Equipments Used in HVDC Transmission System | Electrical Engineering

In HVDC transmission system or HVDC back-to-back coupling station, ac is converted into dc by means of a combination of converter-transformers and concerter valves. The conversion of ac into dc is called rectification. At the receiving end of the HVDC transmission line, dc is converted back to ac by means of converter valves and and converter transformers. The conversion of dc into ac is called the inversion.

HVDC link comprises an ac substation and conversion substation at each end of HVDC transmission line but in case of back-to-back HVDC link, there is only a conversion substation between two ac substations and there is no dc transmission line.

Thyristor valves are the most important converter station equipment and the other main equipment include converter transformers, dc reactor, harmonic filtering equipment, control equipment and reactive power compensation equipment.

1. Thyristor Valves:

Since, the early 1970’s, the thyristor valves are universally employed in all new HVDC transmission system while earlier mercury-arc valves were used for conversion/inversion purposes. A thyristor valve has a better operating life and higher consistency of performance than those of a mercury-arc valve.

The use of thyristors has resulted in a considerable simplification in the design of conversion substation. The ratings of thyristors have increased remarkably during the last one decade and the current ratings of the thyristors available now are of the magnitudes of line current required for transmission.

However, the voltage capability of individual thyristor is quite small in comparison to the line voltage. Thus for obtaining the proper voltage rating it becomes imperative that a large number (typically 5 to 200) of thyristors are connected in series. A thyristor valve is always provided with a number of additional or redundant thyristors in series so that even if some of the thyristors fail, the valve operation remains unaffected.

The valves used are of indoor design, air-insulated and air/water cooled. Thyristor assemblies are protected from dv/dt and dI/dt protection. Six-pulse or twelve-pulse converters are used in modern HVDC schemes—12-pulse is more preferred as it contains minimum ripple.

There are two 3-phase transformers connecting each 12-pulse converter to the ac bus-bars. One of the transformers is star/star connected and the other is star/delta connected so as to give a phase shift of 30° between the lower and upper 6-pulse bridge of the converter.

The structural design of the converters is very simple. Each thyristor module consists of the thyristors and the converter pole is composed of series connection of several thyristor modules. In addition to the thyristor elements, each module contains damping and over- voltage protection circuitry, voltage dividers and firing control circuits. The modules are mounted in layer form in a box-like structure to form the unit.

Basically, the converters are of 12-pulse operation. However, the use of a single 12-pulse converter per pole as well as multiple valves per pole is available in practice. The former design, though, is very simple in construction and operation but it needs a heavy transformer causing transportation and erection problems. Use of single phase transformers has been attempted in the West to overcome this problem.

Use of four valve units per phase of a 12-pulse converter configuration is common. A typical Quadruple valve comprises four valves placed vertically one above the other to form one limb of the converter. Such an arrangement provides the most compact and economical layout of the valves and the valve housing. Three quadruple valves constitute a 12-pulse converter. Valve surge arresters may be directly mounted on the valve structure.

Thyristors produce heat of 30-40 W/cm² and, therefore, need effective cooling system. The valves are cooled by air, SF₆ gas, oil or a combination. The cooling system and insulation are independent. The temperature of silicon wafer joint should be held below critical value (90°-125° C) to prevent change in characteristics and damage to thyristors.

The schematic unit of a 12-pulse bridge is shown in Fig. 14.5.

In practice two 12-pulse converters per pole (connected in series or in parallel) are used so as to enhance the reliability. In parallel connection, a smoothing reactor is required for each 12-pulse converter and current balancing controls are required to be used. A parallel configuration gives less line loss during the period when full-line currents are not yet established.

The thyristors are required to be triggered in a desired sequence and desired instants. Though conventional firing circuitry is still in use but the most modern method of triggering of thyristors is called the “optronics” which employs light guide system.

In this system the electrical pulses are converted into light signal pulses by fibre-optic techniques. The light pulses are transmitted through glass-fibre optical cables up to the individual thyristor. Each thyristor is provided with a small separate thyristor which is triggered by the light pulse. Thereby the main thyristor is triggered.

For the protection of thyristors the protections provided are dv/dt protection, over-temperature protection, forward voltage firing, forward recovery protection and current sharing protection between two parallel thyristors.

2. Converter Transformers:

Converter transformers are connected between converter valves and the ac bus-bars and their main function is to transform the ac voltage to a suitable value for feeding the converter. The other functions served by the converter transformers is to supply the reactive power to the converter through tap changing; control of fault level by suitable reactance offered by converter transformers and help in harmonic suppression. Twelve- pulse operation is feasible due to transformer connections—by suitable star-star and star-delta connections the required phase shift of 30° for 12-pulse operation is achieved.

Converter transformers are either single phase units, or three-phase units with either two winding type or three winding type. These are specially designed so as to withstand direct voltage stresses. Moreover, in a converter transformer the currents flowing through the winding have high harmonic contents. Thus the eddy current losses are high. Noise due to magnetostriction is higher and may require special design tank to suppress the same.

On load tap changing is normally used to reduce the demand of reactive power in steady-state operation. The number of tap-changer operations is higher in HVDC systems and, therefore, proper quality control of components of tap-changers is required. Converter transformers employed in large HVDC schemes are of high MVA ratings (about 300 MVA or even more).

3. DC Reactor:

In HVDC transmission systems the dc reactor, connected in series with each pole of the converter plays an important role. It prevents commutation failures in inverter by limiting the rate of increase of direct current during commutation in one bridge when the direct voltage of another bridge collapses.

It reduces the incidence of commutation failures in the inverter during dips in the alternating voltage. It reduces harmonic voltages and currents in the dc line, smoothens the ripple in the direct current and limits the current in the rectifier following a short-circuit in the line.

The dc reactor is usually of air-core and oil-cooled type and has the non-linear magnetic characteristics.

The value of inductance of the reactor is very important. With the increase in inductance the current waveform on the dc side improves but the control response slows down and the resonance frequency falls making the stabilization of current control more difficult. The limitation of dc line short-circuit currents is normally the decisive factor and leads to smoothing reactances of between 0.4 H and 1.0 H.

4. Harmonic Filtering Equipment:

The 3-phase bridge converter employed in HVDC transmission should convert pure ac sinusoidal wave form to pure dc form but in practice the operation of converter generates harmonic currents and harmonic voltages on ac side as well as dc side.

These harmonics do not interfere with converter operation but they flow through ac and dc lines and thereby produce several harmful effects such as overheating of capacitors and generators, overvoltages at points in the networks, interference with protective gear, interference with nearby com­munication systems, radio interference and television interference. These disturbances are not confined to the vicinity of the converter station but spread over the ac network and dc line and surrounding residential areas.

An n-pulse converter generates harmonics predominantly of the order of nx ± 1 on ac side and nx on dc side where x is an integer and n is usually 6 or 12. Thus in case of a 12-pulse valve group operation, the orders of main harmonics is 11th, 13th, 23rd and 25th on the ac side and 12th and 24th on the dc side. The amplitudes of harmonics decrease with increasing order. It is necessary to provide filters on both the ac and dc sides.

Harmonic filters provided on the ac side serve the following purposes:

1. Harmonic voltages and currents in the ac power network are reduced to acceptable levels.

2. All or a part of the reactive power required by the converter is provided, the additional reactive power being supplied by the shunt capacitor banks or by the ac power system.

The ac filters employed in HVDC transmission systems are tuned filter which is sharply tuned to a harmonic frequency, damped filter which, if shunt connected, offers a low impedance over a broad band of frequencies (say 11th and harmonics of higher orders). The adoption of 12-pulse valve group operation has resulted in reduction of harmonic current generation and consid­erable simplification in ac filters.

In a 12-pulse valve group operation only two damped filters are employed while 6-pulse valve group operation requires four tuned filters and one damped filter. Damped filters are simpler to design and have a lower risk of the resonant overvoltages and currents in comparison to tuned filters.

In bipolar operation, under ideal conditions, the induced voltages would be negligible and filters are not required. However, in practice, assuming certain unbalance (in transformer reactances etc.), the induced voltage level would increase. So use of high pass dc filter tuned to 12th harmonic is usually made for a 12-pulse converter circuit.

5. Control Equipment:

The control of firing angle is very important in HVDC systems. This is accomplished by optic fibre based hardware circuitry.

An important difference between the operations of a dc system and an ac system is that the power transmitted over a dc link is always controlled. In most practical HVDC systems the valves of the two converter stations are controlled in such a way that the rectifier end controls the current while the inverter end controls the voltage. These controls allow the link to maintain constant power.

6. Reactive Power Compensation Equipment:

HVDC converter stations need reactive power, particularly because of converter control and commutation process. The reactive power requirement of a rectifier varies as the sin of the firing angle (i.e., sin α) whereas the power requirement of the inverter varies as the sin of extinction angle (i.e., sin β). Moreover, converter transformers consume reactive power. The reactive power requirement is usually in the range of 50 to 60 per cent of the transmitted power.

As no reactive power is transmitted over the dc line and maintenance of reactive power balance at the two ends is essential to keep ac voltage within specified limits, instal­lation of reactive power compensation equipment in the HVDC terminal stations becomes imperative. This equipment may consist of ac filters, static shunt capacitors, synchronous condensers, static VAR compensators, etc. A combination of this equipment is also used.