A distance protection element measures the quotient impedance (V/I), considering the phase angle between the voltage V and the current I.
Line distance relays constantly measure the line voltage and current, constantly performing Ohm’s Law calculations to measure the load impedance flowing through the relay at any moment in time.
If the measured impedance is less than the line impedance, there must be a fault on the line and the relay should trip with no intentional time delay.
In the event of a fault, sudden changes occur in measured voltage and current, causing a variation in the measured impedance.
The measured impedance is then compared against the set value. Distance element will trip the relay (a trip command will be issued to the CB associated with the relay) if the measured value of the impedance is less then the value set.
Distance protection is used to protect transmission lines . It is a non unit protection.
Why Distance Protection ? Why not Overcurrent relay?
The reach of over current relay is function of Source Impedance which varies considerably, making it difficult to get fast and Selective tripping. So , distance protection is used as a primary protection.
In Fig. the impedance measured at the relay point A is Zin , where x is the distance to the fault (short circuit), and R and X are transmission line parameters in per unit length. The line length is l in the fig.
Operating characteristics of distance protection elements are usually represented using R-X diagrams.
Various operating Characteristics
Protective relays respond and operate according to defined operating characteristic and applied settings.
Each type of protective relay has distinctive operating characteristic to achieve implementation objective: sensitivity, selectivity, reliability and adequate speed of operation.
Required information for Protective Setting
Line Parameters
Line length, voltage level, conductor size and conductor type used for local as well as remote substations.
This information is used to calculate the parameters (positive and zero sequence resistance, reactance) for each section.
Maximum load current or apparent power (MVA) corresponding to the emergency line which can be obtained from the table of standard conductor rating (available in each utility).
The number of conductors in a bundle has to be taken into consideration.
Conductor Specifications if not available, refer to CEA manual ‘Annexure-V, Table 1(a), Line parameters’.
Transformer Parameters
The manufacturer’s positive and zero sequence impedance test values have to be obtained.
The transformer nameplate normally provides the manufacturer’s positive sequence impedance values.
Voltage ratio, MVA rating, %Impedance value
CT & PT Ratios
Obtain the CT & PT ratios as indicated on the protection diagrams.
For existing circuits, it is possible to verify the ratios indicated on the diagrams by measuring the load currents on site and comparing with a known ratio.
Arc & Tower footing resistance
Used in resistive reach calculation.
Advantages of Distance Protection
Speed and Reliability: Distance protection is fast because it provides a direct measurement of fault location and does not require the relay to rely on time-delay mechanisms or communications with other relays, which can be affected by line conditions or other factors.
Selective Fault Isolation: It allows for selective fault isolation by only disconnecting the faulty section of the line, leaving the healthy portions of the transmission system operational. This enhances system reliability and minimizes disruptions.
No Need for Voltage Reference from the Fault Side: Unlike overcurrent protection, distance protection does not require voltage reference from the faulted side of the line. This makes it particularly useful for protecting long and complex EHV transmission lines.
Coordination with Other Protection Relays: Distance protection can be coordinated with other protection schemes, such as overcurrent, differential, and directional protection, to provide comprehensive protection for the entire transmission network.
Works in Different Fault Conditions: Distance protection is effective for both symmetrical and asymmetrical faults (e.g., line-to-line, line-to-ground faults) and can be configured to provide different fault type settings.
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