Transmission of bulk power from generating stations to the load centers is technically and economically feasible only at voltages in the EHV/UHV range. An EHVAC Transmission line stands for Extra High Voltage Alternating Current Transmission line. EHV lines are able to move large power across a single line so the overall conductor requirement is decreased.
EHVAC transmission in India at voltages 220kv, 400kv, and 765kv by Power Grid Corporation.
Why EHVAC Transmission?
- Used to carry large amounts of power across long distances.
- Increase of Surge Impedance Loading
- Higher voltage levels cause lower losses.
- Transmission efficiency is increased due to lower losses.
- Lesser conductor material is required at high voltages.
- Voltage regulation is improved due to a reduction in percentage line drops.
Advantages of EHVAC Transmission:
- Large amounts of power across long distances:
Transmission power carrying capacity
Where Vs= Sending End voltage
Vr= Receiving End Voltage
X= line reactance
δ= Load angle
- Increase of Surge Impedance Loading:
The load-carrying capability of a line is usually expressed in terms of “surge impedance loading” (SIL).
Surge impedance loading (SIL) is the power that a line carries when each phase is terminated by a load equal to the surge impedance of the line.
- For a transmission line, the surge impedance is given as ZC = √L/C where L and C are respectively the series inductance and shunt capacitance per unit length.
- The surge impedance loading (SIL), for a transmission line, is given as 3V2/ZC where V is the line-to-neutral voltage.
- It is evident that SIL varies as the square of the operating voltage, and, therefore, with the increase in voltage level, SIL itself increases. Thus power transfer capability of the line increases with the increase in voltage level.
Major Components of EHVAC Transmission Stations:
1. Bundled Conductors
2. Ground Return
MVAR management using the following devices
3. Shunt Capacitors
4. Series capacitors
5. Shunt Reactors
6. Synchronous Condenser
7. FACTS Devices
- Series Compensation
- Shunt Compensation
1. Bundled Conductors for EHVAC Transmission:
If two or more cylindrical conductors would be at the same potential with reference to predominantly earth potential far away from the parallel conductors, the configuration of so-called ‘bundle conductors’ is formed, a system extensively applied in E.H.V transmission lines.
Use of Bundled Conductors:
Due to the interaction of the single conductors, the maximum field intensity at the conductors is reduced in comparison to a single cylindrical conductor so that the corona inception voltage can significantly be increased.
Properties of Bundled Conductors:
The sub-conductors of a bundle are uniformly distributed on a circle of radius R.
The above figure shows examples of Conductor configurations used for bundles in E.H.V lines.
Bundle Spacing B= Spacing between adjacent sub-conductors
Bundle Radius R= Radius of the pitch circle on which the sub-conductors are located.
2. Ground Return:
Underbalanced operating conditions of a transmission line, ground-return currents do not flow. However, many situations occur in practice when ground currents have an important effect on system performance.
Some of these are:
(a) Flow of current during short circuits involving ground. These are confined to a single line to ground L-G and double line to ground L-L-G faults
(b) Switching operations and lightning phenomena;
(c) Propagation of waves on conductors;
(d) Radio Noise studies.
3. Shunt Capacitors:
Shunt compensation with capacitive VARs is used to inject reactive power and control the receiving-end voltage. Shunt capacitors are used across an inductive load so as to supply part of the reactive VARs required by the load so that reactive VARs transmitted over the line are reduced, thereby the voltage across the load is maintained within certain desirable limits.
4. Series capacitors:
It consists of capacitors connected in series with the line at suitable locations and thus opposes directly the effect of series inductive reactance of the line. And hence these capacitors reduce the inductive reactance between the load and the supply point
5. Shunt Reactors:
Over long transmission lines, reactive power is generated as an effect of the capacitance between the lines and the earth. The reactive power cannot be used for any application and should be balanced to reduce energy losses. A shunt reactor is an absorber of reactive power and is the device most commonly used for reactive power compensation. The shunt reactors are used across capacitive loads or lightly loaded lines to absorb some of the leading VARs to control the voltage across the loads.
The shunt reactor can be directly connected to the power line or to a tertiary winding of a three-winding transformer. The shunt reactor could be permanently connected or switched via a circuit breaker. To improve the adjustment of the consumed reactive power the reactor can also have a variable rating. Variable shunt reactors allow to continuously adjust the compensation, as loads vary over time. They make switching in and out of fixed-rating reactors unnecessary, which eliminates harmful voltage steps.
Shunt Reactors Control Ferranti Effect:
At low loads, the voltage increases along the transmission line. Also, there is a rise in sending-end voltage whenever the load on the generator is thrown off suddenly.
Voltage rise, due to the Ferranti effect, is controlled by using a shunt reactor at the load end.
We know that with the capacitive load on the line the receiving-end voltage is higher than the sending end voltage. This increase is of the order of 1.5 percent for 160 km, 13 percent for 500 km, and 100 percent for 960 km.
A shunt reactor reduces the voltage increase, keeps the voltage within the desired limits, and contributes to the voltage stability of the system.
6. Synchronous Condenser:
An over-excited synchronous machine produces reactive power and acts as a shunt capacitor. Synchronous condenser runs under no-load conditions. The reactive power can be continuously and simply controlled by varying the D.C Excitation.
7. FACTS Devices for EHVAC Transmission:
Flexible AC Transmission Systems are used for higher controllability in power systems by means of power electronic devices.
The basic application of FACTS devices are
- Greater control of Power on desired route (... Deregulation)
- Secure loading (not overloading) of lines
- Greater inter systems power transfers
- Reactive power compensation
- Prevention of Cascading
- Damping of Power System Oscillations
- Time and Frequency of operation.
1. Series Compensation:
It consists of capacitors connected in series with the line at suitable locations and thus opposes directly the effect of series inductive reactance of the line.
- Increase in transmission capacity
- Improvement of system stability
- Load division between parallel circuits
- Provides better voltage regulation
Thyristor Controlled Series Capacitor: TCSC
With TCSC it is possible to vary the degree of compensation. TCSC basically consists of a series capacitor in parallel to a Thyristor Controlled Reactor (TCR).
By varying the firing angle of the thyristor (on/off switch), TCSC can optimize its degree of compensation.
TCSC Composition with MOV-Metal Oxide Varistor is shown in the above figure.
2. Shunt Compensation:
Shunt compensation with capacitive reactors that is Static VAR and STATCOM devices to provide
1. Grid voltage control
2. Preventing overvoltages in case of loss of load
3. Boosting voltage during under-voltage due to faults
4. Damping active power oscillations
5. To increase voltage stabilization
6. Power quality improvement by harmonic filtering, flicker mitigation, and load balancing.
Shunt Compensation Types:
1. Static VAR compensator:
SVC based on high power Thyristor Technology consists of the following equipment
- Thyristor-controlled reactor (TCR)
- Thyristor-switched capacitors (TSC)
- Thyristor-switched reactor (TSR)
- Harmonic Filters
2. STATCOM-Static Synchronous Compensator
STATCOM is based on high power IGBT-Insulated Gate Bipolar Transistor Technology and IGCT-Insulated Gate Commutated Thyristors. Using these devices Multi-level chain-link Voltage Source converters are formed. VSC is formed by linking H-Bride modules in series with one another as shown below.
Problems Encountered with EHVAC Transmission:
EHV Lines are exposed to electrostatic fields near the lines, audible noise, radio interference, corona losses, carrier and TV interference, high voltage gradients, heavily bundled conductors, control of voltages at power frequency using shunt reactors of the switched type which inject harmonics into the system, switched capacitors, overvoltages caused by lightning and switching operations, long air gaps with weak insulating properties for switching surges, ground-return effects.
Drawbacks of EHVAC Transmission:
- Coronal losses and Radio interference
- Insulation requirements increased
- Heavy supporting structures are needed.
1. Corona Losses:
In High voltage Transmission lines when electric fields exceed the insulating strength of the air in the proximity of the conductors, localized discharge processes may enhance per-unit length (p.u.l.) losses.
2. Radio Interference:
Radio Interference is due to corona development in proximity of EHV lines, affecting the quality of AM radio broadcasting. Radio Interference occurs at higher frequencies, in the range between 0.01 MHz and 30 MHz.
Corona on conductors also causes interference to Carrier Communication and Signalling in the frequency range 30 kHz to 500 kHz.
- Audible Noise:
Corona discharge is emitted in the audible spectrum in the range 20Hz to 20 kHz. The total emitted noise like a buzzing sound is produced which causes additional power loss occurred.
When the wavelength of corona emitted fields reached the visible spectrum of the human eye from 380nm to 780nm a visible violet luminosity is produced.
2. Increased Level of Insulation Requirements:
The level of insulation required depends upon the magnitude of likely voltage surges due to internal causes (switching operations) or due to external causes (lightning etc.)
The lines are usually protected against lightning etc. by use of ground wire and rapid auto-reclosing circuit breakers
The maximum switching surge overvoltage to the ground is taken as 2.5 p.u and the insulation is designed for this voltage. In addition, adequate protection against atmospheric overvoltages is provided.
3. Requirement of Heavy Supporting Structures:
EHV transmission lines have large mechanical loading on towers because of the use of bundled conductors, large air and ground clearances, and wind pressures during storms and cyclones.