Power system protection

Protection function test: 1. OBJECTIVE: To confirm the some of specific protection systems (like differential, distance) are working properly as it required manner. These tests required to be performed before and after commissioning. 2. TEST INSTRUMENTS REQUIRED: Primary injection test kit Clamp meter Multimeter Secondary injection test kit 3. TEST PROCEDURE: 3.1. STABILITY TEST: The stability and sensitivity test shall be performed for unit protection to prove the stability for through fault and sensitivity for the internal faults. The principle of the test is as followed. The protected zone of unit protection is restricted by location of the CTs. For stability test the primary current shall be injected through one set of two CTs such a way one of current inwards and other outwards. At relay terminal no operating current shall be observed for proper connection. Sensitivity test shall be conducted with same set-up with either reversed CT polarity or injecting primary current on only one CT of the set. At relay terminal, operating current shall be observed for proper function. Busbar Differential Protection: Sensitivity Test: The test connection shall be made as shown in Fig 14.1. This is applicable for high impedance differential relay. It is required measure the minimum primary operating current with all CTs are connected to the zone. Inject primary current and increase slowly until the relay operates. All the measurements shall be done as per test form. In case of bias differential protection this shall be done with one set of CTs injected and one of CT secondary reversed. Stability Test: The test connection shall be made as shown in Fig 14.2. Inject primary current of minimum 50 % primary operating current. The CTs secondary current and differential current at relay shall be measured. A zero differential current measurement shall be satisfactory results. Transformer Differential Protection: Stability Test: The test connection shall be done as shown Fig 14.3 , 14.4. The test voltage shall be injected to a winding with another winding shall be shorted after the CT with nominal tap. Note that the test voltage is injected before the diff CT. The CTs secondary current shall be measured. At relay only relay through current will be observed and differential current will not be observed for proper connection. The above test shall be done for minimum and maximum tap. During this test a small differential current will be observed. A sensitivity test shall be performed with same set-up and by reversing one of the side CT secondary in all phases. 3.2. PILOT WIRE PROTECTION END-TO-END TEST: This is the test to confirm the pilot wire protection is connected properly with remote end and ensure the operation with single end feed. This test shall be performed as manufacturer recommendation and the limits are followed.

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MOULDED CASE CIRCUIT BREAKERS: • MCCBs have higher fault current withstand capacities compared to MCBs. • MCBs – 4.5kA, 6kA, 10kA and 16kA (rarely). • MCCBs – 25kA, 36kA, 50kA, 65kA and 100kA. • So, MCCBs are required to be used in places where the fault current rating is high. • MCCBs are available from 20A to 2500A and in different poles such as SP, DP, TP and 4P. • MCCBs are used for the followings purposes: Overload protection. Short circuit protection. Isolation / Disconnection. • Overload protection is achieved by a bimetallic contact as in the case of MCBs. • Short circuit protection is achieved based on the principle of electromagnetism. Characteristics of MCCBs (a) Rated Frame Current (Inm) This refers to the maximum current that the MCCB is rated to handle. This defines the upper limit of the adjustable trip unit. (b) Rated Current (In) This refers to the current that determines when the MCCB trips due to overload protection. This can be adjusted to a maximum of the Inm. (c) Rated Insulation Voltage (Ui) This refers to the maximum voltage which MCCB can resist in lab conditions. (d) Rated Working Voltage (Ue) This value is the rated voltage for the continuous operation of the MCCB. This is normally the same as or close to the system voltage. (e) Rated Impulse Withstand Voltage (Uimp) This determines the ability of the MCCB to withstand transient over voltages. The size of the impulse used for this testing is 1.2/50 µs. (f) Service / Operating Short Circuit Breaking Capacity (Ics) This is the maximum fault current that a MCCB can handle without being permanently damaged. MCCBs are generally reusable after fault interruption operation provided that they do not exceed Ics. The higher the Ics, the more reliable the MCCB is. This is the maximum fault current value that a MCCB can handle. (g) Ultimate Short Circuit Breaking Capacity (Icu) If the fault current exceeds this value, the MCCB will be unable to trip. In other words, if the fault current exceeds Ics but does not exceed Icu, the MCCB can still remove the fault, but may be damaged and require replacement. (h) Overload Relay Trip Current (Ir) • There are two types of trip units for the MCCBs; Electronic Trip Unit (E) and Thermal-Magnetic Trip Unit (TM). • The trip unit in the E type MCCBs can be adjustable in the range of 0.4 – 1.0 times In (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0). • The trip unit in the TM type MCCBs can be adjustable in the range of 0.8 – 1.0 times In (0.8, 0.9 and 1.0). • In = 400A • Ir = 400 x 0.9 = 360A • For the fixed MCCBs, The In = Ir. Categories of the Circuit Breakers As per IEC 60947-2, there are two main selectivity categories. Those are Category A and B. Category A: Mainly MCBs and some MCCBs comes under this category. In the event of a short circuit, they trip immediately. Category B: Mainly MCCBs comes under this category. In the event of a short circuit, they will not trip immediately and hence allow the downstream circuit breaker to trip. • The Category A circuit breakers will be used for the protection of final circuits. • The Category B circuit breakers will be used in MDBs, SMDBs and MCCs / MCPs.

Types of Distribution Boards 1. Main Distribution Board (MDB) 2. Sub Main Distribution Board (SMDB) 3. Distribution Boards (DB) / Consumer Units (CU) 5. Motor Control Centers (MCC) / Motor Control Panels (MCP) 4. Automatic Power Factor Corrections Units (APFCU) Main Distribution Boards (MDB) • There will generally be one MDB for an Electrical Installation such as Apartment, Hospital, Factory, etc. • However, there will be more than one MDBs in large Electrical Installations such as Shopping Complexes, Hotels, etc. • The Low Voltage (LV) output of the Transformer will be directly connected to the MBD. Sometimes, MDB will consist of Automatic Transfer Switch (ATS) also in order to receive power supply from CEB / LECO and Generator. The fault current at MDB level is very high as it is very closer to the Transformer (10 – 15 m distance). • So, it mainly consists of ACBs and MCCBs as the fault current rating of those are very high. • MDBs are generally made out of sheet steel of 1.5 – 2.0 mm thickness. • MDBs are generally Floor Mounted Type. Sub Main Distribution Boards (SMDB) • There will generally be one SMDB per floor / section of an Electrical Installation. • The SMDBs are directly fed from MDBs. • The fault current at SMDB level is lesser than that of MDB. Nevertheless, it is more than the fault current ratings of the standard MCBs. • Therefore, SMDBs mainly consist of MCCBs and MCBs. • SMDBs are generally made out of sheet steel of 1.2 – 1.5 mm thickness. • These generally come as Wall Mounting Type. Nevertheless, there are Floor Mounted Type SMDBs also. Distribution Boards (DB) / Consumer Units (CU) • There will generally be one DB / CU in a designated consumer area such as Apartments, Guest Rooms, Patient Rooms, etc. • These are directly fed from SMDBs. • The fault current at DB / CU level is generally below 10 kA. • Therefore, DBs / CUs mainly consist of MCBs, whose fault current rating comes as 4.5kA, 6kA and 10kA. • DBs / CUs are generally made out of sheet steel of 1.0 – 1.2 mm thickness / Polycarbonate. • These are feeding to all the final circuits such as light points, socket outlets, etc. • These generally come as “surface mounted” type and / or “recessed mounted” type.

There many electrical safety device for motor control centre MCC: 1-Type of load The type of load that a MCC controls affects the choice of electrical safety devices. For example, some loads are resistive, such as heaters and lighting, while others are inductive, such as motors and transformers. Inductive loads require more current to start than to run, and they can generate back electromotive force (EMF) when switched off. Therefore, inductive loads need safety devices that can handle higher inrush currents and suppress voltage spikes. Some common electrical safety devices for inductive loads are magnetic circuit breakers, overload relays, and surge suppressors. 2-Voltage level The voltage level of a MCC determines the insulation and clearance requirements of the electrical safety devices. For example, low-voltage MCCs (below 600 V) can use air circuit breakers or molded case circuit breakers, which have smaller dimensions and lower costs than medium-voltage MCCs (above 600 V). Medium-voltage MCCs require vacuum circuit breakers or gas-insulated circuit breakers, which have higher insulation and arc-extinguishing capabilities. Additionally, medium-voltage MCCs need more protection devices, such as potential transformers, current transformers, and relays, to monitor and isolate faults. 3-Coordination scheme The coordination scheme of a MCC defines how the electrical safety devices operate in the event of a fault. For example, some MCCs use selective coordination, which means that only the device closest to the fault trips, while the rest of the system remains energized. This minimizes the impact of the fault on the production and reduces the downtime. Other MCCs use non-selective coordination, which means that multiple devices trip simultaneously, regardless of their location to the fault. This maximizes the safety of the personnel and the equipment, but it also increases the outage duration. The choice of coordination scheme depends on the criticality of the load, the availability of backup power, and the cost-benefit analysis. 4-Installation location The installation location of a MCC influences the environmental and physical factors that affect the electrical safety devices. For example, some MCCs are installed indoors, where they are protected from moisture, dust, and temperature fluctuations. Other MCCs are installed outdoors, where they are exposed to harsh weather conditions and vandalism. Therefore, outdoor MCCs need more robust and durable safety devices, such as metal-enclosed circuit breakers, weatherproof enclosures, and padlocks. Additionally, the installation location determines the accessibility and maintenance requirements of the electrical. Components such as pT's , CT's, fuses , different protection motors in Mcc, , good configuration of unit trip in incomer of circuit breaker , good capacitor bank, ... #motorcontrolcentre #MCC #lowvoltage

Protection Relay test 1. OBJECTIVE: To verify the physical condition and electrical characteristics of protective relays. 2. TEST EQUIPMENTS REQUIRED: Relay test unit 3phase/ 1phase 3. TEST PROCEDURE: 3.1. MECHNICAL CHECKS AND VISUAL INSPECTION: - Inspect for physical damage / defects. - Check for nameplate information for correctness. - Carryout visual check as per manufacturer recommendation. - Check the wiring connections are as per approved drawing and verify tightness. 3.2. SECONDARY INJECTION TEST: This test is to confirm the electrical characteristics of protective or control relay and the healthiness of the same. This will be carried out with relay alone (without external interfacing). The secondary injection test shall be performed as per manufacturer precommissioning instructions and the forms shall be followed for record. The relay shall be set for final approved setting. 3.3. PROTECTION / CONTROL FUNCTIONAL TEST: This test is to confirm the above tested relay is properly connected / interfaced with system, that required. This will verify all inputs to relay and outputs from the relay are properly connected, as it required. An injection (current/ voltage) shall be made and checked the functions outputs (trip, alarm, indication, control). Inputs shall be initiated from the source and monitored the proper outputs from the relay. 4. APPLICABLE STANDARDS: IEC 60255: Electrical Relays.

Megger testing is a type of electrical test that evaluates how well insulation performs in a part of electrical equipment. A voltage is provided, and then a mega-ohm meter is used to measure the resistance of each phase connection between the motor leads and the ground. Working Principle of Megger: Megger operates on the principle that current flowing through a conductor placed in a magnetic field experiences a torque. Mathematically, the force is proportional to the current and the magnetic flux. Where, Vector Force = force and direction of the current and magnetic field. Case (I): If the resistance has a higher value then the pointer on the device coil will move towards infinity. Case(II): If the resistance value is low then the pointer will indicate zero resistance. The comparison between the known resistance and insulation resistance shows the condition of insulation quality. Compared to the other electrical measuring instruments, Megger provides the highest measurement accuracy.

Disconnecter/Ground switch : 1. OBJECTIVE: To verify the physical condition and electrical characteristics of disconnector / ground switch. 2. TEST INSTRUMENTS REQUIRED: Insulation tester Micro ohmmeter HV test kit 3. TEST PROCEDURE: 3.1. MECHANICAL CHECKS AND VISUAL INSPECTION: - Inspect for physical damage/ defects. - Check nameplate information for correctness. - Check the tightness of all the bolted connections. - Check for smoothness operation. - Check that ground switch is connected to earth bar. - Check the mechanical interlocks. 3.2. INSULATION RESISTANCE TEST: The test voltage shall be between phase to earth and across the poles. - Each phase to earth (or body). - Across the pole for each phase. The applied test voltage limits shall be as per table 4.1. 3.3. CONTACT RESISTANCE TEST: This test is to confirm the resistance of the main contacts. Inject 100A DC current through the main contact by keeping disconnector/ earth switch closed. The voltage drop across the contact is measured and resistance is calculated. In many instruments resistance will be a direct reading. Limits: The obtained values shall be compared with factory test reports or manufacturer claimed values could be taken as reference. 3.4. HIGH VOLTAGE TEST: This test shall be performed as per section explaid befor. 4. APPLICABLE STANDARD: IEC 60265: HV switches 1-52KV

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Electrical safety and why does it matter? Every year, there are around 1,000 accidents at work involving electricity, and most of these are preventable. Did you know that 16% of all work-related fatalities stem from electrical incidents? It's a staggering statistic that demands our attention. In 2005, Michael Adamson passed away. A 26-year-old experienced electrician was involved in a preventable electrical incident. His story serves as a reminder of the potential consequences of neglecting safety measures. Electric shocks or "flashovers" can result in life-changing injuries, leaving lasting scars. We must prioritise risk assessment and minimise hazards when working with electricity. Let Michael's story resonate with us. We understand the gravity of our work, and we are unwavering in our commitment to practising Safe Isolation Procedures. Share your thoughts on electrical safety in the comments below.