A simple electromagnetic relay, such as the one taken from a car in the first picture, is an adaptation of an electromagnet. It consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energised there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.
When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energised, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.
If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.
By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light-emitting diode (LED) coupled with a photo transistor.
Latching relay
A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or "stay" relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remanent core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. A remanent core latching relay requires a current pulse of opposite polarity to make it change state.
Reed relay
A reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which protects the contacts against atmospheric corrosion. The contacts are closed by a magnetic field generated when current passes through a coil around the glass tube. Reed relays are capable of faster switching speeds than larger types of relays, but have low switch current and voltage ratings. See also reed switch.
Mercury-wetted relay
A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) because of their low contact resistance, or for high-speed counting and timing applications where the mercury eliminates contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment. See also mercury switch.
Polarized relay
A polarized relay placed the armature between the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion. The poles were on screws, so a technician could first adjust them for maximum sensitivity and then apply a bias spring to set the critical current that would operate the relay.
Machine tool relay
A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally-open to normally-closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel. Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications.
Contactor relay
A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. Continuous current ratings for common contactors range from 10 amps to several hundred amps. High-current contacts are made with alloys containing silver. The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still a good conductor. Such devices are often used for motor starters. A motor starter is a contactor with overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized. Contactor relays can be extremely loud to operate, making them unfit for use where noise is a chief concern.
Solid-state relay
A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across it. This voltage drop limited the amount of current a given SSR could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 Amperes, have become commercially available. Compared to electromagnetic relays, they may be falsely triggered by transients.
Solid state contactor relay
A solid state contactor is a very heavy-duty solid state relay, including the necessary heat sink, used for switching electric heaters, small electric motors and lighting loads; where frequent on/off cycles are required. There are no moving parts to wear out and there is no contact bounce due to vibration. They are activated by AC control signals or DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls.
Buchholz relay
A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil.
Forced-guided contacts relay
A forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays".
Overload protection relay
One type of electric motor overload protection relay is operated by a heating element in series with the electric motor . The heat generated by the motor current operates a bi-metal strip or melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.
The role of protective relaying in electric-power-system design and operation is explainedby a brief examination of the over-all background. There are three aspects of a power
system that will serve the purposes of this examination. These aspects are as follows:
A. Normal operation
B. Prevention of electrical failure.
C. Mitigation of the effects of electrical failure.
The term Ònormal operationÓ assumes no failures of equipment, no mistakes of personnel,
nor Òacts of God.Ó It involves the minimum requirements for supplying the existing load
and a certain amount of anticipated future load. Some of the considerations are:
A. Choice between hydro, steam, or other sources of power.
B. Location of generating stations.
C. Transmission of power to the load.
D. Study of the load characteristics and planning for its future growth.
E. Metering
F. Voltage and frequency regulation.
G. System operation.
E. Normal maintenance.
The provisions for normal operation involve the major expense for equipment and
operation, but a system designed according to this aspect alone could not possibly meet
present-day requirements. Electrical equipment failures would cause intolerable outages.
There must be additional provisions to minimize damage to equipment and interruptions
to the service when failures occur.
Two recourses are open: (1) to incorporate features of design aimed at preventing failures,
and (2) to include provisions for mitigating the effects of failure when it occurs. Modernpower-system design employs varying degrees of both recourses, as dictated by the
economics of any particular situation. Notable advances continue to be made toward
greater reliability. But also, increasingly greater reliance is being placed on electric power.
Consequently, even though the probability of failure is decreased, the tolerance of the
possible harm to the service is also decreased. But it is futile-or at least not economically
justifiable-to try to prevent failures completely. Sooner or later the law of diminishing
returns makes itself felt. Where this occurs will vary between systems and between parts of
a system, but, when this point is reached, further expenditure for failure prevention is
discouraged. It is much more profitable, then, to let some failures occur and to provide for
mitigating their effects.
The type of electrical failure that causes greatest concern is the short circuit, or ÒfaultÓ as
it is usually called, but there are other abnormal operating conditions peculiar to certain
elements of the system that also require attention. Some of the features of design and
operation aimed at preventing electrical failure are:
A. Provision of adequate insulation.
B. Coordination of insulation strength with the capabilities of lightning arresters.
C. Use of overhead ground wires and low tower-footing resistance.
D. Design for mechanical strength to reduce exposure, and to minimize the likelihood of
failure causable by animals, birds, insects, dirt, sleet, etc.
E. Proper operation and maintenance practices.
Some of the features of design and operation for mitigating the effects of failure are:
1. Features that mitigate the immediate effects of an electrical failure.
A. Design to limit the magnitude of short-circuit current.1
. By avoiding too large concentrations of generating capacity.
B. By using current-limiting impedance.
C. Design to withstand mechanical stresses and heating owing to short-circuit currents.
D. Time-delay undervoltage devices on circuit breakers to prevent dropping loads
during momentary voltage dips.
E. Ground-fault neutralizers (Petersen coils).
2. Features for promptly disconnecting the faulty element.
A. Protective relaying.
B. Circuit breakers with sufficient interrupting capacity.
C. Fuses.
3. Features that mitigate the loss of the faulty element.
A. Alternate circuits.
B. Reserve generator and transformer capacity.
C. Automatic reclosing.
4 Features that operate throughout the period from the inception of the
fault until after its removal, to maintain voltage and stability.
A. Automatic voltage regulation.
B. Stability characteristics of generators.
5 Means for observing the electiveness of the foregoing features.
A. Efficient human observation and record keeping.
B.Automatic oscillographs.
6. Frequent surveys as system changes or additions are made, to be sure that the foregoing
features are still adequate.
FAILURE TO OPERATE
In terms of equipment damage, if not system damage, the failure to operate for a fault is of great
concern to the relay engineer. Local backup can limit damage and the spread of tripping, but loss
of service will certainly be greater than if the relay operated correctly. By the time remote backup
takes place, numerous lines must be cleared. Because fault current is divided between these lines,
the delay in clearing is significant.
The fault shown in Appendix 2 (which occurred on 5/5/96) lasted at least 82 cycles (the fault
recorder stopped recording at that point; more on this topic later). Six lines connecting to six
different stations cleared to remove the fault current. Murphy’s law was strongly evident, as it
took a dispatcher five tries to find the correct breaker to open in order to restore the system. With
minimal fault data available, problems compounded, and it took 35 minutes before lines tripped
on backup could be closed.
Using the same categories and rankings as listed for misoperations, we can group the failure to
trip events as follows:
Setting or coordination failure: 1 instance (7.7%)
Accessory component failure: 10 instances (76.9%)
Human-Caused: 0 instances
Relay design hole: 0 instances
Induced Signal/Noise: 1 instance (7.7%)
Force majoure: 0 instances
Relay component failure: 1 instance (7.7%)
Mystery: 0 instances
FALSE OPERATIONS
Relay Component Failure
1. relay with shorted diode, closed in three times, loss of air pressure in
circuit breaker caused trip times to increase until backup relay (on 230 kV bank)
cleared fault on 34.5 kV feeder.
2. Staged fault caused adjacent 500 kV line to trip by “finding” a faulty component
that removed restraint and caused operation on reverse fault. This sent a direct
transfer trip to the other end.
3. 230 kV line tripped due to leaking capacitor in electromechanical distance relay.
Relay Design Hole
4. Two electromechanical distance relays operated for remote bus fault: “the relay
contacts have a history of drifting closed when the line voltage goes dead.” They
did not cause outage. The line was already dead.
5. Solid-state phase comparison relay tripped for a fault on parallel line. Relays were
tested with no problems found.
6. Electromechanical distance relays tripped on PT failure; line did not trip.
7. Electromechanical transformer differential misoperated during inrush. Relay
tested OK.
8. E/M DCB scheme misoperated at one end of line due to fault detector operating
for external fault and forward looking distance relay “drifting” closed on low
voltage (two occurrences on separate lines for same fault).
9. Electromechanical transformer differential misoperated during inrush. Relay
tested OK.
10. Electromechanical transformer differential misoperated during inrush. Relay
tested OK.
Accessory Component Failure
11. Electromechanical pilot wire differential false trip on bad pilot.
12. Electromechanical pilot wire differential false trip on bad pilot.
13. E/M POTT scheme false tripped on external fault due to e/m aux failure causing
transmitter to stay keyed on.
14. Solid-state bus differential tripped on external fault due to a ground return wire
not installed during addition of new equipment to station.
15. Three transformer banks tripped due to false transfer trip during test of breaker
failure relays. Blocking switches were mislabeled on newly installed equipment.
16. Directional overcurrent relay opened while switching a capacitor, due to a control
wiring problem.
17. Fault on adjacent line damaged pilot wires, causing electromechanical pilot wire
differential relays to trip three lines.
18. Electromechanical pilot wire differential tripped on external fault. Apparently
shorted pilot.
19. Transformer false tripped on first load because CT wired backwards.
20. Same transformer tripped again due to one phase wired incorrectly.
Setting or Coordination Failure
21. Electromechanical pilot wire differential operated on fuse-cleared fault.
Electromechanical pilot wire differential cannot coordinate with fuse, cleared
faults.
22. kV staged fault caused an echo-tripping permissive echo that eventually
caused a false trip on that line. Line tripped again on second staged fault test on
adjacent line.
23. Overfrequency relay tripped on transient caused by line tripping. Relay operated
correctly, given its settings, but incorrectly, given its application.
24. Relay operated for a repeated fault on an adjacent 345 kV line. This was a
“correct” incorrect operation. Could be described as a coordination failure.
25. Transfer trip inadvertently sent during disconnect switching 230 kV line.
26. Electromechanical pilot wire differential tripped after fuse-cleared fault—lack of
coordination.
27. Electromechanical pilot wire differential tripped after fuse-cleared fault—lack of
coordination.
28. Electromechanical pilot wire differential false tripped due to circulating current
when transformers were paralleled.
29. 4.8 kV bus tripped on backup due to slow trip of downstream fault (coordination
failure).
30. Overcurrent relay on transformer tripped on back-up when a fault on a feeder did
not clear; coordination error.
31. Underfrequency relays tripped on the transient when a breaker tripped on low SF6
pressure. Settings error (in my opinion).
32. EM TOC relay tripped on circulating current when bus tie closed for routine
work.
33. Electromechanical pilot wire differential overtripped on fault cleared by fuse
tapped on line.
34. Electromechanical pilot wire differential tripped due to circulating current when
lines paralleled.
35. EM directional overcurrent tripped when line was paralleled.
36. Electromechanical pilot wire differential overtripped on fault cleared by fuse
tapped on line.
37. Transformer relay false tripped on new energization because new settings had not
been applied.
Induced Signal/Noise
38. Staged fault at a 500 kV line caused false trips due to noise induced into phase
comparison relay at same station, which sent a transfer trip to other end.
39. Breaker tripped due to a spike in the dc circuit during a dc ground search. No
relay targets were reported.
40. Electromechanical pilot wire differential relay misoperated due to external
230 kV fault sending “noise spike” into pilot wires, which tripped one end of
34.5 kV line.
41. Fault on nearby line created a voltage spike, causing a pilot wire relay to operate
(line did not have drainage reactor).
42. 500 kV false trip due to microwave noise, causing current differential relay to
operate.
Mystery
43. 230 kV line tripped for fault on reverse line. No targets found on any relay.
44. 230 kV bus tripped during transfer of station service. No targets, no cause found.
Human Caused
45. 500 kV line tripped on transfer trip accidentally sent during maintenance.
46. pilot wire differential false tripped when “a construction crew
was drilling on the adjacent relay panel when the relay was jarred closed.”
47. Transformer tripped when RTU was bumped, causing it to operate. No relay
targets (shows advantage of using relay trip contacts for operation).
48. False trip of transformer due to wiring being dropped into a pool of water during
work on transformer pressure relay.
49. Vandals broke into substation. Tripped 8 breakers. No relay targets. Another
reason to use relays to operate breakers. Break-in at 6:04 pm in May.
Force Majoure
50. Water leaked into Buchholz relay.
51. “Concussion from a large explosion at X caused the relay contact to close” EM
directional overcurrent relay (3 lines).
52. False trip due to rain water leaked into the pressure relay on a LTC.
No comments:
Post a Comment