# Digital Protection

• 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.

1- Transducer

2- Relay

3- Circuit Breaker

4- Battery

• What are the differences between dead tank and live tank CBs ?

Live Tank

SF6 CB

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UntitledCT are in one side of CBCTs can be on both sides
UntitledCTs are stand aloneCTs attached with CB bushings
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• 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 ?
• Level detection
• Current in abnormal path
• Current balance
• Phase angle comparison (directional relay)
• Change of parameters(distance relay)
• harmonic content
• frequency sensing
• Buchholz relay
• Non-electrical quantities

• What are relay design types?
1. Electromechanical relays
1. Magnetic Attraction
1. Plunger (solenoid)
1. attracted armature
1. balanced beam
1. Magnetic Induction
1. induction disk
1. induction cup
1. Solid-State relays

3. Computer 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 functions
• multiple setting groups
• programmable logic
• self-monitoring
• self-testing
• sequence of events recording
• oscillography
• ability to communicate with other relays and control computers
• Cost

They depend on commercially available components

• custom logic schemes
• allow users to develop their own logic schemes
• panel space
• 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?

• Transducers

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.

• Digital input

It provides the status of CBs and isolators to the relay.

• Analogue interface

It includes amplifiers, multiplexers and analog-to-digital (A/D) converters. It uses the output of analog input.

• Microprocessor

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.

• RAM

It is used for storing data temporarily.

• ROM

It is used for storing data permanently.

• Control
• Communitcation
• Digital Output

It transmits the trip output signals to the power system.

• Power supply

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 ?
• Discuss Anti-Aliasing.

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

1. using low pass filter to remove high frequencies
1. 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

• DSP equations
• DSP Examples

# 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.

• DFT equations

non- recursive

same Zh and theta_h

• DFT Example

3rd Harmonic

# 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

• Example :

• What are advantages of least squares technique
1. 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.
1. 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 ?
1. Impedance based methods
• the most used method
• simplest
• low cost
• accuracy limitation due to impedance change, mutual coupling, series compensation and variability of fault resistance.
1. Learning based methods
• doesn't require detailed information about system
• doesn't require complex and expensive devices
• based on trial and error technique
1. 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.
• 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.

1. double end technique

GPS used to synchronize the arrival time

2. Single-end technique

• What are fault types wrt their instance of occurrence ?
1. single fault
1. 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

DEFINITIONS

• 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.
• Sampling frequency

• Error percentage
• min distances

• 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