MODULE - V- EHV TRANSMISSION NOTES

MODULE - V- EHV TRANSMISSION

Introduction

          In 1915, the total installed capacity in India was 105 M.W and by 1947, the independence year, it was 2300 MW.  Now it has grown over 125,000 MW.  As far as transmission voltage is concerned the first 100 KV line was commissioned in Maharasthra in 1911.   The first 132 KV line in1932 and first-220KV line in 1962.  The first 400 KV line was commissioner in 1978.  All 400 KV lines use bundle conductors of 2,3 or 4 sub conductors.  The next higher Voltage chosen was 765/800 KV. Some of the lines constructed with this voltage are Anapara- Unnao and Tehri-Meerut in U.P, Mohakishenpura in Punjab and Vidyachal - Bina- Nagda in M.P. High voltage D.C. transmission line also has been introduced in India.  Voltage selected is 500 KV.  Richand- Dadri and Vidyachal back to back systems are already commissioned.  The Richand - Dadri line transmits more than 1000 mw of power.

          Need for EHV Transmisssion

                   Voltages above 300 KV is termed as EHV and above 750 KV are termed as ultra high voltage (uttv). Necessity for EHV transmission are the following:-

          (1)     For a given amount of power to be transmitted for a given distance, the transmission efficiency increases as the transmission voltage increases.

(2)     In EHV/ UHV lines, the per unit resistance drop reduces and volume of conductor material decreases.

(3)     Power transmission capacity of a line is proportional to the square of voltage.  Hence the transmission capacity increases tremendously as the voltage is increased to the EHV/UHV range.  But cost of towers terminal equipments also increases, but this increase in proportional to the voltage only.   As a result over all capital cost on transmission decreases as the voltage is increased.   

(4)     Large power stations are economically more viable.  EHV/UHV lines were required to transmit this large block of power from these large stations.

(5)     The load carrying capacity of lines are expressed in terms of surge impedance load or natural loading.  At this loading reactive VA consumed by the inductive part of the line is equal to copacitive VA in the line.

          PSIL =   Where  =  Surge impedance =

          (SIL - Surge impedance loading)

          Hence Surge impedance loading increases as the square of voltage.

6        It is not possible to interconnect power systems without EHV Transmission lines

Problems associated with EHV Transmission

(1) Corona and Radio interferences - Corona is largely depending on the system voltage and radio interferences is the consequence of corona.  During bad weather corona loss is very lugh.  To reduce corona spacing between the conductors or size of the conductors has to be increased or bundled conductors are to be used.

(2)     Line supports

          EHV line tower has to with stand heavy mechanical loading due to bundled conductors. Large air and ground clearance has to be provided.  Hence the cost on towers well increase and it will be approximately 30% to 50% of the total cost of the line.

(3)     Erection difficulties - It is necessary to evolve and employ new erection and conductor handling techniques.

(4)     Insulation Requirements : - The insulation level has to be increased to with stand lightning surges and switching surges.  Lightning surges are taken care of mostly by the earth wires.

(5)     Power station and sub station Equipment :- The power station and sub station equipments have to handle the high voltage and high  fault current.  Modern transformers and circuit breakers are capable of handling these high values.  But the cost increases.

Limitations of EHV, A.C. Power Transmission

1        Natural loading

          Consider a line with negligible  .......................resistance and reactance Land capacitance C, per unit length Load current is I amperes.  To keep the sending end voltage and receiving end voltage same, lagging voltamberes should be equal to the leading voltamperers.

          ie (I x L) I   = v or IwL =

           ohm  is called the surge impedance of the line.   behaves like a pure resistance.  Power transmitted per phase is

V x I =  and this is called the surge impedance loading or natural loading of the line.  But normally line is not operated at its natural loading.  While transmitting power at a lagging power factor there will br voltage drop, but at leading p. f. there well be voltage rise.

Voltage K.V.	132	220	380	500
Surge Impedance ZS ohm	350	320	295	276
Natural loading MW	50	150	490	900
Current amp A	220	385	740	1000

          The natural loading do not impose any restriction on the distance of transmission line.   But it restricts the amount of power that can be transmitted for a particular voltage or it fix up an voltage for a certain amount of power.

(ii)      Stability considerations:-  If resistance and shunt leakage of  a transmission line (Rand g) is neglected the power that can be transmitted within stability limit is

P =  sin                         = Sending end Voltage

                                                 = Receiving and voltage      

                                                X = Series reactance in ohm/plan       

                                                 = Load angle - angle between V_s and                                               V_r

When

P =   =   for 3 phase where

                   KV = line to line voltage.

 If  = 90 maximum power can be transmitted.  But  is not allowed to vary more than 20⁰ to 30⁰.  Other wise, the machines will loose synchronism under transient conditions. Consider a case, when x= 0.3475 ohm  per km.

  S = length in km of line.  We have

P =

 If the line in to operate at natural load then

PSIL =  Equating the two

 or S= 1.43 Z km.

          This means that the length of the line that can operate at natural loading without loosing stability is 1.43 (For the assumed value of x, For any other value of X these is another fixed value of S).  This imposes a limit of on the length of transmission line in an A.C. system.  In EHV line exceeding 200 km addtitional equipment like series capacitance and staun reactance is to be installed.

3.Current carrying capacity of conductors

          In an overhead system maximum power transmitted is never dependant on thermal condition.  But due to other consideration as explained in 2 and 3 above.  Hence the current carrying capacity is not fully utilized.

4        Ferrants effect

          If the line is loaded with a leading, reactance volt amperers, there will be a rise of voltage at receiving end. The rise is of the order of 1.5% for 160 Km, 13% for 500km and 100% for 960 km.  There is also rise of voltage at the sanding end when the load is thrown off suddenly.  This may agravate the problem.  Usually shunt reactors are used at the load end to control the voltage rise due to Ferranti effect.

5        Economical and Biological aspects

          Recent researches, show that EHV and UHV lines generate electrostatic and electro magnetic field.  These field can induce current and voltage in animals human beings and birds.  But however these effects are minimum and within tolerable limits transmission lines also produce noice in the audible level.  But it is not serious up to 500 KV.

Reactive compensation of EHV system

          To inject reactive power and to control the load end voltage shunt compensation with capacitive VAR is used.  To encounter ferranti effect shunt reactor compensation is made. Series capacitor compensation made in order to enhance power transfer capacity of transmission lines.

 Commonly used compensators

          (a) Static shunt capacitor Banks - These are installed at suitable points in the bus and switched on as per requirement to maintain bus voltage within limits.

Shunt reactors : These are used to control excess voltage due to ferranti effect.

(c) Synchronus condensors or Synchronous  phase  modifires

          Synchronous condensers are Synchronous motors run for power factor improvement.  When over excited it delivers capacitive current and on under excitation it delivers reactions current.  Hence both lagging and leading VAR is possible.

(d) Static VAR compensators:- This serves the same purpose of synchronous condensers.  But there is no rotary part.  It is basically a controllable shunt reactor in parallel with static capacitors.

(e)  Series compensation:- It is an important method in improving the performance of EHV lines.  It increases the power handling capacity and reduces voltages regulation.

          Power transfer capacity P =

                              Is the load angle or torque angle

                                      (angle between V_S and V_r)

If a series compensator in installed in the having          as the capacities reactance the net reactance in () and power transfer capacity of such a line is given by

P

K=  in known as degree of series compensation.  Obviously X_c  < x_L optimum compensation in obtained with  0.4 to 0.7.  When series compensation is applied power angles   is reduced.  Lower values of load angle increases the system stability.

F.A.C.T.S - Flexible A.C. Transmission system is a new technology by which capacity of transmission lines can be enhanced.  The essential ingredients in FACTS is the rapid and precise switching in and out of capacitor banks.  This is made possible by the use of solid state switches such as thyristors.  This can influence the impedance of a line and power can be routed within a system and  is between system.

H.V.D.C. Transmission

          Due to the limitation enlisted above for EHV, A.C. System recent trend is to go for H.V.D.C transmission when the lines lengths are more.  But D.C. in tired for transmission only.  Generation sub transmission and distribution is A.C. only.  This is mainly because stepping up and stepping down using transformer is possible with A.C. only.  At both ends converting equipments are used.

At one and converting equipment work as a rectifier and  other end it works as an inverter.  Power can be transmitted in both the direction.

  Comparison of H.V.DC and H.V.AC system

	D.C.               (Advantages)	-	A.C.
1	Line construction is simple and  cheaper		Line construction complex
2	Power transmitted per conductor is more hence lesser number of conductor is required.	-	More conductors required.
3	Charging currents is totally absent.  Hence no distance limit	-	Charging current increases with length of line, which imposes a limit on distance
4	Current is uniformly distributed	-	Due to skin effect current is concentrated to perifery
5	Reactive power compensation is not required.  Line operation at unity p.f charging currents are absent. Line drop is purely resistive.	-	Long distance transmission is possible only if reactive compensation is done.
6	Corona loss in proportional to (f+25). Hence for D.C. corona loss in less.	-	Corona loss is more
7	Switching surges are less	-	Switching surges are more
8	Stability problems do not arise since it is in asynchronous operation of the generators	-	Due to stability problem distance is limited.
9	Contribution of D.C. Line to short circuit current is less.		In AC system short current is more
10	Power control is  fast and accurate since there is less inertia due to the absence of rotating parts	-	Power control is slow
11	Unlimited power can be transmitted
	Disadvantages
12	The converters required at  both  ends are very expensive.   Hence D.C lines are  economically  viable only of the distance of the line is more.		Investment in terminal equipment is less.
13	The converters absorb reactive power which must be supplied locally. Since D.C. blocks the transmission of reactive VA. the receiving end  must be capable of supplying the whole of reactive component of power required by load.	-	No such problem
14	Suitable for point to point transmission only	-	Suitable for inter connected system

Principle of HVDC control                  

           In H.V. AC. system, power transfer is governed by voltage difference between sending and receiving  end and angular difference between these.  But in d.c it is determined by the difference of voltage between two ends.  Hence controlling HVDC power is fast and simple.  By proper setting of the inverter and converter, power flow  can be controlled. 

Types of D. C. Links

          H.V.D.C.  links (D.C. link = converter+ D.C. lines+ Inverter system) can be classified as follows:

1)       Monopolar link

          In  this configuration  one conductor  (usually negative) is used and earth  is used as the return path.  Negative polarity is used since it reduce radio interference. 

2.Bipolar Link

          This configuration has two conductors one positive and one negative.  At each terminal two converters of equal voltage ratings are connected in series, the neutral points being earthed- Two poles can operate independently, when both neutrals are grounded.  When the current in the two conductors are equal, the ground current is zero.  Even if one conductor is faulty the remaining one conductor can continue with 50% load.  The voltage rating of a bipolar link is usually expressed as  ... volts. 

3.Homopolar link

          It has two conductor but having the  same polarity (usually negative). The link operates with  ground return.  In case of a fault in one conductor the converter can be connected to deliver power through the other conductor.

Terminal Equipments

          Thyristor valves are the most important terminal equipment.  other equipment include converter  transformer, D.C. reactors harmonic  filters,  reactive power componator and HVD Control  system.  Six pulse or 12  pulse converters are in use.   At the sending end thyrister valves act as rectifiers and at the receiving end they function as invertors.  Power can be transmitted in either direction.  A thyristar valve is formed by a number of thyristors connected in series.

GRAETZ CIRCUIT

          The conversion from AC to DC and  Viciversa in done in a HVDC converter station by using 3 phase bridge converter.  The configuration of the bridge (also called Graetz circuit) is shown in fig 

          Gracty Circuit

           This is a six  pulse converter and 12 pulse converter  is composed  of two bridges in series supplied from two different transformers one of the transformers  is star- Delta, so that, there is 30⁰ angular difference between the  secondary voltage of these two transformers

Economic distance  for D.C. Transmission

          The cost of towers and conductors is less in the case of D.C. than A.C. But the cost of terminal equipment in case of D.C such as converters and filters are more than the equipments used in A. C such as transformers.  Fig shows the total cost of lines, for AC and D.C. slope of line represents the cost of per unit laugh for line construction and the intercept on the y-axis represent the cost on terminal equipments.  At point B the curves cross each other and at this point the total cost for D.C and A.C is equal.  If the distance is more than this the, D.C. transmission will be economical below this distance A.C. transmission will be cheaper.  But new techniques are coming up in the converter and inverted system, which will reduce the cost of terminal equipment.  In future a shorter distance will become economical for D.C. transmission.

Converter Station

          The major components of a HV DC transmission system are converter station where convertion from AC to DC (rectifier station) and from D.C to AC (Inverter station) are performed.  The role of rectifier and inverter station can be reversed by suitable converter control.

          A typical converter station with two 12 pulse converter units per pole is shown above.  The various components of a converter station are  disused  below.

1 Converter Unit

          This usually consists of there phase converter bridges  connected in series  to form a 12 pulse  converter unit.  Total number of values in a such a unit are twelve. Each valve is used to switch in a segment of an A.C. voltage wave form.  The converter is fed by converter transformers connected in star/star and star deta.  The valves are  cooled by air, oil, water or freon.

Converter transformers

          the value side windings are connected   in star and delta with neutral point in grounded.

          On the A.C. side the transformers are star connected in parallel with neutral point grounded.  The leakage  reactance  of the transfomer in chosen, so that the short circuit  current is limited,  The converter transformers are designed to with stand D.C. voltage   stresses and increased  eddy currents due to harmonics .

          Filtors

 There are two types of filter used.

1        A.C. filters:- These  provides low impedance shunt path for A.C. harmonics currents.