The thyristor-type static excitation system, due to its many advantages, excellent response characteristics,easy maintenance and simplified main machine construction, is now extensively used for medium-and large-capacity hydro-or steam-turbine generators.

The voltage regulator and potential source static excitation system functions to control the voltage of an synchronous  generator by directly controlling the generator's DC field current. The static excitation system is composed of the followings:

Thyristor rectifier bridge and thyristor elements:

The 3phase full bridge rectifier circuit has fast response characteristics. A compact cubicle design is realized with the large on-state current and high reversed voltage flapack type thyristor elements, and forced air cooling. The thyristor elements are installed in a tray, and can be exchange during operation. For better cost performance, a tray-less-type can also be manufactured.

Field Flashing

The field flashing circuit is necessary when a generator is started, because of self excitation system. A DC battery is usually used as the initial excitation power supply. An AC power supply can also be adopted by means of rectifiers and a transformer.

Field suppression

The de-excitation function is to reduce rapidly field energy when needed and also to separate the rotor circuit from the excitation system. The DC field circuit breaker is generally used. For better cost performance, a static field circuit breaker system can be supplied. This system reduce the field energy by reversing the excitation voltage by rectifier gate controls.

Over voltage protections

The C-R absorbers and varisters are installed in each AC and DC circuit for over voltage protections of thyristor elements. In large capacitance system, a crowbar circuit is adapted on DC circuit.

Excitation transformer

The excitation transformer reduces the supply voltage to the level required for excitation. A dry-type for small capacity or a oil-type for large capacity is generally used.

Monitoring devices

The alarms for thyristor fuse blown, cooling fan failure and air temperature high are available. A rotor temperature converter and field earth detector can be installed in excitation cubicles optionally.

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source: excitations system

Excitation systems have a powerful impact on generator dynamic performance and availability, it ensures quality of generator voltage and reactive power, i.e. quality of delivered energy to consumers. Following types are common:

• Brushless excitation systems, with rotating exciter machines and Automatic Voltage Regulator (AVR), or
• Static excitation systems (SES), feeding rotor directly from thyristor bridges via brushes.

Main functions of excitation system are to provide variable DC current with short time overload capability, controlling terminal voltage with suitable accuracy, ensure stable operation with network and/ or other machines, contribution to transient stability subsequent to a fault, communicate with the power plant control system and to keep machine within permissible operating range.                      

Controlled discharging of Inductive HV Equipment

 HV equipment need to be discharged prior to any maintenance work.  The discharge is usually done with an earthing rod after verifying that there is no voltage present on them with an non-contact voltage detector.

In the case of inductive equipment such as transformers and motors with high inductance value in their windings, a controlled discharge needs to be carried out.  Inductive components such as windings in transformers and motors have high inductance.  Sudden discharge of these windings will create a high discharge current and a rapid change in the flux which will result in a high voltage pulse (according to Lenz' Law). This can damage the winding insulation.

The controlled discharge is carried out with a discharge rod with a resistor in series.  The resistor used is a special non linear resistor which has a reverse temperature coefficient which means that as the temperature increases the resistance falls.  When the discharge is started, the resistance is high. As current flows through the resistor, the temperature of the resistor rises and its resistance falls.  Thus the current flow is increased.

The resistor ensures that the discharge is gradual.

Arcing ground

Arcing Grounds is a phenomenon which is observed in ungrounded three phase systems.  In ungrounded three phase systems operating in a healthy balanced conditions, capacitances are formed between the conductors and ground.  The voltage across these capacitances is the phase voltage. 

Now, in the event of a ground fault, the voltage across the faulty conductor becomes zero while the voltages across the healthy conductors increase by a factor of 1.732. 

The arc caused between the faulty conductor and the ground gets extinguished and restarts many times, this repeated initiation and extinction of the arc across the fault produces severe voltage oscillations of the order of nearly three to four times the nominal voltage. 

This repeated arcing across the fault due to the capacitances between the conductors and the ground is known as arcing grounds.  Arcing grounds can be eliminated by the use of Peterson Coils and Arc Suppression Coils

Duty cycle:: for motor

The relationship between operating and rest period of a motor is known as duty cycle. It is important while choosing a rating of a motor for our particular application. The international Electrotechnical commission has defined the following standard:

S1	Continuous duty	The motor works at a constant load for enough time to reach temperature equilibrium.
S2	Short-time duty	The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature.
S3	Intermittent periodic duty	Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise.
S4	Intermittent periodic duty with starting	Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise.
S5	Intermittent periodic duty with electric braking	Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods.
S6	Continuous operation with intermittent load	Sequential, identical cycles of running with constant load and running with no load. No rest periods.
S7	Continuous operation with electric braking	Sequential identical cycles of starting, running at constant load and electric braking. No rest periods.
S8	Continuous operation with periodic changes in load and speed	Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods.

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Super Capacitors

Capacitors having very high capacitance reaching up to 500 farads is called super capacitor. They are also called ultra or double layer capacitor. The main principle behind the super capacitor is similar as normal capacitor but are manufactured with nano-technology which enables the dielectric to have a large surface area hence greater quantity of charge is stored. The commonly used dielectric is electrolytic soaked separator and the electrode is activated charcoal.  

              These stores energy just like batteries but advantageous due to lighter in weight, environmentally friendly, can be charged and discharged repeatedly unlike the battery, life is about 1000 times that of the battery. Their uses are in camera, electric automobiles, power conditioner, welders etc. Super capacitors can supply short burst of power and are useful when heavy loads are applied suddenly.  They also charge faster and absorb voltage transients better while the battery supplies the regular power requirement. They are also used in automobiles to where they can be charged easily and are particularly effective in recovering energy from the transmission systems through regenerative braking. The downside of supercapacitors is the low energy to weight ratio as compared to batteries.  Further advances in technology may narrow out this difference.

Difference between salient pole and non-salient pole rotor used in synchronous generator.

synchronous machine can be characterized by the type of rotor used. Rotors used in Synchronous alternators can be classified into:

1)Salient and

2)Non-Salient Pole Rotors or cylindrical rotors

Salient pole rotors are used in application with speeds from 100 to 1500rpm. They are alternative known as "projected pole" type of rotors. The poles mounted on the rotor are made of lamination's made of steel. The poles are connected to the rotor shaft by means of dovetail joints. Each pole has a pole shoe around which the winding is wound. The salient pole rotor is generally used in applications where the prime mover is a hydel turbine or a combustion engine which have low or medium speeds. Salient pole rotors usually contain damper windings to prevent rotor oscillations during operation. Due to low speed, they are constructed with higher no. of pole (ranging from 8-24 or higher) to achieve system frequency so they can be said as higher pole machine. Almost all hydro power uses salient pole rotor synchronous generator.

                               Non-salient pole rotors are generally used in application which operate at higher speeds, 1500rpm and above. The prime movers in these applications are generally gas or steam turbines. These are sometimes known as "drum rotors". The rotor is a cylinder made of solid forged steel. The slots on which the windings are fixed are milled on the rotor. The number of poles is usually 2 or 4 in number. Since these rotors are cylindrical, the windage loss is reduced. The noise produced is also less. These rotors have higher axial length. These rotors do not need damper windings. Due to high speed, they are used with gas turbine and high speed steam turbine in nuclear power plant and thermal power plant.

Design of transmission line

STEP 1              
Selection of appropriate voltage level:
 The guiding factor for selection of voltage level of a transmission line are:
(a) Economical factors
(b) Technical requirements
(c) standard voltage levels
(d) Existing scenario. 


a) Economical factor:
For three phase Nc ckt transmission line: Pl =3*Nc*Il^2*Rc and P= sqrt(3)*Vll**Ill*cos(x)*Nc 
Rc=pl/A
therefore Pl/P=P/(Nc*V^2*cos^2(x))*pl/A=percentage power loss

For same percentage power loss: A=K* P.l/(V^2*Nc) =>this shows that the percentage power loss decreases by increasing no of circuit or by increasing the system voltage or by increasing the conductor cross section area. Conductor volume=3*Nc*A*l = K1*P*l^2 / V^2 => The conductor cost is independent of the no of ckt for same percentage power loss.

Veconomical=5.5[Lt/1.6+(P*1000)/Nc*cos(x)*150]^.5

The standard voltage level nearest of the Veconomical is chosen. Then it is checked for technical requirement fulfillment.

b)Technical requirement:

Plimit =sqrt(3)*V*Imax*cosx
Plimit=K*V^2/(x.l)
where V=system voltage
Imax=maximum current that the line can carry, depends upon thermal capability.
x = inductance per unit length
l = length of transmission line
If the power to be transmitted (P)< Plimit then the transmission line is said to satisfy the technical requirement.

It is checked by the following method:
surge impedance loading (SIL)= V^2 /Zc
Plimit/SIL = (KV^2/l)/(K1* V^2) = K2/l = m-factor
Zc=400 ohm for normal line, for double circuit Zc=400/2

length (km)                 m-factor limit
80                                   2.75
160                                 2.25
240                                 1.75
320                                 1.35
480                                 1.0
640                                 0.75
To calculate the Plimit, first we calculate SIL and then m-factor for the given length of the transmission line from the above table. If required value not found on the table use interpolation. Then Plimit= SIL*m-factor
If Plimit>P (power to be transmitte) => satisfied.. otherwise repeat with another voltage level or double ckt

c) Standard voltage is chosen because all the measurement instruments and the switchgear are available at standard voltage level.

d) Existing scenario also influence the selection of voltage level. If there is one line with 66 KV then if we need to connect this line with other station, we chose 66 kv and check for single or double ckt.

STEP 2
AIR CLEARANCE DESIGN:

It includes air clearance design and design of physical insulator at towers.  In air clearance design: insulation must withstand normal system voltage continuously and overvoltages for short times. The overvoltages for short times are:

i.              Temporary overvoltage – due to unsymmetrical fault, Ferranti effect (10% of system voltage up to 220 KV and 5% for voltage greater than 220 KV) and Ferro resonance.

ii.             Switching overvoltage

iii.            Lightning overvoltage

The factors affecting the air clearance between charged conductor and earth metallic structure of the tower:

# Voltage level – phase voltage, peak voltage

# maximum system voltage – highest system voltage at any point along the lines of all condition

Line configurations:

i.              Single circuit

ii.             Double circuit

iii.            Horizontal

iv.           Vertical

v.            Triangular

a= 1cm per KV and 20 cm margin=(Vll*√2*1.1/√3 )+ 20   cm

Cl = a(1+tan x )       

l=asec x

b=1.5 a

y= (l+a)/sqrt(1-(x/y)^2*(l+a)^2/Cl^2)

d=√3 *(Cl +b/2)

d=√3 *(Cl) for double circuit line

STEP3

INSULATOR DESIGN

 No of disc for different voltage level: bit complex because the voltage distribution for different discs along the string are not equal. Dielectric strength of disc varies from different voltage level. It should withstand for sudden abnormal voltage: to minimize the cost of insulator, minimization of overvoltage at lower level is required.

1.       External overvoltage -  lightning overvoltage

2.       Internal overvoltage – short term  and long term

i.                     Unsymmetrical fault: The phase voltage is given by Vph= K*Vll    the value of k varies between (1/√3) to 1. The higher the value of k => cost of insulator is high.  Lower the value of k => grounding cost is high. K=0.8 is taken for Nepal.

ii.                   Ferranti effect:  1.1*Vll up to 220 KV and 1.05* Vll for greater than 220 KV

iii.                  Vxc= Ix*Xc

Ix=V/(2Xtr-Xc)

iv.                 Switching overvoltage Vs/w=K1*E , E is the instantaneous voltage at the instant of switching. If line is assumed lossless and there is no trap charge K=2. If line is assumed lossless and there is –Vpeak trap charge, K=3. Since line has certain resistance, value of k decreases slightly. In practical case K=2.8 is chosen.

v.                   Lightning overvoltage: Lightning current=natural phenomena =>10KA from probability distribution of lightning current. Lightning voltage= depends upon system voltage and tower footing resistance.

Vlightning= I*Rt+Vll*√2*1.1/√3

                                         The standard voltage which an insulator must withstand known as basic insulation level (BIL) of that insulator.  Low voltage lines are very much prone to lightning whereas high voltage lines are dominated by switching over voltages. These facts are greatly considered for designing a transmission line.

Protective ratio (α) = BIL of L.A/ Temporary O/V≈3

Therefore for 11KV, BIL=3*.8*11*1.1

Switching surge ratio (SSR) = Maximum overvoltage/ system peak voltage

Therefore Vs/w(max)=SSR*Vpeak

                                                    KV                                SSR

                                                     220                                2.8

                                                     400                                2.4

                                                           765                                1.8

The insulation voltage is in increasing order for Lightning arrestor, transformer and Line insulator.

why do we need interconnected system of power supply?

The requirement of interconnected system is due to the following reason:
1. To stabilize the frequency of power supply. The interconnected system is very less prone is change in frequency due to some load changes.
2. To continuous supply of electricity. The power supply will be reliable. If one generation station fails or goes to maintenance other will continue to supply at the particular area.
3. Fluctuation of voltage is low.
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Why Electrical Energy?

Although the Electrical energy is not used directly, it is the most popular form of energy. It is the most common and world wide famous form of energy. It is due to the following reasons:
1. In many cases transport of electrical energy may be cheaper than other form of energy.
2. Control of the electrical energy is very easy and convenient.
3. Conversion of electrical energy to other energy can be done easily and efficiently.
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