HVDC Substation

HVDC Substation

The central equipment of a d.c. substation is a thyristor converter. There are two such thyristor converter units. As a separate pole is used for positive and negative (return path) of d.c., there are two poles and the configuration is called bipole d.c. substation.

The converter transformers are used to transform a.c. system voltage to which d.c. system is connected. This ensures derivation of correct d.c. voltage by converter bridges. The converter transformers are generally located in switchyard while the converter bridges are located inside a valve hall. The connection between the two is made with phase isolated busbars or with wall bushings. When wall bushings are used at HVDC level of 400 kV or greater then wall bushings must be designed perfectly and with the care to avoid external or internal insulation breakdown. The harmonic filters consisting L and C are required on a.c. and d.c. side. The d.c. reactors are included in each pole of a converter station. They assist d.c. filters in filtering harmonic currents and smooth the d.c. side current so that a discontinuous current mode is not reached at low load current operation. This makes the commutation process of d.c. converter, more robust.

Surge arresters across each valve in the converter bridge, across each converter bridge and in the d.c. and a.c switchyard are necessary to protect the equipment from all overvoltages regardless of their source. Modem HVDC substations use metal-oxide arresters. The Fig. 7.26.1 shows the layout of a typical HVDC substation 

1. Terminal Equipments of a DC Transmission Line

For proper operation of dc transmission system, various additional auxiliary equipments are required.

These equipments include

1. DC line inductors,

2. Harmonic filters on DC side,

3. Converter transformers

4. Reactive power source,

5. Harmonic filters on AC side,

6. Ground electrodes

7. Microwave communication link between the converter stations.

The dc transmission line with these auxiliary equipments is shown in the Fig. 7.26.2.

a. Inductors and Harmonic Filters on DC Side

On dc and ac side of the dc transmission system harmonics are produced. Normally 6th and 12th harmonic currents are produced. If these currents are allowed to flow through line, it may produce undesirable noise in neighbouring telephone lines. Thus to eliminate these harmonic currents, harmonic filters are used. This filter consists of two inductors and a shunt filter which short circuits the harmonic currents to ground by providing low impedance path.

With the use of these inductors the dc line current is prevented from increasing rapidly under faulty condition.

The inductors connected in series with the line are used to smoothen the dc current output of a converter. An air cored magnetically shielded reactor is used for this purpose.

b. Converter Transformers

The converter transformer is used to provide ac voltage as required by the converter. Three phase transformers of the type star-star or star-delta may be used. A third winding called teritiary winding may sometimes be added for direct connection to source of the reactive power.

It is required to keep dc line voltage constant from no load to full load. Also for reducing the reactive power absorbed by converter the firing angle a should be kept small. It indicates that the ratio between input AC voltage and output DC voltage of the converter is fixed. But as dc line voltage is fixed, the input ac line voltage must also be fixed.

But it may happen that the line voltage on input ac side may go on varying. Thus the converter transformers on rectifier side are provided with tappings which will maintain the ac input voltage nearly constant.

The taps are automatically switched by a motorised tap changer. The taps are also needed on converter transformer on inverter side.

c. Reactive Power Source

The variable static capacitors or synchronous capacitors are required for absorbing reactive power by the converters. The amount of reactive power required increases with the firing angle a of a rectifier and the extinction angle y of the inverter. This power requirement is about 50 % to 60 % of real power transfer. The reactive power consumption is provided by capacitors, filters or synchronous compensators. As the active power transmitted goes on varying, the reactive power must also be varied.

d. Harmonic Filters on AC Side

The three phase, 6-pulse converters produce 5th, 6th, 11th and 13th order harmonics on AC side. These currents are undesirable from the point of view of telephone interference. These currents are bypassed through low impedance filters connected between three phase lines and ground. The filters for each frequency are connected in star and the neutral point is grounded.

e. Ground Electrode

Proper attention must be given towards the ground electrode at each end of dc line. DC currents in the ground have a corrosive effect on pipes, cables and metallic structures. In order that the dc ground current does not produce any local problem around the station, the actual ground electrode is located away from converter station. 

At the grounding site, special means are used to minimize electrode resistance. When bipolar system is temporarily used as monopolar system, the ground current may exceed which may produce excessive heat then this electrode resistance factor is important.

f. Communication Link

For Controlling purpose of the converters at both the ends of the line, a communication link between them is necessary, e.g. to maintain the current margin AI, the inverter side must know what is rectifier current setting is. This information is continually relayed by a high speed communication link between the two converters.

Review Questions

1. Draw the schematic layout of HVDC substation and explain.

2. Discuss the main components of HVDC system.