EHVAC Vs HVDC TRANSMISSION

 

EHVAC Vs HVDC TRANSMISSION

HVDC vs HVAC Transmission Systems

HVDC stands for High Voltage Direct Current while HVAC stands for High Voltage Alternating Current. These are typically the rate of voltage, either DC (HVDC) or AC (HVAC) that are employed for energy transmission over long distances. HVDC is preferred to be utilized for transmitting energy over long distances, commonly more than 375 miles.

HVDC vs HVAC Transmission Systems (Reference: tdworld.com)

Nowadays, both formats of power transmission are employed all over the world. While these both have some advantages and disadvantages, we will explain each of them briefly in this post below based on different characteristics to discuss the HVDC vs HVAC transmission systems fundamentally.

Cost of Transmission

We understand that energy transmission over long-distance application needs high voltages. The power is used in terminal stations that modify the voltage rates. So, the total price of transmission is based on the cost of the transmission line and the terminal source’s situation.

Terminal Station

The great number of voltages transmitted within electrical terminal sources and their operation are introduced as the voltage conversion. The system employed for voltage conversion at these sources is mainly transformers in the case of AC types that is converted between low and high Voltages. Whereas in the case of DC type, the terminal sources utilize IGBTs or thyristor-based converters for modification between low and high DC voltage.

Since the transformers are less expensive and more reliable than these solid-state converters, the AC terminal sources are cheaper than DC types. Thus, the voltage modification in AC is less costly than DC forms.

Transmission Line

The transmission line price is based on the number of conductors being employed and the price of the transmission tower.

In the aspect of conductors being employed for transmission systems, the HVDC transmission type needs just two conductors, while the HVAC transmission form needs 3 or more than 3 instruments (including the covered conductor according to the harmful effects).

Because of the massive mechanical load on AC tower systems, their operation requires to be stronger and it should be taller and wider than HVDC transmission systems. The transmission line price rises with the distance and it is far higher than the HVDC line per 60 miles of a transmission path.

 

Overall Cost of Transmission

The overall cost of transmission is based on the terminal price (remains fixed) and the line price (rises with distance). As a result, the overall cost of the transmission system increases with distance.

HVDC vs HVAC Transmission Systems Cost (Reference: electricaltechnology.org)

The transmission path at which the total investment cost for HVAC begins to increase is introduced as Break-even Distance. This value is evaluated around 400 – 500 miles. HVDC is a more suitable option for energy transmission in the break-even distance. Although, below this distance, HVAC is more effective than HVDC. This data can be simply understood by the previous diagram.

Flexibility

Because the HVDC transmission is employed for transmission over long-distance applications between two areas, we cannot extract power at any section in-between since it would require an expensive converter to reduce such high DC voltages. In contrast, the HVAC transmission provides flexibility using a simple and cheap device like transformers at several terminal stations to control these high voltages.

Power Losses

The HVAC power transmission format has more power wastes such as Radiation losses, Induction losses, Skin effect, Corona losses, etc.

The radiation & induction losses depend on the magnetic field variation near the HVAC conductor. A massive conductor starts operating as an antenna and radiates some power that cannot recover, while the induction wastes are the energy loss when the current is produced in close conductors based on the continuously magnetic field variation. Since DC has an identical magnetic field, the HVDC type is free from these losses.

The alternating current produced in a conductor is separated in such a method that the current density wants to compose largest at the top of the conductor and minimum at the middle; this is known as the skin effect. Because much of the cross-sectional surface is ineffective and we understand that the resistance is straightly related to the cross-sectional surface, the resistance value of the conductor rises. The DC in the system is uniformly spread because the skin effect is just based on the frequency. So, only HVAC type experience power waste according to the skin effect in this aspect of HVDC vs HVAC transmission systems.

When the voltage rises more than a specific limit, the air close to the conductor begins the ionizing process and produces sparks that lose some power; this is introduced as corona discharge. The losses of corona discharge are also based on the frequency and because DC systems have zero frequency, the corona waste in HVAC is almost 3 times higher than that in HVDC.

The Skin Effect in Detail

The skin effect forces the conductor to keep most of the current at its top and less current at the center. It is based on the frequency and directly proportional to it. It reduces the efficiency of the conductors being employed. Thus, in order to provide a larger current, the cross-sectional section of the conductor requires to be increased.

Skin Effect in HVDC vs HVAC Transmission Systems (Reference: electricaleasy.com)

So, the HVAC requires a larger diameter device to carry the equal value of current as compared to the HVDC type employing a shorter diameter conductor.

Current & Voltage Ratings of Cable

As we discussed before, the current and voltage ratings of a cable are the optimum allowable range that it can be passed. The AC systems have a peak current and voltage that is practically 1.4 times greater than its average (the average practical energy delivered) or its DC norm. But in DC type, the average and peak values are equal.

Peak and Average Values for Transmission Systems (Reference: electrical4u.com)

The conductor should be evaluated for the peak voltage and current for the HVAC type which loses almost 30% of its ideal capacity in comparison with HVDC form, which uses the complete capacity of the conductor. So, a conductor with an identical size can be more preferred in HVDC vs HVAC transmission systems.

Right-of-Way

The right of way is the right method to use the land to and from another section of land. In the exploration of HVDC type, it includes a narrower right-of-way since it can employ smaller kinds of towers with fewer conductors being applied, i.e., two in DC types and 3 in 3-phase AC systems. Also, the insulators used in the main towers should be rated for peak voltages in AC systems.

The right-of-way influences the prices of substances used and fabrication requirements for the different transmission systems. We can conclude that HVDC types have a narrower right-of-way than HVAC transmission systems.

Submarine Power Transmission

We use cables in order to move power offshores employing submarine power transmission. At the same time, the cables provide certain capacitance generated between two conductors that operate in parallel arrangement to transmit power over long-distance applications.

The capacitance value is just based on the variation in voltage which is continuously happening in AC types, and only during switching mode in DC systems. The cable does not provide energy due to such capacitance (at the receiving section) before being completely charged. The cable is discharged and charged continuously in alternating current (50 or 60 times per second) which forces the system to lose a huge power. In contrast, the cable is charged only once in DC type. As a result, the submarine power system employed HVDC for energy transmission.

Controllability of Power Flow

When we discuss the HVDC vs HVAC transmission systems in the case of controllability, HVDC form uses particular converters of IGBT semiconductors which can be switched off and on several times in a period and control the total system, while HVAC has not a controllable part for Power flow. While the converters used in HVDC are complex, they help in controlling the distribution of energy to the entire setup and also increase the harmonic performance. These developed electronic converters provide fast protection against line errors and fault clearance contrary to the HVAC types.

Circuit Breaker

The Circuit breaker is a highly important section of power transmission systems. It can cease the whole circuit operation for reaction to any fault or maintenance. The circuit breaker requires arc-extinguish abilities in the current to stop the power supply.

The direction and value of the current modify continuously in HVAC systems and the arc is typically extinguished based on the presence of several zero currents in a second that present various chances to stop the arc. Whereas in DC form, the current is fixed and there are no zero currents, so artificial zero currents should be produced employing particular circuitry to stop the arc.

As a result, in the comparison of HVDC vs HVAC transmission systems, the circuit breakers for HVAC are easy to modeling according to the “self-arc-extinguish” feature. This is while for HVDC, the circuit-breaker modeling is relatively complex and they are more costly than HVAC types.

Generating Interference

The AC systems produce a magnetic field with continuously variable values that can cause interference with the conductors in the nearby communication. Because DC types have a constant magnetic field, they do not cause such problems.

Key Differences to Contrast Hvdc vs Hvac Transmission Systems

At what follows, key differences between HVDC vs HVAC transmission systems are summarized:

• Skin effect is zero in DC systems. Also, corona wastes are especially lower in DC type. An HVDC path has noticeably lower wastes in comparison with HVAC over long-distance applications.
• HVDC transmission path would cost lower than an HVAC type.
• Based on the absence of inductance value in DC type, an HVDC path provides better voltage monitoring. Also, HVDC supplies greater controllability in comparison with HVAC.
• AC power grids are normalized for 60 Hz in some regions and 50 Hz in others. It is impractical to combine two power grids operating at different frequencies using an AC junction. An HVDC port makes this possible for power grids.
• Interference with close relative lines is lower in HVDC types than an HVAC overhead line.
• The short circuit current rate in the receiving setup is great in longer distance HVAC transmission system. An HVDC type does not chip in such circuit current of the AC form.

 

Conclusion

In a general comparison for HVDC vs HVAC transmission systems, HVDC transmission types have many more benefits over HVAC types, including controllability, stability, etc. HVDC systems are more cost-effective for distances greater than the break-even point. Submarine HVDC instruments can be more reliable for use in offshore wind farms as they are less expensive than undersea HVAC wires. As a result, there is an increasing tendency to choose the HVDC transmission type. However, HVAC systems are also employed because they have their particular advantages in distribution and transmission, such as they can be simply stepped down and stepped up which is an important matter in certain applications. HVDC is practically a supplement for AC forms rather than an opponent.

 

Proximity effect

 Definition: When the conductors carry the high alternating voltage then the currents are non-uniformly distributed on the cross-section area of the conductor. This effect is called proximity effect. The proximity effect results in the increment of the apparent resistance of the conductor due to the presence of the other conductors carrying current in its vicinity.

When two or more conductors are placed near to each other, then their electromagnetic fields interact with each other. Due to this interaction, the current in each of them is redistributed such that the greater current density is concentrated in that part of the strand most remote from the interfering conductor.

If the conductors carry the current in the same direction, then the magnetic field of the halves of the conductors which are close to each other is cancelling each other and hence no current flow through that halves portion of the conductor. The current is crowded in the remote half portion of the conductor.

When the conductors carry the current in the opposite direction, then the close part of the conductor carries, the more current and the magnetic field of the far off half of the conductor cancel each other. Thus, the current is zero in the remote half of the conductor and crowded at the nearer part of the conductor.

If DC flows on the surface of the conductor, then the current are uniformly distributed around the cross section area of the conductor. Hence, no proximity effect occurs on the surface of the conductor.

The proximity effect is important only for conductor sizes greater than 125 mm2.Correction factors are to be applied to take this fact into account.

If Rdc – uncorrected DC level of the core
Ys – skin effect factor, i.e., the fractional increment in resistance to allowing for skin effect.
yp – proximity effect factor, i.e., the fractional increment in resistance to allowing for skin effect.
Re –  effective or corrected ohmic resistance of the core.
The allowance for proximity effect is made, the AC resistance of the conductor becomes
The resistance Rdc is known from stranded tables.

Factors Affecting the Proximity Effect

The proximity effect mainly depends on the factors like conductors material, conductor diameter, frequency and conductor structure. The factors are explained below in details

1. Frequency – The proximity increases with the increases in the frequency.
2. Diameter – The proximity effect increases with the increase in the conductor.
3. Structure – This effect is more on the solid conductor as compared to the stranded conductor  (i.e., ASCR) because the surface area of the stranded conductor is smaller than the solid conductor.
4. Material – If the material is made up of high ferromagnetic material then the proximity effect is more on their surface.

How to reduce Proximity Effect?

The proximity effect can be reduced by using the ACSR (Aluminum Core Steel Reinforced) conductor. In ACSR conductor the steel is placed at the centre of the conductor and the aluminium conductor is positioned around steel wire.

The steel increased the strength of the conductor but reduced the surface area of the conductor. Thus, the current flow mostly in the outer layer of the conductor and no current is carried in the centre of the conductor. Thus, reduced the proximity effect on the conductor.

Comparison between Overhead Lines and Underground Cables

 

Comparison between Overhead Lines and Underground Cables

	Overhead Line	Underground cable
Fault location	As the overhead line is visible, it is easy to find the location of the fault.	As the underground cable is invisible, it is very difficult to find the location of the fault.
Initial cost	There is no requirement of digging, manholes, and trench. So, the overhead line system is cheaper than the underground system.	The initial cost of the underground transmission system is more compared to the overhead line because it needs digging, trenching, etc.
Chance of fault	As overhead line exposed to the environment, the chances of faults are more.	The cables are not exposed to the environment, there is less chance of fault.
Safety	This system is less safe as the conductors placed on the towers.	This system is safer as the cables placed underground.
Useful life	In this system, useful life is approximately 20 to 25 years.	Useful life is approximately 40 to 50 years.
Appearance	The general appearance of this system is not good because of all lines are visible.	The general appearance of this system is good because of all lines are invisible.
Maintenance cost	In this system, no need to dig at the time of maintenance. Hence, for the same number of faults, the maintenance cost is less.	In this system, to find the fault, digging is compulsory. It increases labor cost. Hence, for the same number of faults, the maintenance cost is more.
Flexibility	This system is more flexible. Because the expansion of the system is easily possible.	This system is not flexible. The expansion cost is nearly equal to the new erection of the system.
Conductor size	The conductors placed in atmosphere. So, the heat dissipation is better. Therefore the size of the conductor is small compared to the underground system.	Because of the poor heat dissipation, the size of the cables is more.
Interference with the communication line	The communication lines are run along the transmission line. In this case, it is possible to cause electromagnetic interference.	In this case, there is no chance of interference with communication lines.
Proximity effect	The distance between the conductor is very high. So, proximity effect does not affect.	As the distance between cables is very less, the proximity effect is very high.
Application	The cost of this system is low. Therefor overhead lines used in the long transmission system and in rural areas for the distribution system.	Because of the high cost, it uses in the short distance and in populated areas. Where space is a major problem for the overhead transmission line.

Standardization of Transmission System Voltage

 Standardization of Transmission System Voltage

There is much variation in transmission voltages in different countries. Each country have different voltage level as per there requirments. Earlier individual makes  an attempts to fix voltage levels for higher power transmission but such an attempt had resulted in wastage of time and higher cost because of designs of varied nature. Hence, the transmission voltages had to be standardized. The various advantages of standardization of transmission voltage are:

1.      Standardization provides better facilities for research and development.

2.      The equipments can be manufactured with greater economy and reliability.

3.      Systems are easily interconnected.

Hence standardization enables to carry out joint efforts to tackle Extra High Voltage (EHV) or Ultra High Voltage (UHV) problems. By standardizing, the voltage level can be adopted for a reasonable period of time before next change. The choice of the highest system voltage for a country is a matter of great significance. It is not merely the economic factors that influence the next higher voltage but the site of power station, location and density of load, and the technological developments are also kept in mind. The next higher voltage level should also be selected on the basis of future load enhancements. The interval between the existing and the proposed voltage level should be judiciously spaced, as too small interval between the voltages will result in a short life of the proposed voltage level. At the same time too large interval would lead to heavy expenditure. It is therefore desirable that the next voltage selected should be at least two steps higher than the existing one.  

The various AC voltages adopted by different countries above 220 kV are 275, 345, 380, 400, 500, 735, 765, 1000, 1100, 1200 kV etc. The AC transmission voltages adopted in India are 220 kV, 400 kV and 765 kV. The next higher AC transmission voltage selected is 1200 kV.

Smart Grids

     A Smart Grid is an electricity Network based on Digital Technology that is used to supply electricity to consumers via Two-Way Digital Communication. This system allows for monitoring, analysis, control and communication within the supply chain to help improve efficiency, reduce the energy consumption and cost and maximize the transparency and reliability of the energy supply chain.

    Smart grid is a large ‘System of Systems’, where each functional domain consists of three layers: 

(i) the power and energy layer, 

(ii) the communication layer, and 

(iii) the IT/computer layer.

     Layers (ii) and (iii) above are the enabling infrastructure that makes the existing power and energy infrastructure ‘smarter’.

It uses smart meters and appliances, renewable and efficient energy resources.

• The system delivers electricity via 2-way digital communication. It allows consumers to interact with the grid.
•  It reduces energy consumption and reduces cost to the consumers by smart means. Electric supply companies make efficient usage of energy and consecutively will be able to meet the varying load demands of the consumers.

Features of Smart Grid

    Smart grid has several positive features that give direct benefit to consumers:

• Real time monitoring.
• Automated outage management and faster restoration.
• Dynamic pricing mechanisms.
• Incentivize consumers to alter usage during different times of day based on pricing signals.
• Better energy management.
• In-house displays.
• Web portals and mobile apps.
• Track and manage energy usage.
• Opportunities to reduce and conserve electricity etc.

Benefits of Smart Grid Deployments

• Peak load management, improved QoS and reliability.
• Reduction in power purchase cost.
• Better asset management.
• Increased grid visibility and self-healing grids.
• Renewable integration and accessibility to electricity.
• Satisfied customers and financially sound utilities etc.

Smart Grid Architecture Components

The figure depicts generic Smart Grid Network Architecture components or modules with different reference points. As shown typical smart grid network consists of following components.

• Grid domain (Operations include bulk generation, distribution, transmission)

• Smart meters

• Consumer domain (HAN (Home Area Network) consists of smart appliances and more)

• Communication network (Connects smart meters with consumers and electricity company for energy monitoring and control operations, include various wireless technologies such as zigbee, wifi, HomePlug, cellular (GSM, GPRS, 3G, 4G-LTE) etc.

• Third party Service providers (system vendors, operators, web companies etc.)

Structure of power systems

        Electricity is generated at central power stations and then transferred to loads (i.e, Domestic, Commercial and Industrial) through the transmission and distribution system. A combination of all these systems is  known as an Electric Power System.

     A power system is a combination of central generating stations, power transmission system, Distribution and utilization system. Electric power is produced at the power stations which are located at favourable places, generally quite away from the consumers. It is then transmitted over large distances to load centres with the help of conductors known as transmission lines. Finally, it is distributed to a large number ofsmall and big consumers through a distribution network,

Energy is generated (transformed from one to another) at the generating stations. Generating stations are of different type, for example, thermal, hydro, solar power stations, nuclear. The generated electricity is stepped up through the transformer and then transferred over transmission lines to the load centres.

Electric power is generated at a voltage of 11 to 25 kV which then is stepped up to the transmission levels in the range of 66 to 400 kV (or higher). As the transmission capability of a line is proportional to the square of its voltage, research is continuously being carried out to raise transmission voltages. Some of the countries are already employing 765 kV. The voltages are expected to rise to 800 kV in the near future. In India, several 400 kV lines are already in operation. One 800 kV line has just been built. . The transmission of electric power at high voltages has several advantages including the saving of conductor material and high transmission efficiency. It may appear advisable to use the highest possible voltage for transmission of electric power to save conductor material and have other advantages. But there is a limit to which this voltage can be increased. It is because the increase in transmission voltage introduces insulation problems as well as the cost of switchgear and transformer equipment is increased. Therefore, the choice of proper transmission voltage is essentially a question of economics. Generally, the primary transmission is carried at 66 kV, 132 kV, 220 kV or 400 kV.

Transmission System And Distribution System

The large network of conductors between the power station and the consumers broadly divided into two parts viz., can be transmission system and distribution system. Each part can be further subdivided into two — primary transmission and secondary transmission and primary distribution and secondary distribution.

Primary transmission.

The first stepdown of voltage from transmission level is at the bulk power substation, where the reduction is to a range of 33 to 132 kV, depending on the transmission line voltage. The electric power at 132 kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. This forms the primary transmission.

Secondary transmission

The primary transmission line terminates at the receiving station (RS) which usually lies at the outskirts of the city. At the receiving station, the voltage is reduced to 33kV by step-down transformers. From this station, electric power is transmitted at 33kV by 3-phase, 3-wire overhead system to various sub-stations (SS) located at the strategic points in the city. This forms the secondary transmission.

Primary distribution

The secondary transmission line terminates at the sub-station (SS) where voltage is reduced from 33 kV to 11kV, 3-phase, 3-wire. The 11 kV lines run along the important road sides of the city. This forms the primary distribution. It may be noted that big consumers (having demand more than 50 kW) are generally supplied power at 11 kV for further handling with their own sub-stations.

Secondary distribution

In the last stage in a Power System, the electric power from primary distribution line (11 kV) is delivered to distribution sub-stations (DS) or Distribution Transformer. A typical pole mounted distribution transformer is shown in Fig. 5. These sub-stations are located near the consumers’ localities and step down the voltage to 400 V, 3-phase, 4-wire for secondary distribution. The voltage between any two phases is 400 V and between any phase and neutral is 230 V. The single-phase residential lighting load is connected between any one phase and neutral.

EXHAUST FAN POWER CONSUMPTION CALCULATION

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 Let us assume that Exhaust Fan for Kitchen/Bathroom which runs 10 hours per day, 

 According to its nameplate details, the fan consumes 30 watts for one hour.

  Per day Electricity consume by the Exhaust Fan = 30 x 10 / 1000 = 0.3 kWh 

As we know that one unit is equal to kWh. 

 Per day = 0.3 units.

hence per day consumption will be, .3 unit

 Per month = .3*30=9 unit

Hence, the monthly consumption will be 9 units.