In most cases, the dc power is obtained from large ac power systems by employing converting mechanisms like synchronous or rotary converters, solid-state converters and motor-generator sets and so forth. The reasons why power generation is done in ac form rather than dc are:
AC voltage can be efficiently and conveniently raised or lowered for economic transmission and distribution of electric power respectively. Contrariwise, dc power has to be generated at comparatively low voltages by units of relatively low power ratings. So far, there isn’t an efficient method of raising dc voltage for transmission and lowering it for distribution.
It is possible, to construct large high-speed ac generators of capacities up to 500 MW. Such generators are efficient both in the matter of cost per kWh of electric energy produced as well as in operation. On the other hand, dc generators cannot be built of ratings higher than 5 MW because of the commutation problem. Additionally, since they must operate at low speeds, it requires large and heavy machinery.
A general layout power system for obtaining dc power from ac power is illustrated in Figure 1.0 below:
Figure 1.0: Obtaining dc power from ac power. Two 13.8 kV alternators running in parallel, supply power to the station busbars. The voltage is stepped up by 3-phase transformers to 66 kV for transmission purposes and is again stepped down to 13.8 kV at the substation for distribution purposes.
In reference to Figure 1.0 above, three techniques are employed in converting ac power to dc power at the substation:
6-phase mercury-arc rectifier gives 600 V dc power after the voltage has been stepped down to an appropriate value by the transformers. This 600-V dc power is generally used in electrolytic processes and for electric rail lines.
A rotary converter gives 230 V dc power.
A motor-generator set converts ac power to 500/250 dc power for 3-wire distribution systems.
In dc systems, power may be fed and distributed either by:
2-wire system.
3-wire system.
2-wire DC System
The system consists of 2 wires: One is the outgoing or positive wire and the other is the return or negative wire. The loads such as lamps, motors, and other electrical equipment are connected in parallel between the two wires as illustrated in the figure below:
Figure 1.1: The 2-wire dc system
The potential difference and current have their maximum values at feeding points Fp1 and Fp2. The standard voltage between the conductors is 220 V.
This system is rarely used for transmission because of low efficiency as compared to the 3-wire system; however it may be used for distribution of DC power.
3-wire DC System
This system consists of two outers and a middle or neutral wire which is earthed at the generator end. The voltage between the outers is twice the voltage between the outer and neutral wire i.e. if the potential difference between the outers is 460 V, then potential difference of positive outer is 230 V above the neutral and that of negative outer is 230 V below the neutral.
Figure 1.2: The 3-wire dc system
Motors that need high voltage are connected across the outers whereas lighting and heating circuits requiring less voltage are connected between any one of the outers and the neutral.
Why use High Voltage DC (HVDC)?
HVDC transmission lines require only two conductors and the normal working voltage equals the rated voltage of the line. However, it is necessary to develop an adequate ac/dc/ac converter in order to benefit from the lower cost of dc lines and cables in ac system environs.
Some of the key reasons why HVDC may be selected as a means of interconnecting two power systems (or elements of power systems) are:
The frequency or phase angle variation between the two terminals of the interconnection may render an ac link impractical. In an extreme case, the ac busbars at the terminals of the link may operate at different frequencies. Even if they are synchronized, it doesn’t guarantee that reliable ac transmission can be established, because variations in relative phase angle between the two busbars caused either by variation in load or by network disturbances, may result in unacceptable power flow severe enough to cause frequent tripping. Therefore, it may prove efficient to use HVDC for a zero length (back-to-back) transmission or in parallel with an existing ac transmission path.
The transmission distance may be so long that the cost savings arising from the use of relatively cheaper HVDC conductor systems is more than sufficient to outweigh the costs of the extra terminal equipment required for HVDC.
Additionally, the key advantages that a HVDC system may provide beyond those provided by an ac interconnection include:
Offers the facility to interconnect two systems which have different operational procedures for frequency or voltage control.
Provides predetermined and controlled power transfer. Power flow in an ac interconnection is controlled by phase relationships which, being relatively uncontrolled, can cause inadvertent overloading or underutilization during normal or disturbed operation. In the case of HVDC, two utilities can preset the limits of power by which at any time they can assist each other and power will change automatically up to those limits in response to predetermined conditions such as a frequency change.
Enhance transient stability of the interconnected systems by modulating synchronizing or damping power to reduce inter-machine swings.
To prevent excitation of sub-synchronous resonance as might occur in the case of series capacitor applications in an equivalent ac interconnection.
Allows staged development of a state’s or country’s overall power system in a more controlled and hence less expensive manner by providing the means to use generation in geographically independent systems, compared to what could be done by a purely ac transmission development.
Distributes the available power more efficiently and thus delay the introduction of new power stations and major transmission reinforcements.
HVDC does not contribute to the ac system fault current. The contribution to the system fault current by an ac interconnection may necessitate the replacement of the existing switchgear.
In conclusion, in addition to the utilization of HVDC to connect two systems which cannot by synchronized, it can also be considered as one of the possible alternatives whenever improvements are required to make an ac interconnection appealing i.e. the use of series compensation, variable shunt reactive compensation, phase shift boosters, and so on.
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John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.
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