Hvdc advantages and disadvantages pdf

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hvdc advantages and disadvantages pdf

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The electrical power generated in the form of AC power and most of the power utilized in the form of AC. Because it is difficult to generate bulk DC power. This power transmitted for long distances by the HVAC transmission system.

Advantages and disadvantages of hvdc transmission pdf

A high-voltage, direct current HVDC electric power transmission system also called a power superhighway or an electrical superhighway [1] [2] [3] uses direct current DC for the bulk transmission of electrical power, in contrast with the more common alternating current AC systems.

Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks. High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires.

For a given quantity of power transmitted, doubling the voltage will deliver the same power at only half the current. Since the power lost as heat in the wires is directly proportional to the square of the current, doubling the voltage reduces the line losses by a factor of 4. While power lost in transmission can also be reduced by increasing the conductor size, larger conductors are heavier and more expensive.

High voltage cannot readily be used for lighting or motors, so transmission-level voltages must be reduced for end-use equipment. Transformers are used to change the voltage levels in alternating current AC transmission circuits.

Transformers made voltage changes practical, and AC generators were more efficient than those using DC. These advantages led to early low voltage DC transmission systems being supplanted by AC systems around the turn of the 20th century. Practical conversion of power between AC and DC became possible with the development of power electronics devices such as mercury-arc valves and, starting in the s, semiconductor devices as thyristors , integrated gate-commutated thyristors IGCTs , MOS-controlled thyristors MCTs and insulated-gate bipolar transistors IGBT.

The first long-distance transmission of electric power was demonstrated using direct current in at Miesbach-Munich Power Transmission , but only 1. This system used series-connected motor-generator sets to increase the voltage. Each set was insulated from electrical ground and driven by insulated shafts from a prime mover. The transmission line was operated in a 'constant current' mode, with up to 5, volts across each machine, some machines having double commutators to reduce the voltage on each commutator.

Fifteen Thury systems were in operation by Various other electromechanical devices were tested during the first half of the 20th century with little commercial success.

One technique attempted for conversion of direct current from a high transmission voltage to lower utilization voltage was to charge series-connected batteries , then reconnect the batteries in parallel to serve distribution loads.

A modern battery storage power station includes transformers and inverters to change energy from alternating current to direct current forms at appropriate voltages. First proposed in , [18] the grid controlled mercury-arc valve became available for power transmission during the period to Mercury arc valves require an external circuit to force the current to zero and thus turn off the valve.

In HVDC applications, the AC power system itself provides the means of commutating the current to another valve in the converter. Consequently, converters built with mercury arc valves are known as line-commutated converters LCC. LCCs require rotating synchronous machines in the AC systems to which they are connected, making power transmission into a passive load impossible. Mercury arc valves were common in systems designed up to , the last mercury arc HVDC system the Nelson River Bipole 1 system in Manitoba , Canada having been put into service in stages between and The mercury arc valves were decommissioned on 1 August , ahead of commissioning of replacement thyristor converters.

Since , new HVDC systems have used only solid-state devices , in most cases thyristors. Like mercury arc valves, thyristors require connection to an external AC circuit in HVDC applications to turn them on and off. Development of thyristor valves for HVDC began in the late s. Service interruptions of several years were a result of a civil war in Mozambique. Line-commutated converters have some limitations in their use for HVDC systems.

This results from requiring the AC circuit to turn off the thyristor current and the need for a short period of 'reverse' voltage to effect the turn-off turn-off time. An attempt to address these limitations is the capacitor-commutated converter CCC which has been used in a small number of HVDC systems. The CCC differs from a conventional HVDC system in that it has series capacitors inserted into the AC line connections, either on the primary or secondary side of the converter transformer.

The series capacitors partially offset the commutating inductance of the converter and help to reduce fault currents.

However, CCC has remained only a niche application because of the advent of voltage-source converters VSC which completely eliminate the need for an extinction turn-off time. By the end of , this technology had captured a significant proportion of the HVDC market.

They have extended the use of HVDC down to blocks as small as a few tens of megawatts and overhead lines as short as a few dozen kilometers. There are several different variants of VSC technology: most installations built until use pulse-width modulation in a circuit that is effectively an ultrahigh-voltage motor drive.

Multilevel converters have the advantage that they allow harmonic filtering equipment to be reduced or eliminated altogether.

By way of comparison, AC harmonic filters of typical line-commutated converter stations cover nearly half of the converter station area. With time, voltage-source converter systems will probably replace all installed simple thyristor-based systems, including the highest DC power transmission applications. A long-distance, point-to-point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme.

HVDC conversion equipment at the terminal stations is costly, but the total DC transmission-line costs over long distances are lower than for an AC line of the same distance. HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no skin effect. Depending on voltage level and construction details, HVDC transmission losses are quoted at 3.

HVDC transmission may also be selected for other technical benefits. HVDC powerflow between separate AC systems can be automatically controlled to support either network during transient conditions, but without the risk that a major power-system collapse in one network will lead to a collapse in the second.

HVDC improves on system controllability, with at least one HVDC link embedded in an AC grid—in the deregulated environment, the controllability feature is particularly useful where control of energy trading is needed.

The combined economic and technical benefits of HVDC transmission can make it a suitable choice for connecting electricity sources that are located far away from the main users. Long undersea or underground high-voltage cables have a high electrical capacitance compared with overhead transmission lines, since the live conductors within the cable are surrounded by a relatively thin layer of insulation the dielectric , and a metal sheath. The geometry is that of a long coaxial capacitor.

The total capacitance increases with the length of the cable. This capacitance is in a parallel circuit with the load. Where alternating current is used for cable transmission, additional current must flow in the cable to charge this cable capacitance. This extra current flow causes added energy loss via dissipation of heat in the conductors of the cable, raising its temperature. Additional energy losses also occur as a result of dielectric losses in the cable insulation.

However, if direct current is used, the cable capacitance is charged only when the cable is first energized or if the voltage level changes; there is no additional current required. For a sufficiently long AC cable, the entire current-carrying ability of the conductor would be needed to supply the charging current alone.

This cable capacitance issue limits the length and power-carrying ability of AC power cables. Although some leakage current flows through the dielectric insulator , this is small compared to the cable's rated current.

The capacitive effect of long underground or undersea cables in AC transmission applications also applies to AC overhead lines, although to a much lesser extent. Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the line capacitance can be significant, and this reduces the capability of the line to carry useful current to the load at the remote end.

Another factor that reduces the useful current-carrying ability of AC lines is the skin effect , which causes a nonuniform distribution of current over the cross-sectional area of the conductor.

Transmission line conductors operating with direct current suffer from neither constraint. Therefore, for the same conductor losses or heating effect , a given conductor can carry more power to the load when operating with HVDC than AC. Finally, depending upon the environmental conditions and the performance of overhead line insulation operating with HVDC, it may be possible for a given transmission line to operate with a constant HVDC voltage that is approximately the same as the peak AC voltage for which it is designed and insulated.

Because HVDC allows power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and to separate, would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network.

The magnitude and direction of power flow through a DC link can be directly controlled, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone. HVDC is less reliable and has lower availability than alternating current AC systems, mainly due to the extra conversion equipment.

Single-pole systems have availability of about The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line for the same distance.

Operating an HVDC scheme requires many spare parts to be kept, often exclusively for one system, as HVDC systems are less standardized than AC systems and technology changes more quickly.

In contrast to AC systems, realizing multi-terminal systems is complex especially with line commutated converters , as is expanding existing schemes to multi-terminal systems. Controlling power flow in a multi-terminal DC system requires good communication between all the terminals; power flow must be actively regulated by the converter control system instead of relying on the inherent impedance and phase angle properties of an AC transmission line.

HVDC circuit breakers are difficult to build because of arcing : under AC the voltage inverts, and in doing so crosses zero volts, dozens of times a second. An AC arc will "self-extinguish" at one of these zero-crossing points, because there cannot be an arc where there is no potential difference.

DC will never cross zero volts and never self-extinguish, so arc distance and duration is far greater with DC than the same voltage AC. This means some mechanism must be included in the circuit breaker to force current to zero and extinguish the arc, otherwise arcing and contact wear would be too great to allow reliable switching. Conversely, semiconductor breakers are fast enough but have a high resistance when conducting, wasting energy and generating heat in normal operation.

The ABB breaker combines semiconductor and mechanical breakers to produce a "hybrid breaker" with both a fast break time and a low resistance in normal operation. The hybrid breaker is based on a conventional semiconductor breaker the "main breaker" , with the characteristic rapid break time, full voltage and current tolerance but also the characteristic resistance when conducting. This main breaker is placed in parallel with a "load commutator": a small semiconductor breaker the "load commutation switch" in series with a fast mechanical switch the "ultra fast disconnector".

While neither element of the load commutator can break the full voltage of the line, the load commutator can safely carry the normal operating current with lower resistive losses than the main breaker. Finally, there is a slow mechanical switch to fully disconnect the line. It can't be opened when the line is energised, but will fully disconnect the line with no current leakage and no heat generation. In normal operation, all switches are closed on , and most current flows through the low-resistance load commutator instead of the higher resistance main breaker.

When disconnection is required, the first step is disconnecting the load commutator: the low-voltage semiconductor breaker opens, and this diverts almost all of the current through the main breaker. The main breaker is still conducting, so the load commutator does not see the whole voltage of the line, only the voltage drop caused by the high-voltage main breaker not being a perfect conductor. Because the load commutation switch is open, the ultra-fast disconnector is not exposed to high current, and can open without being damaged by arcing.

The mechanical switch being opened, the load commutator is now fully disconnected: no heat is being generated in the semiconductor switch, and even the full line voltage cannot pass through it. All current is now passing through the main breaker. Now the main breaker opens, breaking the current. This drops the current to near-zero, but increases the voltage across the main breaker and load commutator to nearly the entire line voltage. Had the load commutation switch not been mechanically disconnected earlier, this voltage would damage it.

HVDC Advantages, Disadvantages Over HVAC Transmission System

A high-voltage direct current HVDC is a transmission system which uses direct electric current DC to transmit electricity. It is more efficient over long distances than the usual high-voltage alternating current HVAC. HVDC was first demonstrated in in Germany. The technology was further developed during the s in Sweden and Nazi Germany. Early commercial uses were in the Soviet Union and Sweden in

A high-voltage, direct current HVDC electric power transmission system also called a power superhighway or an electrical superhighway [1] [2] [3] uses direct current DC for the bulk transmission of electrical power, in contrast with the more common alternating current AC systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks. High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, doubling the voltage will deliver the same power at only half the current. Since the power lost as heat in the wires is directly proportional to the square of the current, doubling the voltage reduces the line losses by a factor of 4.

Recieve free updates Via Email! Home Electrical machines Power system Ask a question Contact electricaleasy. HVDC vs. Share: Facebook Twitter Linkedin. Electrical power needs to be transmitted over longer distances from generating stations to electrical substations to be distributed to the consumers. Though DC transmission system was the first to born, soon it was replaced by AC transmission system. Earlier DC systems developed by the Edison's company could not transmit the power more than a couple of kilometers.

There is no transfer of fault energy from one AC system to another if they are connected by a DC tie line. The current carrying capacity of HVDC cable is considerably large due to reduced dielectric losses. The corona power loss and radio interference are less as compared to AC transmission.

HVDC Advantages, Disadvantages Over HVAC Transmission System

The electricity we consume travels very long distances before reaching us. The power transmission is done using very high voltage in order to reduce the line losses. Both types of transmission are affected by several factors but the HVDC transmission has more of the advantages than its disadvantages.

High-voltage direct current

Advantages and Disadvantages of HVDC transmission system

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They are listed below with detail while comparing with hvac. Dc transmission is an effective means to improve dynamic system performance. Hvdc disadvantages hvdc is generally less reliable and has lower availability than hvac. Hvdc transmission advantage and disadvantage, hvdc.

For long-distance power transmission, HVDC lines are less expensive, and losses are less as compared to AC transmission. It interconnects the networks that have different frequencies and characteristics. In AC transmission, alternating waves of voltage and current travels in the line which change its direction every millisecond; due to which losses occur in the form of heat. HVDC lines increase the efficiency of transmission lines due to which power is rapidly transferred. Then, the DC voltage is inverted to AC at the receiving end, for distribution purposes. Thus, the conversion and inversion equipment are also needed at the two ends of the line. HVDC transmission is economical only for long distance transmission lines having a length more than kms and for underground cables of length more than 50kms.


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