Power converters make possible the exchange of power between systems with different constant or variable frequencies. The most common converter stations are ac-dc converters for high-voltage direct current (HVDC) transmission. HVDC offers frequency- and phase-independent short- or long-distance overhead or underground bulk power transmission with fast controllability.

Two basic types of HVDC converter stations exist: back-to-back ac-dc-ac converter stations and long distance dc transmission terminal stations. Back-to-back converters are used to transmit power between nonsynchronous ac systems. Such connections exist, for example, between the western and eastern grids of North America, with the ERCOT system of Texas, with the grid of Quebec, and between the 50-Hz and 60-Hz grids in South America and Japan.

With these back-to-back HVDC converters, the dc voltage and current ratings are chosen to yield optimum converter costs. This aspect results in relatively low dc voltages, up to about 200 kV, at power ratings up to several hundred megawatts. Figure 5.1 shows the schematic diagram of an HVDC back-toback converter station with a dc smoothing reactor and reactive power compensation elements (including ac harmonic filters) on both ac buses. The term back-to-back indicates that rectifier (ac to dc) and inverter (dc to ac) are located in the same station.

Long-distance dc transmission terminal stations terminate dc overhead lines or cables and link them to ac buses and systems. Their converter voltages are governed by transmission efficiency considerations and can exceed 1 million V (±500 kV) with power ratings up to several thousands of megawatts.

Typically, in large HVDC terminals, the two poles of a bipolar system can be operated independently, so that in case of component or equipment failures on one pole, power transmission with a part of the total rating can still be maintained. Figure 5.2 shows the schematic diagram of one such bipolar HVDC sea cable link with two 250-MW converter poles and 250-kV dc cables.

Most HVDC converters of today are line-commutated 12-pulse converters. Figure 5.3 shows a typical 12 pulse bridge circuit using delta and wye transformer windings, which eliminate some of the harmonics typical for a 6-pulse Graetz bridge converter. The harmonic currents remaining are absorbed by adequately designed ac harmonic filters that prevent these currents from entering the power systems.

At the same time, these ac filters meet most or all of the reactive power demand of the converters. Converter stations connected to dc lines often need dc harmonic filters as well. Traditionally, passive filters have been used, consisting of passive components like capacitors, reactors, and resistors.

More recently, because of their superior performance, active (electronic) ac and dc harmonic filters as a supplement to passive filters — using IGBTs (insulated gate bipolar transistors) have been successfully implemented in some HVDC projects. IGBTs have also led to the recent development of self commutated converters, also called voltage-sourced converters. They do not need reactive power from the grid and require less harmonic filtering.

The ac system or systems to which a converter station is connected significantly impact its design in many ways. This is true for harmonic filters, reactive power compensation devices, fault duties, and insulation coordination.

Weak ac systems (i.e., with low short-circuit ratios) represent special challenges for the design of HVDC converters. Some stations include temporary overvoltage limiting devices consisting of MOV (metal oxide varistors) arresters with forced cooling for permanent connection, or using fast insertion switches.

HVDC systems, long-distance transmissions in particular, require extensive voltage insulation coordination,which can not be limited to the converter stations themselves. It is necessary to consider the configuration, parameters, and behavior of the ac grids on both sides of the HVDC, as well as the dc line connecting the two stations.

Internal insulation of equipment such as transformers and bushings must take voltage gradient distribution in solid and mixed dielectrics into account. The main insulation of a converter transformer has to withstand combined ac and dc voltage stresses. Substation clearances and creepage distances must be adequate.

Standards for indoor and outdoor clearances and creepage distances are being promulgated. Direct-current electric fields are static in nature, thus enhancing the pollution of exposed surfaces. This pollution, particularly in combination with water, can adversely influence the voltage-withstand capability and voltage distribution of the insulating surfaces.

In converter stations, therefore, it is often necessary to engage in adequate cleaning practices of the insulators and bushings, to apply protective greases, and to protect them with booster sheds. Insulation problems with extra-high-voltage dc bushings continue to be a matter of concern and study.

A specific issue with long-distance dc transmission is the use of ground return. Used during contingencies, ground (and sea) return can increase the economy and availability of HVDC transmission. The necessary electrodes are usually located at some distance from the station, with a neutral line leading to them.

The related neutral bus, switching devices, and protection systems form part of the station. Electrode design depends on the soil or water conditions. The National Electric Safety Code (NESC) does not allow the use of earth as a permanent return conductor. Monopolar HVDC operation in ground-return mode is permitted only under emergencies and for a limited time.

Also environmental issues are often raised in connection with HVDC submarine cables using sea water as a return path. This has led to the recent concept of metallic return path provided by a separate low-voltage cable. The IEEEPES is working to introduce changes to the NESC to better meet the needs of HVDC transmission while addressing potential side effects to other systems.

Mechanical switching devices on the dc side of a typical bipolar long-distance converter station comprise metallic return transfer breakers (MRTB) and ground return transfer switches (GRTS). No true dc breakers exist, and dc fault currents are best and most swiftly interrupted by the converters themselves. MRTBs with limited dc current interrupting capability have been developed.

They include commutation circuits, i.e., parallel reactor/capacitor (L/C) resonance circuits that create artificial current zeroes across the breaker contacts. The conventional grid-connecting equipment in the ac switchyard of a converter station is covered in the preceding sections. In addition, reactive power compensation and harmonic filter equipment are connected to the ac buses of the converter station.

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