When a blackout hits a paper mill, the results can be disastrous: paper jams, fire, machinery damage, costly downtime, and sometimes miles of errant paper. Competing in one of industry''s most asset-intensive sectors, paper manufacturers must not only achieve efficient productivity but also maintain consistent throughput from their mega-million dollar mills in order to meet market demand, and one brief power failure can cost tens of thousands of dollars in production delays and repairs.
Abitibi-Consolidated is a global leader in newsprint and uncoated groundwood papers, with ownership interests in 27 paper mills in Canada, the U.S., the U.K., and Asia. Its Iroquois Falls, ON mill, which annually produces approximately 246,000 metric tons of newsprint and 46,000 metric tons of specialty papers, including colored newsprint, construction paper, and nonprinting grades, needed to upgrade to more reliable protection for one of its power generation stations, which feeds power directly to the paper mill via two 12 kV transmission lines -- without power transformation.
Senior power system engineer Don Elliott (P Eng) chose to install a new type of protective relay from Schweitzer Engineering Laboratories, the SEL-311L Line Current Differential System, using their installed fiber-optic cable from plant to generating station.
"We had a unique situation," explains Elliott. "We had power going from our two transmission lines (5 generators) at one end . . . to the12 kV bus, and our process loads at the other end. Because there were no transformers involved, the mill process was exposed to whatever problems nature gave us, such as transients and faults that occurred on our transmission lines. So, transient survival is paramount.
"Our paper machine rolls along at 1,200 meters per minute through a long winding process," Elliott continues. "The slightest little bump or change in some component along the way can cause the paper-making process to be disrupted. There are electrical control systems involving voltage- and frequency-sensitive components that will drop out or stop working if the nominal 60 Hz signal isn''t present.
"Also, there are a lot of motor contactors in our process system, and if they de-energize due to a transient, they''ll stay dropped out until there is intervention to bring them back on line. So, we really need to clear faults quickly. If we don''t, we may have a mild ground fault that may not seem like a big deal, but within 2 or 3 seconds it becomes a 12 kV phase-to-phase fault not far from our bus, which is likely to cause a lot of equipment damage," adds Elliott.
In the past, a transmission line fault would cause a trip -- and the paper machines would shut down. "This was a troublesome situation," Elliott says, "because transients endangered all mill processes, including dozens of pumps synchronized with massive rotating equipment."
The power system at the Iroquois Falls mill was vulnerable because its "resistance grounded" 12 kV transmission lines lacked sufficient protection.
"If we had a ground fault on a transmission line, the older protection was not sensitive enough to detect the fault. The fault would then remain, as an undetected ground fault, and ultimately propagate into a nasty phase-to-phase fault. Generally, then, it would knock out the paper machines, which are synchronized with dozens of pumps operating with substantial inertia. If there was an insulation failure with any equipment, the resulting damage could be extensive, and it could include production losses as well as substantial repair costs."
A Fiber-Based Solution
Approximately two years ago, the Iroquois Falls mill replaced its 12 kV copper transmission lines with a new double-circuit ACSR (aluminum conductor, steel reinforced) cable.
"When the new line was constructed, we included optical ground wire (OPGW) containing 12 single-mode fibers into the circuit," Elliott explains. This enabled the use of fiber communications for protection. Schweitzer''s SEL-311L Current Differential System provided protection for the new lines and cables.
"The SEL-311L was a good choice for line protection and automation," says Elliott. "This relay allows us to use our fiber-optic capabilities to implement efficient line current differential protection. We have to sense ground faults and clear them instantly."
The negative-sequence sensing elements in the SEL-311L protection introduce a new level of fault sensing and high-speed tripping capability not available a few years ago.
With easy-to-apply SEL-311L Relays, users can protect lines and cables by applying three-pole subcycle current differential protection or optional single-pole differential elements for high-speed fault clearing and improved system stability.
This relay offers complete main and backup transmission line protection using line current differential, and a combination of four stepped-distance zones of phase and ground-distance elements in communications assisted schemes, with directional overcurrent element backup protection.
Users can reduce their protection system costs by using the built-in distance and/or overcurrent backup functions. "We''re also using the SEL-311L as a backup," Elliott says.
SEL-311L standard features include programmable four-shot breaker autoreclose with synchronism and voltage check logic for optimal system restoration. Elliott says the relay''s voltage check logic function is especially helpful in situations when the mill is separated from the grid.
"We have a complicated system with large loads tied to our internal generators. If we get separated from the supplier transmission grid and are completely blacked out, we have the ability to black start from our generating station. We will energize our transmission system and start building up our internal network from that point. The SEL-311L is the component that will basically supervise the closing of the line breaker and energize the line from the generating station," says Elliott.
The Iroquois Falls mill also has an SEL-2030 Communications Processor installed, using Ethernet to interrogate the system for records and data from the SEL relays'' Sequential Events Recorder (SER) directly from Elliott''s office, rather than having to visit the remote equipment. The relays connected to the SEL-2030, and the SEL-2030, communicate to the Ethernet system LAN.
"We also have an ''Arbiter 1093B'' satellite clock input to the SEL-2030, so that all relay records and all the relay time bases are time-stamped to one millisecond accuracy. When we have an event, we''ll reconstruct it and analyze exact information from all sources, correctly time-stamped to the same standard GPS-synchronized time," Elliott says.
Standardized on SEL Protection
The current protection scheme at Abitibi-Consolidated''s Iroquois Falls mill includes two SEL-2PG10 Phase Distance Ground Overcurrent Relays, five SEL-321 Phase and Ground Distance relays, one SEL-311C Distance Relay, four SEL-311L Relays, 12 SEL-300G Generator Protection Relays for added primary and backup generator protection, five SEL-351A Distribution Protection Systems, two SEL-501 Dual Universal Overcurrent Relays, and two SEL-547 Distributed Generation Interconnection Relays used for islanding detection. The SEL-2030 can accept IED information on any of its 16 ports, so the relays can be easily accessible to Abitibi''s engineering staff for analysis of events and feature enhancement.
Elliott''s team has standardized on SEL products for protecting their power system. A primary reason is Schweitzer''s high level of service.
"I have a long and stable relationship with Schweitzer," Elliott says. "They have great product coverage and a very high level of service, including field technical support that has been very valuable to me. During our upgrade project, I was able to get very fast and reliable answers to some technical questions I had about configuration choices. Within half an hour I had an email from an SEL field application engineer who explained precisely what choice made the best sense. And that''s simply how good SEL support is. I know we can count on SEL for that level of support every time."