The document discusses static and numerical relays, detailing their operations, merits, and demerits. Static relays utilize static circuits for measurement and provide advantages such as low power consumption and fast response, while numerical relays, driven by advancements in digital technology, offer greater flexibility and reliability through real-time data processing. It also covers the role of data acquisition systems and the significance of signal conditioning in the operation of these relays.

1. 19EE63C SWITCHGEAR AND
PROTECTION
UNIT-II

2. Static Relay
• In a static relay, the comparison or measurement of electrical quantities is performed by a
static circuit which gives an output signal for the tripping of a circuit breaker.
• Most of the present day static relays include a dc polarised relay as a slave relay.
• The slave relay is an output device and does not perform the function of comparison or
measurement. It simply closes contacts. It is used because of its low cost.
• In a fully static relay, a thyristor is used in place of the electromagnetic slave relay.
• The electromechanical relay used as a slave relay provides a number of output contacts at
low cost.
• Electromagnetic multicontact tripping arrangements are much simpler than an equivalent
group of thyristor circuits.

3. Merits of Static Relays
• (i) Low burden on CTs and VTs. The static relays consume less power and in most of
the cases they draw power from the auxiliary dc supply
• (ii) Fast response
• (iii) Long life
• (iv) High resistance to shock and vibration
• (v) Less maintenance due to the absence of moving parts and bearings
• (vi) Frequent operations cause no deterioration
• (vii) Quick resetting and absence of overshoot

4. Merits of Static Relays
• (viii) Compact size
• (ix) Greater sensitivity as amplification can be provided easily
• (x) Complex relaying characteristics can easily be obtained
• (xi) Logic circuits can be used for complex protective schemes

5. DeMerits of Static Relays
The demerits of static relays are as follows:
• (i) Static relays are temperature sensitive. Their characteristics may vary with the
variation of temperature. Temperature compensation can be made by using
thermistors and by using digital techniques for measurements, etc.
• (ii) Static relays are sensitive to voltage transients. The semiconductor components
may get damaged due to voltage spikes. Filters and shielding can be used for their
protection against voltage spikes.
• (iii) Static relays need an auxiliary power supply. This can however be easily supplied
by a battery or a stabilized power supply

6. NUMERICAL RELAYS

7. NUMERICAL RELAYS
 The numerical relay is the latest development in the area of power system
protection and differs from conventional ones both in design and methods of
operation.
 It has been developed because of tremendous advancement in VLSI and
computer hardware technology.
 It is based on numerical (digital) devices e.g. microprocessors, microcontrollers,
digital signal processors (DSPs) etc.
 This relay acquires sequential samples of the ac quantities in numeric (digital)
data form through the data acquisition system (DAS), and processes the data
numerically using a relaying algorithm to calculate the fault discriminants and
make trip decisions.

8. NUMERICAL RELAYS
 In a numerical relay, the analog current and voltage signals monitored through
primary transducers (CTs and VTs) are conditioned, sampled at specified
instants of time and converted to digital form for numerical manipulation,
analysis, display and recording.
 This process provides a flexible and very reliable relaying function, thereby
enabling the same basic hardware units to be used for almost any kind of
relaying scheme..
 The software used in a numerical relay depends upon the processor used and
the type of the relay. Hence, with the advent of the numerical relay, the
emphasis has shifted from hardware to software.

9. Time-bias Type Phase Comparator

10. NUMERICAL RELAYS
 This relay samples voltages and currents, which, at the power system level,
are in the range of hundreds of kilo volts and kilo amperes respectively.
 The levels of these signals are reduced by voltage and current transformers
(transducers).
 The outputs of the transducers are applied to the signal conditioner (also called
‘analog input subsystem’). Signal conditioner is one of the important
components of the data acquisition system (DAS).
 It brings real-world signals into digitizer. In this case, the signal conditioner
electrically isolates the relay from the power system, reduces the level of the
input voltages, converts currents to equivalent voltages and removes high
frequency components from the signals using analog filters.

11. NUMERICAL RELAYS
 The relay is isolated from the power system by using auxiliary transformers
which receive analog signals and reduce their levels to make them suitable for
use in the relays.
 Since the A/D converters accept voltage signals only, the current signals are
converted into proportional voltage signals by using I/V converters or by
passing through precision shunt resistors.
 Anti-aliasing filters (which are low-pass filters) are used to prevent aliasing
from affecting relaying functions.
 The outputs of the signal conditioner (the analog input subsystem) are applied
to the analog interface, which includes sample and hold (S/H) circuits, analog
multiplexers and analog-to-digital (A/D) converters.

12. NUMERICAL RELAYS
 These components sample the reduced level signals and convert their analog
levels to equivalent numbers that are stored in memory.
 The status of isolators and circuit breakers in the power system is provided to
the relay via the digital input subsystem and are read into the
microcomputer/microcontroller memory.
 After quantization by the A/D converter, analog electrical signals are
represented by discrete values of the samples taken at specified instants of
time.
 The signals in the form of discrete numbers are processed by a relaying
algorithm using numerical methods.
 A relaying algorithm which processes the acquired information is a part of the
software.

13. NUMERICAL RELAYS
 The algorithm uses signal-processing technique to estimate the real and
imaginary components of fundamental frequency voltage and current phasors.
 In some cases, the frequency of the system is also measured. These
measurements are used to calculate other quantities, such as impedances.
 The computed quantities are compared with pre-specified thresholds (settings)
to decide whether the power system is experiencing a fault or not.
 If there is fault in the power system, the relay sends a trip command to circuit
breakers for isolating the faulted zone of the power system. The trip output is
transmitted to the power system through the digital output subsystem

14. NUMERICAL RELAYS
 Most numerical relays are available today with ‘self-check’ feature.
 These relays are capable of periodically checking their hardware and software,
and in case a problem is noticed, the relays give an alarm for corrective action.
 It is extremely easy to service a numerical relay as in most of the cases it
requires only certain cards to be replaced.
 If sufficient spares are maintained at the various locations, the down time of a
relay can be substantially reduced.

15. Advantages of Numerical Relays
 Compactness and Reliability
 Flexibility
 Adaptive Capability
 Multiple Functions
 Detailed Logical and Mathematical Capabilities
 Economic Benefits
 Less Panel Space
 Low Burden on Transducers
 Self-monitoring and Self-testing
 Communication Facility
 Metering Facility
 Memory Action

16. DATA ACQUISITION SYSTEM (DAS)
 Data acquisition is the process of sampling of real-world analog signals and
conversion of the resulting samples into digital numeric values that can be
manipulated by a computer.
 The system which performs data acquisition is called data acquisition system
(DAS).
 Data acquisition typically involves the conversion of analog signals into digital
values for processing.
 For numerical relaying, the data acquisition system acquires sequential
samples of the analog ac quantities (voltages and currents) and converts them
into digital numeric values for processing.

17. DATA ACQUISITION SYSTEM (DAS)
 The voltages and currents at the power system level are in the range of
hundreds of kilovolts and kiloamperes respectively.
 The levels of voltage and current signals are reduced by transducers (voltage
current transformers).
 The output of the transducers are applied to the DAS. DAS also employ
various signal conditioning techniques to adequately modify various different
electrical signals into voltages that can then be digitized using an analog-to-
digital converter (ADC).
 The main components of DAS are the signal conditioner (analog input
subsystem) and the analog interface.

18. Signal Conditioner (Analog Input Subsystem)
 Signal conditioner (also called analog input subsystem) is necessary to make
the signals from the transducers compatible with the analog interface.
 Signal conditioning circuitry converts analog input signals into a form that can
be converted to digital values.
 Since the output signals of transducers are often incompatible with data
acquisition hardware, the analog signals must be conditioned to make them
compatible. Common ways to condition analog signals include the following:
 (i) Analog input isolation and scaling
 (ii) Current to voltage conversion
 (iii) Filtering

19. Signal Conditioner (Analog Input Subsystem)
 The relay is electrically isolated from the
power system by using auxiliary
transformers which receive analog
signals from the transducers and reduce
their levels to make them suitable for use
in the relays,.
 Metal Oxide Varistor (MOV) which has a
high resistance at low voltages and low
resistance at high voltages due to highly
nonlinear current-voltage characteristic is
used to protect circuits against excessive
transient voltages.

20. Signal Conditioner
 Since the A/D converters can handle voltages only, the current are converted to
proportional voltages by using current to voltage (I/V) converters or by passing
the current through precision shunt resistors.
 The phenomenon of appearance of a high frequency signal as a lower frequency
signal that distorts the desired signal is called aliasing.
 To prevent aliasing from affecting the relaying functions, anti-aliasing filters
(which are low-pass filters) are used.

21. Aliasing
 Post-fault power system signals contain dc offset and harmonic components, in
addition to the major fundamental frequency component.
 In order to convert analog signals to sequences of numbers, an appropriate
sampling rate should be used, because high-frequency components which might
be present in the signal, could be incorrectly interpreted as components of lower
frequencies.
 The mechanism of a high-frequency component in an input signal manifesting
itself, as a low-frequency signal is called ‘aliasing’.
 It is the appearance of a high-frequency signal as a lower frequency signal that
distorts the desired signal.

22. Aliasing
 For explaining the phenomenon of aliasing, let us consider a signal of 550 Hz
(11th harmonic component) as the high-frequency component, as shown in Fig.
11.4(a).
 If this signal is sampled 500 times a second, the sampled values at different
instants would be as shown in Fig. 11.4(b).
 The reconstruction of the sampled sequence, and its interpretation by an
algorithm, indicates that the signal is of the 50 Hz frequency.
 This misrepresentation of the high frequency component as a low- frequency
component is referred to as aliasing.
 Therefore, for obtaining a correct estimate of the component of a selected
frequency, the sampling rate should be chosen in such a manner that
components of higher frequencies do not appear to belong to the frequency of
interest.

23. Aliasing

24. Aliasing
 Since it is not possible to select a sampling frequency (sampling rate) that would
prevent the appearance of all high frequency components as components of
frequency of interest, the analog signals are applied to low-pass filters and their
outputs are processed further.
 This process of band-limiting the input by using low-pass filter removes most of
the high-frequency components.
 Thus the effect of aliasing is removed by filtering the high-frequency
components from the input.
 The low-pass filter that accomplishes this function is called an anti-aliasing filter.

25. Analog Interface
 The analog interface makes the signal compatible with the processor.
 The outputs of the signal conditioner are applied to the analog interface which
includes sample and hold (S/H) circuits, analog multiplexers and analog to digital
(A/D) converters.
 These components sample the reduced level signals and convert their analog
levels to equivalent numbers that are stored in memory for processing.

26. Sampling
 Sampling is the process of converting a continuous time signal, such as a
current or voltage, to a discrete time signal.
 The selected sampling rate should be as high as is practical taking into account
the capabilities of the A/D converter and the processor.
 Modern numerical relays use sampling rate that are as high as 96 samples per
fundamental cycle.
 The sampling theorem states that in order to preserve the information contained
in a signal, it must be sampled at a sampling frequency fs of at least twice the
largest frequency (fm) present in the sampled information (i.e. fs ≥ 2fm).

27. Analog Interface
 A sample and hold (S/H) circuit is used to acquire the samples of the time
verying analog signal and keep the instantaneous sampled values constant
during the conversion period of ADC.
 A S/H circuit has two modes of operation namely Sample mode and Hold mode.
When the logic input is high it is in the Sample mode, and the output follows the
input with unity gain.
 When the logic input is low it is the hold mode and the output of the S/H circuit
retains the last value it had until the command switches for the sample mode.
 The S/H circuit is basically an operational amplifier which charges a capacitor
during the sample mode and retains the value of the change of the capacitor
during the hold mode.

28. Analog Interface
 An analog multiplexer has many input channels and only one output. It selects
one out of the multiple inputs and transfers it to a common output.
 Any input channel can be selected by sending proper commands to the
multiplexer through the microcomputer.
 Analog to digital (A/D) converters take the instantaneous (sampled) values of the
continuous time (analog) signal, convert them to equivalent numerical values
and provide the numbers as binary outputs that represent the analog signal at
the instants of sampling.
 The signals in the from of discrete numbers are processed by a relaying
algorithm using numerical methods. A relaying algorithm which processes the
acquired information is a part of the software.

29. Numerical OverCurrent Protection
 Overcurrent protection is the simplest form of power system protection. It is
widely used for the protection of distribution lines, motors and power equipment.
 It incorporates overcurrent relays for the protection of an element of the power
system. An overcurrent relay operates when the current in any circuit exceeds a
certain predetermined value.
 The value of the predetermined current above which the overcurrent relay
operates is known as its pick-up value Ipick-up.
 A protection scheme which incorporates numerical overcurrent relays for the
protection of an element of a power system, is known as a numerical overcurrent
protection.

30. Numerical OverCurrent Protection
 A numerical overcurrent relay acquires sequential samples of the current in
numeric (digital) data form through the Data Acquisition System (DAS), and
processes the data numerically using a numerical filtering algorithm to extract
the fundamental frequency component of the current and make trip decision.
 In order to make the trip decision, the relay compares the fundamental frequency
component of the current (I) with the pick-up setting and computes the plug
setting multiplier (PSM), at which the relay has to operate.
 If the fundamental frequency component of the fault current (I) exceeds the pick-
up Ipick-up (i.e., PSM > 1), the relay issues a trip signal to the circuit breaker. The
time delay required for the operation of the relay depends on the type of
overcurrent characteristic to be realised.

31. Numerical OverCurrent Protection
 In case of instantaneous overcurrent relay there is no intentional time delay.
 For definite time overcurrent relay, the trip signal is issued after a predetermined
time delay. I
 n orders to obtain inverse-time characteristics, the relay either computes the
operating time corresponding to the fault current or selects the same from the
look-up table.

32. Numerical OverCurrent Protection
 The numerical filtering algorithms which can be used for extraction of the
fundamental frequency component of the fault current I.
 The fundamental Fourier sine and cosine coefficients (F1 and F2) are
respectively equal to real and imaginary components (IS and IC) of the
fundamental frequency current phasor I.
 I in complex form is given by
 The current from the Current Transformer (CT) is applied to the signal
conditioner for electrical isolation of the relay from the power system, conversion
of current signal into proportional voltage signal and removal of high frequency
components from the signals using analog low-pass filter.

33. Numerical OverCurrent Protection

34. Numerical OverCurrent Protection
 The output of the signal conditioner is applied to the analog interface which
includes S/H circuit, analog multiplexer and A/D converter (ADC). After
quantization by the A/D convertor, along current (i.e., voltage proportional to
current) is represented by discrete values of the samples taken at specified
instants of time.
 The current in the form of discrete numbers is processed by a numerical filtering
algorithm which is a part of the software. The algorithm uses signal-processing
technique to estimate the real and imaginary components of the fundamental
frequency current phasor. The measured value of the current is compared with
the pick-up value to decide whether there is a fault or not. If there is a fault in any
element of the power system, the relay sends a trip command to circuit breaker
for isolating the faulty element.