Insulation coordination

 

  

 

Why we use output choke in bridge rectifier?

Why we use capacitor bank in VFDs

Types of protection devices

Some notes on HVDC

Advantage of ground retuyrn:

• Earth resistance in case of dc for long lines is interdependent of length and this resistance is < that of metallic conductor if used.
• d.c line can be built in two stages.
• reliability increases in case of outage one monopolar out of the bipolar.

Disadvantages of  ground return:

• Electrolytic erosion of buried / immersed metallic structure.
• difficult design
• Ground currents can cause dangerous step and touch voltages.
• Ground current interferes with other services.

Z Bus Calculation

Ohm's Law and kirchoff's law

Ohms law:

•v(t) = i(t) R  - or -  V = I R

•p(t) = i2(t) R = v2(t)/R  [+ (absorbing)]

Kirchoffs law:

•Kirchhoff’s Current Law (KCL)

–sum of all currents entering a node is zero

–sum of currents entering node is equal to sum of currents leaving node

•Kirchhoff’s Voltage Law (KVL)

–sum of voltages around any loop in a circuit is zero

Kirchoffs Current Law :

The sum of currents entering the node is zero:

Analogy: mass flow at pipe junction

KVL Polarity

•A loop is any closed path through a circuit in which no node is encountered more than once

•Voltage Polarity Convention

–A voltage encountered + to - is positive

–A voltage encountered - to + is negative

Electrical Analogies (Physical)

Basic Electrical Quantities

•Basic quantities: current, voltage and power

–Current: time rate of change of electric charge

  I = dq/dt 

1 Amp = 1 Coulomb/sec

–Voltage:  electromotive force or potential, V 

1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb

–Power:  P = I V 

1 Watt = 1 Volt·Amp = 1 Joule/sec

•Active elements can generate energy

–Batteries

–Voltage and current sources

•Passive elements cannot generate energy

–Resistors

–Capacitors and Inductors (but CAN store energy)

An independent source (voltage or current) may be DC (constant) or time-varying, but does not depend on other voltages or currents in the circuit.

The dependent source magnitude is a function of another voltage or current in the circuit.

A motor problem

Q) A 25 HP 6 Pole 50 Hz 3 phase 960 rpm at full load. I 2 = 35 A, Allowing 250 W as Cu loss in SC gear . 1000 W = Mech losses.
How much is r2?
Ans)

Q) 440 V , 50 Hz, 6P, 3 phase, 

Ans) F2 = 100/60 

RL circuit on a AC source.

In the previous post we had seen RL circuit on DC source. Now lets see it on AC source

Transients in power system

Transients in simple circuit

DC Sources:

• Resistance only
• Inductance only
• Capacitance only
• 
  
• RC Circuit
• 
  
• RLC Circuit:

How to control voltage in power system

We have seen total grid collapsing twice recently. How voltage is controlled? Lets see in the above diagram


Sources and sink of Reactive power

• Transmission line
• Cables

• Synch. Machines
• Shunt capacitors and reactors
• Series capacitors

Insulated Cables

Capacitance of three core cables

Grounding

Advantages of neutral grounding:

• Voltage of phases are limited to phase to ground voltage.
• High voltages due to arcing grounds or transient line-ground faults are eliminated.
• Sensitive protective relays against LG faults can be used.
• Overvoltages due to lightening are discharged to ground.

Advantages of isolated neutral

• It is possible to maintain supply with fault on line.
• Interference with communication lines is reduced because of no Z sequence current.

Effective grounding systems:

X0 / X1 < 3

Least Expensive is = maxm Vphase= 0.8 V line

Series or Shunt Capacitor

Voltage Boost:

Shunt Capacitor: Evenly distributed over transmission line.

Series Capacitor: -----------------------I  I-------------------------
                                                   V1            V2
                                     Sudden change of voltage

• Shunt Capacitor improves power factor of the load whereas Series capacitor has little effect on pf.
• For long transmission line, where total reactance is high, series capacitors are effective for improvement of system stability.

How to measure Flow?

Flow Strainers, Impellers, Magnetic sensing pickups. 

Turbine Meter

• good for telemetry
• errors may be caused 'coz of excessive frictional torque.

Electromagnetic flowmeter

• suitable for slurry, sludges etc
• Adv: size adaptability,No pressure drop
• Disad: for heavy slurry, high operating cost.

Hot Wire Anemometers

• more flow > more cooling
• voltage variation
• measured by POT

Using Thermister

• can measure velocity of air

Ultrasonic flow meter

• used mainly for liquid
• useful for bidec tional flow
• Adv:linear relationship
• Adv:insensitive to variation in viscocity, density and temp
• Disadv: complexity
• Disadv: relative high cost

Per Unit Impedance

Some EE Questions

Q) For eleiminating 5th harmonics in a three phase AC system, the winding pitch should be?

Choices:   2/3, 4/5, 5/6, 6/5

Q) Magnetic circuit is saturated, delivering full load current at rated volt and current. n= constant, brushes at interpolar axis. When load is switched off, terminal voltage would?

Choices:  remain same , increase, decrease, will become 0.

Q) Under which of the following conditions dc motor with compensating winding is used?

Choices:
A) Wide range of speed control above normal
B) Wide range of steady load variation with no speed control
C) Wide range of rapid variation in load

Q) What happens when field of DC shunt motor gets opened?

Q) Match?

Interpoles.................Commutation in D C machine.

Three pt. starter.........DC shunt motor

Dummy Coils............Wave Winding

Equaliser Ring...........Lap Winding

Q) The phase sequence of alternator will reverse when

a) If ( field current) is reversed, keeping direction of rotation same.
b) If remains same, direction of rotation is reversed.
c)If reversed, number of poles is doubled.
d) Number of poles doubled, If same.

Some news in Electrical Engineering

• Now Low Fire Hazard Cables are available in India. Uniflex has come up with this product.
• Drawing outsourcing has begun in India as BHEL-Jhansi has done it for 220 kv Power Transformers.
• Rs 400 Crore capacitor industry in India is growing @ 8 % p.a. 
• Use of PTC thermistor in the refrigerators saves lots of energy.
• Now India is equipped for 1200 KV transmission lines.
• After heat shrink, cold shrink technology is also here.
• What are PMUs? Ans) Phasor Measurement Units installed in the power grid. 

Electric Heating and Welding

Q) What is difference between types of electrical heating?
Ans) Resistance Heating is done at power freq. while Induction Heating and Dielectric Heating is done at high freq.

Q) Why adjustable AC voltage is used for Salt Bath Furnace?
Ans)Slat Bath Furnace ( No oxidation while hardening)  is used for Steel Tools Hardening at 20 V, 3000 V AC ( DCcan cause electrolysis of NaCl solution). Since Resistance drops with temperature, Adjustable voltage is needed.

Q) Why Heating Materal/Element should have high sp. resistance?
Ans) So that short length of wire is enough.

Q)Why heating element should be free from oxidation?
Ans) Otherwise life will be short. ( Iron addition means oxidation)

Q) Why heating element should have low temperature coeff of resistance?
Ans) So that R does not change much with T.

Q) What are common heating materials?
Ans) Ni-Chr, Ni-Chr-Fe, Ni-Chr-Al, Ni-Cu.