What is instrument transformer testing?
There are two common designations of instrument transformers: CTs and voltage transformers (VTs) or potential transformers (PTs). The function of an instrument transformer is to reduce the level of voltage or current so that the protective relay (or metering) does not have to be rated for full line voltage or current.
The insulation resistance, transformer ratio, and polarity may be tested in both CTs and VTs. The ratio is the number of turns of wire in the primary winding divided by the number of turns of wire in the secondary winding.
The polarity is determined by which way the wire was wrapped around the iron core. This determines the relationship between the primary winding terminal (H1) and the secondary winding terminal (X1) so that X1 is positive with respect to X2 at the same time that H1 is positive with respect to H2.
The correctness of polarity is important to the correct operation of many relays and metering instruments. CTs often have two additional tests performed: "burden" and "saturation" tests. The burden on a CT is the amount of impedance connected to the secondary winding as a load, usually in the form of protective relays or metering.
The burden test consists of injecting a known current level (usually 1-5 A ac) into the load (usually from the shorting terminal block of the CT) and measuring the voltage at the point of injection. The impedance (or burden) of the circuit is the ratio of the voltage measured to the current injected.
A saturation test is performed to find out the voltage at which the iron in the CT saturates. A known voltage source is connected to the secondary of the transformer and is raised in steps, while the current value is recorded at each step.
When saturation is reached, the given voltage changes cause much smaller changes in current. The saturation test is used in conjunction with the burden test to make sure that the CT is capable of operating the load (usually protective relays) to which it may be subjected.
If the burden on the CT is too high, it may go into saturation and be unable to maintain its proper ratio. When this happens, protective relays may trip too slowly or not at all due to an insufficient level of current from the CT secondary.
LOW VOLTAGE CIRCUIT BREAKER TESTING BASIC INFORMATION AND TUTORIALS
What test to conduct for low voltage circuit breaker?
Low-voltage circuit breakers come in the following three major types:
a) Power (air-frame) circuit breakers;
b) Molded-case circuit breakers;
c) Insulated-case circuit breakers.
Power circuit breakers start with a frame size of 600 A and go up to 4000 A. The sensing unit that operates the breaker on a short circuit or overload may be either an oil-dash pot with springs and copper coils (for older breakers) or may consist of current transformers (CT) and an electronic trip unit.
With the advent of the electronic trip unit, the number of possible settings and trip functions has dramatically increased, making it easier to coordinate circuit breakers with other protective devices.
Molded-case circuit breakers and insulated-case circuit breakers are very similar in mechanical construction and insulation. The circuit breakers' contacts and operating mechanisms are totally enclosed in a molded plastic housing.
The difference between the two is that a molded-case circuit breaker normally has a thermal-magnetic trip unit (i.e., a trip unit made up of two pieces: a thermal unit to sense overload that uses two dissimilar metals and a magnetic unit to trip on short circuit), while an insulated-case circuit breaker has CTs and an electronic trip unit built into the insulated case.
The most thorough test for all three types of circuit breakers is by "primary injection". A special test set that puts out high (fault level) current at low voltage (typically 6-20 V ac) is used to functionally test the circuit breaker.
These test sets have built-in timing functions; therefore, the breaker can be tested at various currents in order to make sure that it operates within the time-current specifications that are provided by the manufacturer and that it is calibrated to perform in conformance with the coordination study.
For circuit breakers that have electronic trip units, it is often possible to do "secondary injection" testing. This is usually done with a special test set that is designed for the trip unit.
It injects low-level test currents into the trip unit, directly testing only the trip unit. For this reason, primary injection testing is a better practice, as it tests the whole circuit breaker (CTs, shunt trip, etc.) in a manner that is similar to how the breaker would operate during a fault.
In addition to testing the tripping characteristics of the circuit breaker by injecting current, it is also normal practice to test the insulation resistance (usually at 1000 V dc) and the resistance of the breaker's contacts. The contact resistance can be measured directly with a low resistance ohmmeter (usually in milliohms) or indirectly by performing a millivolt drop test.
A millivolt drop test is performed by using a primary injection test set to inject rated continuous current through the breaker while measuring the millivolt drop across the breaker's poles. It is a comparative test between each phase of the breaker in which the millivolt reading typically should not differ by more than 50% between phases.
Power circuit breakers have mechanical adjustments and inspections that should also be periodically checked. The manufacturer's instructions list the adjustments for each model.
Low-voltage circuit breakers come in the following three major types:
a) Power (air-frame) circuit breakers;
b) Molded-case circuit breakers;
c) Insulated-case circuit breakers.
Power circuit breakers start with a frame size of 600 A and go up to 4000 A. The sensing unit that operates the breaker on a short circuit or overload may be either an oil-dash pot with springs and copper coils (for older breakers) or may consist of current transformers (CT) and an electronic trip unit.
With the advent of the electronic trip unit, the number of possible settings and trip functions has dramatically increased, making it easier to coordinate circuit breakers with other protective devices.
Molded-case circuit breakers and insulated-case circuit breakers are very similar in mechanical construction and insulation. The circuit breakers' contacts and operating mechanisms are totally enclosed in a molded plastic housing.
The difference between the two is that a molded-case circuit breaker normally has a thermal-magnetic trip unit (i.e., a trip unit made up of two pieces: a thermal unit to sense overload that uses two dissimilar metals and a magnetic unit to trip on short circuit), while an insulated-case circuit breaker has CTs and an electronic trip unit built into the insulated case.
The most thorough test for all three types of circuit breakers is by "primary injection". A special test set that puts out high (fault level) current at low voltage (typically 6-20 V ac) is used to functionally test the circuit breaker.
These test sets have built-in timing functions; therefore, the breaker can be tested at various currents in order to make sure that it operates within the time-current specifications that are provided by the manufacturer and that it is calibrated to perform in conformance with the coordination study.
For circuit breakers that have electronic trip units, it is often possible to do "secondary injection" testing. This is usually done with a special test set that is designed for the trip unit.
It injects low-level test currents into the trip unit, directly testing only the trip unit. For this reason, primary injection testing is a better practice, as it tests the whole circuit breaker (CTs, shunt trip, etc.) in a manner that is similar to how the breaker would operate during a fault.
In addition to testing the tripping characteristics of the circuit breaker by injecting current, it is also normal practice to test the insulation resistance (usually at 1000 V dc) and the resistance of the breaker's contacts. The contact resistance can be measured directly with a low resistance ohmmeter (usually in milliohms) or indirectly by performing a millivolt drop test.
A millivolt drop test is performed by using a primary injection test set to inject rated continuous current through the breaker while measuring the millivolt drop across the breaker's poles. It is a comparative test between each phase of the breaker in which the millivolt reading typically should not differ by more than 50% between phases.
Power circuit breakers have mechanical adjustments and inspections that should also be periodically checked. The manufacturer's instructions list the adjustments for each model.
PROTECTIVE RELAY TESTING BASIC INFORMATION AND TUTORIALS
What is protective relay testing? How to do protective relay testing?
Protective relaying is a very broad subject. Only a brief overview can be given here. There are two major objectives in protective relaying.
First, a protective relay serves to provide equipment protection (i.e., locate and isolate overloads, short circuits, undervoltages, and other types of electrical problems quickly in order to minimize damage).
Second, the protective device that is closest to the problem should operate first to clear the problem, and no other device should operate unless the closest one fails. This concept, known as "selective tripping" or "selectivity", maintains service to as much of the electrical system as possible by isolating only the problem area.
In order to achieve these objectives, each relay must function as it was designed, and the relays must function in conjunction with the other protective devices in the system. Having all the protective devices function as one overall protective system is called "coordination".
Each protective device has specific parameters within which it has been designed to operate. For example, a single element fuse has a value of current above which it opens.
It takes a specific amount of time for a given current to melt the link away and open the fuse. Manufacturers of fuses publish "time-current" curves that show how long it takes a fuse to operate for varying current values.
Generally, the higher the current, the shorter the time.
This same inverse current-vs.-time concept is used for overcurrent relays and for low-voltage circuit breakers. Relays and low-voltage circuit breakers (with internal trip units) have a range of "pickup" operating current that causes them to operate.
In many cases, this value of current is adjustable. By properly selecting the type, characteristic, and/or setting of fuses, relays, or circuit breakers, the system can be coordinated so that the device that is closest to the problem opens before any device upstream of it. It is necessary to select compatible time current characteristics of the devices for the entire system, in addition to selecting the proper settings for the devices.
Prior to performing protective relay testing, a coordination study should be completed to determine the proper settings for the relays to be calibrated. This is usually done by the design engineer when the system is first installed. If there have been revisions or additions to the system, a new study may be necessary.
Once the coordination study has been completed, the relays need to be calibrated to the proper settings. There are special test sets available for this purpose that inject currents and voltages, as necessary, and time the various operations of the relays.
This type of testing is usually performed by a technician who specializes in this area. Depending upon the relay to be calibrated, quite complex test equipment may be required and in-depth training in protective relaying may be needed to properly set the relay.
Protective relaying is a very broad subject. Only a brief overview can be given here. There are two major objectives in protective relaying.
First, a protective relay serves to provide equipment protection (i.e., locate and isolate overloads, short circuits, undervoltages, and other types of electrical problems quickly in order to minimize damage).
Second, the protective device that is closest to the problem should operate first to clear the problem, and no other device should operate unless the closest one fails. This concept, known as "selective tripping" or "selectivity", maintains service to as much of the electrical system as possible by isolating only the problem area.
In order to achieve these objectives, each relay must function as it was designed, and the relays must function in conjunction with the other protective devices in the system. Having all the protective devices function as one overall protective system is called "coordination".
Each protective device has specific parameters within which it has been designed to operate. For example, a single element fuse has a value of current above which it opens.
It takes a specific amount of time for a given current to melt the link away and open the fuse. Manufacturers of fuses publish "time-current" curves that show how long it takes a fuse to operate for varying current values.
Generally, the higher the current, the shorter the time.
This same inverse current-vs.-time concept is used for overcurrent relays and for low-voltage circuit breakers. Relays and low-voltage circuit breakers (with internal trip units) have a range of "pickup" operating current that causes them to operate.
In many cases, this value of current is adjustable. By properly selecting the type, characteristic, and/or setting of fuses, relays, or circuit breakers, the system can be coordinated so that the device that is closest to the problem opens before any device upstream of it. It is necessary to select compatible time current characteristics of the devices for the entire system, in addition to selecting the proper settings for the devices.
Prior to performing protective relay testing, a coordination study should be completed to determine the proper settings for the relays to be calibrated. This is usually done by the design engineer when the system is first installed. If there have been revisions or additions to the system, a new study may be necessary.
Once the coordination study has been completed, the relays need to be calibrated to the proper settings. There are special test sets available for this purpose that inject currents and voltages, as necessary, and time the various operations of the relays.
This type of testing is usually performed by a technician who specializes in this area. Depending upon the relay to be calibrated, quite complex test equipment may be required and in-depth training in protective relaying may be needed to properly set the relay.
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