Power system protection

Metal clad switchgear : 1. OBJECTIVE: To verify the physical condition and proper connections of bus bar. 2. TEST EQUIPMENTS REQUIRED: Insulation test Micro ohmmeter High voltage tester Torque wrench 3. TEST PROCEDURE: 3.1. MECHANICAL CHECKS AND VISUAL INSPECTION: - Inspect switchgear and all components for any physical damage / defects. - Check nameplate information for correctness. - Inspect enclosures for proper alignment, foundation fixing, and grounding and vermin entry. - Inspect all covers, panels section and doors for paintwork and proper fit. - Check all the transport locks are removed. - Check for smooth and proper movement of racking mechanisms, shutter, rollers, rails and guides. - Check proper alignment of the primary and secondary contacts. - Check operation of all mechanical interlocks. - Check tightness of all bolted connections. - Check for correct phasing connection of bus bar. - Perform mechanical check and visual inspection for breaker/ Contactor as per section. - Perform mechanical check and visual inspection for instrument transformers as per section - Perform mechanical check and visual inspection on all disconnect and grounding switches as per section. 3.2. INSULATION RESISTANCE TEST: It includes panel enclosure, busbar, CT & breaker. The following precautions should be taken care, before starting the testing. - A visual inspection will be made to ensure the surface dust and moisture has been removed from the component under test. - Ensure the component is isolated from other connected system, which may feed back to other components or circuits not under test. On testing, voltage shall be applied between one phase and other phases connected with ground, testing shall be repeated for other phases as mentioned above. Test voltage limits mentioned in table 4.1. 3.3. CONTACT RESISTANCE TEST: This test is to confirm the busbar joints are connected properly and verify the tightness. The test connection diagram is as shown in Fig. The test shall be done with CBs inserted and closed. Measure the contact dc resistance between panels by injecting 100ADC. This will include busbar joint, CB contact resistance, CB cluster resistance, and CT primary resistance (if applicable). Limits: The obtained results should be similar for all phases for each set of measurement. Other influencing factors to be considered, like length of the measured path, rating of the busbar, rating of CB, rating of CT and temperature. 3.4 HIGH VOLTAGE TEST: This shall be performed as per section that we see before 9.4. APPLICABLE STANDARD: IEC60298: – AC metal enclosed switchgear and control gear for rated voltage above 1KV to 52KV.

Under voltage relay test: 1. Connect PT Terminals, Aux. Voltage terminal (if required), Trip contact Terminals of your kit to relay 2. Now set AC Normal voltage to 63.5V (P-N) or 110V (P-P) and Aux. Voltage to 24V or 110V DC/AC(If necessary). 3. Now enable trip command in your Kit and make sure you close the locking system of relay on theleft side 4. When you apply Normal voltage Vn= 63.5V (P-N) the flag will be reset 5. Now gradually Increase voltage, at certain stage the relay coil will Pick off and gradually decrease to get drop off voltage. Both voltages will be near the set voltage. Note down the readings 6. %Vn is set to 90 which means when voltage reduces below 90%X 63.5 =57.15V (P-N) the relay will trip at t Sec = 4 Sec. 7. Continue the trail for Different Voltage Setting (%Vn) and for different value of time (t Sec)

Over voltage relay test: 1. Connect PT Terminals, Aux. Voltage terminal (if required), Trip contact Terminals of your kit to relay 2. Now set AC Normal voltage to 63.5V (P-N) or 110V (P-P) and Aux. Voltage to 24V or 110V DC/AC (If necessary). 3. Now enable trip command in your Kit and make sure you close the locking system of relay on the left side 4. Apply Normal Voltage. Now gradually Increase voltage, at certain stage the relay coil will Pick off and gradually decrease to get drop off voltage. Both voltages will be near the set voltage. Note down the readings 5. %Vn is set to 120 which means when voltage increases above 120% X 63.5 =76.2(P-N) the relay will trip at t Sec = 0 Sec Instantaneously. 6. Continue the trail for Different Voltage Setting (%Vn) and for different value of time (t Sec)

distance protection(21): non-unit protection whose operation and selectivity depend on local measurement of electrical quantities from which the equivalent distance to the fault is evaluated by comparing with zone settings. [SOURCE: IEC 60050-448:1995, 448.14.01]

SF6 is a synthetic, odourless gas that's used in the electricity industry to keep networks running safely and reliably. It's highly stable, non-toxic, non-flammable and electronegative, which means it will not form other compounds that will alter its state and effectiveness.

Transformer protection

Phase-to-Ground (SLG ) Fault Through High Resistance: 1- High-resistance faults can occur on EHV systems, and normally such faults are cleared from the system by backup directional ground time overcurrent relays. These types of faults offer a challenge to relaying systems that are based on operating principles using impedance (distance) measurements. In this case, the delayed clearing is acceptable since these high-resistance faults have less impact on the transient stability of the system. 2- Protection of EHV lines should be based on applying redundant ground directional overcurrent protection to provide a guarantee to clear high-resistance ground faults and to sense open-phase conditions. 3-The communication schemes are used nowadays to provide faster tripping for the high resistive fault and open phase (AIDED SCHEME). 4- for the communication scheme, practically I think negative sequence voltage polarization to identify the forward direction of the fault, in many cases more reliable than zero sequence polarization voltage especially in parallel lines since the zero sequence can be affected by mutual coupling and cause the relay to mal-operate during external faults resulting in cascade trippings. What do you think is better to be used in the DEF-aided scheme, negative sequence voltage polarization or zero sequence?

Negative sequence component for directional overcurrent protection: Since negative- and zero-sequence quantities are only present in relatively large values for fault scenarios, they are often used to determine that the fault exists on a power system. Negative-sequence can be used to detect phase-to-phase, phase-to-ground, and phase-to phase-to-ground faults. Zero-sequence can be used to detect phase-to-ground and phase-to phase-to-ground faults. The traditional way to identify the fault direction using the negative sequence element, is by utilizing the following torque equation: torque (T)= |V2|.I2|.cos(<-V2-(

High Voltage and Partial Discharge Test of Cables: High Voltage Testing 1.1. Objective of Test High voltage test is performed to check the strength of cable insulation. Through this test it is to be ensured that cable will operate safely during rated electrical quantity. 1.2. HV Testing Procedure 1. One phase is directly connected to HV terminal of tester 2. Other two phases needed to be shorted together 3. These shorted phases should be shorted with the cable armour 4. Cable armour must be earthed with HV tester “Earth” terminal 1.3. Calculations Test Voltage = 4 x Phase Voltage (Vph) Example: If cable is for 11 kV Test Voltage = 4 x(11 /√3) = 25.4 kV The test voltage of 4 – times of Phase voltage will be applied for 4 – hours to test the cable according to the standard. 1.4. Disadvantage • This test is disruptive type of test. • Repeating this test again and again will degrade the cable insulation. That is the reason this test use to be performed rarely. 1.5. Need of Test • During cable inspection for purchasing decision • After commissioning to validate the cable health 2. Partial Discharge Test This test is performed to test the healthiness of the equipment’s insulation 2.1. Partial Discharge It is the phenomena of localized dielectric breakdown of small portion of electrical insulation under high electrical stress. • Partial Discharge Test is a “diagnostic test” • Partial Discharge output in “Pico-Columb” 2.2. Requirement of PD Test • If the cable is tested through IR test, the output of the test will declare the cable healthy because IR test does not consider voids/ partial discharge phenomena • So, to study the partial discharge on equipment and to assess their insulation, partial discharge test is required 2.3. Procedure of PD Test: 1. Supply will be connected to the conductor 2. The armour will be earthed with tester “Earth” terminal 2.4. Calculations Test Voltage = 1.73 x Phase Voltage (Vph) Test output will be in ‘pC” The value must not exceed “10 pC” if the equipment is new

How can the network be maintained stable and no total separation (black out)? - In the beginning, we have to know that there is an inverse relationship between frequency and loads, where we find that the more loads this leads to a decrease in frequency, hence the idea if we want to maintain the electrical power generated with loads, we have to maintain the frequency of the network, whether it is 50Hz or 60Hz, and any decrease in frequency means that there may have been a shortage in the energy generated or an increase in loads - Therefore, we find that with all SwitchGear distributors on the voltages of (11-13.8-22) palm, an Auto Under Frequency Load Shedding device is installed and the loads are divided on the device for a set of stages at different frequencies and the goal is to remove a group of loads instantaneously from the network in the event of a decrease in frequency to maintain that no units come out of generation or the occurrence of Black Out - Setting varies according to different regions, but if we assume that we are working at a frequency of 50Hz, the stages will often be like this: - First phase at 49.4 HZ Second stage at 48.8 HZ The third stage with a frequency. 48.2 HZ - There is another Protection Function through which it can be inferred that there is a problem in the network, such as: - 1- Under voltage 2- Reverse power flow

Periodic testing should focus on carrying out steps that detect most in-service hardware failures and avoid additional testing that tends to reverify the design, software behavior, or the fundamental installation correctness that were already confirmed. Excessive testing risks accidental introduction of problems and work errors that leave the system unable to protect after the test is complete and the technicians have left the site. This is especially true for invasive testing that calls for taking systems out of service, disconnecting circuits, changing settings, or opening unit cases. Note that for electromechanical relays, as well as solid-state and microprocessor-based devices, users have been accustomed to reverifying pickup characteristics during each periodic test because some internal failures can change these characteristics. Relay technicians open test switches and apply a large set of boundary tests from a computer-operated test set. For microprocessor-based devices with self-monitoring and diagnostics, internal failures have different effects, most of which can be observed during normal operation via data communications or front-panel data checking. For example, metered nonfault data that the processor communicates or displays can show any measurement error that could influence trip characteristics. Although settings could be incorrect, they can also be checked via data communications or the panel. The only element of the tripping chain that might need an overt periodic test is the trip contact and circuit to the breaker, and that can sometimes be tested via communications or the relay front panel. Periodic maintenance tests can thus be minimally invasive, and the risk of problems caused by maintenance activity is reduced. If the scheme is designed and installed with this opportunity in mind, the user may be then able to carry out some or all of the periodic checking without entering the substation. In making efforts to detect every possible failure, the user should balance the risk from a missed element of low failure probability versus the maintenance risk of introducing a disruptive step to check it. 4.Troubleshooting tests following operations It is important to emphasize the value of verifying overall system performance following correct as well as incorrect operations by retrieving and analyzing sequence-of-events and oscillographic records captured from various devices and recorders for nearby disturbances. It is also suggested that correct operations also be studied to verify security and quantify nearness to trip. Some key steps in reviewing performance of relay systems after operation include the following: a)Periodically review the application in light of power system evolution and protection and control system changes. b)Analyze relay or digital fault recorder (DFR) data from disturbances for which the protection system did or did not operate. c)Consider correctness of logic, characteristics, and set points. d)When problems appear, carry out commissioning-like tests to demonstrate continuing suitability or to verify needed changes. e)Make and verify needed changes to the periodic test procedures.