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.

More on Magnetic materials

There are Three types of SOFT MAGNETIC MATERIALS:-

• Solid Core Material :- These are parts of magnetic circuit carrying steady flux as core etc. Made of Soft iron, relay steel;.
• Electrical sheet and strip
• Special Purpose Alloys

Solid Core Materials

• Iron, Low Carbon, Si Steel  ( Carbon is harmful it raises ciercive force & hystersis loss. Silicon is also harmful but with oxygen it becomes good)
• Low Carbon Steel ( Steel has low resistivity and high edddy current losses)
• Cast Iron :- Low relative permeability.
• Gray Carbon:- Magnetically inferior to wrought iron or steel.
• Cast Steel:- Used for those portions of magnetised circuits which carry steady flux and need superior mechanical quantities.
• Soft Steel:-now widely used in place of cast steel.
• Ferro Cobalt:-It has its saturation flux density 10 % higher than that of pure iron. Cost is High. Use is limited to pole pieces where a huge value of induction ( flux density) is desired.

Sheet Steel:-
Electrical Sheet Steel ( Non-Oriented):- Iron with low Carbon was used earlier. It was prone to ageing. Use of Silicon Steel, this is no issue. Advantages of adding Silicon ( More age, less hysterisis loss, increased resistivity, less eddy current loss). Disadvantages ( loss of permeability at higher flux densities & lower ductility). Hence Maxm Si = 5%
Electrical sheet steel can be Hot Rolled or Cold Rollled. In rotating parts we use steels of low Si content and are called "Dynamo Grade Steel" ( since these have to work at high flux densities).
4 to 5 % Si steel = "Transformer grade Steel"( also called high resistance steel (HRS))


CRGOS:- Saturates at a later stage . Constituent crystals, crystalline axis, cold reductions, annealing. Rolling direction = direction of high permeability.Reduces Iron loss.

Special Purpose Alloys:- SPA , HULOB.
They have high flux permeability at very low flux densities. Ni ( 60 to 90%) + Chr + Mb.
MUMMETAL :- Low permeability but high eddy currrent losses. Some Cu too.
PERMALLOY:High Ni (80%) permalloy ( high initial & maxm permeability , Mb + Cr , High resistivity) Low Ni (50%) Permalloy ( low permeability, high resistivity)
SUPERPERMALLOY:- High Purity, Relative permeability 100,000
PERMINVAR:- Permability independent of field strength.
PERMEANDER:-Capable of producing high flux density 49 % Cobalt 2 %, Vn 49 %

Factors to Be considered in Design

Transformer Design

• Calculation of leakage reactance.
• Core dimension.
• Window Dimension.
• Frame Size
• Conductor turns, size.
• Cruciform dia etc.
• Hw/Ww, Kw, Stacking Factor

Motor Design

• Main dimensions. ( alternator)
• Speed Limitations (alternator)
• Air gap length, Tph etc.(alternator)
• SCR L/T (alternator).
• Bar etc of Induction Motor.

A Note On Magnetic Materials

Limitation of design

• Saturation
• Temperature Rise
• Insulation
• Efficiency
• Mechanical Parts
• Commutation
• Power Factor
• Consumers Spec
• Standard Spec

Machine Design Facts

Q) Why electrostatic machines are not used and not electromagnetic machines are used?

Ans) Because of use of maximum energy density funda.

Q) What is Basic Structure of machine:

Ans:

• Magnetic Circuit
• Electrical Circuit
• Dielectric Circuit
• Thermal Circuit
• Mechanical Circuit

considering all above a successful designer brings out an economic and technical compromise.

Electrical titbits

Q) What is efficiency of Electric Locomotive compared to Steam?

Ans) Steam loc 6 %, Electric Loco 45 %

Q) Which load forms sizeable base load for grid?

Ans) Electric Traction

Q) What is effect of low frequency on motor?

Ans) It improves commutation of DC machines and p.f. and efficiency is also increased.

Q) What is advantage of DC series motor vs ac series motor?

Ans) DC series motor develops more starting as well as running torque for same size than ac motor.

What we use for Magnetic calculations

1. Basic Formula etc.

2. Total mmf calculation

• mmf for airgap
• Net length of iron
• mmf for teeth
• real & apparent flux densities

3. Leakage calculation

Some material science facts for Electrical Engineers

Q) Why Silicon ( 3 to 5 % ) is added to steel for making Transformer core lamination?

Ans) Higher content of silicon raises the resistivity of material and thus reduces the eddy current losses.

Q) Why Cold Rolled Steel is used?

Ans) Because it permits high flux densities 1.8 Wb/sq.m

This reduces the amount of material. For HR its just 1.3 Wb/sq.m

Insulations for HR:- Kaolin, Varnish

Insulation for CR:- Phosphate base.

Motor Design

Lighting Basics

Step towards Smart Grid

Through Ditrsibution Automation and Control (DAC), Following needs to be regulated:-

• DSM:- Demand side management.
• DLS:- Discretionary Load Switching = customer load management.
• Peak load pricing
• Load shedding
• Load reconfiguration.
• Voltage regulation
• TLM ( Transforner load management)
• FLM( feeder load management)
• Capacitor control
• Fault detection, isolation.
• Condition and state monitoring
• Load studies
• Customer meter reading
• Remote connect/disconnect.

Smart grids used AI ( artificial intelligence) these days.

Latest Tech Innovations in Electrical Engineering

• Grid - Scale storage

Till now pumped storage or compressed air storage technology was there, which costs about Rs 40,000 per unit.

Now flow batteries, liquid-metal batteries, flywheels and ultracapacitors have come up which costs Rs 10,000 per unit.

• Digital Power Conversion

Silicon carbide/ gallium nitrite high speed switches for high freq. power handling, They are energy efficient and nano and can replace today's bulky transformers.

• Compressorless AC and Electrochromic windows

Thery dehumidify air with desiccants instead of refrigeration cycle. Window shading w.rt temperature is also coming up.

• Clean Coal

Till now carbon capture and sequestratyion (CCS) costs Rs 500,000 per KW. New tech will make 90 % of CO2 capture @ Rs 100,000 per KW.

• Biofuels and Electrofuels

Cane/Corn ethanol are available. Cellulosic and algae based biofuels are also coming up @ Rs 100 per gallon.

• Others:- Gen IV nuke reactors, Small , shrouded wind turbines, Solid-oxide fuel cells, ground-source heat pumps.

What is used to make LEDs?

Material Formula and its type

Magnetism of different materials

Some facts on protection

HRC fuse breaking capacity:- 80 KA

SF6 breaking capacity:- 30 KA

ACB breaking capacity :- 50 KA

hence fuses are used for S.C. protection and Relays for O.L. protection.

T1 and T2 reduce the fault current ( say from 400 A to 10 A ) which is the utility of these transformers.

Protections used in 132 KV/ 6.9 KV switchyard:-

• Lightening arrestor 120 KV, 10 KA

• IDMT O/C Relay + IDMT E/F Relay ( 51 + 51 N)

• IDMT O/C Relay + Instantaneous Relay + IDMT E/F Relay ( 51 + 50 + 51 N)
• Transformer Differential Relay(87)
• Restricted E/F Relay + Standby E/F Relay (64 R + 51 G)

Some notes on SubStation Relays

Trip Circuit Supervision Relay ( VAX ) :- 21 ( After closing only) 31 ( Preclosing and After Closing).
Application :- After Cl;osing or continuous supervision of the trip circuit or circuit breaker.
Detection:-
1.Failure of Trip Supply
2.Open Circuit of trip Coil or trip circuit wiring
3.Failure of mechanism to complete the tripping operation.

A CTZM Relay is used at ACB in PCC which feeds the MCC and the delay settins are provided there at.

When 10 MVA primary side SF6 primary side SF6 breaker operates, there should be an interlock to open the breaker on secondary side ( to prevent closing on load T/F) but this is not provided and is done manually.

CBCT :- Core Balance Current Transformer, Used for Unbalanced Current Protection.

Some questions answered.

Q)What is the difference between Screw Compressor and Reciprocating Compressor?
Ans) Screw Comp. is a constant torque load device. The output( i.e. air flow is also const.). unlike reciprocating comp, current of Screw Comp is much less fluctuating.

Q)Why three intermediate tap positions ( 11A,11B and 11C) are provided?
Ans) For Mechanical convenience.

Q) What re three MODES of CB/Feeder?
Ans) 1.Service :- Both Control and Power Supply will be there.
2.Test :- Only control Supply and no power supply.
3.Isolation:-Neither Control Now Power Supply.

Q)Where you find two diaphragms in transformer?
Ans) In the EXTRA VENT provided just below level og conservator.

A Lecture on Dielectrics

3A - Dielectric Materials

Some Rapid Fire Questions on Electrical Engineering

Q) Which machines have parabolic characteristics?

Ans) Fans and Pumps

Q) Which machine has linear load characteristics?

Ans) Screw Feeders

Q) What is difference between NEMA and IEC?

Ans) NEMA is American and IEC is Indian standard.

Power Management in Factories

There are three types of loads:-

• Load which cannot afford any power failure. ( on a small capacity DG set ;say 250 KVA)
• Load which can tolerate 2 min inturruption. ( on a small capacity DG set ;say 250 KVA)
• Load which can tolerate 30 min interruption. ( on a high capacity DG set ;say 4 MVA)

Motor Facts

Upto 30 KW DOL starters are used, above that, y-Delta starters are used.

For motors rated 55 KW+, motor heaters are used.

For motors rated /. 90 KW, PTC Thermisters are used.

For all motors rated 90 KW +, the vendor should furnish following data soon after submission of T-S curves and G D sqaure value of the driven equipment:- (This data is required for selection of proper motor protection relay).

• Thermal Withstand Curve: Hot and Cold.
• Stator Time Constant.
• Stalling Current.
• Stall Withstand:Hot and Cold
• Starting Current.
• Acceleration Time.