Technology and engineering application of cross area HVDC interconnection system high-precision simulation modeling based on ADPSS

1 Introduction

High voltage direct current (HVDC)transmission has played an important role in the strategy of “west to east transmission and national interconnection”in China,with the advantages of long distance and large capacity transmission and asynchronous interconnection of power grids [1-3].Several large-capacity long-distance ultrahigh voltage direct current (UHVDC)transmission channels have been built in China.At present,more than 20 HVDC transmission projects havebeen completed in China [4-7].The maximum voltage level is 800 kV,and the maximum capacity is 8 million kW.In addition,to implement the action plan of air pollution prevention and control in China,the State Grid Corporation will plan and build four UHVDC transmission channels,which will form an AC and DC hybrid power grid with the highest DC transmission power,highest voltage level,most concentrated drop point,and most complex structure worldwide [8-10].

A large number of HVDC transmission projects have been connected to AC systems.This has started to have an impact on AC/DC hybrid power grids,particularly receiving terminal power grids [11-13].As an HVDC system is a large-scale power electronic integrated nonlinear system that includes a primary system and a control and protection system,the precision and degree of detail of HVDC systems directly affect the actual effect of simulation [14-16].In recent years,in the case of the normal operation and failure of AC power grids,the abnormal fluctuation and even locking of HVDC systems caused by inappropriate strategies of the control and protection system component have strongly influenced power grids.This has significantly affected the safety and stability of receiving power grids and normal operation.In this study,aiming at solving the problem of DC locking caused by AC faults,the transmission process and control technology of DC locking caused by AC faults are analyzed and the integrated call technology of a DC control and protection program is studied.The closed-loop interaction and signal synchronization mechanism of a manufacturer's DC control and protection program and a system mode type are designed.The detailed model of the HVDC system is closed loop cosimulation.The actual engineering HVDC control logic provided by a manufacturer is analyzed and simulated based on the user defined (UD)component library of the ADPSS electromagnetic transient calculation program,and an HVDC control model based on the actual system is established.The entire process of DC key component protection and DC locking caused by AC system faults is simulated.The control and protection action characteristics of DC commutation failure caused by AC network faults and the mechanism of DC key control caused by DC key element control and protection are studied.The accuracy of the DC control custom model based on ADPSS is verified through the simulation of an actual power grid.

2 High-precision control and protection modeling of HVDC system in ADPSS

The electromagnetic transient mathematical model of an HVDC transmission system is constructed using the basic element based on time domain instantaneous value calculation.Valve arm components are composed of a thyristor and a buffer circuit.Six valve arm components and a trigger pulse generator constitute a group of six pulse converters.The complete electromagnetic transient model of the HVDC primary system consists of the six pulse converters,a converter transformer,a DC line,a connection,an earth electrode,a smoothing reactor,an AC/DC filter,and so on.

The overall structure of the HVDC model is shown below.The upper part is a control and protection system consisting of 8 UDM channels.The lower half is a single system composed of 1 station and 2 subcircuits and DC lines.

Fig.1 Model of HVDC system in ADPSS

In the upper half of the 8 UDM channels,the param_settings page sets the page for control parameters and the protection fixed value.OWS sets the interface page for running mode parameters and the page for the command interface of the DC control system and security system.The S1P1_C_P,S1P2_C_P,S2P1_C_P,and S2P2_C_P pages are polar control and protection pages;S1DCC S2DCC is a reactive power control page.This page is separated from the RPC application in polar control,and the bipolar redundancy of the RPC application is cancelled to reduce the amount of simulation calculation.

The bottom half of the 8 UDM channels consists of the S1 and S2 subcircuit pages for stations 1 and 2,including an AC equivalent power supply,an AC filter,a converter valve,a DC filter,and a grounding electrode line.

The primary system consists of the primary systems for stations 1 and 2 and a DC line connecting stations 1 and 2.The systems for stations 1 and 2 are subcircuits with the same structure.

Among them,the AC filter part,polar 1 commutation device,and polar 2 commutation circuit are represented by subcircuits.

The ADPSS system is an open simulation platform that integrates various functions such as electromechanical transient,electromagnetic transient,and small step simulation.The UD modeling function based on ADPSS contains an open UD function invocation interface to provide users with flexible simulation expansion capability.At present,UD functions support a dynamic library invocation mode (referred to as the DLL library in Windows and for so library in Linux).The basic interface scheme is as follows:

The external library of the ADPSS system uses the existing UD library of ETSDAC to integrate a user's external functions into a new UD function block.Then,it uses the UD function block in the program to realize UD external functions.

Any external function,which can be equivalent to an external function box,(External Function or External Function 2)is mapped to a new UD function block,which can be linked to a third party library function block(Equivalent Block UDM 1 or UDM 2).After the mapping is completed,the new UD function block is exactly the same as any UD functional block in ADPSS,and it can copy the movement and connect with other UD function blocks outside.Function blocks can also be used in another UD function area of ADPSS.

A UD function is mapped to the UD linked third-party library function block,as shown in Fig.2.

Fig.2 UD DLL interface

The equivalent circuit of the HVDC transmission line of a bipolar DC transmission line with two terminal DC systems is shown Fig.3.

Fig.3 HVDC diagram

Vdr and Vdi indicate the DC voltage of the rectifier side and inverter side,respectively;Ldr and Ldi indicate the inductance of the rectifier side and inverter side flat wave reactor,respectively;Ld and Rd denote 1/2 DC line inductance and resistance,respectively;Cdc is the total capacitance of the DC transmission line;Vdor and Vdoi indicate the principle of the rectifier side and inverter side,respectively.The DC voltage is no-load voltage;α is the trigger delay angle of the rectifier,and β is the trigger advance angle of the inverter.Rcr and Rci are the equivalent commutation resistors on the rectifier side and inverter side,respectively.

Xcr and Xci denote the reactance of the rectifier side and inverter side,respectively.

The dynamic equations can be listed as follows [7]:

where Vdr is the line voltage of the two sides of the rectifier side converter transformer,and Vdi is the line voltage of the two sides of the inverter side converter transformer [6-11].

3 Simulation verification and effect analysis in ADPSS

The simulation example uses the ADPSS electromechanical and electromagnetic hybrid simulation.The electromagnetic side consists of two HVDC systems.The operation mode is the east-area power grid in the light load state,and voltage is high.The effect of changing the trigger angle of the DC system at the receiving end from 19° to 29°in 5s is analyzed.

3.1 Trigger angle regulation of HVDC simulation

The following conclusions can be drawn based on the above waveforms:The turn off angle is increased 10°,the trigger angle is reduced from 6°,and reactive AC and DC power is transmitted from the receiving grid to surplus reactive power.This can significantly improve the reactive power and voltage in the receiving power grid and reduce the exchange of AC and DC reactive power.Under light load,the voltage of the receiving end grid is high and surplus reactive power is evidently improved.

When the HVDC system is under low power operation,HVDC bus voltage is reduced by increasing the arc angle or changing the converter transformer gear.However,the reactive power absorbed under these methods differs significantly.Owing to the nonlinearity of reactive power consumption,increasing the extinction angle of the inverter side can increase the reactive power absorbed by the converter.The variation in the reactive power caused by changing the converter transformer tap is almost negligible compared to the amount of reactive power absorbed by the converter.

Under the low power operating mode of the HVDC system,the reactive power consumed by the HVDCsystem can be changed by adjusting the arc angle of the inverter side and the tap gear.Then,the reactive power exchanged between the DC system and AC system under the low power operation mode is changed.However,in practical engineering,the angle of the inverter side and the adjustment of gear position are limited by factors such as the loss of the system and the capacity of the exchange valve.Therefore,it is necessary to analyze the possible adverse effects of these factors to ensure the feasibility and economy of the adjustment measures.

Fig.4 HVDC Partitioning in ADPSS

Fig.5 HVDC electromagnetic model in ADPSS

Fig.6 Change in DC trigger angle

Fig.7 Change in DC turn off angle

3.2 Analysis of DC locking accident caused by AC system fault

According to the analysis of the above HVDC transmission principle,a fault in the HVDC power supply terminal or the AC power grid will have an evident influence on the HVDC side that is connected to it.The HVDC control and protection system will respond accordingly.Therefore,an improvement scheme is proposed by studying the action strategy of the HVDC control and protection system in the case of AC power grid faults.This will help maximize the reliability of HVDC transmission systems and prevent unnecessary outages.

Fig.8 Voltage value of A phase on inverter side

Fig.9 Reactive power variation at DC receiving end

Based on the ADPSS HVDC electromagnetic transient simulation example and the integrated manufacturerprovided control and protection module of the HVDC system,the response process of the HVDC system under different fault types of the terminal AC system and different fault durations is simulated and analyzed,and the conditions that cause HVDC locking are studied.

The single phase metal grounding fault at the receiving end of the AC side lasts 1.3s.This causes DC system commutation failure numerous times.During the fault,DC transmission active and reactive power decreases and DC voltage reduces.The latch signal is delayed 150 ms after the failure of the second phase commutation,time delayed 1.5s of fault action.The type of locking can be seen:commutation failure,single bridge protection and double bridge protection have no action,and the second frequency harmonic protection action leads to the blocking of the DC system at the receiving end.

Fig.10 Voltage on inverter side

Fig.11 Trigger angle variation waveform

Fig.12 Extinguishing angle waveform

Asymmetrical faults in AC systems are the most common cause of negative sequence voltage in these systems.Under the action of the converter,the basic frequency negative sequence voltage in the AC system results in 2 harmonic potentials in the DC system.The 2 harmonic potentials generate 2 harmonic currents on the DC side.According to the transformation relationship function between AC and DC harmonics,the 2 harmonic currents on the DC side can be converted to 3 harmonic currents on the AC side.

The fault in the AC power grid causes the key electrical quantity on the AC side of the DC system,such as the change in AC line voltage and frequency.In addition,the response of the DC control and protection system also causes a few key electrical quantities such as DC voltage,DC current,and ignition angle.

The faults in the communication system may occur close to or far away from the converter station.Hence,converter station AC bus voltage reduces or even completely disappears.This results in voltage wave distortion and the offset of the line voltage zero point.The failure of certain AC systems may lead to a substantial increase in AC voltage.

The decrease in AC voltage causes the DC voltage drop of the converter bridge and even forces the commutation bridge to stop working.In addition,the fault in the inverter side AC system is the main reason for commutation failure on the inverter side.An abnormal rise in AC voltage affects the effective control of reactive power at the converter station.In addition,an abnormal change in the voltage of the AC power grid or a change in frequency affects the synchronous voltage of the converter and the normal operation of the nuclear lock phase ring of the ignition pulse generator of the DC control and protection system.According to the analysis of the above HVDC transmission principle,AC power grid faults cause the DC system controller to respond,and DC voltage changes accordingly.Therefore,the influence of AC power grid faults on the DC system can be considered from two aspects,i.e.,DC control system and DC protection.

The coordination of the control and protection system under AC system faults can be analyzed as follows:Commutation voltage is proportional to the AC voltage of the change network side.The DC control and protection system detects the instantaneous value of AC voltage at the current time and calculates its amplitude.If AC voltage amplitude decreases at several consecutive testing times,it is possible to determine the drop in AC voltage at this time.The reference value of the extinction angle should be increased,the trigger angle should be reduced,and a trigger pulse should be sent in advance to prevent valve commutation failure.

Excessive DC current leads to an increase in the overlap angle,a decrease in the extinction angle,and the lack of commutation margin.This may cause commutation failure.The DC current control system detects DC current.If DC current increases at several continuous detection times,the abrupt increase in DC current can be determined at this time.The reference value of the arc angle should be increased,the trigger angle should be reduced,and a trigger pulse should be sent in advance to prevent the failure of the phase change of the valve.

When the commutation side communication system fails,the valve group sends out the communication low voltage alarm and performs the pole control VDCL action.After a certain delay,the pole protection sends out the 100 Hz protection action.The pole control sends out the descending stream instruction after a certain delay if the AC system fault has not been cleared yet.The 100 Hz is protected from the Y protection,and the Y latching instruction is controlled.

When the inverter side AC system fails,the valve group protection first sends out the AC low voltage alarm signal.The AC system fault is typically the failure action of any bridge.The valve group protection sends out the bridge commutation failure alarm.The valve control sends the commutation failure alarm and increases the GAMMA angle.The current criterion commutation failure prediction action is performed,followed by the polar control VDCL action.After a certain delay,the pole protection sends out a 100 Hz protection cutting system (the delay time of any bridge commutation failure cutting system is longer than that of the 100 Hz protection cutting system).When the fault has not been cleared,the valve group protection sends out a bridge commutation failure protection Y latch,the polar control Y latch,the 100 Hz protection current,and the Y latch.The delay is longer than the commutation failure of any bridge,and the 100 Hz protection is the backup protection for commutation failure.When commutation fails,the 100 Hz protects current and Y latching.

4 Conclusion

The paper proposed and achieved the HVDC control and protection logic simulation in practice based on the UD component library of the ADPSS electromagnetic transient calculation program.The accuracy of the DC control custom model based on ADPSS was verified through the simulation of an actual power grid.The following conclusions were drawn:the trigger angle of HVDC systems can improve the voltage and reactive power surplus of the receiving end.The single-phase fault in the near area communication system causes the failure of DC system commutation.The conventional AC single-phase fault durations of 100 ms,300 ms,700 ms and even 1.1s do not cause DC system locking.A fault duration above 1.3s causes the two harmonics to exceed the limit of the DC system.In addition,the duration exceeds the harmonic protection setting value,and DC harmonic protection is implemented.The operation setting value is less than commutation failure protection.Hence,the DC system harmonic protection action leads to DC blocking.

Acknowledgements

The paper was supported by National Key Research and Development Program of China-High performance analysis and situational awareness technology for interconnected power grids.