In the earlier stage dc system was employed for generation,transmission and distribution of the electric power.with the introduction of transformers and 3 phase ac system the electric power generation,transmission and distribution is shifted towards ac system.The first 110V dc central Electric station was installed by Edision in New york in 1882.mercury arc converters were used in earlier dc system.with the development of high power thyristors in early 1970s the HVDC transmission has become technically and commercially viable alternative to EHV/UHV AC transmission for distance bulk power transmission,cable transmission and system interconnection .HVDC technology is used to transmit electricity over long distances by overhead transmission lines or submarine cables.It is also used to interconnect separate power systems where traditional HVAC connections cannot be used.
Principle of HVDC System operation
when using direct current to provide an asynchronous link between two ac systems,it is necessary to have two converter stations one at each end connected by dc transmission line.Transformer and thyristor valves are used in converter stations. chockes and filters are provided at each end to ensure smooth direct current and suppress harmonics.At sending end the thyristor acts as rectifiers to convert ac into dc which is transmitted over the line.At the receiving end the thyristor valves act as inverters to convert dc into ac which is used at receiving end.
The power dispatched from the generating station Ps less than the power received at the receiving end PR(Ps-PR) represents the power losses due to the conversion and transmission.The converter at the sending end acts as a rectifier and is suitable for power frequency on its ac side while the converter at the receiving end acts as an inverter and its frequency is determined by the frequency of the load system.
In the converter at the sending end the voltage is stepped up to appropriate value by step-up tarnsformer and then converted into the direct current by the thyristor valves.The rectified current flows through the transmission line to the receiving end converting station where it is converted into 3-phase ac current by the thyristor valves and then stepped down by the step-down transformer to the low voltage for the distribution.
Types of HVDC Links
- Monopolar links
- Bipolar links
- Homopolar links
- Back-to-back links
Monopolar DC Link
- It uses one conductor .
- The return path is provided by ground or water.
- Use of this system is mainly due to cost considerations.(As the monopolar line is more economical than a bipolar line because the ground returns saves the cost of the one metallic conductors and losses in it).
- A metallic return may be used where earth resistivity is too high.
- This configuration type is the first step towards a bipolar link.
- This type of configuration is used for low power factor rating and mainly for cable transmission.
- Monopolar is not economical with the length longer than 25Km and power rating of about 250Mw.
2.Bipolar DC Links
- It uses two conductors, one positive and the other negative
- Each terminal has two converters of equal rated voltage, connected in series on the DC side
- The junctions between the converters is grounded
- Currents in the two poles are equal and there is no ground current
- If one pole is isolated due to fault, the other pole can operate with ground and carry half the rated load (or more using overload capabilities of its converter line)
3.Homopolar DC Link
- It has two or more conductors all having the same polarity, usually negative
- Since the corona effect in DC transmission lines is less for negative polarity, homopolar link is usually operated with negative polarity.
- The return path for such a system is through ground.
- Homopolar dc link has limited applications.
COMPONENTS OF HVDC TRANSMISSION SYSTEMS
- Smoothing reactors
- Harmonic filters
- Reactive power supplies
- DC lines
- AC circuit breakers
- They perform AC/DC and DC/AC conversion.
- They consist of valve bridges and transformers.
- Valve bridge consists of high voltage valves connected in a 6-pulse or 12-pulse arrangement.
- The transformers are underground such that the DC system will be able to establish its own reference to ground.
- converter transformers are either single-phase units or three-phase units with either two-windings type or three-winding type.
- converter transformers employed in large HVDC system are of high MVA ratings.
- They are high reactors with inductance as high as 1 H in series with each pole.
- They serve the following:
- They decrease harmonics in voltages and currents in DC lines.
- They prevent commutation failures in inverters.
- Prevent current from being discontinuous for light loads.
The 3-phase bridge converter employed in HVDC transmission should convert pure ac sinusoidal waveform 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.
- Converters generate harmonics in voltages and currents. These harmonics may cause overheating of capacitors and nearby generators and interference with telecommunication systems,over voltages at points in the network.
- Harmonic filters are used to mitigate these harmonics.
- Harmonic filter provide on ac side serve following :
- Harmonic voltages and currents in the ac power network are reduced to acceptable levels.
- 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.
Reactive power supplies
- Under steady state condition conditions, the reactive power consumed by the converter is about 50% of the active power transferred.
- Under transient conditions it could be much higher.
- Reactive power is, therefore, provided near the converters.
- For a strong AC power system, this reactive power is provided by a shunt capacitor.
- Electrodes are conductors that provide connection to the earth for neutral. They have large surface to minimize current densities and surface voltage gradients.
- They may be overhead lines or cables.
- DC lines are very similar to AC lines.
AC circuit breakers
- They used to clear faults in the transformer and for taking the DC link out of service.
- They are not used for clearing DC faults.
- DC faults are cleared by converter control more rapidly.
Advantages of HVDC over HVAC
a)HVDC can transmit more power per conductor per circuit than HVAC.
b)HVDC has no skin effect.
Under a.c. conditions, the current is not uniformly distributed over the cross section of the conductor. The current density is higher in the outer region (skin effect) and result in under utilization of the conductor cross-section. Skin effect under conditions of smooth d.c. is completely absent and hence there is a uniform current in the conductor, and the conductor metal is better utilized.
e)Less corona and Radio-interference in HVDC.
Since corona loss increases with frequency (in fact it is known to be proportional to f+25), for a given conductor diameter and applied voltage, there is much lower corona loss and hence more importantly less radio interference with d.c. Due to this bundle conductors become unnecessary and hence give a substantial saving in line costs.
g)Asynchronous interconnection is possible in HVDC.
With a.c. links, interconnections between power systems must be synchronous. Thus different frequency systems cannot be interconnected. Such systems can be easily interconnected through HVDC links.
In addition, different power authorities may need to maintain different tolerances on their supplies, even though nominally of the same frequency. This option is not available with a.c. With d.c. there is no such problem.
h)HVDC has lower circuit fault levels.
When an a.c. transmission system is extended, the fault level of the whole system goes up, sometimes necessitating the expensive replacement of circuit breakers with those of higher fault levels. This problem can be overcome with hvdc as it does not contribute current to the a.c. short circuit beyond its rated current.
- Depends on length
- The break-even distance for overhead lines is around 600 km and for submarine lines it is around 50 km
- HVDC is better/ more economic for bulk power distance > break even distance
- Better voltage regulation.
- Absence of charging current and limitation of cable length
- No reactive compensation needed
Disadvantages of HVDC
- Converters generate harmonics on both ac and dc sides so filters are required.
- Converter stations are expensive.
- More maintenance of line insulators required.
- Circuit breaking in multi terminal DC system is difficult and costlier.
- Voltage transformation not easy hence cannot be employed for distribution, sub-transmission and backbone transmission.