Necessity of a protection system.

Electrical power system operates at various voltage levels from 415 V to 400 kV or
even more. Electrical apparatus used may be enclosed (e.g., motors) or placed in
open (e.g., transmission lines). All such equipment undergo abnormalities in their
life time due to various reasons. For example, a worn out bearing may cause
overloading of a motor. A tree falling or touching an overhead line may cause a
fault. A lightning strike (classified as an act of God!) can cause insulation failure.
Pollution may result in degradation in performance of insulators which may lead to
breakdown. Under frequency or over frequency of a generator may result in
mechanical damage to it's turbine requiring tripping of an alternator. Even
otherwise, low frequency operation will reduce the life of a turbine and hence it
should be avoided.

It is necessary to avoid these abnormal operating regions for safety of the

equipment. Even more important is safety of the human personnel which may be
endangered due to exposure to live parts under fault or abnormal operating
conditions. Small current of the order of 50 mA is sufficient to be fatal! Whenever
human security is sacrificed or there exists possibility of equipment damage, it is
necessary to isolate and de-energize the equipment. Designing electrical equipment
from safety perspective is also a crucial design issue which will not be addressed
here. To conclude, every electrical equipment has to be monitored to protect it and
provide human safety under abnormal operating conditions. This job is assigned to
electrical protection systems. It encompasses apparatus protection and system
protection.
System protection deals with detection of proximity of system to unstable
operating region and consequent control actions to restore stable operating
point and/or prevent damage to equipments. Loss of system stability can lead
to partial or complete system blackouts. Under-frequency relays, outof-step
protection, islanding systems, rate of change of frequency relays, reverse
power flow relays, voltage surge relays etc are used for system protection.
Wide Area Measurement (WAM) systems are also being deployed for system
protection. Control actions associated with system protection may be
classified into preventive or emergency control actions.

What is a Relay?

Formally, a relay is a logical element which processes the inputs (mostly voltages
and currents) from the system/apparatus and issues a trip decision if a fault within
the relay's jurisdiction is detected. A conceptual diagram of relay is shown in fig
1.2.
In fig 1.3, a relay R1 is used to protect the transmission line under fault F1. An
identical system is connected at the other end of the transmission line relay R3 to
open circuit from the other ends as well. To monitor the health of the apparatus,
relay senses current through a current transformer (CT), voltage through a voltage
transformer (VT). VT is also known as Potential Transformer (PT).

The relay element analyzes these inputs and decides whether (a) there is a
abnormality or a fault and (b) if yes, whether it is within jurisdiction of the relay.
The jurisdiction of relay R1 is restricted to bus B where the transmission line
terminates. If the fault is in it's jurisdiction, relay sends a tripping signal to circuit
breaker(CB) which opens the circuit. A real life analogy of the jurisdiction of the
relay can be thought by considering transmission lines as highways on which
traffic (current/power) flows.

If there is an obstruction to the regular flow due to fault F1 or F2, the traffic police
(relay R1) can sense both F1 and F2 obstructions because of resulting abnormality
in traffic (power flow). If the obstruction is on road AB, it is in the jurisdiction of
traffic police at R1; else if it is at F2, it is in the jurisdiction of R2. R1 should act
for fault F2, if and only if, R2 fails to act. We say that relay R1 backs up relay R2.
Standard way to obtain backup action is to use time discrimination i.e., delay
operation of relay R1 in case of doubt to provide R2 first chance to clear the fault.

Evolution of Relays :

If we zoom into a relay, we see three different types of realizations:

 Electromechanical Relays
 Solid State Relays
 Numerical Relays

Electromechanical Relays:

When the principle of electromechanical energy conversion is used for decision

making, the relay is referred as an electromechanical relay. These relays represent
the first generation of relays. Let us consider a simple example of an over current
relay, which issues a trip signal if current in the apparatus is above a reference
value. By proper geometrical placement of current carrying conductor in the
magnetic field, Lorentz force is produced in the operating coil.
This force is used to create the operating torque. If constant 'B' is used (for
example by a permanent magnet), then the instantaneous torque produced is
proportional to instantaneous value of the current. Since the instantaneous current
is sinusoidal, the instantaneous torque is also sinusoidal which has a zero average
value. Thus, no net deflection of operating coil is perceived.

On the other hand, if the B is also made proportional to the instantaneous value of
the current, then the instantaneous torque will be proportional to square of the
instantaneous current (non-negative quantity). The average torque will be
proportional to square of the rms current. Movement of the relay contact caused by
the operating torque may be restrained by a spring in the overcurrent relay. If the
spring has a spring constant 'k', then the deflection is proportional to the operating
torque (in this case proportional to ) .When the deflection exceeds a preset
value, the relay contacts closes and a trip decision is issued. Electromechanical
relays are known for their ruggedness and immunity to Electromagnetic
Interference (EMI).
Solid State Relays:

With the advent of transistors, operational amplifiers etc, solid state relays were
developed. They realize the functionality through various operations like
comparators etc. They provide more flexibility and have less power consumption
than their electromechanical counterpart. A major advantage with the solid state
relays is their ability to provide self checking facility i.e. the relays can monitor
their own health and raise a flag or alarm if its own component fails. Some of the
advantages of solid state relays are low burden, improved dynamic performance
characteristics, high seismic withstand capacity and reduced panel space.

Relay burden refers to the amount of volt amperes (VA) consumed by the relay.
Higher is this value, more is the corresponding loading on the current and voltage
sensors i.e. current transformers (CT) and voltage transformers (VT) which
energizes these relays. Higher loading of the sensors lead to deterioration in their
performance. A performance of CT or VT is gauged by the quality of the
replication of the corresponding primary waveform signal. Higher burden leads to
problem of CT saturation and inaccuracies in measurements. Thus it is desirable to
keep CT/VT burdens as low as possible.

These relays have been now superseded by the microprocessor based relays or
numerical relays.

Numerical Relays:

The block diagram of a numerical relay is shown in fig 1.5.

It involves analog to digital (A/D) conversion of analog voltage and currents
obtained from secondary of CTs and VTs. These current and voltage samples are
fed to the microprocessor or Digital Signal Processors (DSPs) where the protection
algorithms or programs process the signals and decide whether a fault exists in the
apparatus under consideration or not. In case, a fault is diagnosed, a trip decision is
issued. Numerical relays provide maximum flexibility in defining relaying logic.

Numerical Relays

The hardware comprising of numerical relay can be made scalable i.e., the
maximum number of v and i input signals can be scaled up easily. A generic
hardware board can be developed to provide multiple functionality. Changing the
relaying functionality is achieved by simply changing the relaying program or
software. Also, various relaying functionalities can be multiplexed in a single
relay. It has all the advantages of solid state relays like self checking etc. Enabled
with communication facility, it can be treated as an Intelligent Electronic Device
(IED) which can perform both control and protection functionality. Also, a relay
which can communicate can be made adaptive i.e. it can adjust to changing
apparatus or system conditions. For example, a differential protection scheme can
adapt to transformer tap changes. An overcurrent relay can adapt to different
loading conditions. Numerical relays are both "the present and the future". Hence,
in this course, our presentation is biased towards numerical relaying. This also
gives an algorithmic flavour to the course.
A Circuit Breaker (CB) is basically a switch used to interrupt the flow of current. It
opens on relay command. The relay command initiates mechanical separation of
the contacts. It is a complex element because it has to handle large voltages (few to
hundreds of kV's) and currents (in kA's). Interrupting capacity of the circuit
breaker is therefore expressed in MVA.

Power systems under fault behave more like inductive circuits. X/R ratio of lines is
usually much greater than unity. For 400 kV lines, it can be higher than 10 and it
increases with voltage rating. From the fundamentals of circuit analysis, we know
that current in an inductive circuit (with finite resistance) cannot change
instantaneously. The abrupt change in current, if it happens due to switch opening,
will result in infinite di/dt and hence will induce infinite voltage. Even with finite
di/dt, the induced voltages will be quite high. The high induced voltage developed
across the CB will ionize the dielectric between its terminals. This results in arcing.
When the current in CB goes through the natural zero, the arc can be extinguished
(quenched). However, if the interrupting medium has not regained its dielectric
properties then the arc can be restruck. The arcing currents reduce with passage of
time and after a few cycles the current is finally interrupted.

Usually CB opening time lies in the 2-6 cycles range. CBs are categorized by the
interrupting medium used. Minimum oil, air blast, vacuum arc and SF6 CBs are
some of the common examples. CB opening mechanism requires much larger
power input than what logical element relay can provide. Hence, when relay issues
a trip command, it closes a switch that energizes the CB opening mechanism
powered by a separate dc source (station battery). The arc struck in a CB produces
large amount of heat which also has to be dissipated.

Protection Paradigms - Apparatus Protection

Objectives:
 Principle of overcurrent protection.
 Principle of directional overcurrent protection.
 Principle of distance protection.
 Principle of differential protection.
 For simplicity in explaining the key ideas, we
consider three phase bolted faults. Generalization of
different fault types
Overcurrent Protection

This scheme is based on the intuition that, faults typically

short circuits, lead to currents much above the load current.
We can call them as overcurrents. Over current relaying and
fuse protection uses the principle that when the current
exceeds a predetermined value, it indicates presence of a fault
(short circuit). This protection scheme finds usage in radial
distribution systems with a single source. It is quite simple to
implement. Fig 2.1 shows a radial distribution system with a
single source. The fault current is fed from only one end of
the feeder. For this system it can be observed that:
 To relay R1, both downstream faults F1 and F2 are visible i.e. IF1 as well
as IF2 pass through CT of R1.
 To relay R2, fault F1, an upstream fault is not seen, only F2 is seen. This
is because no component of IF1 passes through CT of R2. Thus,
selectivity is achieved naturally. Relaying decision is based solely on the
magnitude of fault current. Such a protection scheme is said to be non-
directional.
Directional Overcurrent Protection:

In contrast, there can be situations where for the purpose of selectivity, phase angle
information (always relative to a reference phasor) may be required. Fig 2.2 shows
such a case for a radial system with source at both ends. Consequently, fault is fed
from both the ends of the feeder. To interrupt the fault current, relays at both ends
of the feeder are required.

In this case, from the magnitude of the current seen by the relay R2, it is not
possible to distinguish whether the fault is in the section AB or BC. Since faults in
section AB are not in its jurisdiction, it should not trip. To obtain selectivity, a
directional overcurrent relay is required. It uses both magnitude of current and
phase angle information for decision making. It is commonly used in
subtransmission networks where ring mains are used.

Distance Protection:
Consider a simple radial system, which is fed from a single source. Let us measure
the apparent impedance (V/I) at the sending end. For the unloaded system, I = 0,
and the apparent impedance seen by the relay is infinite. As the system is loaded,
the apparent impedance reduces to some finite value (ZL+Zline) where ZL is the
load impedance and Zline is the line impedance. In presence of a fault at a per-unit
distance ‘m', the impedance seen by the relay drops to a mZline as shown in fig
2.3.

The basic principle of distance relay is that the apparent impedance seen by the
relay, which is defined as the ratio of phase voltage to line current of a
transmission line (Zapp), reduces drastically in the presence of a line fault. A
distance relay compares this ratio with the positive sequence impedance (Z1) of the
transmission line. If the fraction Zapp/Z1 is less than unity, it indicates a fault. This
ratio also indicates the distance of the fault from the relay. Because, impedance is a
complex number, the distance protection is inherently directional. The first
quadrant is the forward direction i.e. impedance of the transmission line to be
protected lies in this quadrant. However, if only magnitude information is used,
non-directional impedance relay results. Fig 2.4 and 2.5 shows a characteristic of
an impedance relay and ‘mho relay' both belonging to this class. The impedance
relay trips if the magnitude of the impedance is within the circular region. Since,
the circle spans all the quadrants, it leads to non-directional protection scheme. In
contrast, the mho relay which covers primarily the first quadrant is directional in
nature.
Principle of Differential Protection

Differential protection is based on the fact that any fault within an electrical
equipment would cause the current entering it, to be different, from the current
leaving it. Thus by comparing the two currents either in magnitude or in phase or
both we can determine a fault and issue a trip decision if the difference exceeds a
predetermined set value.

Differential Protection for Transmission Line

Differential Protection for Transformer:

Consider an ideal transformer with the CT connections, as shown in fig 2.8. To

illustrate the principle let us consider that current rating of primary winding is
100A and secondary winding is 1000A. Then if we use 100:5 and 1000:5 CT on
the primary and secondary winding, then under normal (no fault) operating
conditions the scaled CT currents will match in magnitudes. By connections the
primary and secondary CTs with due care to the dots (polarity markings), a
circulating current can be set up as shown by dotted line.

Differential Protection for Busbar:

Overview of Power System Dynamics:

Usually, system protection requires study of the system dynamics and control. To
understand issues in system protection, we overview dynamical nature of the
power system. Power system behavior can be described in terms of differential and
algebraic system of equations. Differential equations can be written to describe
behaviour of generators, transmission lines, motors, transformers etc. The detailing
depends upon the time scale of investigation

Figure 3.1 shows the various time scales involved in modelling system dynamics.
The dynamics involved in switching, lightening, load rejection etc have a high
frequency component which die down quickly. In analysis of such dynamics,
differential equations associated with inductances and capacitances of transmission
lines have to be modelled. Such analysis is restricted to a few cycles. It is done by
Electromagnetic Transient Program (EMTP).

At a larger time scale (order of seconds), response of the electromechanical

elements is perceived. These transients are typically excited by faults which disturb
the system equilibrium by upsetting the generatorload balance in the system. As a
consequence of fault, electrical power output reduces instantaneously while the
mechanical input does not change instantaneously. The resulting imbalance in
power (and torque) excites the electromechanical transients which are essentially
slow because of the inertia of the mechanical elements (rotor etc).

Detection and removal of fault is the task of the protection system (apparatus
protection). Post-fault, the system may or may not return to an equilibrium
position. Transient stability studies are required to determine the post fault system
stability. In practice, out-of-step relaying, under frequency load shedding,
islanding etc are the measures used to enhance system stability and prevent
blackouts. The distinction between system protection and control (e.g. damping of
power swings) is a finer one. In the today's world of Integrated Control and
Protection Systems (ICPS), this distinction does not make much sense. In this
lecture, we discuss these issues from distribution system perspective. In the next
lecture, a transmission system perspective will be discussed.

System Protection Relays:

Consider a medium voltage distribution system having local generation (e.g.,

captive power generation) as shown in fig 3.2 which is also synchronized with the
grid. During grid disturbance, if plant generators are not successfully isolated from
the grid, they also sink with the grid, resulting in significant loss in production and
damage to process equipments. The following relays are used to detect such
disturbances, its severity and isolate the inplant system from the grid.

 Underfrequency and over frequency relays.

 Rate of change of frequency relays.
 Under voltage relays.
 Reverse power flow relays.
 Vector shift relays.

Underfrequency Relay and Rate of Change of Frequency Relay:

In case of a grid failure (fig. 3.3), captive generators tend to supply power to other
consumers connected to the substation. The load-generation imbalance leads to fall
in frequency. The underfrequency relay R detects this drop and isolates local
generation from the grid by tripping breaker at the point of common coupling.
After disconnection from the grid, it has to be ascertained that there is load-
generation balance in the islanded system. Because of the inertia of the machines,
frequency drops gradually. To speed up the islanding decision, rate of change of
frequency relays are used.

Undervoltage Relay :

Whenever there is an uncleared fault on the grid close to the plant, the plant
generators tend to feed the fault, and the voltages at the supply point drops. This
can be used as a signal for isolating from the grid.

Reverse Power Relay:

Distribution systems are radial in nature. This holds true for both utility and plant
distribution systems. If there is a fault on the utility's distribution system, it may
trip a breaker thereby isolating plant from the grid. This plant may still remain
connected with downstream loads as shown in fig 3.4 and 3.5. Consequently,
power will flow from the plant generator to these loads.

If in the prefault state, power was being fed to the plant, then this reversal of power
flow can be used to island the plant generation and load from the remaining
system. This approach is useful to detect loss of grid supply whenever the
difference between load and available generation is not sufficient to obtain an
appreciable rate of change of frequency but the active power continues to flow into
the grid to feed the external loads.

Example

In fig 3.4, consider that the plant imports at all times a minimum power of 5 MW.
Studies indicate that for various faults in utility side, minimum power export from
the plant generator is 0.5 MW. Deduce the setting of reverse power relay. If the
plant generator is of 50 MW capacity, what is likelihood of underfrequency or rate
of change of frequency relay picking up on such faults? Ans: Reverse power flow
relay can be set to 0.4 MW. Since minimum reverse power flow is 1% of plant
capacity, it is quite likely, that utility disconnection may not be noticed by
underfrequency or the rate of change of frequency relays. Vector shift relays and
system protection schemes in transmission systems will be discussed in more
details in later lectures.

Lightning Protection:

Many line outages result from lightning strokes that hit overhead transmission
lines. Lightning discharges normally produce overvoltage surges which may last
for a fraction of second and are extremely harmful. The line outages can be
reduced to an acceptable level by protection schemes like installation of earth
wires and earthing of the towers. Lightning overvoltages can be classified as
follows:

 Induced overvoltages which occur when lightning strokes reach the ground
near the line.
 Overvoltages due to shielding failures that occur when lightning strokes
reach the phase conductors.
 Overvoltages by back flashovers that occur when lightning stroke reaches
the tower or the shield wire.
 The most commonly used devices for protection against lightning surges are
the following:
  Shielding by earth wires: Normally, transmission lines are equipped with
earth wires to shield against lightning
 discharges. The earthwires are placed above the line conductor at such a
position that the lightning strokes are intercepted by them. In addition to
this, earthing of tower is also essential.
 Lightning Arrestors: An alternative to the use of earthwire for protection of
conductors against direct lightning strokes is to use lightning arrestors in
parallel to insulator strings. Use of lightning arrestors is more economical
also.
 ZnO varistor is commonly used as lightning arrestor because of its peculiar
resistance characteristic. Its resistance varies with applied voltage, i.e, its
resistance is a nonlinear inverse function of applied voltage. At normal
voltage its resistance is high. But when high voltage surges like lightning
strokes appear across the varistor, its resistance decreases drastically to a
very low value and the energy is dissipated in it, giving protection against
lightning.

Review Questions

1. Describe various system protection relays in use.

2. What are the functions of an underfrequency relay?

3. Explain the functioning of reverse power flow relay.

4. How transmission lines are protected against lightning?

5. Explain the functioning of ZnO varistor.

Desirable Attributes of Protection:

Objectives

In this lecture we will learn the following desirable attributes of protection

system viz:

 Dependability.
 Security.
 Sensitivity.
 Selectivity.
 Reliability.
 Necessity of speed in relaying.
 Speed vs. accuracy conflict.
 A protection system is characterized by following two important parameters:
 Dependability
 Security
Necessity of Speed in Relaying (contd..):

Fig 4.5 shows the pre and post fault characteristics for the single machine infinite
bus system shown in fig 4.4. Initial operating point A is on the pre fault
characteristic. Occurrence of fault reduces Pe to 0. The power generation
imbalance accelerates generator and hence its (power angle) increases. At point C
the fault is cleared by tripping the faulted line and the system moves to post fault
characteristics. The power output jumps to point D. Now Pe > Pm and the machine
decelerates.

At point E, is equal to zero and the extreme point of swing is reached. As Pe >
Pm, the deceleration continues and hence the rotor starts retarding. At point O, Pe
= Pm the acceleration is zero, but machine speed is lower than nominal speed .
Consequently, the angle continues to fall back.

However, as reduces further, Pe also reduces, therefore Pm - Pe > 0 and the

generator starts accelerating. This arrests the drop in at point F and the swing
reverses, again a consequence of acceleration. In absence of damping, these
oscillations will recur just like oscillation of a simple pendulum. However, because
of damping provided by generator, the oscillations reduce in magnitude and finally
system settles to equilibrium at point O.
It should be obvious that interval BC is dependent on fault clearing time of the
protection system. The shaded area ABCC1 is the acceleration area and area
C1DEE1 the deceleration area. As per equal area criteria, the post fault system
reaches stable equilibrium if accelerating area equals to the decelerating area. The
limit point for deceleration is defined by point G the intersection point of Pm0 and
the post fault characteristic.
If the swing of generator exceeds beyond point G, the generator moves from
deceleration to acceleration region. Then, its angle continues to rise indefinitely,
and the machine is said to go out-of-step. If any machine goes out-of-step with rest
of system it has to be islanded. Out-of-step condition in a multi machine system
can be simulated by transient stability program. Detection in real-time is a much
more challenging task and it is dealt by ‘out-of-step relaying' schemes. When a
multi machine system is islanded in to different sub-systems, then for stable
operation of each sub-system, it is necessary that each sub-system should have
generation load balance. Fig 4.6, however it should be obvious by now that from
the stability perspective, transmission system protection should be made as fast as
possible. As the fault clearing time increases, the stability margin (area EE1G)
reduces. The fault clearing time at which the stability margin reduces to zero is
called the critical clearing time.

Speed Vs. Accuracy Conflict:

Intuition tells us that quickness is an invitation to disaster. The possible

consequences of quick tripping decisions are:

 Nuisance Tripping
 Tripping for faults outside the relay jurisdiction.

Nuisance tripping is the tripping when there is no fault, e.g. an overcurrent relay
tripping on load. It compromises faith in the relaying system due to unnecessary
loss of service. On the other hand, tripping on faults that are outside the relay's
jurisdiction also cause an unwarranted loss of service in the healthy parts of the
system.

It has to be mentioned that speed and accuracy bear an inverse relationship. The
high-speed systems tend to be less accurate for the simple reason that a high speed
system has lesser amount of information available at it's disposal for making
decision.

Thus, the protection engineer has to strike a balance between these two
incompatible requirements. Innovations in protection are essentially driven by such
requirements. The ways to tackle this conflict will become clear as we proceed into
future lectures.

Review Questions

1. How is reliability achieved in a protective system?

2. Distinguish between dependability and security of a relay

. 3. How is selectivity criteria provided in (a) Overcurrent protection scheme (b)

Differential protection scheme.

4. Why is high speed system said to be less accurate?

5. The performance of a distance relay was monitored over a period of 2 years. It

was found that it operated 15 times, 12 were desired trips due to faults in its
jurisdiction. It was found that relay failed to issue trip decision on 2 occasions.
Compute dependability and security for the relay.

6. Define the following terms (a) % Dependability (b) % Security (c) % Reliability