Transmission Characteristics of DC Transmission System and AC-DC Hybrid Transmission System

Transmission Characteristics of DC Transmission System and AC-DC Hybrid Transmission System
  1. Transmission characteristics of DC transmission system

Figure 1 shows the structure of a DC transmission system.
The transmitting end converts AC to DC through a thyristor rectifier, which is called the rectifier side; the receiving end converts DC to AC through a thyristor inverter, which is called the inverter side. The DC transmission system relies on the adjustment of the firing angle of the thyristor to realize the control of the transmission power. The power transmission direction can be reversed, that is, the original inverter side is changed to the rectifier side, and the original rectifier side is changed to the inverter side at the same time.

Figure 1 - Schematic diagram of the structure of the DC transmission system
Figure 1 – Schematic diagram of the structure of the DC transmission system

The power transmission system with thyristor rectifier will generate harmonics and absorb the reactive power of the AC system when the firing angle is not zero, and its magnitude is related to the firing angle. The role of AC filter and reactive power compensator is to eliminate harmonics on the AC side and perform reactive power compensation to maintain the voltage stability of the AC power grid.
Figure 2 shows the equivalent circuit of a DC transmission system. The direction of the DC voltage is determined by the firing angle of the thyristor. The figure shows an operating situation: one side generates a DC voltage U1, whose direction is the same as the positive direction of the rectifier, called the rectifier side; the other side generates a DC voltage U2, whose direction Opposite to the positive direction of the rectifier, it is called the inverter side.

Figure 2 - Equivalent circuit diagram of a DC transmission system
Figure 2 – Equivalent circuit diagram of a DC transmission system

The condition to keep the DC transmission system running continuously is U1>U2.
The formula for the current I12 is (3)

In the formula, I12 is the current of the transmission circuit; U1 and U2 are the DC voltages generated by the rectifier side and the inverter side, respectively; ΔU is the voltage difference between the two sides, ΔU=U1-U2; R12 is the resistance of the transmission circuit.
Since the direction of the voltage U1 generated by the rectifier side is the same as that of I12, power is sent out; the direction of the voltage U2 generated by the inverter side is opposite to that of I12, so it absorbs power. It can be seen that in the DC transmission system, the active power is transmitted from the high voltage end to the low voltage end.

Adjusting the firing angle of the thyristor can reverse the direction of the DC voltage generated on both sides. When only the DC transmission line is connected between the two AC systems, the two systems can run in an asynchronous state, and the direction and size of the DC voltage at both ends are controlled by the thyristor to control the direction and size of the power transmission between the two systems.

  1. Transmission Characteristics of AC-DC Hybrid Transmission System

With the increasing competitiveness of DC transmission, HVDC has been increasingly applied in large power systems, making modern power systems an AC-DC hybrid system.
As a case of the AC/DC hybrid power transmission system, Figure 4 shows the structure of the AC/DC parallel power transmission system: both ends of the AC transmission line and the DC transmission line are connected in parallel, connecting the AC system 1 and the AC system 2 together.

Figure 4 - Simplified equivalent diagram of AC-DC parallel transmission system
Figure 4 – Simplified equivalent diagram of AC-DC parallel transmission system

In 2001, China Southern Power Grid’s Tianshengqiao-Guangzhou 500kV DC transmission bipolar was put into operation, and then China Southern Power Grid built a west-to-east power transmission trunk channel composed of multiple AC and DC transmission lines, forming an AC-DC parallel operation state.

The two AC systems connected by the AC/DC parallel transmission line must operate in a synchronous state, and the operating frequency and the total power transmitted by the AC/DC parallel transmission line are determined by the power generated by the two AC systems and the load. If the DC transmission line performs constant power control, on the condition that the frequencies of the AC systems at both ends are equal, the total power that should be transmitted by the AC-DC parallel transmission system minus the power transmitted by the DC transmission line is the power that the AC transmission line should transmit.

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