Comparison Of HVAC and HVDC Transmission Network

What is HVAC vs HVDC

Electrical power needs to transmit a distance from its generation station to transmission and distribution between users. Initially, transmission system born as a DC system, but finally stopped it and grew as an AC system. Now again retaining and developing HVDC for power transmission.

The feature comparison between HVDC and HVAC is as below:

Investment cost:

Considering the power carrying conductors, DC transmission requires fewer conductors than AC transmission - 2 conductors per DC circuit whereas three conductors per 3 phase AC circuit. 

HVDC needs smaller tower for line support thus, HVDC transmission line would cost lesser than an HVAC line. 

But, the terminal converter stations in HVDC are much more expensive which are not required for HVAC transmission. 

Over a certain line length what is called break-even distance, HVDC line becomes cheaper than HVAC. The break-even distance for overhead lines is around 600 km and for submarine lines, it is around 50 km.

Losses:

There are no skin effect losses in the DC system. Corona losses also significantly lower in the case of the DC system comparing the AC system. An HVDC line has considerably lower losses compared to HVAC over longer distances.

Controllability:

There is better voltage regulation in the HVDC system due to the absence of inductance. Also, the HVDC system offers greater controllability compared to HVAC.

Asynchronous interconnection:

We worldwide there are 50 Hz or 60Hz AC system is existed, it is not possible to the interconnection between different frequency 50 Hz and 60Hz AC power grids. But in the DC system, there is no issue about frequency.

Interference with nearby communication lines:

Interference with nearby communication lines is lesser in the case of HVDC overhead line than that for an HVAC line.

Short circuit current:

In longer distance HVAC transmission, short circuit current level in the receiving system is high. An HVDC system does not contribute to the short circuit current of the interconnected AC system.

What are the advantages of HVDC Power system?

The major advantages of the HVDC power transmission system are as below:

i) The use of this system reduces the number of conductors and insulators in the power system. As a result line cost is reduced.

ii) As a result of using this system, the amount of Phase to phase and ground to ground clearance is less.

iii) As a result of using this system pole or tower can be installed very cheaply.

iv) The amount of corona loss in this system is much less.

v) The power loss of the line is greatly reduced.

vi) No reactive power is generated or absorbed in the HVDC system. So no reactive power compensation is required in this system.

vii) HVDC system does not have any frequency of current so skin effect and proximity effect does not occur in the transmission line.

What are the disadvantages of an HVDC transmission system?

The major disadvantages of an HVDC transmission system are as below:

i) The cost of circuit breakers used in HVDC systems is much higher.

ii) No transformer is used to change the voltage level in this system.

iii) The cost of this system is much higher as the converter substation is connected to the sending end and receiving end terminals of the transmission line.

iv) The inverter used in the converter substation has limited capacity.

v) The two terminals of the inverter and rectifier device create harmonic, which is used to reduce the active filter. Using an active filter is very expensive.

vi) A heat loss is created in the converter substation, which is reduced using an active cooling system.

Most powerful underground power transmission cable system- 640 kV extruded HVDC cable

The world recognized cable manufacturer NKT claims that their 640 kV extruded DC cable system is the latest result of the intensive investment in research and development in the field of HVDC transmission technology. It sets a new world record for extruded HVDC cable technology enabling the integration of renewable power and connecting energy markets.

They said by using cables, efficient power delivery can also be made through densely populated or environmentally sensitive areas. A single pair of 640 kV extruded HVDC cables could, for example, transmit enough green power to supply three million households.

Underground Power Cable: Environment and Health Issues

Power Plant Cooling Tower

Impact of Power Cable Uses in Environmental and Human Health:

Technology changing every day, every moment! High voltage electricity transmission underground cable system also taking over instate of the overhead power line. Though underground high voltage electricity transmission more technically challenging and expensive than installing overhead lines; but to get enough space for an overhead line, especially in an urban area is very difficult now. So, it’s the demand of time for the underground cabling system. Of course, we have to keep in mind the potential impact on the environment and the health of cable installations.

The overhead conductor is more harmful to human and other animals than underground cables.

When a vehicle is parked under a high voltage transmission line an electrostatic field is developed in it. When a person who is grounded touches it a discharge current flows through the human being. In order to avoid this parking, lots are located below the transmission lines the recommended clearance is 17 m for 345 kV and 20 m for  400 kV lines.

EFFECT ON ANIMALS

Many researchers are studying the effect of the Electrostatic field on animals. In order to do so, they keep the cages of animals under a high Electrostatic field of about 30 kV/m. The results of these Experiments are shocking as animals (are kept below high Electrostatic field their body acquires a charge & when they try to drink water, a spark usually jumps from their nose to the grounded Pipe) like Hens are unable to pick up grain because of the chattering of their beaks which also affects their growth. 

EFFECT ON PLANTS

High power transmission lines affect the growth of plants.

Physiological parameter was primarily due to the effect of reduced cell division and cell enlargement.

From various practically study it was found that the response of the crop to EMF from 110 kV and 230 kV Power lines showed variations among themselves. Based on the results the growth characteristics like shoot length, root length, leaf area, leaf fresh weight, specific leaf weight, shoot/root ratio, total biomass content and total water content of the four crop plants were reduced significantly over the control plants.

SHORT TERM HEALTH PROBLEM

• Headaches.
• Fatigue.
• Insomnia.
• Prickling and/or burning skin.
• Rashes.
• Muscle pain

LONG TERM HEALTH PROBLEM

• Risk of damaging DNA.
• Risk of Cancer.
• Risk of Leukemia.
• Risk of Neurodegenerative Disease.
• Risk of Miscarriage.

Insulating in Overhead Power Line and Underground Cable:

Conductors, no matter they are overhead or underground that transmit electricity must be electrically insulated. But there is one major difference between overhead lines and underground cables is the way they are insulated. Overhead lines are insulated by open-air surrounding conductor whereas underground cable conductors are fabricated wrapped in layers of insulating material.  Air insulation is the simplest and free from nature for overhead current-carrying conductor. Air also flow through the bare overhead conductors and remove the produced heat. On the other hand for cable system, conductors are buried into the ground which high-quality insulation is required to withstand the very high voltage. Produced heat dissipation into the environment for underground cable conductor and their surrounding insulation in different installation method is a major issue for impact on nearby flora and fauna.

Technology for Cable Insulation System:

Fluid filled or Gas Insulated Cables: Mainly the first generation cables are fluid-filled, in these cables insulation is provided by paper, impregnated with fluid or insulated oil under pressure, wrapped around the central copper conductor. Lead or Aluminium sheath uses to cover this paper layer and to prevent corrosion of the sheath PVC, MDPE or HDPE outer the jacket is used.

Alternatively gas insulation lines are used for insulation where Aluminium conductors are supported by insulators contained within sealed tubes which are pressurized with a Nitrogen or Sulphur Hexafluoride (SF6) gas to provide the main insulation. The main advantage is that SF6 is a greenhouse gas and the system is cost benefit for sealing end system.

XLPE Insulated Cables: As a result of change and advance in cable technology fluid-filled cables are moving back and XLPE cables are coming at the front everywhere. In these smart cables, the core conductor is insulated by XLPE means Crossed Linked Polyethylene material. It requires less maintenance and no auxiliary fluid equipment to monitor and manage for XLPE cables.

Underground Cable Installation Methods, Requirement and Impact:

For high voltage or extra high voltage underground cable installation mainly three types of method are following:

Direct Buried;

Deep Tunnels;

Surface or Trench.

Direct Buried is the traditional method for high voltage cables installation system where trenches are approximately 1.5m wide and 1.2m deep for each circuit. To maintain the rated current carrying capacity of cable thermally stable backfilling is required with properly graded sand. Space required for cable easement corridor: 20m – 40m, 55m width; Joint bays intervals:  500m – 800m; working space required for join bays, power roller for excavating cable trench, sand bed, backfilling material, handling cable drum in site etc.

Deep Tunnel is generally used in urban locations where direct bury installation would cause unacceptable disruption. Typically 4m and 12m for joint bay diameter depth of a tunnel around 25m to 30m and maintains proper slope to provide free drainage. Two head house buildings order of 16m x 16m x 7m high require for maintenance and for installation of the cables at each end. A cooling and ventilation system and rail-mounted access vehicle is required for emergency exit, inspection, maintenance and repair. River or railway crossing can be made in this system.

Surface or Trench is excavated and is constructed concrete trenches where cables are laid directly within the trench which is capped with reinforced concrete covers laid flush with the ground surface. Surface or trench are used in urban situations or and within the substation compounds needed to take reduced land.

Environmental Impact on Underground Cable Installation:

The major environmental issues associated with the installation of direct buried or cut and cover tunnel cables are the disruption to traffic, noise, vibration, visual intrusion and dust generation and deposition due to the excavation of trenches along the route. Heavy goods vehicle traffic will also be generated by the work, cable crossing major roads, removing spoil and bringing in plant and materials, including backfill, to trenches. About 30m x 80m land area required for each circuit of high voltage cable sealing end compound and a significant space required for a terminal tower construction.

Nearly all installations require joints at specific intervals along the route and joint bays maybe 30m to 40m in length and 5m in width. The majority of the jointing accessory assembling is carried out on site. Sometimes cooling system pipe is laid alongside cable and pumping equipment and heat exchangers are required above ground and these must be sited which cause noise on the locality. Direct buried cable produce heat which harmful for on and near vegetation and insects.

EMFs or Electric and Magnetic Field Effect of Underground Cable:

The conductor which energized or having the presence of voltage there is electric field present surround it, when current flow continues magnetic field also present there surround the conductor, this mainly happens for the overhead electric line. But underground cable able to eliminate its electric field totally as it is screened out by the sheath around the cable; but it still produces magnetic fields. EMFs field strength is stronger to the centre of the field or near to the conductor and gradually weakened at the outer point from the centre. In a general sense overhead line is farther than an underground cable to human, so it should overhead line is safer than an underground cable. However, as the individual cables are installed much closer together than the overhead conductors which result in the magnetic field from cables falling more quickly with distance than the magnetic field from overhead lines. The overall result is that the cable produces a lower field than the overhead line; means the EMFs effect is smaller for underground cable than the overhead line.

Transposition Tower for Overhead Electrical Transmission Lines

Transposition Tower

What is Transposition Tower?

The basic definition for transposition of the transmission line is to rotate the conductors which result in the conductor or a phase being moved to the next.

In electrical power transmission, a transposition tower is a transmission tower that changes the relative physical positions of the conductors of a transmission line. A transposition tower allows these sections to be connected together while maintaining adequate clearance for the conductors.

What is the purpose of the transposition tower?

The transposing is necessary as there is the capacitance between conductors, as well as between conductors and ground. In electrical power transmission, a transposition tower is a transmission tower that changes the relative physical positions of the conductors of a transmission line in a Polyphase system. A transposition tower allows these sections to be connected together while maintaining adequate clearance for the conductors.

What is the suitable distance for transposing in transmission lines?

Ensuring a good transposing of a three-phase system, the transposing is applied after each (1/3n) distance of the line. Where n is the number of transpositions across each line length.

(1/3)distance to get equal average reactances(XA=XB=XC) overall distance of transmission line and then the symmetric system.

For single-circuit lines, you can get a balanced operation if you transposed for each 1/3 of the overall distance. This statement, however, is only valid if you did not tap (load) the line anywhere along its length except at the end.

For double-circuit lines, you can either transpose both circuits at 1/3 of the line length or transpose one circuit at 1/6 of the line length, yet, surprising as it may seem, this will achieve balanced voltage the line ends.

What are the advantages and disadvantages of transposition?

There are actually no disadvantages in power line transposition system, but some advantages are as below:

1. When conductors are not transposed at regular intervals, the inductance and capacitance of the conductors will not be equal.
2. When conductors such as telephone lines are run in parallel to transmission lines, there is a possibility of high voltages induced in the telephone lines. This can result in acoustic shock or noise. Transposition greatly reduces this undesired phenomenon.
3. In practice, however, conductors are not transposed in the transmission lines. The transposition is done in the switching stations and the substations.

A transposition tower is important to allow transposition sections to be connected together while maintaining adequate clearance for the conductors. 

Why Need Power Cable Test After Installation

Power Cable Test After Installation

HV/EHV power cable testing at the field after installation is the common requirements of cable user. While MV-normal voltage, HV-high voltage and EHV-extra high voltage power cables are carefully tested by the manufacturer before consignment with AC-alternating or DC-direct voltage. 

Some defects may not be detected or, more likely, damage may occur during shipment handling, carrying to storage and site, or during installation. To make over-sure additional testing carry out after installation and prior to being placed in service, including joints and terminations, may be conducted. 

Moreover, many users find that gradually cable systems degrade and service failures which become troublesome for them. Due to that, they desire to reduce or eliminate those failures by performing periodic tests after some time in service. Determining the economic replacement of the downgraded cable, users need a special diagnostic test.

Power cable test in field after installation may be broadly divided into the following two categories:

A.   Field tests are intended to detect defects in the insulation of a cable system in order to improve the service reliability after the defective part is removed and appropriate repairs performed. These tests are usually achieved by application of relatively elevated voltages across the insulation for prescribed duration.

B.   Field tests are intended to provide indications that the insulation system has deteriorated. Some of these tests will show the overall condition of a cable system and others will indicate the locations of discrete defects which may cause the sites of future service failures.  Both varieties of such tests may be categorized as “pass/fail” or “go/no-go” and are usually performed by means of moderately elevated voltages applied for a relatively short duration, or by means of low voltages.

Applied Voltage Type for Cable Test:

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Both of Direct Current Voltage and Alternating Current Voltage has been accepted for many years as the standard method.

DC test for Power Cable: 

DC testing method is performing for a long time as high voltage tests on cable insulation systems, especially for a field test. But recent research has shown that dc testing is the causes of certain types of defects and that it can make worse the deteriorated condition of some aged cables insulated with extruded dielectrics and affected with water trees.

AC test for Power Cable: 

Alternating voltage tests at alternating voltages are highly acceptable since the insulation is stressed in a similar way to normal operation and the test is similar to that used in the factory on new reels of cable. But high voltage AC tests equipment normally heavy, bulky and expensive test transformers which may not be readily transportable to a field site.

During high voltage test carry out of power cable in site after installation the grounding system should be sufficiently, due to long periods of time following such tests can experience dangerous charge build-ups and cause an accident. To avoid any mistake recommended grounding procedure and appropriate working rules should be followed.

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