Power System Protection

Fault Vs Abnormal condition:

Abnormal condition in a power system, as the word itself states, is a non-desirable state in a power system.
Best example is 'Overload' in a power system, where the load current is above the nominal rating of the system it is designed for.

Whereas Fault is an abnormal condition which is detrimental to the power system , and will be dangerous if not eliminated.
Best example is a short-circuit, whereby two phase conductors touch each other between the load and the generator - so that a very high current flows in the circuit.

Transformer getting fire due to fault

Image from: www.seeclab.com

Sequence analysis:


Balanced system is where in a 3-phase power system - the current flowing in all three phases are equal.

Unbalanced system is where in a 3-phase power system - the current flowing in all three phases are unequal.


Any unbalanced system can be represented by balanced systems containing 3 elements. Those are,

1. +ve sequence
2. -ve sequence
3. zero sequence

Vector addition of the above 3 elements will result in the former unbalanced system.

Protection Equipments:

Fuse:

1. Semi-enclosed
-found in domestic installations, rewirable.

rewirable fuse holders

2. Cartridge type
-found in electrical apparatus such as UPS etc, not re-wirable.

cartridge fuse








3. High Rupturing Capacity (HRC)
-normally found in the secondary (LV) side of transformers. This has high current capacity before breaking.

Knife-edged Low volatge HRC fuse links








Drop-down-lift-operate (DDLO) fuse:

These are sometimes called fusible cutouts.

This is a type of expulsion fuse, which is normally found in the HV side of transformers. When high current flows in the primary side (HV) of transformers - the fusing element (a special metal wire) will melt so that the connection will be cut-off due to gravity.

a DDLO in opened position









Miniature circuit breaker (MCB)

This is the  protective device seen in modern homes, replacing older fuses. available in low current versions.
for e.g. 6A, 10 A, 16A etc. These MCBs normally have two tripping phenomena. one is magnetic coil used for instantaneous tripping ,and thermal bimetallic strips used for inverse-time overcurrent tripping.

These are available in different classes - considering the load current characteristics such as high inrush (starting) currents. High starting currents are caused when loads such as fluorescent lamps and motors are switched on.

single pole , three pole , four pole versions are available depending on the number of wires.






Moulded case circuit breaker (MCCB)
These are high current versions of domestic MCBs. Normally found in factories, utility bulk supply (3-phase) entry points.

3-phase currents in the orders of even 500A can be handled by these MCCBs.

Earth leakage circuit breaker (ELCB)

This is the older version  of RCCBs. These devices detect the leakage current to earth and trip if that current exceeds a threshold, for e.g. 30mA, 100mA.

It should be noted that ELCB is a (residual)-voltage-operated device.

Residual current Devices (RCD)/ Residual current circuit breaker (RCCB)

This is a modern version of an ELCB, which works by comparing the residual current (resultant) produced - by means of checking the current difference between live and neutral wires .

It should be noted that RCCB is a (residual)-current-operated device.

Protective Relays:


1. Overcurrent (O/C) relays
2. Overload  (O/L)  relays
3. Earthfault  (E/F) relays
4. Under frequency  (U/F) relays
5. Overvoltage (O/V) relays
6. Distance relays
7. Differential relays
8. Reverse power (R/P) relays
9. Bucholz  relays
10. Directional relays
11. Overspeed  (O/S) relays


Arcing Horns/ gaps:
These are sometimes employed in transformer HV terminals or between the terminals of an HV insulator, to protect them from lightning surges. This works on the simple principle of HV rod-gap breakdown.

High Voltage Circuit breakers:
This was earlier explained in the post - "Grid sub stations"

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Thermal power stations

When we say thermal power stations, it really means a coal-fired power plant with steam turbines(ST). But , in common language, it may include diesel engine(DG) power plants, gas-turbine(GT) powered plants or even combined-cycle(CCY) power plants.

Coal-fired steam turbines:
Since coal is expected to be more abundant than other fossil fuels such as diesel, petrol, natural gas - and because of it's low cost , coal power plants are installed in most power systems as base load catering plants.

A coal power plant,  showing cooling tower

The turbine of a modern steam-turbine-generator

Gas-turbines:

The main elements of a gas-turbine are,
1.Compressor
2.Combustor
3.Turbine

A General Electric gas-turbine which was used in a US military jet-aircraft


Combined-cycle power plants:
In order to increase the efficieny of gas turbines, combined cycle plants were introduced. These plants utilise the still hot exhausts left out to the atmosphere to heat water pipes. this effects to be a steam turbine and significant efficiency improvement is done.

Model diagram of a CCY plant, showing a gas turbine and a steam turbine



Diesel-Engines:
Diesel engine powered plants are a kind of Internal combustion engines. Normally petrol-based (gasoline) engines are spark-ignitioned. This means every combustion cycle is initiated by a HV spark provided by spark-plugs. But big diesel-based engines are compression-ignitioned. The compression of air produces heat and, makes the incoming tiny particles of diesel get ignited.

main elements in a diesel engine are,
1. cylinder
2. Piston
3. nozzles
4. diesel pump
5. set of valves
6. air, oil, fuel filters
7. exhaust opening
8.silencer
etc...

Principle of diesel engine

from: www.rkm.com.au

3 major liquids are required for the correct operation of any diesel engine. Those are;
* good fuel (slag-free, water-free, dust-free diesel)

* good engine oil
*good cooling water (particle-free, calcium compound-free purified water)

Power & Power factor

In simple terms power is the work done in a unit time.
Its SI unit is Watts (W). Basic equation of electric power = P = V*I

Other main expression related to voltage/ current/ power is 'rms'.This means 'root-mean-square'. This is derived by first taking square - then getting the average - and finally taking the square root for a formula representing certain electrical quantity.

Other expressions similar to rms are, average and peak values of an electrical quantity. The rms, peak & average values of a Sine wave is illustarted below.

V      = Vpk*Sin(θ)
Vrms= Vpk/(root 2)


Electric power is viewed as 3 different quantities, but related to each other.

1. Apparent power (S)
2. Active/Real power (P)
3. Reactive power (Q)

where,
S= V*I ;
P= V*I*Cos(φ)    ; φ - phase angle difference between the current and voltage waveforms
Q=V*I*Sin(φ)

It is notable that:
S^2= P^2 + Q^2   ; ^2 - square of that quantity

Technically speaking, Cos(φ) is called the 'power factor' (p.f.) of that particular electrical system.

In a nutshell, active power relates to the actual work done, while reactive power is a loss to the system - BUT it is inevitable.

As we know the power factor is Cos(φ), it's further differentiated into leading power factor and lagging power factor. Leading occurs when the current waveform leads the volatge waveform (in time/angular axis); and lagging p.f. occurs at the opposite occasion.

Here Inductors and Capacitors come into play. Whatever conductor coiled spatially provides an inductance, and whatever two electrical terminals - having an insulator (air, paper etc) in between, provides capacitance.

Both Inductors and capacitors are reactive loads. But pure capacitors offer leading power factor while pure inductors offer lagging power factor to the supply side of the system.

Normally, a power system is inductive. This is because, when it is loaded - winding in transformers, windings in induction motors make the most of the power system. Hence the inductance present in these loads would offer leading power factor to the system.
(Note: Induction motors make up very significant portion of an industrialised power system, which are used in elevators, factories, industrial pumps etc.)

But, it should be noted that - long extra high voltage transmission lines are found to be capacitive; means those offer leading power factor. Similarly it is claimed that unloaded transmission lines have a tendency to show leading power factor.

Image obtained from: www.nationalgrid.com

Technically speaking, how does a transmission line gets capacitance?
1. capacitance between every phase and earth
2. capacitance between each of the phases
3. capacitance between each phases and trees/objects etc.

Overhead(O/H) Power Distribution

Power Distribution is the last part of the delivery of electrical energy to consumers. It is normally done in low voltage, for example 400V/230V denoting 3phase/1phase. In some cases this can be, medium voltages like 11kV/ 33kV.

This can be done by conductors, either drawn OverHead (O/H conductors) or buried underground (Insulated cables). Each have their own advantages and drawbacks. for e.g.: In densely populated areas - undergroung cables are preferred. At the same time, cables are costly than O/H conductors. In addition, faultfinding is a cumbersome process in underground cables. On the other hand, O/H cables are mostly vulnerable to lightning surges, bad weather conditions etc.


Here we will mostly consider about O/H power distribution and its main components:

It mainly consists of Line supports (poles), O/H conductors, Insulators, Earth wire, small distribution transformers etc.


Insulators:
Used to protect the phase conductors reaching the grounded terminals, and as supporting elements. those are classified as:

1. Strain

Image from: www.made-in-china.com

2. Pin

Image from: www.made-in-china.com

3. Post

Image from: www.made-in-china.com

4. Suspension

Image from: www.made-in-china.com

5. Shackle

Image from: www.made-in-china.com

Poles:
1. Steel poles

2. Wooden poles

3. Reinforced concrete (RC) poles

O/H Conductors:


1. All aluminium conductors (AAC)

2. All aluminum alloy conductors (AAAC)

3. Aluminum conductor, Steel reinforced (ACSR)

4. Arial bundled cables (ABC)

Grid Sub-Stations (GSS)

This is an arrangement of electrical conductors, towers, protective equipment, transformers etc for the operation and maintenance of a power transmission network.

Flow of equipments in a Typical Grid SubStation in USA
Image from: www.osha.gov

Most common equipments found in a GSS with few abbreviations are:

1. Power transformers
2. Circuit Breakers (CB)
3. Isolators
4. Current transformers (CT)
5. Potential transformers (PT)/ Capacitive voltage transformers (CVT)
6. Bus bars (BB)
7. Surge arrestors (SA)
8. Line trap (for PLC communication)
9. Earthing transformers
10. Auto-Recloser
11. Overhead earth wire
12. Underground earthing system

Power transformers:

When classified depending on the voltage levels in both sides of a transformer:
1. step-up type (used in voltage increase from alternator-> transmission line)
2. step-down type (used in voltage decrease from transmission line->distribution)

Hermetically sealed Mineral oil-filled transformer 
Image from: www.toppowertransformer.com

Classified depending on the insulation medium:
1. mineral oil-filled type
2. dry-type

Circuit Breakers :
These are normally classified according to the arc-quenching medium around the contacts:

SF6-filled High Voltage Gas Circuit Breaker
Image from: www.made-in-china.com

1. Air circuit breakers; Air Blast circuit breakers (ACB)
2. Vacuum circuit breakers (VCB)
3. Oil circuit breakers (OCB)
4. Gas circuit breakers (eg: SF6 breakers)

Isolators:
These are mechanical devices used to open an electrical path. This is particularly vital as a visual indication of isolating high voltage components, which is not provided by a CB.

Air-break Isolator 
Image from: www.yashexports.tradeindia.com

These can be operated only in Off-Load condition.

Current transformers:
A type of transformer used to reduce the magnitude of the flowing current in a conductor, so that current can be handled safely for measurement & instrumentation.

Primary side is the current measured and secondary side will have the reduced current. Reduction in magnitude will be determined by the turns ratio (e.g.: 400/5, 1000/5, 2000/5 etc. ).

110 kV High-voltage Current tranformer in a grid substation



It should be noted that the secondary side of a CT is NEVER open-circuited. This is to avoid the dangerous high voltage present in the secondary side of the CT.
for e.g. :
If a 400A/5A CT is used in a transmission line rated at 132kV & 400A, secondary voltage will become 132*400/5 kV (= 10,560 kV) – if it is left open circuited.

Potential transformers:
A type of transformer used to reduce the magnitude of the voltage in a conductor, so that voltage can be handled safely for measurement & instrumentation.

Like CTs, reduction in voltage will be determined by the turns ratio of a PT.

Busbars:
These are normally made of hollow Copper/Aluminium rods. The reason is to account for the high current flow so that normal cables would be unable to withstand the electrical stress produced.

Busbars as seen in a GSS

Surge arrestors/ Lightning arrestor:
These are devices made for the protection of a power system arising from dangerous surges. These surges (high voltage impulses of shorter duration) are either from lightning or load switching.

Surge/lightning arrestor

Image from: www.electrical-res.com

These SA’s work allow the normal power frequency waves (50 / 60 Hz) but yield a grounding to surges (have very high frequency in the order of 10000 Hz).

Gas Insulated Sub-Stations (GIS):
A modern development is to make grid substation indoors. introduction of SF6 gas as insulation medium with very desirable charctersitics.

A gas-insulated indoor substation by ABB Inc

These type of substations occupy little space compared to the conventional outdoor stations, making them useful in densely populated cities or harshly-polluted areas.

For a deatiled study visit wikipedia.