Power system protection

Why Use Lockout/Tagout Procedures? Lockout/Tagout procedures prevent equipment from unexpectedly starting up. If you don’t properly lockout/tagout your equipment, it can unexpectedly start-up while you are servicing or maintaining it, resulting in injuries and even death during service and maintenance of equipment and for those involved with industrial cleaning. Many injuries occur when a co-worker starts-up machinery while another employee is still working on it. Coworkers see machinery unplugged or a switch facing the wrong direction and they think they’re doing the right thing by reenergizing the equipment. They don’t realize that they are placing you in danger. Locking and tagging out machinery will ensure that this doesn’t happen.

Types of transformers: The Transformers are largely divided into Power and Distribution Transformers and Step up or step down Transformers. Depending upon utility, the following other types of classification is also in use: (1) Generator Transformer (2) Unit Auxiliary Transformer (3) Station Transformer (4) Transmission Sub-Station Transformers (5) Auto Transformers (6) HVDC Converter Transformer (7) Arc Furnace Transformers (8) Traction Transformers (9) Earthing Transformer or the Zig-Zag (10) Generator Neutral earthing Transformer (11) Series and Shunt Reactors (strictly not a transformer)

Types of Power Connections • LT < 50KW. • HT > 50KW. • Independent feeders 11KV. •High Voltage Transmission/ Distribution is Preferred •High Voltage Transmission/ Distribution is Preferred because of Low losses & Reduced size of Conductors during transmission. • The power is transmitted at high voltage and low current. • The heat generated during transmission is proportional to current transmission. Selection of Circuit Breaker: - Power (KW/KVA/HP) - Rated Current. - System Voltage - Breaking Capacity -Making Capacity -Selection of Circuit Breaker -Making Capacity - Category A or B -Suitability to site Condition (Indian Condition) -Maintenance Definitions -Effect of harmonics System for HT/ LT connection: • HVPN Supply • GO Switch • HT Metering • VCB • Transformer • LT Panel ( Power control centre, Power factor panel, AMF) • Distribution Panel • Sub Distribution panel Selection Of GO Switch : Gang Operated Switch Comprises of :- Lightning Arresters, GO (gang operated) switch, Drop off Fuse(DO), Plate Earthings two Nos. for LA (lightening arrestor) & GO switch. Selected at 11KV Only Selection of CT & PT •CT (Current Transformer /5A) Selected as per the full load current of Transformer. •PT ( Potential Transformer) As per the system Voltage.(11Kv/220v/110v/24V)AC •Calculation of HT Current for CT (KVA/11KV/1.732) •Example:- Transformer rating ( HT current for CT/PT selection) 500Kva/11KV/1.732 = 26.24 •Tri vector Meter selected as per HT Current. VCB Panel: • Vacuum Circuit Breaker • For Switching carrying Normal Current and Breaking abnormal current at High Voltage. • 630A ( Short circuit current) and Above. • Over Load, Short Circuit & E/F protections. • Measurement of current, Voltage etc. Transformer: • 11KV( input)/433V ( Output) • Selection of Transformer as per Total load. • Buckle relay and winding temperature for safety of Transfer. These help to trip VCB. • Silica gel to check moisture content. If blue- Ok, If white or light pink- replace it. LT Panels: PCC--- Power Control Center. MCC- Motor Control Center. AMF- Auto Mains Failure Panel APFC- Automatic Power Factor Correction Distribution Panels.

Siemens Circuit-Breaker 3AP1 FG for rated voltage 245 kV.

3.6. WINDING RESISTANCE TEST: (OPTIONAL TEST) This test is to measure the both primary and secondary winding dc resistance and to confirm there is no winding discontinuity, abnormality at all taps. The test connection shall be made as shown in Fig 8.3. The test shall be conducted at all taps by four-wire method or double bridge method. Test to be done for primary and secondary winding phase by phase. This test shall be the last test to avoid DC flux remaining in core and giving erroneous values to the other tests. Limits: The test values shall be compared with factory test reports after temperature correction. Note: The test values are influenced by temperature, so the oil temperature should be noted during test. 3.7. BUSHING CT TEST: This test is to confirm the CT electrical characteristics and connectivity of CT circuits. The following tests to be performed. Insulation test as per SEC 4.2.3 Polarity test as per SEC Winding /loop resistance as per SEC Ratio test: This shall be done by primary injection at test terminal (if provided) otherwise the primary current can be driven through BCT by injecting 3 phase test voltage at primary with short circuited secondary or vice versa. Limits: Test results should match with nameplate specification or factory test results shall be taken as reference. 3.8. TRANSFORMER OIL TEST: This is to confirm the dielectric medium electrical characteristics and condition. Oil samples shall be taken from top and bottom in both tank and OLTC (if applicable). BDV test shall be performed for all the samples and recorded. One more set of samples shall be sent for Dissolved gas analysis to laboratory. Limits: BDV value for oil should match with the manufacturer instruction manual. 3.9. TRANSFORMER AUXILIARIES: Buchholz relay: Operation verification shall be done gas injection as per supplier’s instruction manual. Alarm and trip contacts wiring shall be checked for proper operation. Oil / Winding temperature indicator: - OTI and WTI shall be calibrated with standard thermometer as per supplier’s instruction manual. - For WTI shall be tested for thermal image by injecting current at current terminals as per supplier’s instruction manual. - Check alarm / trip contacts for proper function. Cooler fans: - Insulation resistance of fan motor to be checked. - Fan starting current and running current shall be noted and direction of rotation to be checked. - The overload relays should be properly adjusted. OLTC mechanism: - Operation test shall be carried out for OLTC. - Check Operation interlock. 4. APPLICABLE STANDARD: IEC-60076 Power Transformer IEC-60726 dry type power transformer #powertransformer #transformers

Power Transformer test: 1. OBJECTIVE: To confirm the physical condition and electrical characteristics of Power transformer installed in the installation. Ensure the power transformer is connected to system properly in all respect (primary and secondary). 2. TEST EQUIPMENTS REQUIRED: Insulation tester 3-phase voltage source, multimeter Digital low ohmmeter/ double bridge BDV tester 3. TEST PROCEDURE: 3.1 MECHANICAL CHECKS AND VISUAL INSPECTION: • Inspect for physical damage/ defects and oil leakage. • Verify transformer nameplate ratings in accordance with customer drawings and specifications. • Check the impact recorder records for any abnormal impacts during transit, if applicable. • Verify that positive pressure is maintained on nitrogen-blanketed transformers, if applicable. • Check tightness of all bolted connections (torque-wrench method). • Check that all grounding is securely connected (including neutral grounding). • Check that piping to Buchholz relay has proper slope. • Check the transformer wheel stoppers installed. • Top up the oil to the tank if required and drying out oil. • Check oil in the tank, conservator and bushing for proper level. • Release trapped air at the bushing turrets and tank top. • Check that valves between the tank and the radiators are open. • Check condition (colour and quantity) of silica gel in breather and oil in bath level. • Check the OTI and WTI thermal probes are fixed in the oil pockets and the oil pockets are filled with oil. 3.2 INSULATION RESISTANCE TEST: Above-mentioned precautions section4.3 shall be made for this also and primary/ secondary winding ground connection to be isolated. The voltage shall be applied between - Primary to secondary plus ground. - Secondary to primary plus ground. - Primary winding to secondary winding. Test voltage limits mentioned in table4.1. Test shall be conducted for 10min. The reading for 1 min and 10min will be noted and PI value can be calculated as PI= reading at 10min/1min. Condition of insulation indicated by PI values are listed in table 4.2. 3.3. VOLTAGE RATIO TEST: This test is to verify the voltage ratio of the transformer with its nameplate values. Test circuit connection shall be made as shown in fig 8.1. The test voltage single-phase 220V shall be applied across one of the phase primary winding (in case of star connected RN, YN, BN or in case of delta connected RY, YB, BR or RB, YR, BY) and voltage shall be measure at respective secondary winding. This test shall be done phase-by-phase for all taps. Limits: Compare the trend of voltage ratio with reference to nameplate or previous factory test results. 3.4. MAGNETISATION CURRENT TEST: This test is to verify there is no abnormality in no load magnetising current and there is no internal fault on transformer. Test circuit connection shall be made as shown in Fig 8.1. This test shall be done phase-by-phase. Apply test voltage across one of the phase of primary or secondary (preferably at secondary side) and other side kept open circuited. Measure the magnetising current drawn by the winding. This can be done during ratio test for each tap. Limits: Comparison shall be done with previous test results. 3.5. VECTOR GROUP TEST: This test is to confirm the vector relation between primary and secondary as shown in the nameplate. This will ensure the polarity and terminal marking on the transformer. The test connection shall be made as shown in Fig 8.2. Apply 3-phase test voltage on transformer primary with secondary open circuited. Short ‘R’ phase primary and ‘r’ phase secondary. Measure voltage on several points and record. Check the criteria for specified vector group with recorded results. Limits: The result should match with nameplate specification.

RCCB (Residual Current Circuit Breaker): An RCCB is essential current sensing equipment used to guard a low voltage circuit from the fault. It comprises of a switch device used to turn off the circuit ,when a fault occurs in the circuit. RCCB is aimed at guarding a person from the electrical shocks. Fires and electrocution are caused due to the wrong wiring or any earth faults. This type of circuit breaker is used in situations where there is a sudden shock or fault happening in the circuit. For instance, a person suddenly enters in contact with an open live wire in an electrical circuit. In that situation, in the absence of this circuit breaker, a ground fault may occur and an individual is at the hazardous situation of receiving a shock. But, if the similar circuit is defended with the circuit breaker, it will tour the circuit in a second therefore, avoiding a person from the electric shock. Therefore, this circuit breaker is good to install in an electrical circuit.