An electric power system should ensure the availability of electric power without interruption to every load connected to the system. One of the sources of trouble to continuous supply is the shunt fault or short circuit which produces a sudden and sometimes violent change in the system.
Protective relays and relaying system detect abnormal conditions like faults in electric circuits and operate automatic switchgear to isolate faulty equipment from the system as quickly as possible.
Over Current Relay
A relay that operates or picks up when it’s current exceeds a predetermined value (setting value) is called Over Current Relay.
Over current protection protects electrical power systems against excessive currents which are caused by short circuits, ground faults, etc. Over current relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.
Over current includes short-circuit protection. Short circuits can be Phase faults, Earth faults, Winding faults, Differential and distance protection.
Over current protection is useful for the following:
Type of Over Current Relay
Application of over current relay: Motor protection, Transformer protection, Line protection, Distribution protection
Universal Relay Torque Equation
The universal relay torque equation can be given as
where I = RMS value of current in current coil
V = RMS value of voltage fed to the voltage coil
ϕ = Electrical angle between V and I
T = The maximum torque angle K1, K2 and K3 = Relay constant
K = Mechanical restraining torque
Differential Protection
It is used for transformer and generator protection. It simultaneously compares the phaser difference & magnitude of the current entering & leaving the protected zone. Differential protection is a unit protection, relay works on the principle of Kirchoff's current Law. Measuring element is Current Transformer. The differential current measured between the incoming current and Outgoing current must be negligible current during stable and through fault condition. In case of in zone fault or unstable condition (due to CT saturate) relay will sense the differential current and issue the trip signal.
Applications of Differential protection: Transformer, Generator and Cable Protection. Distance Protection: Distance protection is widely used in the transmission network, its also called impedance protection because relay operates with respect to fault impedance of the transmission line (Z=V/I).
It calculates the apparent impedance of a line with the help of voltage & current input connected to the relay. If measured impedance falls below set impedance trip command is issued to clear the fault.
Measuring element is Current Transformer and Voltage Transformer. Usually relay having 4 zones of the transmission line.
Zone1, Zone2, Zone3 will be forward zones (Towards the Line) and zone 4 will be Reverse zone (Towards the Source).
Impedance Relay
From the universal torque equation, putting K3 = 0 and giving negative sign to voltage term, it becomes,
T=K1I2+K2V2
(neglecting spring torque)
An impedance relay is a voltage restrained over current relay.
Reactance Relay
In universal torque equation, putting k2 = 0
The reactance relay is direction restrained over current relay.
Mho Relay
In this relay, the operating torque is obtained by the V-I element and
The mho relay is voltage restrained direction relay i.e., this relay has property of inherently directional.
Time Multiplier Setting (TMS)
The time multiplier setting for an inverse time relay is defined as,
Where T = Required time of operation
Tm = The time obtained from the relay characteristics
Plug Setting Multiplier (PSM)
Current setting is adjusted by means of a topped plug bridge, hence known as PSM
Distance protection Scheme
A distance relay has the ability to detect a fault within a pre-set distance along a transmission line or power cable from its location. Every power line has a resistance and reactance per kilometer related to its design and construction so its total impedance will be a function of its length. A distance relay therefore looks at current and voltage and compares these two quantities on the basis of Ohm’s law.
Zones of Protection
In Zone of Protection, Careful selection of the reach point settings and tripping times for various zones of measurement enables correct coordination between distance relays on a power system. Basic distance protection will comprise one instantaneous (Zone 1) and one or more time delayed zones (Zone 2, Zone 3, Zone4 …). Typical reach and time settings for a 3-Zone distance protection are shown below:
Impedance Characteristic
If the relay’s operating boundary is plotted on an R-X diagram, its impedance characteristic is a circle with its center at the origin of the coordinates and its radius will be the setting (the reach point) in ohms. The relay will operate for all values less than its setting i.e. for all points within the circle. This type of relay, however, is non-directional. It can operate for faults behind the relaying point. It takes no account of the phase angle between voltage and current. It is also sensitive to power swings and load encroachment due to the large impedance circle.
Mho Characteristic
The limitation of the impedance characteristic can be overcome by a technique known as self polarization. Additional voltages are fed into the comparator in order to compare the relative phase angles of voltage and current, so providing a directional feature. This has the effect of moving the circle such that the circumference of the circle passes through the origin. Angle 𝜃 is known as the relay’s characteristic angle. It appears as a straight line on an admittance diagram.
Significance of R-X Diagram
The subscript ‘p’ represents primary and ‘s’ represents secondary quantities. In terms of the secondary quantities of voltage and current transformers, the relay sees Zfp & Zfs as
where ni and ne are the current transformer (CT) and voltage transformer (VT) turns ratios.
Sampling Comparator
There are two methods of comparison: the amplitude and phase comparison techniques.
In amplitude comparison technique, the compactor produces an output whose amplitude is proportional to the amplitude difference of the input quantities;
While in phase comparison technique, the Comparator the phase angles of the input quantities and produces pulses whose width is proportional to the phase difference of the input quantities. The amplitude Comparator can be used as phase Comparator and vice versa, if certain modifications are made.
Instantaneous Comparator (Directing Amplitude Comparator) – Averaging Type
This is then compared with the peak value of operating signal, which may or may not be rectified but is smoothened. The tripping signal is provided if the operating signal exceeds the level of the restraint. Since this method involves smoothening, the operation is slow. A faster method is phase splitting the wave shapes of instantaneous amplitude comparator are shown in fig below before rectification and the averaging circuit can be eliminated.
Phase Comparator
Phase comparison technique is the most widely used one for all practical directional, distance, differential and carrier relays. If the two input signals are S1 and S2 the output occurs when the inputs have phase relationship lying within the specified limits.
Both the input must exist for an output to occur. The operation is independent of their magnitudes and is dependent only on their phase relationship. The figures below show that the phase comparator is simple form. The function is defined by the boundary of marginal operation and represented by the straight lines from the origin of the S-plane.
The condition of operation is β1 < θ < β2.
θ is the angle by which S2 lands S1. If β1 = β2 =90o, the comparator is called cosine comparator and if β1=0 and β2=180o, it is a sine comparator.
In short, a phase comparator compares two input quantities in phase angle (vertically) irrespective of the magnitude and operates if the phase angle between them is < 90o.
The Carrier-Current Protection
Under certain situations like internal faults, the generator has to be quickly isolated (shut down), while problems like loss of field problem requires an ‘alarm' to alert the operator. Following is a descriptive list of internal faults and abnormal operating conditions.
Abnormal Operating Conditions
Protective Relays Used
Protection Schemes Used
Stator Earth Fault Protection
Earth fault protection must be applied where impedance earthing is employed that limits the earth fault current to less than the pick-up threshold of the overcurrent and/or differential protection for a fault located down to the bottom 5% of the stator winding from the starpoint. The type of protection required will depend on the method of earthing and connection of the generator to the power system.
Step-up Transformer: Differential Protection
The generator stator and step-up transformer can be protected by a single zone of overall differential protection. This will be in addition to differential protection applied to the generator only. The current transformers should be located in the generator neutral connections and in the transformer HV connections. Alternatively, CT’s within the HV switchyard may be employed if the distance is not technically prohibitive. Even where there is a generator circuit breaker, overall differential protection can still be provided if desired.
Internal faults in oil filled transformers
In oil filled transformers, internal faults may be classified as follow:
These faults may be the consequence of external lightning or switching over voltage.
Depending on the type of the transformer, there are two kinds of devices able to detect internal faults affecting an oil filled transformer.
Unit Transformer Differential Protection
The current taken by the unit transformer must be allowed for by arranging the generator differential protection as a three-ended scheme. Unit transformer current transformers are usually applied to balance the generator differential protection and prevent the unit transformer through current being seen as differential current.
Overloads and internal faults in dry type transformers
The dry type transformers are protected against over-heating due to possible downstream overloads by a dedicated relay monitoring thermal sensors embedded in the windings of the transformer.
The internal faults, mainly inter turns and phase to earth short circuits occurring inside a dry type transformers are cleared either by the circuit breaker or the fuses installed on the primary side of the transformer. The tripping of the circuit breakers when used is ordered by the phase to phase and phase to earth over current protections.
Inter turns faults need a dedicated attention:
Busbar Protection
Busbars have often been left without specific protection, for one or more of the following reasons:
Types of Protection Scheme
A number of busbar protection systems have been devised:
Single-Busbar Frame-Earth Protection
This is purely an earth fault system and, in principle, involves simply measuring the fault current flowing from the switchgear frame to earth. A current transformer is mounted on the earthing conductor and is used to energize a simple instantaneous relay.
A circuit breaker should be capable of opening on the occurrence of a fault and clearing the fault. It should be closed on to a fault. It should be capable of carrying fault current for a short time while another circuit breaker is clearing the fault.
A circuit-breaker rated at In = 125 A for an ambient temperature of 40°C will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). The same circuit-breaker can be used at higher values of ambient temperature, however, if suitably “derated”. Thus, the circuit-breaker in an ambient temperature of 50°C could carry only 117 A indefinitely, or again, only 109 A at 60°C, while complying with the specified temperature limit.
Derating a circuit-breaker is achieved, therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit-breakers (derated as described) to operate at 60°C (or even at 70°C) ambient.
Restriking voltage
Where V = Restriking voltage
Vm = Peak voltage at the instant of are interruption
L = Inductance per phase of the system
C = Capacitance per phase of the system
Natural frequency of oscillation
Rate of Rising of Restriking Voltage (RRRV)
RRRV is maximum
at
Resistance Switching
Frequency of damped oscillation,
if is known as critical resistance
if there will be oscillation
if there will be no transient oscillation
Breaking Current
The symmetrical breaking current of a circuit breaker is the current which it will interrupt at a power factor of 0.15 for railing up to 500 MVA and a power factor of 0.3 for 750 MVA with a recovery voltage of 95% of normal voltage.
Asymmetrical breaking current of a circuit breaker is the current which it will interrupt when there is symmetrical component, its peak value let then asymmetrical breaking current (I) may be calculated as
Making Capacity
The (peak) making current of a circuit breaker is the peak value of maximum current in the first cycle of current after the current is closed by the current breaker.
Rated (peak) making current = 2.55 × rated asymmetrical breaking capacity.
Short Time Operated Duty
Any Circuit breaker must be capable of the following duty B – 3’ – MB – 3’ – MB
Where B = breaking operations
MB = making operations
3’ = 3 = min time interval
Types of Circuit Breaker
Electric Arc Model for SF6 Circuit Breaker
The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV. These days for higher transmission voltage levels SF6 Circuit Breakers are largely used. OCB and ABCB have almost become obsolete. In fact, in many installations, SF6 CB is used for lower voltages like 11 kV, 6 kV etc..
Sulphur Hexafluoride symbolically written as SF6 is a gas which satisfies the requirements of an ideal arc interrupting medium. So SF6 is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kV upto 765 kV class. In addition to this SF6 is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of SF6 which is the reason of its extensive use in circuit breakers
(Transient Recovery Voltage) TRV is the voltage across the opening contacts of a fault-interrupting circuit breaker (CB) immediately after the electric arc is extinguished For calculation of the initial part of TRV, the modeling of the arc resistance in a SF6 CB is important, because it has a significant impact on the TRV. Black box models use a mathematical description of the electrical behavior of an electrical arc. These types of models do not give full representation of the physical processes taking place inside the CB. Recorded voltage and current traces during the "thermal period" are used to obtain the CB parameters that are later on substituted in differential equations.
Vacuum Circuit Breaker
Medium Voltage Oil Circuit Breakers
Air Blast Circuit Breaker
The other type of circuit breaker that we discuss here is Air Blast Circuit Breaker(ABCB). This type of breakers is also becoming obsolete. Once Air Blast type of breakers was preferred in Extra High Voltage substations. Now it is difficult to find new HV/EHV substations equipped with Air Blast Circuit Breakers.
Note: One should not be confused between Air Circuit Breaker and Air Blast Circuit Breaker.
See the Sketches below illustrating the arc extinction process of the axial blast type breaker.
Advantages of the Air Blast Circuit Breaker(ABCB)
Minimum Oil Circuit Breaker
The simplified constructional diagram of a Minimum Oil Circuit Breaker (MOCB) is shown in the figure. It consists of two oil-filled chambers namely upper chamber and lower chamber, which are separated from each other.
The arc extinction process is carried out in the upper chamber. So, it is called as an arc extinction chamber or current interruption chamber of Minimum Oil Circuit Breaker (MOCB).This chamber houses an arc control device, an upper fixed contact, and a ring-shaped lower fixed contact.
The arc control device is fitted to the upper to the upper fixed contact. The moving contact slides through the lower fixed contact such that a physical (or electrical) maintained between them. The entire assembly of upper fixed contact. lower fixed contact and arc control device is enclosed in a glass fiber enclosure which is surrounded by oil.
Applications of Minimum Oil Circuit Breaker
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1. What is power system protection and why is it important? |
2. What are the main components of switchgear? |
3. How does switchgear provide protection in a power system? |
4. What are the different types of switchgear? |
5. How is switchgear tested for reliability and performance? |
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