Published by Carelabs (Carelabz), Website: carelabz.com
Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth.
From time to time, disturbances in the normal operation of a power system occur. These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of electrical service, to limit injury to people, and to limit damage to equipment when problem situations develop. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system.
Why Protective Device Testing is Done?
Protection systems play a key role for the safe and reliable operation of today’s electricity power systems. Properly working protection devices help to maintain the safety of the system and to safeguard assets from damage. In order to ensure reliable operation, protective relays as well as recloser controls must be tested throughout their life-cycle, from their initial development through production and commissioning to periodical maintenance during operation. Our test equipment is ideal for each of these life-cycle phases and for any environment. As a reliable long-term partner, we offer state-of-the-art testing solutions which are continuously being developed and maintained to help you to keep pace with the increasingly complex requirements of your systems.
What is Done During Protective Device Testing?
Lightning protection is a means to protect equipment, facilities and people from the effects of nearby or direct lightning events. Whereas, surge protection provides protection to equipment from the effects of more distant lightning events or power system anomalies. Five basic procedures are employed to test protection devices.
Clamping Voltage Tests
When a transient occurs, the SPD resistance changes from a very high stand-by value to a very low conduction value. The transient is absorbed and clamped at a defined level, protecting sensitive electronic circuits and diverting the transient energy to ground. A normalised current impulse of 8/20us is defined in the standards IEC 61643-1 and IEC61180-1.
Surge Withstand Tests
Surge withstand tests are intended to assess the maximum peak current carrying capability of varistors. The surge withstand capability is approximately proportional to the varistor disk size (diameter). Energy levels are much higher than for the clamping voltage tests with impulse levels in the tens of kilo amps range.
Energy Absorption Tests
High energy surges are usually generated by inductive discharges of motors and transformers. Energy absorption in an SPD is the integral current flow through and the voltage across an SPD. Surge currents of relatively long duration are required for testing maximum energy absorption capacity of an SPD. A rectangular wave of 2ms duration is sometimes used instead of the double exponential waveforms.
Combination Wave Tests
Surge events can be generated by lightning phenomena, switching transients or the activation of protection devices in the power distribution system. A surge itself is influenced by the propagation path taken so that impulses from the same event may have different forms depending upon where a measurement is taken. Combination Wave Generators (CWG) simulate a surge event in power lines close to or within building.
Duty Cycle (Flammability) Tests
A series of pulses is applied to the varistor to assess maximum rated dissipation. Exceeding the maximum rated dissipation will cause the protection device to be destroyed. A flammability risk could occur. The 8/20us current impulse is superimposed on the mains power supply
How do We Conduct Protective Device Testing?
Basic devices have the ability to recognize and define fuses, protective relays, breaker trip devices, and surge suppressors and to understand their differences and uses. A common mistake for relay testers is to use spare outputs, displays, and/or LEDs for their pickup and timing tests and ignore the in-service output logic, believing that they are using the same elements in their test equations as the final logic.
Depending on the protective device the tests varies accordingly:
They conduct the flow of current as long as a nominal value of current is flowing through the circuit to the load attached to it. Even at the slightest contact, current conduction occurs. But as soon as the breaker senses an excessively large amount which does not lie in its operating range (which can be checked through the ratings of the circuit breaker), the trip unit actuates the bimetallic strip and the contact breaks and immediately further flow of current is stopped. In addition to the safety operation, it also provides a kind of voltage insulation to the circuit and retains the flow after the current retains its appropriate value.
The field-testing and calibration of solid-state trip units can be performed by either primary current injection method or secondary current injection method. A coordination study is an organized effort to achieve optimum electrical distribution system protection by determining the appropriate frame sizes, ampere ratings and settings of overcurrent protective devices. When an overcurrent occurs in a properly coordinated distribution system, only the protective device nearest the fault will open. The secondary injection test is performed using a specially designed power supply unit. It should be noted that the secondary injection method only tests the solid-state trip unit logic and does not test the current sensors, wiring, or the breaker current handling components. Most solid-state trip units have terminal blocks that are equipped with test plug terminals for making the calibration test. The test set allows checking of the solid state trip unit operation without using primary current. The test set will pass enough current to check any desired calibration point. The breaker must be de-energized before checking the operation of the solid-state trip units. If the test set shows that the solid-state trip unit is not functioning properly, the trip unit should be replaced.
The primary current injection method is usually preferred because this method verifies the sensors and wiring, as well as the conduction path in the breaker. It is recommended that the primary injection test be performed simultaneously on all three phases when testing breakers with solid-state trip units. If three phase primary injection testing is not practical, then it is recommended that the sensors and wiring should be tested separately. This testing should be performed per NETA and the National Electrical Manufacturers Association (NEMA) procedures, and in accordance with manufacturer recommendations. Coordination Time-current curves are used to show the amount of time required for a circuit breaker to trip at a given overcurrent level.
A relay is an automatic device which senses an abnormal condition of electrical circuit and closes its contacts.
The first electrical test made on the relay should be a pickup test. Pickup is defined as that value of current or voltage which will just close the relay contacts from the 0.5 time-dial position. Allowing for meter differences, interpretation of readings, etc., this value should be within ±5% of previous data. Generally, one or two points on the time-current curve are sufficient for maintenance purposes. Reset the relay to the original time-dial setting and two points that could be checked and 3 and 5 times pickup. Of course, other points could be used, but the important thing is to always use the same point(s). The instantaneous unit should be checked for pickup using gradually applied current for reasons previously discussed. Wherever possible, current should be applied only to the instantaneous unit (to avoid over-heating the time unit).
There are different types of relays:
Relays can include phase overcurrent, current balance, negative sequence, zero sequence, thermal, and ground fault.
This is the first generation oldest relaying system and they have been in use for many years. They have earned a well-deserved reputation for accuracy, dependability, and reliability.
Contact function – Manually close (or open) contacts and observe that they perform their required function, i.e. trip, reclose, block, etc.
Pickup – Gradually apply current or voltage to see that pickup is within limits. Since preventive maintenance is the guide post, gradually applied current or voltage will yield data which can be compared with previous or future data and not be clouded by such effects as transient overreach, etc.
Dropout or reset – Reduce the current until the relay drops out or fully resets. This test will indicate excess friction. Should the relay be sluggish in resetting or fail to reset completely, then the jewel bearing and pivot should be examined. A 4X eye loupe is adequate for examining the pivot, and the jewel bearing can be examined with the aid of a needle which will reveal any cracks in the jewel. Should dirt be the problem, the jewel can be cleaned with an orange stick while the pivot can be wiped clean with a soft, lint free cloth. No lubricant should be used on either the jewel or pivot.
Test should be made to check that the overcurrent unit operates only when the directional unit contacts are closed.
Directional and Power Relays
Directional overcurrent relaying refers to relaying that can use the phase relationship of voltage and current to determine direction to a fault.
Power Directional Relays provides protection against excess power flow in a predetermined direction. And are used for anti-motoring protect on of AC generators.
The simplest pickup test for a directional unit is an in-phase test – i.e. current and voltage in phase. This test will eliminate the need for a three-phase supply, phase shifter, and phase-angle meter. However, it must be kept in mind that such a test is usually far from the angle of maximum torque (usually 60° lag for ground relays) and thus, small changes in components can yield large variations in in-phase pickup. As long as this fact is recognized and the pickup value is within limits, an angle of maximum torque check would not be necessary. Clutch pressure must always be measured in the same manner. For example, some instruction books express clutch settings in both grams and current/voltage levels. Portable pre-calibrated reactance-resistance test boxes are available for many of these tests. The use of such equipment, properly applied, will yield results which will exceed in accuracy those obtained with conventional phase-angle meters, ammeters, voltmeters, etc. In addition to the tests previously described for the overcurrent relay, the directional unit should be tested for minimum pickup, angle of maximum torque, contact gap, and clutch pressure. Further, a test should be made to check that the overcurrent unit operates only when the directional unit contacts are closed. . Either test is valid, but to have comparative data, the same method, either grams or electrical quantities should be employed each time
Voltage Relays Secondary Injection Test
Measure the relay auxiliary supply to ensure it is within the nameplate rating allowable range.
Creep or Pickup Test
- Connect the voltage injector’s output.
- Adjust relay’s set points to the plant / substation recommended settings, if necessary.
- Set Red phase of voltage injector to 120% of relay setting. Note: Set Yellow and Blue phase to zero.
- Inject and reduce slowly the Red phase injector’s voltage in order to monitor and record the relay’s pickup voltage.
- Repeat step 2.3 and 2.4 for other settings, if any.
Trip Time Test
- Set Red phase of injector’s voltage to 80% of relay setting. Note: Set Yellow and Blue phase to zero.
- Inject Red phase voltage through the relay in order to record the tripping time. Check test results against the tripping curve characteristics of the relay.
A test of minimum pickup should be performed. The differential characteristic (slope) should be checked and where applicable the harmonic restraint should be tested. Generally, differential relays are extremely sensitive devices and require some special consideration. For example, those relays employing ultra-sensitive polarized units as sensing devices are slightly affected by previous his-tory such as heavy internal or external fault currents. To eliminate previous history and truly perform a maintenance test, it is the usual practice to disregard the first pickup reading and use the second reading for comparison with previous and future data. By “disregard” it is not meant to imply that the initial reading be forgotten; rather it is meant that this reading not be used for comparison purpose.
The fuse is a reliable overcurrent protective device, primarily used as a circuit protection device for over currents, overloads and short-circuits.
A time-current characteristic curve, for any specified fuse, is displayed as a continuous line representing the average melting time in seconds for a range of overcurrent conditions.
NFPA 70B recommends checking fuse continuity during scheduled maintenance, but testing to assure proper operation and protection against overcurrent conditions is not required. Fusible switches and fuse blocks require maintenance, such as tightening of connections and checking for signs of overheating as recommended per NFPA 70B.
In all cases, though, the idea is to send a small current through the fuse; if it passes through the fuse the fuse is good. If it does not the fuse is blown and needs replacement. This means that a battery is necessary to provide that small current and every fuse tester will have a battery in it. If a tester shows that a fuse is blown, the next step is to check the tester. This is accomplished by touching the test leads together or, in the case of testers without leads, to put a piece of metal (wire, coin, dinner spoon, anything metal) across the probes. If it does not indicate “good” the battery probably needs replacing.
- Using a Continuity Tester
Continuity testers will have two test leads and a small light that will light up if the leads are touched together. To test a fuse simply touch one lead to each of the electrical contacts on the fuse; if the light bulb lights up the fuse is good.
- Testing a Fuse with a Multimeter
A multimeter again has two leads just like a continuity tester. However, there are many settings on a multimeter to measure amperage, voltage and resistance in several different ranges. Some multimeters are auto ranging (no need to choose a range), some are digital and some are analog meters with a needle to indicate the reading. With all multimeters the first step is to set it to measure resistance, or Ω. If different ranges are available, choose the lowest range (K means thousand on the dial, so 2K equals 2000) – usually around 200. Like a continuity tester, touch one probe to each contact on a fuse and observe the reading. A very low w reading of 1 ohm or less means the fuse is good; if it is blown the reading will be infinite, or the maximum the meter will display. An intermediate reading of several ohms probably means you aren’t making good contact; wriggle the probes on the fuse contacts or clean them and try again.
Motor Management Systems
Microprocessor-Based Motor Protection Takes Protecting and Monitoring Electric Motors into the Digital Age
Before microprocessor relays, electromechanical and solid state relays were tested on an element by element basis. This was a coherent approach, allowing individual parts of the relay to be calibrated and proven. When microprocessor relays arrived, many continued this approach and tested individual elements within the relay, while others found alternative methods to test. Developing automated testing procedures for microprocessor relays can be classified into three categories:
- Element testing,
- Functional testing, and
- Black box testing.
The black box testing method, is adequate in terms of NERC compliance.
Whether functional or black box testing, the use of dynamic testing software is the logical choice to perform the testing. Dynamic tests drive relaying test sets to run in a series of defined sequences called states-such as pre-fault, fault and post-fault..
The use of element testing for microprocessor relays is likely to decline because, in part, to its noted shortcomings. The choice of functional vs. black box testing is less clear because both have their advantages and disadvantages. One thing is clear, however, regardless of the testing method employed-documentation of testing is critical, especially if the relay application is under the NERC umbrella. Tracking of testing intervals, previous test dates and last test dates are all part of the data required to be submitted during an audit. A detailed account of the testing on a subset of the full listing will often be requested. Maintaining this data by paper copy can result in much time spent tracking dates and data gathering. The larger the number of relays to track, the more daunting this task can be. Storage of all this data into a centralized database, with the ability to extract data and run audit reports, is quickly becoming a necessity to prepare for NERC audits. These reports can prevent a last minute crisis of discovering relays that were missed by tracking testing dates on a continual basis-and also provide the data needed for audit submissions. There are many different relay database programs, some home grown, others commercial. Regardless of NERC regulations, the reliance on these databases will only grow.
Resetting Overcurrent Protective Devices
Circuit breakers are sometimes selected over fuses because circuit breakers can be reset where fuses have to be replaced. The most time consuming activity that results from the operation of the overcurrent protective device is typically investigating the cause of the overcurrent condition.
A known overload condition is the only situation that permits the immediate resetting or replacement of overcurrent protective devices per OSHA. If the cause for the operation of an overcurrent protective device is not known, the cause must be investigated.
Thus, having a device that can be easily reset, such as a circuit breaker, possibly into a fault condition, could be a safety hazard and a violation of OSHA regulations.
Because a fuse requires replacement by a qualified person, it is less likely to violate OSHA. Also, when an opened fuse is replaced with a new fuse in the circuit, the circuit is protected by a new factory calibrated device.
Generally, overload conditions occur on branch-circuit devices. Typically this is on lighting and appliance circuits feed from circuit breaker panel boards, where resetting of circuit breakers may be possible. Motor circuits also are subject to overload considerations.
However, typically the device that operates is the overload relay, which can be easily reset after an overload situation. The motor branch-circuit device (fuse or circuit breaker) operates, as indicated in NEC® 430.52, for protection of short-circuits and ground-fault conditions. Thus, if this device opens, it should not be reset or replaced without investigating the circuit since it most likely was a short-circuit condition.
The technical excellence and many unique features of the Protection Device Testers translate directly into benefits for the user:
- optimum return on investment
- Standard control unit, reduces user training
- Impulse reproducibility
- Accurate measurement system delivers information about the SPD
- Integration into existing test facilities saves engineering costs
- Pass / Fail indication for individual samples, speeds up production
- High degree of automation, reduces operator workload
- Save operator time with the automated test routines and test report facility
- Easy integration into a full test suite
- Unparalleled reliability and system up-time