MI-270032 VT’s and CT’s Testing

PURPOSE

VT’s and CT’s are Instrument transformers that are used both to protect personnel and apparatus from high voltage, and to allow reasonable insulation levels and current-carrying capacity in relays, meters, and instruments such as voltmeters, ammeters, wattmeter’s, and time/overcurrent relays. Instrument transformer performance is critical in protective relaying, because the relays are only as accurate as the instrument transformers. Voltage transformers (VTs), which were also formerly referred to as potential transformers (PTs), or current transformers (CTs) testing are critical to the performance of the above listed devices (e.g., meters and relays). Figure below illustrates how instrument transformers are used (connected) in a typical circuit. In USA, standard instrument transformers are rated at 60 Hz and 5 amperes for the CT secondary output and at 120 volts for the VT secondary output.

The major criterion for selecting a CT is the continuous current rating of the protective equipment and the secondary winding of the CT itself. In general practice, with normal load current flowing through the phase relays, the ratio of a CT is selected so that the secondary current output is 1/2 to 2/3 of 5 amperes at the maximum primary load current. Based on this selection criterion, the CT is usually sized at 150-200% of normal full-load amperes. Where delta-connected CTs are used in the protection scheme, for example, for differential protection of delta-wye connected transformers, the factor must be included in the CT ratio selection process. Figure 3 is an illustration of a window-type CT that is used in a modern-day protection system.

Voltage transformers (VTs), which in USA were formerly called potential transformers (PTs), are typically selected according to two criteria: the system voltage level and the basic impulse level

(BIL) that is required by the system on which the VTs are to be used. The two nominal secondary line-to-line voltages for VTs are 115 and 120 volts; the corresponding line-to-neutral voltages are 66.4 V (115/ 3 ) and 69.3 V (120/ 3 ). Most protective relays have standard voltage ratings of 120 V or 69V, depending on whether they are to be connected line-to-line or line-to-neutral. Figure 4 is an illustration of a VT that is used in a protection system.

PRECAUTIONS AND LIMITATIONS

Enviroment

Aspect Impact What To DoUse of maintenance chemicals Resource depletion 1. Proper handling of chemicals

with MSDS2. Use of PPE

Generation of materials laden with chemicals (rags, gloves, contact cleaner)

Air, land and water pollution 1. Follow guidelines on industrial waste management

Health and Safety

Hazard Hazardous Event What To Do What To WearWork positioning Working in limited

spaceFollow correct ergonomics positioning

1. Correct working clothes2. Required PPE

Sharp or pointed objects

Suffer cuts/abrasions Comply with Safety Protection Program

1. Correct working clothes2. Gloves3. Goggles4. Hard Hat5. Safety Shoes

Live power supply Electrocution 1. Comply with Electrical Safety on Live equipment.

2. Switch off power supply.

3. Apply lock-out/tag-out system.

1. Correct working clothes2. Required PPE.3. Use the appropriate insulated tools.

Operational Isolation

Follow Safety Work Permit System and lock-out tag-out procedures prior to commence work execution. Rack out or switch off the breaker or switchgear as per plant operation switching procedure.

ROLES AND RESPONSIBILITIES

End-User ResponsibilityPlanning Plan Program Activities and Material Requirement

Issuance of Maintenance Job Card Updating of Equipment History Record Closing and Encoding of Work Orders

Electrical Technician Receive/Close PTW Ensure all appropriate tools, testers and other consumables are

available. Observe and follow safety procedures at all times Perform the required activity or coordinate/assist the third party. Accomplish and sign-off service report Fill-up and close job card Inform Supervisor upon completion of work activity

Electrical Supervisor Supervise the whole activity. Ensure that the maintenance manuals, single line diagrams and

schematic drawings are updated and available. Ensure that all safety procedure is complied at all times. Check and sign service report. Check and sign filled-up job card. Advise Operation on work completion.

Operation PTW Issuance and Closing. Check area condition before and after the maintenance activity

Fire and Safety Monitor Safety and ensure any other accessory works such as scaffoldings, lifting, etc., are securely done. When scaffold are required, it should be properly designed, erected and inspected by a Certified Scaffolding Inspector.

Stop the work if found non conforming to the safety, health and environmental requirement.

EQUIPMENT/TOOLS NEEDED

Basic Insulated Electrical Hand Tools Multi-amp tester Fluke Multimeter Flashlight

CONSUMABLES

Gloves / Rags Soft Paint Brush Electrical Tape Contact Cleaner

REFERENCES

OEM Maintenance Manual Vendor’s Operating manual

APPLICATION

System: Generator Transformer Protection

Plant Item: VT’s and CT’s

Tag No:

Task: VT’s and CT’s Testing

PTW: Required

Frequency: Annual or as the need arise.

Step Procedure Done Not Done Remarks 1.

2.

Preparatory Works:

-Prepare all necessary tools, Electrical testing equipment and consumables to be needed.\-Coordinate with the Unit Operator if there is a possible maintenance shutdown of the Station Transformer. Coordinate with the THIRD PARTY in the testing of CT’s and VT’s.

Actual Works:

1. Check the VT accurate secondary voltage values. It should be directly proportional to the primary voltage, thus it is important to verify the ratio and phase accuracies at commissioning and then on a routine basis.

2. Check the VT coil resistance.3. Check the insulation resistance of the VT,

high side to low side and both coil to ground.

3.

CT Ratio and Phase Error

1. Check the CT accurate secondary current values. It should be directly proportional to the primary current, thus it is important to verify the ratio and phase accuracies at commissioning and then on a routine basis.

2. Check the CT coil resistance.3. Check the insulation resistance of the CT

against the ground.

For the IEC standard, accuracy classes for various types of measurement are set out in IEC 60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important especially in power measuring circuits, and each class has an allowable maximum phase error for a specified load impedance. Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults.

Work Completion:

a. Fill up and sign corresponding Job Card and endorse to the Supervisor.

b. Inform Electrical Switching Operator of completed activity, remove padlock and tag and close the work permit.