or the constant-gamma controller.

Fig.4 reflects the V-I characteristic curve of the HVDC control system model.The HVDC system normally operates at point X,as shown in Fig.4.However,during a severe contingency producing a voltage drop on the AC system connected to the rectifier side,the operating point of the HVDC system will move to point Y.Consequently,the rectifier’s firing angle will be forced to reach its setting minimum value,and the inverter’s control mode will switch to the constant-current mode from the original constantvoltage or constant-gamma mode.Similarly,a voltage drop on the AC system feeding the inverter will force a control mode to change to the gamma regulation.It is worth noting that the gamma reference can be revised by the delta gamma error(DGE)provided by the constant-current controller.The above-mentioned HVDC control system model is built in ADPSS using the classical parameters offered by CIGRE,which can be achieved in [9].

Fig.5.Comparison between the simulation and fault recording waves of Udand Idunder rectifier-side bus ground fault:(a)simulation results and(b)fault recording waves.

Fig.6.Comparison between the simulation and fault recording waves of Udand Idunder inverter-side bus ground fault:(a)simulation results and(b)fault recording waves.

4.Simulation Analysis of the Real±500 kV HVDC Project

On the basis of the electromechanical transient simulation model,the load flow calculation was carried out at a special time,and the HVDC project’s actual bipolar transmission power was about 1175 MW,and the per-unit value was 0.39 p.u.Then,Udrefand Idrefcould be set as 1 p.u.and 0.39 p.u,respectively.To estimate the availabilities of the built HVDC hybrid model,the AC system fault and the DC line fault were simulated,and further the simulation results were compared with the fault recording waves from a ±500 kV/3 kA HVDC project based ABB control[10].

4.1 AC Bus Ground Fault Simulation

Supposing that the single-phase(phase A)ground fault happened at the AC buses connected to the rectifier and inverter at t =5 s respectively,and the fault duration was 100 ms and the ground resistance was 0.04 Ω.Fig.5 and Fig.6 respectively show the comparison between the simulation and fault recording waves of Udand Idunder the two fault conditions.From these figures,it is found that the hybrid model’s direct voltage/current simulation waves can have approximately the same dynamic trend as fault recording data under the grounded short-circuit happening at the AC bus connected to the converter,and the HVDC simulation model’s effectiveness is proved to a certain extent.With regard to the above two fault cases,the inverter side’s control regulator would both switch to the constant-current mode for maintaining the direct current.

4.2 DC Line Ground Fault Simulation

Assuming that the ground fault happened at the DC line connected to the inverter’s output end at t = 5 s,the fault duration was 200 ms,and the ground resistance was 4 Ω.Fig.7 shows the comparison between the simulation and fault recording waves of Udand Idunder this fault condition.Once the DC line fault occurred,affected by an actual HVDC control system,an emergency dephasing operation would be performed on the rectifier/inverter according to the DC traveling-wave protection,and the firing angles of the two converters will be regulated to their own maximum values for bleeding the DC fault current.

Fig.7.Comparison between the simulation and fault recording waves of Udand Idunder DC line ground fault:(a)simulation results and(b)fault recording waves.

Since the protection strategy was not constructed in the HVDC control system simulation,there were a few differences between the direct current’s simulation and fault recording waves.Compared with the condition that the recording DC fault current was cleared and its amplitude would become zero,the simulated DC fault current was limited to a lower level.After the line fault was moved,the HVDC simulation model could return to its initial steady-state quickly and in time.

5.Conclusions

Aiming at a practical ±500 kV HVDC power transmission project,its electromechanical-electromagnetic transient hybrid simulation model was built based on ADPSS,and the running characteristics under different fault conditions were imitated.By comparing the simulation results with the real fault recording waves,it was observed that the simulation model could describe the selected ±500 kV HVDC system’s dynamic response well under some special fault conditions.When the AC bus fault happened,the direct voltage and current simulation waves could have approximately the same dynamic change trend as the recording data.In case that the DC line fault occurred,there would be a few differences between the two kinds of waves,and in contrast with that the recording DC fault current was cleared,the simulated DC fault current was limited to a lower level.To more fully imitate the HVDC project,the building of protection strategy in the control model will be performed in future.

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