Simulation Analysis of HVDC Transmission Control S

EMTDC

Yong Chen YanHui Wu

Introduction

Power demand is growing, the power quality also put forward higher requirements. Because of the characteristics of low power loss, high controllability and high stability, HVDC has been widely used in the fields of long-distance large-capacity transmission, submarine cable transmission and system interconnection. In view of this, high-voltage DC transmission will be in China's east to east and power system national network project play an important role. Therefore, the use of computer simulation method, the study of DC transmission system operating conditions and control methods is extremely necessary. In this paper, a typical back-to-back (BTB) converter station is built by using the electromagnetic transient simulation software PSCAD / EMTDC. The current and voltage waveforms of the rectifier side and the inverter side when the normal operation and the inverter side AC system are faulty are given. Analysis of the operation and control of the DC transmission system provides a reliable basis.

Keywords: PSCAD / EMTDC; HVDC, transmission

1 Structure and Control Principle of HVDC Transmission System

A typical DC transmission system shown in Figure 1, the work process is as follows: AC system 1 power through the transformer, the rectifier station will be AC power into DC power, through the DC transmission line DC power to the inverter station, And then by the inverter station will be DC power into AC power to the transformer 2 after the transformer, sent to the AC system 2.

For the typical DC transmission system shown in Figure 1, the equivalent circuit shown in Figure 2. Figure 2 from the equivalent circuit can be obtained when the steady-state current DC current:

I d=V doz cosα−V don cosβ

（1）

dγz+R+d dγ

Where the trigger hysteresis for the rectifier is the angle of the trigger for the inverter.And there are：

V doz=3√2E1

π=1.35E Z，V don=3√2E2

π

=1.35E n（2）

(1) and (2) show that the trigger hysteresis angle of the rectifier or the triggering angle of the inverter is adjusted to adjust the phase of the trigger pulse applied to the control valve or gate of the converter valve, and the DC current The purpose of DC power, in order to achieve the control of the DC transmission system.

Fig 1 Typical DC transmission system

Fig 2 Equivalent circuit of typical DC transmission system

2 Control Mode of DC Transmission Control System

High-voltage DC transmission and AC transmission compared to a significant feature is through the rapid adjustment of the two ends of the converter to control the DC line transmission power size and direction to meet the entire AC / DC combined system operating requirements.

The basic control characteristics of the rectifier is the constant current control mode, as shown in Fig 3. From (1) and (2), it can be seen that the angle is a straight line inclined to the lower right, and the straight line is moved down with the angle. The control characteristic of the constant current is a straight line perpendicular to thehorizontal axis. When the AC side voltage is changed, the operating point is moved up and down along the straight line by adjusting the angle to keep the current constant.

Fig 3 basic control characteristics of the rectifier

The inverter is controlled by the constant gamma (extinguishing angle) angle and also by the constant current control. Stable operation, the use of angle control, the inverter side of the trigger angle can be compared by the arc angle measurement and the difference between the rated value, and then a PI controller to control the output. However, when the measured DC current and current set value difference is large, the inverter current controller will replace the angle control. In the case of abnormal AC voltage, the inverter controls the current. At this time the rectifier side operation is controlled by αmin.

The control of the rectifier side also incorporates the VDCL (V oltage Dependent Current Limit) control, that is, when the DC voltage is too low, the controller will not continue to keep the current constant, but the appropriate to reduce the current, to avoid the system Continuous commutation failure. The characteristic diagram of low voltage current limiting is shown in Fig 4.

Fig 4 low-voltage current limiting link when the characteristics of the converter When the AC side of a serious failure, the AC voltage will drop to a very low level, then the inverter commutation failure is inevitable. For this rectifier side toincrease the CD segment in Figure 4 characteristics, so that low current current drop, to prevent the occurrence of the above situation. The EF section is also set on the inverter side so that the characteristic curves on both sides do not intersect. DG and FH segments are low current minimum current limiting characteristics. With this feature, in the DC line short circuit failure, both sides of the converter can still maintain the appropriate current to keep the AC voltage in the appropriate range.

3 Model building and simulation

The basic parameters are as follows: the number of pulses is 12, the rated voltage of the DC link is 500kV, the rated current is 2kA, the DC line inductance (including the flat wave reactor) is 0.5968, and the DC voltage is 50kV. H, the DC line resistance is 2.5, the inverter commutation reactance is 9.522, the rectifier commutation reactance is 21.4245, the reactive device is the fixed capacitor, the filter adopts the damping filter.

DC side of the DC control model using the above model, the structure shown in Figure 5. Using the simulation software PSCAD to build the model, the following simulation experiments were carried out. In the process of building the model, all the output values were tailored. The simulation time is set to 1.0s and the EMTDC iteration time is set to 0.06s. The output is rectified DC current, voltage, inverter DC current, voltage. Simulation analysis of three cases of normal operation, single - phase earth fault and three - phase ground fault.

Fig 5 high voltage direct current transmission standard model structure diagram It can be seen from Figure 6, the normal operation, the rectifier side, the inverter side of the current, voltage waveform after about 0.06s EMTDC iterative stability, the per unit value is 1, the simulation results are ideal. Figure 7, the inverter side of the AC system in 0.3s single-phase ground fault occurs, at 0.5s when the failure to lift. It can be seen from the simulation results, the ground fault after the inverter side of the current control, then rectifier side running in the αmin control. According to equations (2) and (3), the system will reduce the operating voltage and operating current, which can be clearly seen from Figs. (6), (7), (8). Therefore, the control model based on the above control principle is feasible, with good dynamic characteristics, and after the fault removal, the system quickly into the normal operation state, there is no violent waveform oscillation, but in the event of a moment there is a greater impact value. Figure 8, the inverter side of the AC system in 0.3s three-phase ground fault occurs, at 0.5s when the release. It is clear that the system is back to normal for 0.65 seconds, and the system oscillates for about 150ms after the fault has disappeared. And the oscillation period of the rectifier side, the inverter voltage amplitude is about 1.5 times normal, the current amplitude is normal when about 2.4 to 2.8 times, which is a great test of the system insulation performance.

Fig 6 normal operation of the DC side of the simulation waveform

Fig 7 single-phase ground fault DC simulation waveform

Fig 8 DC simulation waveform when the three-phase fault is faulty

4 Conclusion

From the simulation results can be seen, the normal operation of the system, the DC voltage, current in the per unitary value of 1, in a stable working condition. In the

event of a single-phase earth fault or a three-phase earth fault, the system can quickly go to a newly stabilized state that has been set and can be quickly restored to the rated operating state after a fault cut. Therefore, the use of PSCAD / EMTDC HVDC transmission system to establish the control model is ideal. In the rectifier side to take a constant current control has a very good effect, but in the inverter side AC system single-phase ground fault or can not prevent the system's greater impact current. In the system inverter system occurs when the three-phase short circuit, the system to go through about 6 to 7 cycles of oscillation, and the oscillation current, voltage amplitude is greater, the harm to the system is even more serious. In this regard, you can add fault monitoring and control links in the system, according to the voltage and current changes to select the appropriate firing angle, in order to achieve the purpose of increasing system stability.