Methods for reducing Harmonics in System

There are so many methods available by which harmonics can be reduced in system and power quality can be improved. There are following methods of reducing the harmonics in system:-

(i)                  DC choke

(ii)                By reducing harmonics at loads

(iii)               By using filters

Now let’s discuss about them:-

(i)                 DC choke:-

VFD’s are main source of harmonics generation in a plant. DC choke is used in VFD’s to reduce harmonics. DC choking is an inductance in series with semiconductor bridge circuit. DC choke provides reduction in 5th and 7th Harmonics. DC choke is comparable to an equivalent AC-side line reactor, although the %Total Harmonic Distortion (THD) is somewhat less.

(ii)               By reducing Harmonics levels at Loads and Source:-

In electrical Transmission and distribution system there are lot of equipment’s connected in distribution so there will be harmonics getting in system . So it will become even more important to minimize the harmonics.

If there are harmonics get generated in plant i.e. at source end then it is always best to counter these harmonics at source end. As stated earlier in a plant source of harmonics are VFD’s so harmonics can be reduced installing a Delta-Star Transformer in parallel with Delta- Delta Transformer. This is done to convert two synchronized 6-Pulse VFD’s to a 12-Pulse application. This is done as to reduce 5th and 7th harmonics as there will phase shift of 30 degree in 12 pulse applications instead to 60 degree in 6-pulse application. Delta connected Transformer block the zero sequence harmonics so triple Harmonics are eliminated.

(iii)             By using filters:-

Filters are used to minimize the harmonics levels in distribution systems.  These are used in Plants also. There are following types of filters.

(a)    Passive

(b)   Active

Now let’s discuss about these 2 types of filters

(a)   Passive filters:-

 These types of filters are generally inductors and capacitors. These filters are used to block Harmonics. These are even used to shunt the Harmonics to ground. For a particular type particular application these filters are designed accordingly.

We know that as frequency increase impedance of an inductor increases on the other hand impedance of capacitor deceases with increased frequency.

There are some disadvantages associated with these filters are that these become ineffective if harmonics changes due to variation in loads.

In AC drives usually line reactors and Transformers are used to reduce the harmonics. If DC bus choke is not used then inductor of suitable capacity is used to reduce harmonics level to significant levels.

Thus passive filter may contains series/ Shunt capacitor/ Inductor network and a series inductor or Transformer. Usually now days in electrical system passive filters are integral part of drive system.

(b)   Active filters:-

Unlike passive filters in which harmnocis are blocked or shunted away , active filters are used to condition the power lines and also known as power line conditioners. In active filters these are condition monitors which sense the harmonic currents electronically and generate the counter waveform corresponding to harmonic current generated. So by this way harmonics currents are cancelled out.

Active filters are used to reduce the harmonics levels as per limited specified by IEEE. So by using these filters power factor of system get improved.

Working of Active filters:-

Active filters are installed in parallel to the loads so as to offset the generated harmonics. In electrical systems Current Transducers are used to provide the control logic according to current waveform to counter harmonics. In Three phase system 2 Current transducers are sufficient for these filters working.

Active filters remove the fundamental frequency from this waveform. The resulting waveform after removal of fundamental frequency is inverted and used to direct the firing of the IGBTs. This inverted waveform is injected back into the ac line.

This will leads to cancellation of cancellation of current Harmonics in electrical systems.

As we know that voltage harmonics are due to current harmonics so since we have redcued current harmonics voltage harmonics are also get reduced.

There are following components of Active filters:-

(a)    Power semiconductors

(b)   DC link capacitors and Bus Bars

(c)    Fuses.

Power semiconductor consists of IGBT pulse width modulation (PWM) at an appropriate switching speed. An internal filter in active filters blocks this frequency from entering the ac lines and decouples the active harmonic filter from the rest of the system so no harmful interaction occurs.

In active filters Rated output current= square root of the sum of the squares of the harmonic and reactive currents at the bus.

If total harmonics > Rating of an active harmonic filter, then additional units can be installed in parallel.

For knowing what are Harmonics and effects on Electrical Systems:-
http://electrialstandards.blogspot.com/2016/09/methods-for-reducing-harmonics-in-system.html

Harmonics and its effects on electrical systems

Harmonics in electrical systems:-

Before discussing about consequences of Harmonics you must know what are harmonics:-

Harmonics are electric current and voltages that causes power quality problems. These are generated due to electronic equipment’s which draws non linear loads with abrupt pulses. These pulses causes distorted current and voltages causes of which will be discussed in this article.

Harmonics are classified according of level of harmonics these are multiple of frequencies such as

2^nd level, 3^rd level, 4^th level, and upto 11^th levels

Where 2^nd level means Harmonics with frequency of 100 HZ

3^rd level means Harmonics with frequency level of 150 HZ

And similar for other levels

Now 2^nd level, 5^th level, 8^th level, and 11^th level will have –ve sequence of currents

3^rd level, 6^th level and 9^th level will have zero sequence of currents

4^th level, 7^th level, 10^th levels will have +ve sequence of currents

Harmonics are now integral part of electricity. Harmonics is also known as pollution in electricity. Now days while power availability is not a big concern so intention now shifts towards quality of electiricity.

There is an increase in harmonics levels in electricity there are following few reasons for harmonics in electricity:-

1.       Increase in non-linear loads such as power electronics equipment’s. Such as VFD, SMPS etc where power is chopped and used as per requirements at low or other frequencies.

2.       There are various other devices which leads to Harmonics in electrical systems such as :-

(a)    Arcing devices

(b)   Ferromagnetic devices

(c)    Appliances

(d)   Electronic switching power converters

There will be following effects due to harmonics in electrical systems:-

(a)    Overheating of electrical equipment’s as more eddy currents are produced due to these harmonic levels.

(b)   Due to overheating and other losses this will leads to reduction in life of equipment

(c)    These harmonics will leads to premature failure of equipment’s

(d)   Harmonics will leads to malfunctioning of equipment’s

(e)   Harmonics will leads to more losses in systems

(f)     Harmonics will leads to interference in communication signals

(g)    Harmonics will leads to Nuisance tripping of circuit breaker and blowing off the fuses.

(h)   Harmonics will leads to more vibrations in motor and other equipment’s.

(i)      This will leads to flickering of computers.

There are so many effects of Harmonics on various equipments

When there are harmonics in systems than we can’t be able to get 1 Power factor as there are so many reactive powers associated with these harmonics which will not get compensated using capacitor banks. Even when we try to compensate the same by installation of more and more capacitors then we lose a hafety amount on installation of capacitors and also this will leads to higher losses due to capacitor banks.

Earlier we known that Power factor=  Voltage X Current X Power factor

                                                                         Voltage X Current

Now days due to these Harmonics there are Distortion power factors which will be calculated as below:-

Where THDi Stands for Total harmonics distortion in currents

And THDv Stands for Total Harmonics distortion in voltage.

Now Total Power factor is given as = Power factor X Distortion power factor

As Distortion power factor will never will be unity so Total power factor of system will never will be unity.

                               

There are following effects of harmonics on Transformers and Motors:-

1.       There will be more eddy current loses in Transformers.

2.       Harmonics will leads to more losses due to skin effect.

3.       Harmonics will leads to overheating of coils and that will cause premature failure of Transformers

4.       Harmonics will leads to more Hysteresis and eddy current losses in motors as Hysteresis losses are directly proportional to frequency and eddy current losses are directly proportional to square of frequency.

5.       There will be more winding eddy current losses, High frequency Rotor and stator losses, and tooth pulsations in motors due to harmonics

6.       More harmonics will be leads to more I²R losses in both Transformers and motors

7.       There will be increased over temperature due to these losses and leads to premature failure

There are following effects of harmonics on Cables and Capacitors:-

1.       There will be Higher Proximity effects and Skin effects in cables

2.       There will be additional losses in cables

3.       These will cause additional overheating of cables

4.       This will decrease insulation level of cables

5.       Harmonics will cause increased Resistance

6.       Harmonics will cause derating of cables

7.       Harmonics will leads to higher neutral currents in cables and leads to unbalanced loads on cables.

8.       Harmonics will leads to resonance with 7^th Harmonics in capacitor banks which will leads to lower capacitive reactance.

9.       Harmonics will leads to more electricity bill due to increased losses due to harmonics.

10. Harmonics will leads to more KVA demand as stated earlier due to increased losses.

For reducing harmonics visit link:-
http://electrialstandards.blogspot.com/2016/09/methods-for-reducing-harmonics-in-system.html

                                                           

Electricity from Pototes and Other Fruits and vegetables

Electricity works on an idea that whenever two dissimilar metals bring in contact with an electrolytic solution then there will electron movement in these metals when these metals are connected using a conductor.

All plants consist of fluid which is electrolyte for metals. So using above principle of inserting metals in a fluid will leads to electricity generation.

We have done during childhood for generating electricity from potatoes.

It works on principle that Potatoes contains a liquid in which many water soluble chemicals are present. So these may cause a chemical reaction with one or both of our electrodes. So we will get some electricity from that.

Best result will be obtained while using zinc and copper electrodes.

Let’s discuss about the same experiment:-

For conducting experiment let’s take 8 potatoes. We will take 8 potatoes to get enough electricity to light up a LED.

Now insert different metals of any shapes into these potatoes.

Now by using small wires all potatoes are connected in series. Series connection can be done soldering wires at metal ends or by making after series connections one wire is connected to LED which is taken from last potato and other wire is taken from 1^st potato.

Now this last wire which is connected at one end acts as phase and other wire which is taken from 1^st potato acts as neutral.

You will see that LED starts glowing.

Electricity generated during this process in DC in nature.

We can generate higher amount of electricity using lemons as Lemons are having acidic nature which will leads to metals to react quickly so they will generate more electricity.

So for conducting same experiment for generating electricity half the quantity of potatoes.

When you measure voltage generated by one potato using digital multi-meter you will find that it will shows 1.2 volts so in order to get enough value for glowing a LED at-least 8 potatoes must be used.

Now if you keep connecting LED for long time then you will find that when you cut the potato it will be found black.

Similar experiment can also be applicable for fruits such as oranges and apples.

Electricity from Boiled Potatoes

Recent experiments has also shown that if boiled potatoes are used in experiments than they will produce electricity ten times than raw potatoes.

As boiled potatoes break the resistance which will leads to more smooth passage for electrons.

If you cut the potatoes into 4-5 pieces then efficiency will be increased considerably.

Even you can light up a room using boiled potatoes for over one month.

For knowing about discovery of electricity visit link"-
http://electrialstandards.blogspot.in/2016/07/discovery-of-electricity-how.html 

Synchronous Generators Prime movers; Steam turbine; Hydraulic turbine & diesel engines

Synchronous machines construction depends upon type of prime mover used in machines. There are following types of prime movers used in power generation:-

1.       Steam turbines

2.       Hydraulic Turbines

3.       Diesel Engines

Let’s discuss them :-

1.     Steam turbine:-

These types of synchronous machines have high speed.  Generators driven by steam turbine are also called Turbogenerators. Maximum speed of Turbogenerators is 3000 RPM as per formula

Frequency= PN/ 120;

Where P is no. of poles

N is no. of revolutions

In 2 Pole machine at 50 HZ frequency Speed of Turbogenerator comes out to be= 50X120/ 2= 3000 RPM

With Such high speed lower value of armature diameter is to be designed. Lower diameter is selected to limit the centrifugal forces which have very much influence on the generator design.

Peripheral Speed of a Machine is given by formula as below:-

Peripheral Speed(V)= πDn  m/Sec

Where D is diameter of rotor in meters

n=speed of rotor in revolutions per second

As we have seen from above formula that Peripheral Speed increases with increase in diameter as both are directly proportion to each other, With increase in diameter centrifugal forces increases. So diameter should be kept low.

Due to this high speed diameter of 2 pole machine is limited to 1.2 meter which will give peripheral speed of about 175 m/s.

Also for turbogenerators cylindrical rotor construction is to be used instead of salient pole as in Salient pole construction is impractical in turbogenerators due to high mechanical forces.

In these alternators efficiency is very high. Turbo-generators are available upto rating of 1000 MW.

2.     Hydraulic Turbines:-

Power plants using hydraulic turbines are Hydro Power plants these power plants uses Synchronous generators driven by Hydraulic turbines. Hydraulic turbines are driven by water heads. There are 3 types of hydraulic turbines due to different levels of water head availability.

Types of Hydraulic turbine used according to head height are as below:-

(i)                  For Water heads of 400 Meter and above- Pelton Wheel design to be used

(ii)                For water heads upto 380 meter – Francis turbine design to be used

(iii)               For water heads upto 50 meter- Kaplan turbine design to be used

As water heads not so much high so speed of turbine is low varies from 50 to 500 rpm. So get such low rpm salient pole alternators are used for the same. No. of poles in these generators are 12 onwards.

Also further Pelton type turbine design can be Horizontal or vertical shaft type. Usually Horizontal Shaft design is more commonly used.  For high power low speed synchronous generators are usually installed for low water heads power plants are generally built with vertical shaft.

There are following fundamental types for vertical shaft Hydro generators:-

(i)                 Suspended Type:-

In this type thrust bearing is at upper bracket above alternator rotor

(ii)               Umbrella Type:-

In this type bearing is mounted on lower bracket on the turbine cover, Mostly umbrella type construction is used to reduce generator weight and height of power plant. As in suspended type where bearing is used on upper bracket it has to be bigger in size as it has to withstand both generator and turbine load, also it has to withstand water reaction. This will leads to higher generator weight and height of power plant.

These type of generators have rating upto 750 MW.

3.       Diesel Engines:-

These are most widely used in small commercial and industrial establishments also known as Diesel generators. They are usually of small rating. These are horizontal type,  Since these are slow speed machines so salient pole construction is used for the same. Torque generated during its operation is non-uniform so it makes the synchronous generator sensitive to torque variations.

Why Power isn't generated at Higher Frequency i.e. greater than 50/ 60 HZ??

There is more often a question arises while you are dealing with electrical systems.
As you know that for alternators (Synchronous generators)
Frequency=  No. of Pole X Speed of Machine

                               120
So increasing the frequency of power generation you have to either increase the no. of poles or increase the speed of alternator.
Now if you increase no. of poles than diameter of machine will increase considerably to accommodate increased no. of poles. Also if machine diameter is increased to accommodate no. of poles then you can't rotate the machine at higher speed as centrifugal forces and vibration forces will be very high. Mechanical strength requirements of machines will also be very high.

Now for increasing speed of alternator you have to provide higher input which will leads to very high maintenance as deterioration happens at such a higher input. So alternator should be mechanically very strong .

Why Armature is place on Stator in Synchronous machines

In case of synchronous machines  Armature is always placed on stator and field winding on armature as there are following advantages:-

1.     Better economy:-

It is always economical to place armature winding on Stator and field winding on rotor in synchronous machines as this can be illustrated by example as below:-

Consider a 3-Phase Star connected 500 MVA, 11KV synchronous machine, now its line current will be = 500X10³

                               11 X

Which Will be = 26244 A

Now If we place armature winding on rotor than we will required 3 slip rings each will be capable of handling such a high current of 26244A. Further Slip rings needed to be insulated from shaft for a voltage= 11/ = 6.35 KV.

Also Star point needed to be brought out by using fourth slip ring which will be grounded through a resistance.

Now consider a case where we will place field winding on rotor and armature winding on  Stator, in that case field winding need to handle lower current and voltages.  Now consider 2 MW at 500V then current required to handle field current of= 2000/0.5= 4000A . So we needed 2 brushes to handle 4000A each instead of 4 brushes carrying such a high current as in earlier case.

Also Slip rings needed to be insulated for voltage of 500V only. Thus we see that how it will be cheaper to have armature winding on stator.

2.     Low insulation levels required:-

As we have seen in both cases if we place armature winding on stator than slip rings needed to be insulated for lower voltages. Thus this type of construction will allow construction even upto 33KV synchronous machines

3.     Lower losses:-

When armature winding is placed on stator than we will require only 2 carbon brushes which means lower losses due to slip rings.

4.     Higher Output power:-

Since field winding of lighter weight is placed on field winding so this will leads to low centrifugal forces. So higher RPM of rotor will be possible and which will result into increased output for given dimensions.

5.     More rigid and easy construction:-

When armature winding is place on stator than it will be capable of handling higher voltages and currents , also it will be easier to place heavy winding on Stator slots. It will easier to install water tubing for cooling on stator than rotor. These all add to rigidness and convenience of synchronous machines.

6.     Higher Armature tooth Strength:-  

Higher current synchronous machines will require more armature copper for each slot.  For accommodating more copper by making slots deeper on armature.  It will be easier to cut deeper slots in when armature is on stator than on rotor. As if armature winding to be placed on rotor than rotor would have narrower and weaker teeth.

Stronger the teeth lower will be the noise and vibration.

7.     Lower rotor weight and lighter bearings:-

Field winding on rotor will leads to lower quantity of copper and insulation on rotor. This means lower weight of rotor and which means low priced bearings which means longer life due to minimum wear and tear.

Fault locating methods for High and low tension cables

In electrical systems its usual that cable may get faulty there are following methods of checking the type of fault in cable:-

1.      Using Meggar for LT Cables:-

Meggar is most commonly used method for checking the fault in cables. If Meggar is used at 500V/ 1000V DC then cable shows infinite value If cable is Ok.  if value lies above 100 Mohm then cable is also acceptable it indicates only that there cable is having some moisture which will be dried out when cable is put on load but if value falls below this value upto 50 Mohm then it will be used where there were light loads.

If there is value is near to zero than cable should not be used as if cable insulation level between two leads is zero this means cable is short circuited. If Cable insulation with respect their leads is ok but between lead and earth is zero than this indicates cable get short circuited with armored.

In Meggar we can check resistance value also in this 2 leads one is placed on resistance and other is placed on common. 500/1000V is applied to two terminals of cable and resistance value is measured.

2.      Using Hi-Pot For testing High tension cables

Hi-Pot test is used for testing High tension cables in this test DC voltage is applied to cable for 5-15 min’s. According to IEEE-400 hi-pot voltage for a 15-kV class cable is 56 kV for an acceptance test and 46 kV for a maintenance test.

There are fault locators which are used to identify the location of fault in a cable as Most of HT cables are laid underground so it becomes very much necessary to find out the location of fault as in this case cable replacement will not be easy and also it will leads to lot of cost of replacement of whole cable.

1.           Divide rule:-

This method is very old one but still used in LT system, in this method cable is cut

down into 2 pieces and check for insulation resistance one piece of cable will give perfect value other will give low values so higher valued cable will be used and other faulty part will be again tested used divide rule.

    2.  High voltage test:-

This is another old method, in this method high voltage is applied to faulty cable this will leads to loud noise which you can hear above ground. In method very high voltages upto the level of 25-32 KV needed to be applied to cable so that noise can be hear able.   This will also leads to heating of cable as high currents generated during that process. Thus it will cause degradation of cables.

     3. Time domain reflectometry 

This is new technology for detection of faults in under ground HT cables. In Time domain reflectometry  there is a device which sends a low-energy signal through the cable if cable is perfect than cable returns that signal in a known time and in a known profile. 

There following limitations of TDR:-

(i)         It does not Provide accurate location of  pinpoint faults. This method is accurate upto 1% of testing range.

(ii)        It cannot see faults-to-ground with resistances much greater than 200 ohms. So, in the case of a "draining fault" rather than a short or near-short, TDR is blind.

     4. High-voltage radar methods:-

There are following types of this method:-

(i)         Arc reflection

(ii)        Surge pulse reflection

(iii)       Voltage pulse reflection.

Lets discuss them one by one :-

(i)      ARC Reflection Method:-

This method consists of  TDR with a filter and thumper (High voltage).  In this method filter is used to limit current and voltage due to surge on cable which means minimal stress to the cable. This method provides an approximate distance to the fault

(ii)     Surge pulse reflection method

This method uses a current coupler and a storage oscilloscope with a thumper. The advantage of this method is its superior ability to ionize difficult and distant faults. Its disadvantages are that its high output surge can damage the cable, and interpreting the trace requires more skill than with the other methods.

(iii)    Voltage pulse reflection method

This method uses a voltage coupler and an analyzer with a dielectric test set or proof tester. This method provides a way to find faults that occur at voltages above the maximum thumper voltage of 25kV.

Effects of Higher and lower voltage on Induction motors

Motor name plate consists of voltage range for operating motors. Operating the motors both lower and higher than motor name rating will leads to reduced efficiency and may also leads to premature failure of motors.

Effects of low voltage on Induction motors 

When voltage applied to motor get lower than name plate lower range of voltage than for fixed amount of power at lower voltage current drawn by motor get increased. Higher current will leads to exceeding motor name plate current rating that will leads to heating up the motor and even burning out the motor if over-current persists.

Power in motor is = Voltage X Current X Power factor

So lower the voltage higher will be current drawn by motor as power factor is same for motor.

Effects of high voltage on Induction motors:-

 There is often assumption made from Power relation that Since low voltage increases the current drawn by motors, then high voltage must reduce the current draw this lower current will leads to lower heating effect of motor. But results are somewhat different High voltage on a motor tends to push the magnetic portion of the motor into saturation. Usually this happens when motor voltage exceeds certain limit of voltage level of motor. Thus Higher voltage will cause the motor to magnetize the iron beyond the point where magnetizing is practical by drawing higher current.

This higher current drawn by motor will leads to overheating of motor and thus will leads to shortening life of motor.

Usually  motors rated at 220 and 440V with voltage tolerance band of 5-10% . If voltage increases on decreased than this band level then there will be drastic effects on performance and efficiency of motors. Also this will not means that you will continue to operate motors at extreme levels of this voltage level otherwise this will shorten life of motors.

Low voltage can lead to following:-

a.      Overheating

b.      Shortened life

c.       Reduced starting ability and

d.      Reduced pull-up and pullout torque.

The starting torque, pull-up torque, and pullout torque of induction motors all change, based on the applied voltage squared.

A 10% reduction from nameplate voltage would reduce the starting torque, pull-up torque, and pullout torque by a factor of .92.9.

The resulting values would be 81% of the full voltage values.

At 80% voltage, the result would be .82.8, or a value of 64% of the full voltage value.

Let us assume that the load torque remains constant. With reduction in voltage the torque produced, T will reduce ( T is proportional to V^2). So the torque slip (T-s) characteristic at reduced voltage will have all its amplitudes reduced. So in order to to get the same torque required by the load the slip s will increase.

There are following facts about Higher and lower voltages effects on motors:-

1.       Effects on Single and Three Phase motors

a.      It has been found that Single phase motors are more sensitive to overvoltage than three phase.

b.      U-frame motors are less sensitive to overvoltage than are T-frames.

c.       Premium efficiency motors known as Super-E are less sensitive to overvoltage than are standard efficiency motors.

2.      Effects of No. of poles

6 and 8 Pole motors are more sensitive to higher voltages than Two- and 4-pole motors.

3.      Effect of over-voltage at Light loaded Motors

Over-voltages can cause higher current and temperature of motors even at light loaded motors. It means that even at lighter loads motor life get reduced.

4.      Effect  on efficiency

Motor Efficiency drops with both high or low voltages.

5.      Effects of voltages on Power factor

Motor Power factor improves with lower voltage and drops sharply with higher voltage.

It has been found that current will increase in same proportion to voltage decrease. This means that 5% decrease in voltage will cause 5% increase in current. Motor will not get damaged until motor current exceeded the rated current of motor. If this current exceeded the rated current of the motor than this will damage the motor.

On lightly loaded motors with easy-to-start loads, reducing the voltage will not have any appreciable effect, except that it might help reduce the light load losses and improve the efficiency under this condition. This is the principle behind some add-on equipment whose purpose is to improve efficiency.