There are certain limitations in the transmission of power by EHV ac system.
These limitations and design aspects are discussed below:
1. Stability Considerations:
Power transferred is expressed as:
if resistance and shunt leakance of the transmission line are neglected.
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For equal terminal voltages (i.e. VS = VR = V)
Maximum steady state transfer of power takes place theoretically for δ = 90° but in actual practice δ is kept within limits of 20° to 30° at full load. Taking X = 0.3475 Ω/km and δ = 30°, we have,
If the line operates at natural load given by the expression:
Thus we see that the theoretical length of a transmission line in km that can be operated on its natural load and load angle of 30° without loss of stability only is 1.43 times surge impedance (which comes out to be about 500 km). The transmission distance in EHV ac systems is, therefore, limited. This limitation is overcome by use of special equipment such as series capacitors or shunt reactors.
2. Current Carrying Capacity:
The loading of overhead line conductors does not depend on the thermal considerations. However, for overhead transmission lines operating at voltage up to 220 kV, the conductor size is determined on the basis of its continuous and short-term current carrying capacity (i.e., on the basis of thermal considerations).
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In case of underground cables, however, both thermal current limit and charging current are very important and that is why the underground cable is never operated beyond its natural load. Also the cost of cable is very high in comparison to that of an overhead line; therefore use of underground cables is limited in high voltage systems.
3. Ferranti Effect:
We know that with the capacitive load on the line the receiving-end voltage is higher than sending-end voltage. This increase is of the order of 1.5 per cent for 160 km, 13 per cent for 500 km and 100 per cent for 960 km. Also there is a rise in sending- end voltage whenever the load on the generator is thrown off suddenly. Care is also to be taken from these aspects in case of ac transmission system.
Voltage rise, due to Ferranti effect, is controlled by using shunt reactors at the load end.
4. Surge Impedance Loading:
Surge impedance loading (SIL) of a transmission line is defined as the load at the receiving end which is equivalent to √L/C. The surge impedance loading for a transmission line is given as V2/Z0 watts per phase. SIL is also called the natural load. When the line carries natural load, the voltage along the entire length of line is the same.
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However, in practice a line is not always operated at its natural load. In such a case there will be a voltage drop along the line for inductive load and a voltage rise for capacitive load. Although the reactive voltage drop/rise and the natural load do not impose any restriction on the distance over which power may be transmitted but fix the voltage that may be employed for the transmission of a certain amount of power.
Typical values of surge impedance and natural load for overhead transmission line are given below in tabular form:
5. Mechanical Vibrations and Oscillations:
With the increase in number of sub-conductors in bundled conductor transmission lines, there may be a considerable effect on vibrations of conductors. Hence the mechanical design of the system (tower dimensions, phase spacing, sub-conductor spacing, conductor height etc.) needs modifications so as to counter the problem of conductor vibrations. These modifications affect the electrical design such as voltage gradient.
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Other problems include Aeolian vibrations and galloping. The problem of galloping can, however, is controlled by applying weights at different locations of the line span.
6. Environment and Biological Aspects:
The biological effect of electric field of EHV lines and EHV substations has been studied extensively during 1970s. The electrostatic and electromagnetic fields produced by EHV/UHV transmission lines induce currents and voltages in animals, birds and human beings. EHV/UHV lines are designed such that maximum electrostatic field gradient does not exceed 9 kV/m at mid span under the line near the ground level.
It has been found that very low frequency magnetic fields even of weak intensities can affect certain cellular processes. Induced current densities below 10 mA/m2 have no significant biological effect. Currents 10-100 mA/m2 cause minor biological effects. Current densities exceeding 100 mA/m2 cause health hazards.
Safe line to ground clearance of 20 m at mid span is recommended for 400 kV lines and 24 m for 1,100 kV lines. This permits movement of vehicles safely.
7. Audible Noise:
EHV transmission lines and substations also produce audible noise. The sources of noise generation are corona, humming of transformers, cooling systems and mechanical and electrical auxiliaries.
The audible noise (AN) generated by a transmission line is a function of the following:
(i) Voltage gradient on the surface of a conductor.
(ii) Number of sub-conductors in a bundle.
(iii) Conductor diameter.
(iv) Atmospheric conditions.
(v) Lateral distance between the line and the point of measurement of noise.
The audible noise is caused by vibrations produced in air due to change in the air pressure.
Below 500 kV, audible noise (AN) levels do not exceed the permissible noise but the lines of 500 kV and above must be designed for a satisfactory audible noise level.
The width of right-of-way for the line corridor has a reference to the decision about AN. Line geometry is based upon 50 dB at the edge of ROW.
8. Radio Interference and Television Interference:
Operation of EHV transmission lines and substations can cause radio interference. Since radio noise is associated with corona, it mainly depends on the potential gradients at the conductors. It is also influenced by all those factors which influence corona, such as air density, humidity, wind contaminants, imperfections and precipitation. The radio noise in bad weather is about 10-25 dB higher than the radio noise during fair weather. The other causes for radio interference are partial discharges on insulators and spark across gaps.
The noise due to corona in EHV lines do not cause a serious problem for frequency modulated (FM) radio-receivers or TV receivers. However, the TV reception in the immediate vicinity of a UHV line may get affected especially during bad weather.
Radio interference and television interference is caused by electromagnetic waves in the frequency range of broad cast frequencies.
Radio interference: 0.5-1.6 MHz
Television interference: 54-216 MHz
The line is designed such that radio noise within the width of the line corridor should be below permissible limits; say 40 dB at 1 MHz. Radio interference is more important factor in the design of line and in deciding right of way. The radio interference can be eliminated and/or minimised by appropriate design of line conductor and hardware. By using bundled conductors, surface voltage stress, corona and radio interference can be reduced.
9. Route Clearance:
EHV/UHV transmission lines are generally constructed over different terrain including dense forests, hilly areas and agricultural land, and clearance of minimum ROW is very important. Generally the trees are not taller than 10 m. Only dry trees need to be cleared from the ROW of transmission lines because a live tree is found to be well grounded through its sap, has small power loss and little chance of burning.
It has also been found that the dry tree burning can occur if an induced voltage exceeding 100 kV occurs and potential of 100 kV can exist within a radius of 6 m from 765 kV line. So a minimum clearance of 6 m from 765 kV line conductors is essential.
10. Conductor Material:
ACSR conductors are universally employed for overhead transmission lines.
Necessary data about ACSR bundle conductors used for ac transmission lines is given below in tabular form:
11. Span, Number of Circuits, Conductor Configuration:
For a high voltage line the economical value of span is 200-300 m, whereas for 400 kV line it is 350-400 m.
If two three-phase lines are on the same tower and are identical in construction and electrically in parallel then they have the same reactance X. The inductive reactance of the single equivalent circuit becomes X/2 and hence the power transfer capability is doubled. Double-circuit lines are in common use. They have additional advantage that continuity of supply is maintained over one line, though with reduced capability, while the other line is out of service for maintenance or repair.
Each circuit of a double circuit line is usually designed for 75% of the line capacity. In India, both single and double circuit lines exist in the high voltage and extra high voltage class (66, 132, 220 and 400 kV). The number of circuits is determined from the SIL.
Vertical configuration has been found the most economical for double circuit lines. For single circuit lines, horizontal or L type configurations are most suitable.
12. Insulation Coordination:
Transmission lines and substations are subjected to overvoltages due to switching, lightning, faults, resonance and other causes. The surge arrestors are provided at strategic locations to protect the line insulation and substation equipment from transient and temporary overvoltages.
In general there are three types of overvoltages viz. switching overvoltages, temporary overvoltages and overvoltages due to lightning. Switching overvoltages are caused due to energization of lines, reclosing, fault interruption, load throw, out of phase switching and sudden switch-off of a line. The temporary overvoltages at power frequency are due to sudden loss of load, disconnection of inductive loads, connections of capacitive loads, etc.
For EHV and UHV transmission lines the switching overvoltages become the limiting factor in the design of insulation levels and arrester characteristics. Once the line is designed adequately against switching overvoltages, it is capable of encountering temporary overvoltages. It has also been found that overvoltages due to lightning are not very important for EHV/UHV transmission lines.
The waveshapes of switching surges are practically infinite. Since the waveshape has a definite influence on the flash-over strength, a standard waveshape has been selected for analysing the line performance. This standard switching surge has a crest time (i.e. the time to attain the peak value) of 175 µs, and a tail time (i.e. the time to attain half the peak value on the tail) of 3,200 µs. It has been found desirable and possible to reduce the switching surges for EHV systems to about 2.5 pu and insulation of EHV system is usually designed for this strength. The temporary overvoltages can be limited to 1.5 pu if shunt reactors are employed.
The insulation characteristics of various equipment is defined in terms of rated voltage, standard switching impulse, standard lightning impulse, power frequency withstand voltage etc. These are co-related with characteristic of surge arresters.
The insulation levels of various substation apparatus and the transmission line should be coordinated with various surge arresters such that the overvoltages are discharged to earth without causing damage to the equipment insulation.
13. Series Compensation:
Series compensation is an important method of improving the performance of EHV transmission lines. 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.
It increases the power handling capacity and reduces the voltage regulation as explained below:
The power transfer capability of a transmission line is given as:
With the insertion of capacitors having capacitive reactance XC in the line, the net reactance of the line becomes (XL – XC) and the power transfer capability of the line is given as:
From above Eqs. (13.1) and (13.2) it is obvious that for the same magnitudes of VS, VR and δ, P’ is much higher than P and the increase in power transfer capability is given as:
where K is the= degree of compensation and equals XC/XL.
For example if the line is compensated to an extent of 50% i.e. K = 0.5, the power transfer capability of the line becomes 1/ (1 – 0.5) i.e. double of that without series compensation.
Economic Considerations:
The growing cost of ROW, aesthetic considerations calling fewer transmission lines, increase in the lengths of transmission lines and in transmission voltages, decrease in series capacitor costs, have all contributed to an increase in the rate of use of series capacitors in India and other countries.
In view of the line cost, the effect of lines operating in parallel and problems of system protection, the economic degree of compensation is found to be in the range of 0.4 to 0.7.
Improvement of System Stability:
From the power flow equation it is evident that for the transfer of same amount of power and the same values of sending-end and receiving- end voltages VS and VR, the power angle δ is lower for compensated line as compared to that in case of an uncompensated line. A lower value of power angle means better system stability.
Various measures have been adopted for improvement of power system stability. These include reducing the reactance of synchronous generators and transformers, increasing the number of parallel lines, use of bundled conductors and series compensation. Series capacitors usually offer the most economical solution with respect to both the steady state and transient stability.
Location of Series Capacitors:
Series capacitors can be located either along the lines, in intermediate substations or switching stations. Line location has many advantages such as better voltage profile along the line, reduced short-circuit current through the capacitor during a fault. The small short-circuit current contributes to a much simpler protection of the capacitor. The capacitor stations are generally unattended type.
Installation of series capacitors in intermediate attended type substations or switching stations may appear advantageous, since staff for maintenance and service, auxiliary power etc. are available. However, these advantages must be weighed against the higher cost for protection and control and sometimes also for the space required as well as the increase of transmission losses.
Capacitor Bank:
A capacitor bank is built up of a number of capacitor units connected in series and in parallel. The units for series capacitors are designed, manufactured and tested with due regard for the specific service conditions, such as high overvoltages and capacitor discharge currents.
A capacitor unit consists of a number of capacitor elements in a common container. The units are equipped with fuses, which may be either external ones for each unit or internal individual element fuses. The capacitor units are mounted in simple frames, called the racks, placed on supporting insulators and stacked on top of each other.
Problems Associated with Series Compensation:
There are some major problems associated with series compensation.
These problems are summarized below:
(i) Sub Synchronous Resonance:
The series capacitor introduces a sub-synchronous frequency, proportional to the square root of the compensation in the system. In some cases this frequency may interact with turbo-generator shafts and develop high torsional stresses. The risk of sub-synchronous resonance in hydro-generators is quite small as the torsional frequencies are about 10 Hz or even less.
(ii) Ferroresonance:
When a lightly loaded transformer is energized through a series compensated line, ferroresonance may occur. This can be suppressed by using shunt resistors across the capacitors or by short-circuiting the capacitor temporarily through an isolator or bypass breaker.
(iii) Maloperation of Line Protection:
Series compensation may lead to maloperation of the protection system leading to unnecessary trippings.
(iv) High Recovery Voltage:
High recovery voltage may develop across the circuit breaker contacts due to series compensation.
14. Shunt Compensation:
In EHV transmission systems, shunt compensation is invariably essential. Shunt compensation with capacitive VARs is used to inject reactive power and control the receiving-end voltage whereas shunt reactor compensation is used to neutralize the Ferranti effect. For shunt compensation, static compensation employing capacitors and reactors or synchronous compensation using synchronous phase modifier may be employed.
Static compensation is being preferred to the synchronous compensation because of its inherent advantages of higher speed of response, absence of fault indeed to the system, lower maintenance, low cost, greater reliability and simpler erection. In addition to shunt capacitors, shunt reactors and phase modifiers, a thyristor controlled static shunt compensator to meet reactive power generation and absorption demand has appeared in recent years.
One primary reason for using shunt reactors or reactive control devices on EHV lines is to control steady-state overvoltages when energizing the long EHV lines or when operating under light-load conditions. If the shunt reactors are not employed, the reactive power generated by the capacitance can cause high voltages at the receiving end of the line. To restrict insulation stresses caused by overvoltages following sudden load rejection a substantial part of the shunt reactive compensation is usually left permanently connected.
It should be noted that with shunt reactors, the power transfer capability of the line is reduced owing to increase of transfer reactance X. Thus at fault load when stability is most important, the shunt compensation reduces the maximum power limit of the long line.
In case line shunt reactors are switched-out under heavy load conditions, the maximum power transfer capability can be considerably increased, but voltage variations due to sudden load thrown off are likely to be unacceptably high. In actual practice some of the shunt reactors are kept connected permanently so as to avoid voltage increase due to sudden fall in load from heavy load conditions.
15. Series and Shunt Compensation:
In EHV long transmission lines both series capacitors and shunt reactors are provided. The purpose of providing series capacitors is to artificially reduce series reactance of the line, so as to improve stability and efficiency of transmission. The shunt reactors are provided to artificially reduce the susceptance of the line so as to improve voltage regulation under light load conditions.
Practical and economical reasons lead to concentration of the compensating elements at a few points along the line. For a given total MVARs of series and shunt compensation of a transmission system, maximum power transfer capability, voltage control conditions and efficiency of power transmission depend on the number, location and circuit schematic of the series capacitor and the shunt reactor stations. Series and shunt compensation schemes. There are also other possible schemes.