Thursday, 29 September 2016

Circuit Breaker and Arc Phenomenon


What is Circuit Breaker?

Circuit Breaker is switch capable of making or breaking the circuit under no-load as well as on-load condition. It can make or break circuit either manually or by remote control. A Circuit Breaker in conjunction with Relay can break the circuit under fault condition.

You may also like to read, Basic Principle of Relay Operation

Operating Principle of Circuit Breaker:

A Circuit Breaker CB consists of two contacts which are called electrodes, one of which remain fixed, called fixed contact and another moving contact. Under normal operating condition, this contact will remain closed to supply power but as soon as fault is sensed by the Relay, trip coil of Circuit Breaker energizes and the moving contact of CB is pulled apart by some mechanism to open the CB.



When contacts of CB are separated under fault condition, an arc is stuck between the fixed and moving contacts. The current is thus able to continue till the arc persists. The production of arc not only delays the current interruption but it also produces huge amount of heat which if exceeds a limit may damage the system or CB itself. Therefore, the design of CB is done in such a way to minimize the arcing period so that

1)   Heat produced during arcing may not exceeds the dangerous value.

2)   To have fast fault clearing.

It is worth here to mention that a typical Breaker opening and closing time remain around 30-35 ms and 60-70 ms respectively. Notice that CB opening time is less than the closing time to ensure fast fault clearing.

Arcing Phenomenon in Circuit Breaker:

When a short circuit occurs, heavy current flows through the contacts of circuit breaker before they are opened by the protective system. At the instant when the contacts begin to open after getting trip command from the Relay, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium in between the contacts is sufficient enough to ionize the medium. This ionized medium acts as a conductor and arc is stuck in between the contacts of the circuit breaker. It shall be noted here that the potential difference between the fixed and moving contacts is quite small and just enough to maintain the arc. This arc provides a low resistance path to the current and thus due to arcing the current in the circuit remain uninterrupted as long as arcing persists.



During the arcing period the current flowing through the contacts of circuit breaker deepens upon the arc resistance. The greater the arc resistance the smaller will be the current flowing through the contacts of CB. The arcing resistance depends upon the following factors:

Degree of Ionization:

The more the ionization of medium between the contacts, the less will be the arcing resistance.

Length of Arc:

The arc resistance increases as the length of arc increases i.e. as the separation between the contacts of Breaker increases the arcing resistance also increases.

Cross Section of the Arc:

The arcing resistance increases with decrease in the cross sectional area of the arc.

Principle of Arc Extinction:

As we discussed earlier in this post, ionization of medium in between the contacts and potential difference across the contacts are responsible for the production and maintenance of arc. Thus for arc extinction, we can increase the separation between the contacts to such an extent that potential difference across the contacts is not sufficient enough to maintain the arc. But this philosophy is impractical as in EHV (Like 220 kV, 400 kV, 765 kV etc.) system; the separation between the contacts to extinguish the arc will be many meters which is not practically achievable.


Another way for extinction of arc is to demonize the medium in between the contacts. If the arc path is demonized the arc extinction will definitely be facilitated. This may be achieved by cooling the arc or by quickly removing the ionized particles from the space in between the contacts. This principle of arc extinction is used in all modern Circuit Breakers.

Introduction and Architecture of Numerical Relay


Most of us are aware of Electromagnetic Relays and Static Relays but most us may not be well acquainted with Numerical Relay. If I define a Numerical Relay, honestly speaking it will seem to be quite tough but in reality they are very user friendly and easy to implement different types of protection scheme. However I am going to define a Numerical Relay.

Numerical Relays are device in which measured electrical quantities are sequentially sampled and then converted into numerical data which are mathematically or logically processed to take decision for issuing trip command.



Numerical Relays are basically Digital Relays for which manufacturers have developed specified hardware which can be used in conjunction with suitable Software o meet different protection needs. 



A Digital Relay comprises both Hardware and Software. The Hardware part is briefly described below.

CPU: CPU stand for Central Processing Unit which is responsible for the processing of protection algorithms and digital filtering.

Memory: Memory is of two types. One is RAM (Random Access Memory) and ROM (Read Only Memory). RAM serves for the purpose of retaining the input data to the Relay and processing the data during the compilation of algorithm.

ROM is used to store Software needed for the working of Relay. ROM is also needed for storing Event and Disturbance data. Event and Disturbance Recording is a must feature for a digital relay because these data are used for troubleshooting any event. A typical Numerical Relay can store as much as 520 Events and 50 Disturbances. The most attractive feature of such relay is that it works on FIFO (First In, First Out). Suppose if it happens to be the number of disturbances exceeds 50 then the Relay will delete the last Disturbance and will register new disturbance.

Input Module: The analog single from the Power System is stepped down using Current Transformer and Potential Transformer and then fed to the Numerical Relay using low pass filter. Low pass filter is incorporated in the input module to eliminate any noise single induced in the line due to corona or induction effect of nearby high voltage line. The output from the Filter is then fed to Sample and Hold (S/H) circuit.

A Sample and Hold (S/H) circuit is used to keep the rapidly changing instantaneous value constant during the period of conversion for processing.

In addition to the analog input, Numerical Relays are designed to accept digital input too. Separate terminals are provided for the analog and digital inputs.

Multiplexer and Analog to Digital Converter:

The CPU accepts the input in digital form but the input from Current Transformer CT and Potential Transformer PT are analog in nature. Therefore and A/D converter is used to convert the analog signal to digital signal. In case more than one analog quantity is to be converted into digital form, Multiplexer is used for selecting any analog input at a time to convert into digital form.



Output Module:

Output module provided in Numerical Relay is digital contacts which are actuated when a trip decision is taken by the CPU. These output digital contacts are a pulse which is generated as a response signal. The timing of pulse can be changed by the user.

Digital Input / Communication Module: Numerical Relays are provided with serial and parallel ports for the interconnection with control and communication system of the substation. Digital output contacts of Numerical Relays are used for wiring with the Auxiliary Relays to extend tripping command to the Circuit Breaker.

Software: Numerical Relays are equipped with software to communicate with external device to program to Relay or one can program by navigating through the Relay Menu.

Hardware for Metering: In principle, the hardware setup discussed above can be used for both measurement and protection function. However, considering the order of difference between current magnitudes in case of fault and load, there can be loss of accuracy during metering applications. Consider a hypothetical case where in maximum load current is 100 A and maximum fault current is 20 times this load current i.e. 2000 A. Let a 12 bit unipolar A/D converter be used for sampling current signal. This implies that resolution of A/D converter is 2000/(212-1)=0.488 A. This resolution may be inadequate for metering purposes.

One solution is to increase resolution i.e. the number of bits in A/D converter. For example, one may use 16 bit A/D converter in place of 12 bit A/D converter.

However, increasing the number of bits of A/D converter also affects the selection of processor. A good design guideline is to choose a processor with double the number of bits of A/D converter. This ensures that truncation and numerical precision problems associated with finite precision arithmetic do not cause significant loss of accuracy. For example, with 16 bit A/D converter, 32 bit processor is the natural choice. Alternatively, a variable gain amplifier can be used along with the A/D converter. At low currents, high gain setting is used and at high currents low gain setting is preferred. However, during the change from one setting to another, loss of information can take place. Therefore, a simple solution would be to keep metering and protection functionality separate.


In the next post we will be discussing about some interesting features of Numerical Relays. So be there and follow ETRICAL.

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Wednesday, 28 September 2016

Basic Principle of Relay Operation


Relay is a switch which senses fault in a system and once fault is sensed by the Relay, it issues trip command to the Circuit Breaker, CB to isolate the faulty section of the network from the healthy section.

The Relay detects the abnormal condition by continuously monitoring electrical quantities which are different for healthy and faulty condition. The electrical quantities which may change during fault condition are voltage, current, frequency and phase angle. If one or more of the above electrical quantities change, that signals the presence, type and location of the fault to the Relay. After detecting the fault condition, Relay pick-up, its contact will change from NO to NC or vice versa. So we can wire up a particular kind of Relay contact to Breaker tripping circuit. So whenever, the Relay picks up, the tripping of Breaker will take place.


A simplified Relay circuit is shown in figure below. Figure below shows one of the three phase system for simplicity.



As shown in the figure above, Current Transformer CT secondary winding is directly connected to the Relay coil. Under normal condition, the current through the Relay coil is not sufficient enough to pull the plunger and close the circuit of Breaker Tripping Coil. Notice here that Breaker Tripping coil is solely responsible for the tripping of Circuit Breaker. If trip coil of breaker fails, then tripping of Breaker will not take place. This is the reason, two trip coils are normally provided in Circuit Breaker to get reliable operation of Breaker. Not only two Trip Coils are provided in CB rather a Trip Coil monitoring Relay is also used. If case of fault i.e. if it happens to be any open circuit in Trip Coil, then the Trip Coil Supervision Relay will be flagged to attract the attention of the operator.

In case of fault, the current through the CT secondary will go up which will cause increased current through the Relay coil. If it happens that the current through the Relay coil exceeds the setting value or pick-up value then the coil will get produce sufficient magnetic pull to the plunger and thus plunger will complete the CB trip circuit. As soon as the CB trip circuit is complete, current will start flowing in the Trip Coil which in turn will pull a lever to trip the Circuit Breaker CB.

In the above figure, it is shown that Relay coil is directly pulling the plunger to complete the Breaker Trip Coil circuit but in actual practice, Relay coil when picked up will change its contact status. Let us say Relay Normally Open (NO) contact is wired to the Breaker Trip Coil Circuit. Therefore when the Relay coil is in de-energized state, the circuit of Trip Coil of CB is not complete and hence no tripping of the CB. During fault condition as the current through the Relay coil exceeds the pick-up value, the Relay coil will get actuated which in turn will force its contact to change over i.e. NO contact will change to Normally Close (NC) thereby closing the Trip Coil circuit of the Breaker.




Since Trip Coil circuit of Breaker is complete, current will flow through the Trip Coil causing CB to trip.

Single Line to Ground Fault Analysis


In earlier posts we discussed, Concept of Symmetrical Components and Calculation of Symmetrical Components. It is recommended to go through these topics before analyzing Single Line to Ground Fault.


Let us consider a three phase system with earthed neutral as shown below.



Let us assume that a ground fault takes place in A phase (In many industries and numerical relays, normally the phases are said as A, B and C instead of R, Y and B, though they represent the same thing i.e. A phase means R phase, B means Y phase and C means B phase). Ea, Eb and Ec are the Generator terminal voltage per phase. Bold letters here represent vector form. 



Because of ground fault in A phase, the voltage at the point of fault will become zero and current through the other phases i.e. B and C phases will become zero. Therefore we can write as

Va = 0

Ib = 0

Ic = 0

Therefore, from the Calculation of Symmetrical Components, we can write as

Ia0 = (Ia + Ib + Ic)/3

      = Ia/3

Ia1 = (Ia + λIb + λ2Ic)/3

      = Ia/3

and,

Ia2 = (Ia + λ2Ib + λIc)/3

      = Ia/3

From the above expressions of positive, negative and zero sequence components of current in faulted phase A, we observe that all the sequence currents are equal in magnitude and phase. Thus for faulted phase, in case of Single Line to Ground Fault, we can write

Ia0 = Ia1 = Ia2 = Ia/3

Calculation of Fault Current:

First thing which must be understood at this point, that fault current is completing its path through the grounded neutral. If there were no any grounded neutral, no fault current would have been flow.

We will apply Kirchhof’s voltage law here to find the fault current. As fault current is only flowing in the faulted phase A, therefore we are only interested in finding Ia.

Ea = Va + Ia0Z0 + Ia1Z1 + Ia2Z2

     = 0 + Ia0Z0 + Ia1Z1 + Ia2Z2

Ea = Ia0Z0 + Ia1Z1 + Ia2Z2

But Ia0 = Ia1 = Ia2 = Ia/3 so,

Ea = Ia/3 Z0 + Ia/3 Z1 + Ia/3 Z2

Ia = 3Ea / (Z0 + Z1 + Z2)

From the above expression of fault current, it is quite clear that positive, negative and zero sequence impedance are connected in series for Single Line to Ground Fault and the equivalent circuit may be represented as shown below.



The above expression for fault current has been derived assuming that the neutral of the system is solidly grounded. For a system where neutral is grounded through some finite resistance, say Z, then the fault current would be given as

Ia = 3Ea / (Z0 + Z1 + Z2 + Z)

It shall also be noted that, for ungrounded system or isolated neutral system as there is no path for neutral current to flow, therefore the impedance seen by zero sequence current will be infinite (as only zero sequence current flows through the neutral) and hence the value of zero sequence component of fault current will be zero.

You may like to read Concept of Neutral Grounding.

Calculation of Voltage of Healthy and Faulty Phases:

Since the generated voltage of Generator is positive sequence voltage, therefore

Ea0 = 0

Ea1 = Ea

Ea2 = 0

Now, the positive sequence voltage at the point of fault

Va1 = Ea Ia1Z1

      = Ea - Ea Z1/ (Z0 + Z1 + Z2)

Va1 = (Z2 + Z0)Ea / (Z0 + Z1 + Z2)

Similarly,

Va2 = 0 – Ia2Z2

        = - Ea Z2/ (Z0 + Z1 + Z2)

and

Va0  = 0 - Ia0Z1

       = Ea Z0/ (Z0 + Z1 + Z2)

Thus we have calculated the sequence components of voltages of faulty phase. For calculating the voltages of healthy phase we will apply the concept of symmetrical components as shown below.

Vb = Vb0 + λ2Vb1 + λVb1

Vc = Vc0 + λVc1 + λ2Vc1


In this way, we can calculate the level of earth fault current in Single Line to Ground fault and voltages of different phases.

Tuesday, 27 September 2016

Concept of Series and Shunt Faults


Electrical Faults can be classified into two categories: Shunt Faults and Series Faults. Shunt faults include power conductor or conductors to ground or short circuit between the conductors.

Series type of fault is basically unbalance in system. Suppose we have used Fuse / Breaker to protect the circuit. If one or two phases open while the third phase remain in circuit, such kind of fault is called Series Fault. Notice that Series Fault may also occur in case of one or two Broken Conductor. Here broken conductor is like breaking of jumper on the tower of transmission line which is not touching the grounded tower body.


Shunt faults are characterized by increase in current and decrease in voltage and frequency whereas Series faults are characterized by increase in voltage and frequency and decrease in current in the faulted phase.
Shunt faults are classified as:
1)    Line-to-Ground Fault
2)    Line-to-Line Fault
3)    Double Line-to-Ground Fault
4)    Three phase fault
Of the above faults, first three faults are unsymmetrical fault as the symmetry is disturbed in one / two of the phases. The method of Symmetrical Components shall be applied for the analysis of such unbalance and fault.


Three phase fault is balanced fault which can also be analyzed using concept of symmetrical components.
Series faults are classified as:
1)    One Open Conductor
2)    Two Open Conductors
These faults also disturb the symmetry and therefore these faults are unbalanced faults and hence shall be analyzed using concept of symmetrical components.
Neutral Voltage during Fault:
The potential of neutral when it is grounded through some impedance or is isolated will not be at ground potential under unbalance condition as in unsymmetrical fault rather it will have some finite value with respect to ground.
The potential of neutral is given as Vn = -InZn where Zn is neutral grounding impedance and In is neutral current. Notice the negative sign before the expression of neutral voltage Vn, it indicates the flow of current from ground to the neutral point and therefore the potential of neutral point will be less than the ground potential.
For a three phase system we know that,
Ia + Ib+ Ic = 3Ia0
You may also like to read Calculation of Symmetrical Components
Therefore,
Vn = -3Ia0Zn
Notice that only zero sequence current flows through the neutral and therefore voltage drop across neutral will be only due to zero sequence currents.

Calculation of Symmetrical Components


We are now aware of the concept of Sequence components of current / voltage. If you have miss this concept, please read Concept of Symmetrical Components.

Now we are at a stage to calculate the zero, positive and negative sequence components of current / voltage. As already discussed any three phase unbalanced voltage / current can be resolved into three set of balanced vectors. Thus we will use this concept to calculate the positive, negative and zero sequence components of voltages. Mind that the same philosophy is applicable for current also.



Before going into the calculation part, let us introduce ourselves with an operator λ. λ is an operator which when multiplied to any vector quantity, rotates the vector by an angle of 120° in anticlock wise direction without changing the magnitude of the vector. This means that λ must have a magnitude unity. From this definition we can write λ as below.

λ = ei2π/3

   = Cos(2π/3) + jSin(2π/3)

   = -0.5 + j0.866

Why not to explore more properties of λ? Sure, we must…

λ2 = ei4π/3

   = Cos(4π/3) + jSin(4π/3)

   = Cos(2π - 2π/3) + jSin(2π - 2π/3)

   = Cos(2π/3) - jSin(2π/3)

   = -0.5 - j0.866

and

λ3 = ei6π/3 = ei2π

    = Cos(2π) + jSin(2π)

    = 1

λ3 – 1 = 0

(λ + 1)(1 + λ2 + λ) = 0

As (λ + 1) cannot be zero, therefore

1 + λ2 + λ = 0

Thus to summarize the properties of operator λ,

λ3 = 1

λ4 = λ3. λ = λ

1 + λ2 + λ = 0

Consider the figure below where a three phase unbalanced voltages Va, Vb and Vc are resolved into three set of balanced voltages.




Va = Va1 + Va2 + Va0  …………………(1)

Vb = Vb1 + Vb2 + Vb0 ………………….(2)

Vc = Vc1 + Vc2 + Vc0 …………………..(3)

But taking Va1 reference and applying the concept of operator λ,

Vb1 = λ2Va1

Vc1 = λVa1

Similarly for Negative Sequence we can write as

Vb2 = λVa2

Vc2 = λ2Va2

Fortunately for Zero Sequence,

Va0 = Vb0 = Vc0

Thus from equation (2) and (3),

Vb = λ2Va1 + λVa2 + Vb0  ………………(4)

Vc = λVa1 + λ2Va2 + Vc0  ……………….(5)

Now, multiplying equation (4) by λ and (5) by λ2 and adding them to equation (1), we get

Va + λVb + λ2Vc

= Va1(1+ λ3+ λ3) + Va2(1+ λ2+ λ4) + Va0(1+ λ + λ2)

= 3Va1 + Va2(1+ λ + λ2)

= 3Va1

Va1 = (Va + λVb + λ2Vc ) / 3  …………………(6)

For getting negative sequence component, multiply equation (4) by λ2 and (5) by λ & add them to equation (1),

Va + λ2Vb + λVc

= Va1(1+ λ4+ λ2) + Va2(1+ λ3+ λ3) + Va0(1+ λ + λ2)

= 3Va2

Va2 = (Va + λ2Vb + λVc) / 3  ……………………(7)

For Zero Sequence component, add equation (1), (4) and (5),

Va + Vb + Vc

= Va1(1+ λ+ λ2) + Va2(1+ λ+ λ2) + 3Va0

= 3Va0

Va0 = (Va + Vb + Vc) / 3  ……………………(8)

Therefore from equation (6), (7) and (8), we have completely calculated the positive, negative and zero sequence voltages.

In the same way, we can calculate the three components of currents. For currents we can write as below.

Ia1 = (Ia + λIb + λ2Ic ) / 3 

Ia2 = (Ia + λ2Ib + λIc) / 3 


Ia0 = (Ia + Ib + Ic) / 3