High voltage direct current (HVDC) technology overview

There are a lot of advantages to using high voltage direct current technology such as an ability to control active and reactive power flow, connection of an unsynchronized AC systems, and efficient use of energy by submarine cables which make the system attractive for use at the international level. Wind power stations are often located at a distance of 10-12 km from the shore (and sometimes more), and require underwater cables and synchronization.

On the one side power converters significantly increase the cost of the transmitted energy. On the other side, above a certain distance (about 50 km for underwater cables, and about 600-800 km for overhead lines), lower cost of HVDC electrical conductors outweighs the cost of electronics. When electrical energy is to be transmitted over long distances, such as in remote areas of Siberia, Canada and the Scandinavian north, the choice more often is in favor of the lower-cost HVDC line Modern power electronics also provides the ability to effectively manage the grid by controlling power flow, which gives the potential increase in the stability of the power system.

It is well known that AC power lines can connect only synchronized AC systems, which operate on the same frequency and phase. Many areas in the world that are willing to share power have unsynchronized electrical networks. UK, northern Europe and continental Europe grids are not united into a single synchronized power grid (see Figure 6.17).

Figure 6.17. HVDC projects developed by ABB Group.

Development of the insulated-gate bipolar transistor (IGBT) and locked thyristor (GTO) made small HVDC systems cost effective. They can be installed in existing AC power system to stabilize the power without increasing the short-circuit current, as in the case of an additional AC power line. Such devices are developed by ABB and Siemens and are called «HVDC Light» and «HVDC PLUS» respectively. The use of such devices led to the expanded utilization of HVDC units of tens of MW lines and a few kilometers of power line. The difference between the two technologies - the

notion of an autonomous voltage inverter (VSI), whereas «HVDC Light» uses pulse width modulation, «HVDC PLUS» is made on a multi-level inverter [25].

6.8.1 Direct current circuit

Figure 6.18 shows a simplified equivalent circuit of a DC circuit for one pole of the DC system.

The current and the direction of power is controlled by setting the difference between the voltages Ud1 and Ud2. The current direction is constant, the power direction is changed depending on the polarity of the DC voltage source.

Figure 6.18. Equivalent DC circuit.

6.8.2 Bipolar scheme for transmission over long distances

Bipolar scheme is a combination of two poles, wherein there is only small unbalance current in one of the poles under normal operating condition. This scheme is used when the required transmission capacity can not be ensured by monopolar line. A feature of the scheme is that in case of failure of one pole more than 50% of the transmission capacity of the circuit remains. The advantage over the use of two monopolar schemes is that costs on return conductor are reduced, on the other hand, the common return conductor impacts on the two poles of the circuit.

6.8.3 Bipolar scheme with ground return through electrodes

The most common scheme used for bipolar transmission. In an accident on one of the poles the damaged pole is isolated and intact pole continues to operate normally as the current returns either by the grounded electrode or, in case of an accident on the very converter station, through non-operating pole, which acts as the return metal conductor (see Figure 6.19).

Figure 6.19. Bipolar scheme with return through the electrodes in the normal mode, the unipolar mode with return through the electrodes in an accident on one of the poles, unipolar mode with return through inactive pole in case of an accident on the converter station.

6.8.4 Bipolar scheme with a metallic earth return conductor

If there is any restriction and is not possible to apply the return electrodes or distance is relatively small, low voltage metallic conductor can be used as an alternative to electrodes (see Figure 6.20) [26].

Figure 6.20. Bipolar scheme with a metallic earth return conductor.

6.8.5 Advantages and disadvantages of HVDC technology

HVDC is very effective in case of the presence of long sea straits or asynchronous systems or systems operating at different frequencies. There are a lot of advantages when using HVDC:

 This technology removes the concept of the limit of static stability (no need in stability calculation).

 The limit on transmission capacity is considerably greater than that of the AC power lines.

 Construction of DC lines is easier than AC: the number of insulator strings and cost of conductors are lower.

 It takes less conductors compared to AC power lines.

 The voltage in such lines more stable, since in the steady state DC lines don't generate reactive power.

 Asynchronous AC systems or systems operating with different frequencies can be connected by DC lines without any constraints.

 The power flow direction can be changed (reverse lines).

Disadvantages:

 The need for the construction of complex terminal substations with a large number of voltage converters and auxiliary equipment. It is known that the rectifiers and inverters strongly distort the voltage waveform on the AC side.

Therefore, it is necessary to install a powerful smoothing device and filters which greatly reduces the reliability;

 The DC lines requires polarity and voltage at both ends of the line to be approximately the same before operation starts.