ح#4 الفرق بين اجهزه الوقايه الرقميه والكهرومغناطيسيه.

What is a Relay?
Definition: A device which detects the faulty conditions and initiates the circuit breaker operation to isolate the defective element from the rest of the system is known as a relay. A relay can detect many faults such as over current, over and under voltage, over and under frequency, short circuit, earth fault, etc.
Protective relays can be categorized as:
Electromechanical relays
Numerical relays
The following block diagram shows how a relay is installed in a distribution network.
Fig. - How a relay is installed in a distribution network.
As a relay is connected to the power system through a current transformer, so its designing or programming is to be done according to the parameters of the CT rather than the power system itself. The CT has a secondary voltage of 1A or 5A usually, so relays are also rated at either 1A or 5A.
👉🏼 You may be interested in an article on the Current Transformer (CT). Go through it if you want to know its operation and application in power systems.
A zone of protection is the portion of the power system which is secured by a particular protection device. For example, the figure below is showing protection zone of a differential relay. If any fault occurs in the region bounded by two CTs of differential scheme, there is significant effect on the CT ratios and the differential current (difference between secondary currents of both CTs) flowing through the relay becomes non zero. So, the relay issues trip signal and thus the equipment within the bounded region of CTs is always protected and any external fault i.e. outside the zone of protection goes undetected by the relay.

Protective Relay Settings and Parameters
Some parameters must be defined or set before the operation of a relay. They are as following:
Pick-up Current
It is the minimum amount of current at which the relay starts to operate. It is the sensing point of a relay. Pick-up current can be calculated as:
Ipick-up = Rated CT output × Current settings
Current Settings
These are used to adjust the pick-up current up to any desired value. It is usually achieved by the use of tapping on the relay coil. For example, the current flowing through a circuit is 80A, and CT connected to the circuit is of 100A/5A, then the current settings of CT will be at 80% so that it may correctly detect the 80A current as normal.
CT Ratio
In some cases, CT ratio is unknown, or its output is rated according to the relay. Then CT ratio can be selected according to the current flowing through the main circuit. For example, rated CT output is 1A and the current flowing through a circuit is 105A then possible standardized CT ratio can be 100A/1A having current settings of 105%.
Plug Setting Multiplier (PSM)
It tells us about the severity of fault and decisions regarding tripping can be made based on PSM. Its minimum value is 1 and can be calculated as:
PSM = (Fault current × 1/CT ratio) / Ipick-up
Time Setting Multiplier (TSM)
It gives the value of actual time of operation for a relay. It is a value that is used to set the tripping speed of the relay. Higher the value means increased time delay and for reducing time delay lesser will be the value of TSM. It is expressed in percentage.
Time of Operation:
It is the total amount of time taken by a system to make a trip. There are predefined graphs available for time vs PSM. For calculated PSM value, its corresponding time is selected from the graph and then the time of operation is evaluated i.e.
Time of Operation = TSM × Time from ‘time vs PSM’ curve
ELECTROMECHANICAL AND NUMERICAL RELAYS
What is Electromechanical Relay?
Definition: Electromechanical relay is a switch that is electromagnetically operated i.e. a small amount of current is used to induce magnetic field inside a magnetic core which is then used to operate a switch for controlling a large amount of current.
What is Numerical (Microprocessor) Relay?
Definition: Numerical relays are those in which electrical (analog) quantities are sampled and changed into numerical or digital data, that is treated arithmetically and then a tripping decision is issued accordingly.
#coursefree #electrical #mvsg #protection #الوقاية #power
#مهندس_عمرو_عفيفي #كورس_فري