- Lec1 : introduction to protection
- Lec2 : introduction to digital protection
- Lec3 : DSP (recurrsive and non recurrdive )
- Lec4 : DFT
- Lec5 : Wavelet
Lecture 1 : Introduction to Protection
What is Relaying ?
Relaying is the branch of electric power engineering concerned with the principles of design and operation of relay that detects abnormal power system conditions, and initiates corrective action as quickly as possible (few ms) in order to return power system to its normal state.
Mention the elements of a protection system.
3- Circuit Breaker
Define : Reliability, Selectivity, Speed of Operation, Simplicity and Economics. What are the basic objectives of system protection.
Reliability : Assurance that the protection will perform correctly
Dependability : operate correctly for all the faults for which they are designed to operate
Security: will not operate incorrectly for any fault.
Selectivity: Maximum continuity of service with minimum system disconnection
Speed of Operation : Minimum fault duration and consequent equipment damage and system instability
Instantaneous : operate as soon as a secure decision is made. No intentional time delay.
Time Delay : An intentional time delay inserted
Simplicity :Minimum protective equipment and associated circuitry to achieve the protection objectives.
duplicate, backup or breaker-failure protection systems.
Economics: Maximum protection at minimal total cost
How to judge relaying performance?
Correct : (99% of operations are correct)
- At least one of the primary relays operated correctly.
- None of the backup relays operated
- fault isolated in the time expected.
Incorrect (due to)
- misapplication of relays
- incorrect setting
- personnel errors
- equipment problems or failures (relays, breakers, CTs, VTs, station battery, wiring, pilot channel, auxiliaries, and so on).
No Conclusion :
one or more relays have or appear to have operated, but no cause can be found. No evidence of a power system fault
What ate the methods of fault detection ?
Current in abnormal path
Phase angle comparison (directional relay)
Change of parameters(distance relay)
What are relay design types?
- Electromechanical relays
- Magnetic Attraction
- Plunger (solenoid)
- attracted armature
- balanced beam
- Magnetic Induction
- induction disk
- induction cup
- Magnetic Attraction
- Solid-State relays
3. Computer relays
- Electromechanical relays
Current and Voltage Transformers (Lec 2)
Nonpilot overcurrent Protection of TLs
Directional current relays
Lecture 2 : Microprocessor-based relays
What are microprocessor relays ?
They are relays that utilize software based numerical measuring techniques.
What are the benefits of μp-based relays?
- multiple setting groups
- programmable logic
- adaptive logic
- sequence of events recording
- ability to communicate with other relays and control computers
They depend on commercially available components
custom logic schemes
- allow users to develop their own logic schemes
- required much less space for the hardware integration
- and the multiple functionality of one device
burden on instrument transformers
- have less burden on CT and VT
- can detect CT saturation
- require fewer CT or VT (as zero seq. current can be derived numerically)
sequence of events and oscillography
to analyze the performance of relays and system disturbances
self-monitoring and self-testing
What are the shortcomings of μp-based relays?
short life cycle
because every generation has better functionality that the previous
susceptibility to transients
setting and testing complexity
proper procedures are to be followed to insure that correct settings and logic are activated
What are the major functional blocks of up-based relay?
reduce voltage and current to 67 V and 5 A
Analogue input :
isolates the relay from the power system, reduces the level of the input voltages, converts currents to equivalent voltages and remove high frequency components from the signals using input filters.
It provides the status of CBs and isolators to the relay.
It includes amplifiers, multiplexers and analog-to-digital (A/D) converters. It uses the output of analog input.
It processes the acquired information to decide whether the power system is experiencing a fault/abnormal operating condition or not. If it is, the relay sends a command open one or more CBs for isolating the fault zone of the power system.
It is used for storing data temporarily.
It is used for storing data permanently.
It transmits the trip output signals to the power system.
It is used to provide energy to the relay during normal and abnormal operating conditions of the power system.
What are the subsystems of a relaying computer?
Sketch the signal flow diagram of a numerical relay.
How does the scaling of measured current and voltage happen ?
What's the process of multiple signal sampling?
1- single ADC with multiplexed input
2- Sample and hold added to each channel
3- Separate ADC for each channel
How to calculate the phase shift between two signals x, y ?
Aliasing is the misrepresentation (during the conversion from Analog to Digital) of the high frequency component as a low frequency component.
Method to Minimize Aliasing
- using low pass filter to remove high frequencies
- digital filtering can be added as well
sampling rate and aliasing are interviewed by Nyquist Criterion
anti aliasing filter c/cs
Lecture 3 : DSP Technique
Lecture 4 : DFT Technique
Mention a case at which we need DFT
for 3ph or LL fault, where there is a DC offset, that would cause the current rms increase, causing overreaching, so we use the fundamentals rms to detect fault.
same Zh and theta_h
Lecture 5 : Least Squares Method
What's Least Error Squares used for?
it's used to estimate the phasors of fundamental and harmonics. of voltages and currents
it's based on
minimizing the error between actual and assumed waveforms
How is the assumed waveform modeled ?
an exponentially decaying DC and fundamental and harmonics
from it with taylor expansion
and from expanding the summation
from trigonometric relations
What are advantages of least squares technique
- method is that the sampling rate does not need to be an integer number of samples per cycle. However, the sampling rate needs to be proportionally adjusted when the power system frequency changes, to minimize the magnitude and angle errors introduced by this change.
- the description of the curve to be fitted does not need to be a sine function, in this sense the least squares method is more general.
Lecture 6 : Wavelet
What are fault allocation methods ?
- Impedance based methods
- the most used method
- low cost
- accuracy limitation due to impedance change, mutual coupling, series compensation and variability of fault resistance.
- Learning based methods
- doesn't require detailed information about system
- doesn't require complex and expensive devices
- based on trial and error technique
- Travelling wave fault location
- the most accurate fault location method
- not affected by inception angle, mutual coupling or fault resistance
- but as it propagates with the speed of light it requires high sampling rate and accurate timing and synchronization devices.
- Impedance based methods
how does traveling wave fault location work?
when a disturbance occurs in a transmission line or cable a forward and backward, high frequency traveling wave signals will be generated propagating toward both transmission line ends.
by determining the arrival time of those signals the fault location could be determined accurately.
- double end technique
GPS used to synchronize the arrival time
2. Single-end technique
What are fault types wrt their instance of occurrence ?
- single fault
- simultaneous faults
simultaneous faults can't locate using previous techniques
How to discriminate between single and simultaneous faults
if it's single fault
if it is simultaneous
What's discrete wavelet transform
Wavelet transform is a mathematical technique used in signal analysis.
works a high and low pass filter, so it can represent the high frequency signals in both time and frequency domains accurately.
Discuss parameters effect on the performance of fault location algorithm.
fault inception angle
static var compensator
Explain the modified algorithm for simultaneous fault with time shift.
supposing we know x, then T1 to end A and T2 to end B
for phase fault
for ground fault