Fault-pressure relays protect transformers from internal electrical faults that other protective relays, like differential or overcurrent, can’t always detect. If the transformer’s protective relaying scheme is old, it might have a GE model HAA relay installed. This is likely in nuclear power plants of a late 1960s or early 1970s vintage.
Prior to the 1960s, sudden pressure (or fault pressure) relays were too unreliable to use in transformer protective circuits, but this began to change in the 1960s. As the relays evolved, other shortcomings were uncovered. GE developed an auxiliary relay specifically meant for use in conjunction with its sudden-pressure relays. In some nuclear plants, those circuits are still in operation.
Internal transformer faults can have sudden and detrimental results. Like this article says, “Internal arcing in an oil-filled power transformer can instantly vaporize surrounding oil, generating gas pressures that can cause catastrophic failure, rupture the tank, and spread flaming oil over a large area.” Damage to nearby equipment or people is likely.
Certain internal faults can create fault current magnitudes that are too low for external protective relays (differential or overcurrent) to detect. If they fail to detect these faults fast enough, then the transformer can fail catastrophically.
The sudden pressure (also called rapid-rise or fault-pressure) relays use mechanical principles to detect these electrical faults. Internal sparking causes quick pressure increases as the oil vaporizes. Faults that can lead to this situation are turn-to-turn faults, high eddy currents in the laminations, and high-resistance faults, just to name a few. The internal arcing creates a pressure wave that the relay detects. It is tuned to ignore most slow-rising pressure waves that occur with normal operation.
The relays developed in the 1960s were more reliable and began to be used in trip circuits instead of as alarm-only triggers. One problem they suffered from, however, was inadvertent tripping due to voltage arcs across the relay’s normally open contact.
GE’S SPECIALLY CRAFTED HAA RELAY
GE developed a relay for specific use in the sudden-pressure circuit, the HAA16B and HAA16C models. They differ from the HAA15 variants because they are special high-speed models. Because the sudden-pressure relays actuate only momentarily (they do not usually latch on their own) and quickly, the auxiliary relays must be fast enough to detect the relay trip.
Additionally, the HAA relay incorporated extra contacts to enable latching, as well as a “target.” The target makes quick visual identification possible, giving operators a means of determining which relay tripped. This can be especially important in a panel full of relays. The extra contacts allow the relay trip circuit to seal-in, remaining actuated even after the momentary action of the sudden-pressure relays has passed. It also minimizes the contact count since there’s no critical need to use a contact to illuminate an indicator light; this reduces the relay’s footprint and the panel space required because there is no need to account for a separate light.
A simplified schematic of the HAA 16B relay is shown below, taken from an IEEE working group report (Figure A-3):
Another peculiar characteristic is the relay’s inclusion of two extra resistors. The first is usually 350 ohms, but it ranges down to 75 ohms depending on the relay’s specified operating voltage. The second is called an external resistor and is only present on some models. For the 125 VDC relay, the external resistor is 650 ohms, and on the 250 VDC model it is 1,650 ohms.
The resistors serve two different purposes.
The smaller internal resistance is meant to prevent accidental relay tripping caused by voltage surges. It works in tandem with the sudden-pressure relay’s normally closed contact, as explained in Appendix A of a sudden-pressure relay report prepared by an IEEE working group: “The [sudden pressure relay’s] normally closed contact prevented operation of [the HAA auxiliary] relay (and tripping) for arcing of the [sudden pressure relay’s] normally open contact due to DC surges. The 350-ohm resistor prevented shorting the DC supply should the [sudden pressure relay’s] normally open contact arc over.”
In other words, the 350-ohm resistor ensured that an arc-over on the normally closed contact caused by a voltage surge wouldn’t short out the power supply. The normally closed contact shunts the current from the voltage surge to ground through a resistance instead of simply shorting out the DC power supply. Shorting the power supply could cause a protective device to operate, cutting out the DC supply and any other circuits attached to it.
The larger external resistor is meant to limit the current through the relay coil at higher voltages. This probably reduced the need to produce too many coil variants which would have saved cost by allowing one coil part number to be used in multiple relay models. We can see this when we examine the currents produced by each load. For example, for each of the 48, 125, and 250 VDC relays, the coil resistance is given as 95 ohms. Each relay has an internal resistance of 350 ohms. The 125 VDC relay, however, adds a second (external) resistor rated at 650 ohms, and the 250 VDC relay’s external resistor is rated at 1,650 ohms. The currents through the three relays are:
I1 = 48 VDC ÷ 445 ohms = 0.108 amps
I2 = 125 VDC ÷ 1,095 ohms = 0.114 amps
I3 = 250 VDC ÷ 2,095 ohms = 0.119 amps
The designed coil current averages out to approximately 114 mA.
Finally, the HAA relay also incorporated a slight time delay to guard against pressure waves caused by normal operations or other kinds of shocks. The waves are produced during normal operation but are shorter in duration than the rapid pressure rise created by internal faulting; the time delay prevents the relay from confusing the two. The sudden-pressure relays are also susceptible to external shocks; they are usually mounted external to the transformer, so they can be bumped by personnel during walkdowns or maintenance — not to mention earthquakes.
Regarding the configuration of its control circuit, the HAA relay’s contacts are used to seal itself in and possibly actuate a second auxiliary relay if the sudden-pressure trip is incorporated into the transformer protection scheme. If this is the case, then the auxiliary pressure relay (HAA) will probably be used to actuate a lockout relay, perhaps a GE HEA ,which is another high-speed, multi-contact auxiliary relay used in conjunction with differential and overcurrent relay trips.
Older transformer control circuits that use sudden-pressure fault protection relays may also use GE HAA auxiliary relays to perform an alarm function (by ringing in an annunciator). The HAA relay may also be used to actuate a trip circuit by tripping a third relay. The third relay is a lockout relay, which may be a GE HEA relay or similar.
The HAA relay provides three main features:
- Fast operation to ensure it can detect the fast-acting sudden-pressure relays.
- Supplies a target for quick operator identification at the relay control panel.
- Implements a slight time delay to guard against false trips due to pressure waves created by normal transformer operation.
The HAA relays are used to activate alarms and/or transformer trip circuits. They may have two resistors: one (350 ohms) for preventing the power supply from shorting out upon an arc-over of the normally open contact; and a second (650 ohms or greater) for regulating the current through the trip coil at higher voltages.
The sudden-pressure relay’s normally closed contact, by being wired in parallel with the auxiliary relay trip coil, is used to guard against false trips by shunting potential contact arc-overs to ground.