In this article we will discuss about:- 1. Introduction to HVDC Transmission 2. Principle of HVDC Transmission 3. Power Control 4. Ground Return 5. Earth Electrode and Station Earth 6. Transient Overvoltages and Insulation Coordination 7. Corona 8. Circuit Breaking 9. Comparison with EHV-AC Transmission 10. Limitations.
Contents:
- Introduction to HVDC Transmission
- Principle of HVDC Transmission
- Power Control in HVDC System
- Ground Return of HVDC Transmission
- Earth Electrode and Station Earth of HVDC Transmission
- Transient Overvoltages and Insulation Coordination in HVDC Systems
- Corona in HVDC Lines
- HVDC Circuit Breaking
- Comparison between EHV-AC and HVDC Transmission
- Limitations of HVDC Transmission
1. Introduction to HVDC Transmission:
ADVERTISEMENTS:
In the initial stages dc used to be employed for generation, transmission and distribution of electric power. The first 110 V dc Central Electric Station was installed by Edison in New York in 1882. With the introduction of transformers and the 3-ɸ ac system the situation changed in favour of ac, especially where the electric energy used to be harnessed from water power which is usually available far from the load centres.
For many reasons, such as facility of transformation of voltages from one value to another, better performance of ac motors and superiority of ac generators in comparison to dc generators, the power was generated, transmitted, distributed and utilized in the form of ac.
The supporters of dc, however, did not forget the advantages of dc transmission and extensive research has been carried out in this field and as a result it has staged a sort of comeback to the field of electric power transmission at high and extra high dc voltages.
The earlier dc schemes used mercury-arc convertors. Extensive research has been carried out especially in Sweden for the development of high voltage converters. With the successful development of high power (50 kV, 100 A) thyristors in early 1970’s the HDVC transmission has become technically and commercially viable alternative to EHV/UHV AC transmission particularly for distance bulk power transmission, cable transmission and system interconnection.
ADVERTISEMENTS:
For these applications HVDC transmission systems have a distinct superiority over EHV AC transmission systems and are being increasingly preferred. However, HVDC transmission cannot be substitute of ac transmission because for backbone ac network, generation, transmission and distribution are definitely superior.
The choice of transmission systems and operating voltages for a transmission line is made from HV-AC (up to 220 kV), EHV-AC (between 400 kV and 765 kV), UHV-AC (exceeding 765 kV) and HVDC (up to 1,600 kV) on the basis of technical and economical studies for each particular line and associated ac systems.
The first dc link was set up in 1954 between Swedish mainland and the island of Gotland. This was a monopolar, 100 kV, 20 MW, cable system making use of sea return. The England-France cross-channel dc link was commissioned in 1961. This was a bipolar, ± 100 kV, 160 MW, cable system over a distance of about 65 km. Since then more and more HVDC systems have been set up in the world.
HVDC transmission has also been introduced in India. ± 500 kV has been selected as the voltage for HVDC transmission. A ± 500 kV, 1500 MW, 810 km bipolar HDVC line has already been set up between Rihand and Delhi. It is designed to operate in the bipolar, monopolar-ground return and monopolar metallic return modes. Another HVDC system set up in India is Vindhyachal HVDC back to back system.
ADVERTISEMENTS:
This back to back link is for exchange of power between Northern and Western regions. Each block of 250 MW is capable of operating independently in either direction and can transfer power in the range of 25-250 MW depending on the conditions of the system. In addition, Chandarpur back to back project (2 × 250 MW), Chandarpur-Padghe bipolar system (± 500 kV, 1,500 MW, 736 km), Jeypore back to back project (2 × 250 MW) and Mau back to back project (2 × 250 MW) are also proposed.
At present the world has over 50 HVDC schemes in operation for a total capacity of more than 50,000 MW and the capacity is increasing by about 2,000 MW every year.
2. Principle of HVDC Transmission:
When using direct current to provide an asynchronous link between two ac systems, it is necessary to have two convertor stations one at each end, connected by a dc transmission line. The main equipment in a convertor station is transformers and thyristor valves.
ADVERTISEMENTS:
Chokes and filters are provided at each end to ensure smooth direct current and suppress harmonics. At the sending end the thyristor valves act 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 utilized at the receiving end.
Single line diagram of a HVDC transmission system is shown in Fig. 14.1, where A and B are the two converter stations. Converter station A is supplied from the generating station G. In converter station at the sending end the voltage is stepped up to appropriate value by step-up transformer and then converted into direct current by the thyristor valves.
Thus at the start of transmission line, we have high voltage direct current. This rectified current flows along the transmission line to the receiving-end converting station B, where it is converted into 3-phase ac current by the thyristor valves and then stepped down by the step- down transformer to low voltage for further distribution.
ADVERTISEMENTS:
The power dispatched from the generating station PS less the power received at the receiving end PR i.e., (PS – PR) represents the power losses due to conversion and transmission. The convertor at the sending end acts as a rectifier and is suitable for power frequency (i.e. frequency of generator) 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. This frequency is independent of the sending-end frequency provided the two ends A and B are not additionally connected by the 3-phase lines.
The dc output voltage magnitude is controlled by varying the firing angle of the thyristor valves in the converter. In rectifier the firing angle is between 0° and 90° while in inverter it is between 90° and 180°. As the dc output voltage is a function of cosine of the firing angle hence the converter voltage becomes negative when firing angle α exceeds 90°. This makes the converter to operate as an inverter. The two converters at sending end and receiving end are identical and whether they have to work as rectifier or inverter is determined by the direction of power flow.
In practical HVDC converter stations three-phase bridge converters are employed at both ends (sending as well as receiving ends). Reversible operation of converters as well as bidirectional power flow in HVDC link is possible simply by the control of firing angle.
3. Power Control in HVDC System:
In dc systems the power transferred from one station to another station is governed only by the magnitudes of terminal dc voltages at the two ends while in ac transmission systems the power transfer is governed by phasor difference (magnitude as well as phase) of the voltages at the two ends. Thus the controllability of HVDC power is fast and stable. The current flows from higher voltage to lower voltage which is set by adjustment of firing/extinction angles of the two converters (rectifier and inverter).
If VS is the voltage at the sending and VR is the voltage at the receiving end then the line current is given as:
Idc = (VS – VR) / R … (14.1)
where R is the resistance of the entire transmission link
The sending-end voltage is given as:
and the receiving-end voltage is given as:
where α is the firing angle of rectifier, β is the extinction angle of inverter, Vac S is the ac side line-to-line rms voltage at the sending end, Vac R is the ac side line-to-line rms voltage at the receiving end, XCS is the commutation reactance at the sending end and XCR is the commutation reactance at the receiving end.
Thus, the power transferred is given as:
Tap-changers on the ac side take care of voltage variations on ac side and dc power is controlled by controlling the sending-end and receiving-end voltages VS and VR which is possible by control of firing and extinction angles α and β respectively
4. Ground Return in HVDC Transmission:
A ground return means the ground or sea water or both as the return conductor. Most dc transmission lines use ground return path for reasons of economy and reliability. The monopolar link and homopolar link use the ground return continuously for carrying the return current whereas the bipolar link uses ground return for short duration of emergency (when one of the lines is under repair or maintenance). For the same length of transmission the resistance offered by the ground in case of dc is much less than that in case of ac transmission.
This is because the direct current in the earth, unlike alternating current, does not follow closely the route of the line but spreads over a very large cross-sectional area in both depth and width. The resistance of ground returns path in case of dc is independent of the length of lines (for long lines) and is essentially the sum of grounding resistances of the earthing electrodes at the two ends of the line.
Besides the advantage of economy, there are two more definite advantages of using ground return. The first advantage is that a dc line can be built in two stages—the line can be built as a monopolar line with ground return in the initial stages and may be converted into a bipolar line on a later date when the load requirement increases.
Thus a considerable part of the total investment can be deferred until the second stage. The second advantage is the reliability of the system i.e. in the event of an outage of one conductor of the bipolar line, it can be operated temporarily at almost half of its rated power, by using the healthy line and the ground.
Ground return lines have some problems also. The main problems are designing of ground electrodes for low resistance and low cost of installation and maintenance, location and screening of electrodes so that ground currents cause negligible electrolytic corrosion of buried or immersed metallic structures, prevention of interference of the ground currents with the operation of other services such as ac power transmission, railway signals, ship’s compasses etc. and control of surface gradient near the electrodes for the safety of persons, livestock and fish.
5. Earth Electrode and Station Earth in HVDC Transmission:
The mid-point of converters, called the neutral point, in each station is grounded with a suitable switching arrangement. The earthing is independent of the station earthing. This electrode earthing is through electrode installed at a safe distance (about 5 to 25 km) from the terminal station, major pipe lines, substations and populated areas. The earth electrode is installed away from the substations and major pipe lines so as to avoid the galvanic corrosion of the substation earthing system, underground pipes, buried cables and structures.
Earth electrode is an array of conducting elements placed in the earth or sea which provide a low resistance path between the dc circuit and the earth and which is capable of carrying continuous current for some extended period. The earth electrode at one of the stations acts as an anode while at the other station it acts as a cathode. The terminal station is connected to the earth electrode through an insulated cable, called the earth electrode line.
Station earth is an array of conducting elements placed in earth at the substation location and which provides connection between the earthed parts of the station equipment and neutral of converter transformers and the earth. Station earth is provided for the protection of equipment from overvoltages and safety of personnel. There is no direct metallic connection between earth electrode and station earth.
Graphite electrodes buried in a pit filled with crushed coke are used. The electrode transfers current to coke which distributes it to earth. This arrangement reduces the loss of material due to corrosion to a negligible value. Iron and graphite cannot be employed as material for earth electrode because iron has high rate of corrosion and graphite when buried in earth causes significant loss of material due to corrosion.
The design aspects of earth electrode are low earth resistance, low current density (not exceeding 1.5 A/m2) at electrode surface, temperature rise of electrode and surroundings not exceeding 60° C and step voltage on the ground surface above the electrode not exceeding the safe limits.
An earth electrode may be a straight electrode, a ring electrode or a radial star electrode buried horizontally in earth at a depth of about one metre. The radial star arrangement is commonly employed as it uses land area more effectively.
6. Transient Overvoltages and Insulation Coordination in HVDC Systems:
Convertor overvoltages are due to transient in converter itself, disturbance on dc side, disturbance on ac side, and transient due to external agents.
Transient overvoltages may occur in the converter itself. This occurs due to convertor’s own problems like commutation failure or repetitive misfire. Sudden blocking of inverter firing may also cause overvoltage across the converter.
Disturbance on dc side that causes overvoltages may be due to short-circuit of one pole in a bipolar link and transient overvoltage induced in the healthy pole. Re-energization of thyristor bridges can cause large transient overvoltages.
Severe overvoltages may be caused, on the ac side, due to resonance between the filter and the system for a low order harmonic. The other factors that may cause overvoltages on the ac side, are transformer switching, load rejection, ac system faults etc. Transformer saturation due to harmonic current may also result in overvoltage.
Lightning and switching surges are external agents that may cause transient overvoltages in the HVDC systems. Also the switching surges and lightning surges in the ac system can be transferred to the dc side through the converter transformers.
Zinc oxide surge arresters (ZnO arresters) are connected in dc yard, valve hall, ac yard filter circuits, etc. HVDC system requires a large number of arresters. These are coordinated with surge arresters in ac yard.
The rated voltage switching impulse withstand characteristics, lightning impulse withstand characteristics, test voltages, test procedures, connection surge arresters, etc. form a part of insulation coordination studies.
7. Corona in HVDC Lines:
In dc corona, the charges released from one conductor are carried to the ground or the other conductor because of the opposite polarity. Thus the movement of charged particles is characterised by electric field distribution rather than the surface gradient. In case of ac voltage, the peak value of voltage wave is √2 times the rms value but in case of dc there is no such factor and, therefore, in HVDC lines the corona loss is smaller.
In case of HVDC lines, corona over insulators and metal surface are negligible. Corona loss is HVDC lines during bad weather conditions do not increase abruptly as in case of ac line. Corona loss in case of dc lines does not increase with the increase in operating voltage as rapidly as in case of ac lines.
In case of ac lines, the ions going away from the conductor surface due to like polarity of charging during a half cycle get attracted towards the conductor during the next half cycle of ac wave. But in case of dc lines, the ions with the same polarity as that of the conductor get a time to go away from the conductor. Thus the corona phenomenon acts differently with ac and dc lines.
However, the corona loss in HVDC system is an important factor and cannot be neglected in the design of the line. The corona behaviour of a monopolar HVDC line is different from that of a bipolar line. This is due to the difference in release of the charge from the vicinity of the conductor surface. Corona loss in the monopolar HVDC line is minimum.
The critical line voltages VC for monopolar and bipolar HVDC line are given by Eqs. (14.5) and (14.6) respectively.
For monopolar HVDC line,
where EC is critical stress in kV/cm (14kV/cm in case of dc), m is the surface irregularity factor (0.8-0.92), h is the height of the conductor above earth in cm, r is conductor radius in cm and D is the pole to pole spacing in cm.
The corona loss in a bipolar HVDC line is given as:
Pdc = P0 + K (E – EC)2 kW/km … (14.7)
where P0 is corona loss at critical gradient (0.3-0.5 kW/km), E is actual surface stress in kV/cm and K is a constant.
Protection Aspect:
The malfunctioning of a converter may occur due to the following reasons:
1. Misfire and back fire in converter valves.
2. Commutation failure, the disturbance that usually occur in inversion operation.
3. Short-circuit in converter station.
Many of the valve faults are of temporary nature and can be eliminated by interrupting conduction temporarily. The faults can be eliminated by the control action used in controlling firing angle.
The faults (short-circuit or earth faults) occurring on ac side of the HVDC system cause dip in the voltage at the terminals of the converter and are required to be cleared in the shortest possible time duration. It is necessary for keeping dc power transfer normal. Inverse relays and differential protections are employed for clearing the faults that are generated in the filter banks.
8. HVDC Circuit Breaking:
The process of arc extinction in ac circuit breakers is easier because of natural zero value of alternating currents. But direct current being a steady unidirectional current does not have a natural zero and, therefore, it is difficult to interrupt large direct currents at high voltages. The lack of dc circuit breakers has limited the network of dc lines.
Almost all dc transmission projects till this date are two terminal projects and it is not difficult to interrupt the fault currents. The transient faults can be cleared using valve control methodology while for clearing permanent faults combination of valve control, fault locators and isolating switches is employed.
A vacuum interrupter is employed for breaking direct currents in HVDC systems. An auxiliary device is attached to the vacuum interrupter such that the arc current is opposed by the auxiliary device current (oscillatory discharge of capacitor). However, in practice, the fault current is shaped before being interrupted.
Commutation of load current from a saturable reactor to an R-C circuit is made such that after commutation the current is opposed by the high inductance of the reactor. Researches on HVDC circuit breakers are still in progress and use of these circuit breakers is limited to multi-terminal dc systems only.
9. Comparison between EHV-AC and HVDC Transmission:
Comparison between EHV-AC and HVDC for long distance high power transmission is given in tabular form in Table 14.1.
The dc transmission links so far used mostly are based on the factors like long river crossings, frequency conversions and asynchronous ties between large ac systems. To give an approximate idea of economic distance for HVDC transmission is that for a distance of 400 km the power of at least 100 MW is required to be transmitted. With the development of converting equipment, the break-even distances will be reduced.
10. Limitations of HVDC Transmission:
Although dc transmission has so many advantages over ac transmission, yet it will never completely overtake ac transmission, at the most it will work in conjunction with it.
The problems associated with and limitations of dc transmission are given below:
1. Costly Terminal Equipment:
The converters required at both ends of the line have proved to be reliable but they are much more expensive than the conventional ac equipment.
The converters have very little overload capacity and absorb considerable reactive power— the reactive power can be as high as 25% of true power.
It is obvious that no reactive power can be transmitted by HVDC transmission system and, therefore, equipment such as static or synchronous capacitors are installed at the receiving end to generate reactive power (MVAR) according to the demand of the load and converting apparatus.
The converters produce lot of harmonics both on dc and ac sides which may cause interference with the audio-frequency communication lines. Filtering and smoothing equipment are provided at the convertor stations in order to remove ripples from the dc output. It is also possible to provide filters on the ac side to absorb the harmonic currents. Thus there is considerable increase in cost of the converter substation.
HVDC converters need complex cooling systems.
HVDC circuit-breaker system comprises several components including the main circuit- breaker capacitors, reactors etc., and the total cost is likely to be several times that of an ac circuit breaker of equivalent rating.
2. More Maintenance of Line Insulators:
Maintenance of insulators in HVDC transmission line is more.
3. Circuit breaking in multi-terminal dc systems is difficult and costlier.
4. Voltage Transformation:
Voltage transformation is not easier in case of dc and hence it has to be accomplished on the ac side of the system. DC system cannot be employed for distribution, sub-transmission and backbone transmission.