Self-healing fiber ring stymies squirrels.
Thanks to a hungry (or angry) squirrel with sharp teeth and strong jaws, the Springfield [Oregon] Utility Board quickly found out that its new self-healing ring fiber-optic communications network was reliable--so reliable in fact, that several days went by before it realized that the cable in one of the fiber-optic rings had been severed.
"We suspected a bad splice or jumper cable and were more than a little surprised when an optical time domain reflectometer (OTDR) reading showed a break in the middle," says Tom Weller, substation and metering engineer.
"We sent out a technician to investigate and, sure enough, several feet of the cable had literally been chewed to pieces by what we assume to be a squirrel. The damage to the cable was remarkable. But we found out the self-healing ring worked."
SEARCH FOR RELIABILITY
Spurred by a need for more reliability because of the harsh environment, Springfield began looking in late 1996 into installing a new communications network to link its substations with the master communications center. It would continue to use its existing SCADA (supervisory control and data acquisition) software. At that point, Springfield communicated with its substations over leased telephone lines. Because of the leased line's vulnerability to ground faults, reliability was a constant concern, according to Weller.
"When it rained, the Mt. Vernon substation leased line went down like a bowling pin, and it took the total communications network with it," he explains. "This substation includes some old underground lines installed in the '60s, and they would always ground out when it rained. With 48 average inches of rainfall per year this was a continuing concern. This situation was especially stressful because this substation is a delivery point from Bonneville Power. In some instances, with the leased line system, we would be down for two or three days. That made scheduling power a real guessing game."
Springfield's technicians tried several stop-gap efforts to keep the old system functional. For example, to prevent one node failure from disabling the entire network, Springfield technicians split the bridged network into two and had the bridge location brought back to the master station. This fix helped the overall situation but still only maintained communications to one-half of the substations when a fault occurred. The faulted line was then removed by flipping a toggle switch and allowing normal communications to resume to the remaining RTUs (remote termination units) while waiting for the telephone company to repair the faulted line.
Springfield looked at several communication network alternatives. Using fiber optics would eliminate ground fault problems and the self-healing ring would insure system integrity by automatically rerouting communications in the event of a cable or device failure. Furthermore, the fiber modems would include antistreaming/ antijabbering to prevent a single node failure from blocking out communications on the entire data path.
Springfield's proposal specified RS-232 multidrop fiber-optic modems, supporting asynchronous data rates from 1.2 to 19.2 kbps. Other specifications included 24VDC power, ST fiber-optic connectors, and DB25 female electrical connectors. The specified loss budget for each unit was 15dB. Separate self-healing (counter-rotating) fiber-optic communication rings were designated for both the Springfield electrical department and water department (Figure 1).
[Figure 1 ILLUSTRATION OMITTED]
MULTIDROP FIBER-OPTIC MODEMS
Springfield opted to purchase Model TC2100 fiber-optic multidrop modems from TC Communications in Irvine, Calif. The TC Communications' fiber modems were designed with harsh environments in mind and met all of Springfield's requirements including reliability, future expansion and flexibility, and reasonable cost.
To validate both of the rings' integrity, the master modem constantly sends out a ring-monitoring signal to both rings A and B. In this manner, it can detect faults upstream and downstream from a remote device. If the cable breaks or a device fails, the master unit will not receive the monitoring signal and immediately will initiate an alarm condition. LEDs on the master unit's front panel show the status of the ring-monitoring signal.
Because the TC Communications' multidrop fiber-optic modems are transparent to communication protocols, the Quindar Quicks IV, Telegyr, and TCP/IP protocols used with Springfield's RTUs and PLCs posed no special problems.
The fiber modems also enabled RTU alarm conditions to be transmitted to the SCADAhost via dry contact alarms at each fiber-optic modem. By looping back the dry contacts to the RTU, the operation can isolate exactly where the problem is occurring. Standard alarms for the electrical department include equipment operation, voltage levels, breakers, and temperatures.
Springfield will add two optional features in the near future. The "Quick-Talk" Voice Adapter can turn a fiber-optic network or RS-232 circuit into a voice network simply by plugging a telephone set into an RJ-11 (or DB25) connector. It is compatible with most PBXs or key systems. Another feature is a second fiber-optic RS-232 interface, enabling two master units to be placed at different locations, each polling different slave units on the same fiber-optic ring. Springfield is already making plans to use the second fiber-optic interface to enable two master fiber modems to use one cable. This will maximize use of the bandwidth and, ultimately, enable Springfield to merge the four fiber rings currently in use into two fiber rings.
THE SELF-HEALING RING NETWORK
Springfield's fiber network is extensive. It includes 33 miles of 48-strand and 96-strand bundled single mode fiber-optic cable, mostly aerial, across its existing network of power poles. The longest run between substations (fiber modems) is about four miles. The measured optical loss over this segment is 12dB, comfortably within the 15dB loss budget of the fiber-optic modems.
Currently, Springfield is using four separate fiber-optic ring topologies--two for the electrical department and two for the water department.
FIBER-OPTIC MODEM NETWORK BENEFITS
One of the primary benefits of the fiber system has been a consistent flow of information for scheduling power deliveries. When the communications system was down, vital information about load levels was unavailable. As a result, the utility board potentially could have to buy more power than originally allotted--and at a higher rate--to protect itself from shortages.
The Springfield Utility Board is looking at a variety of ways to continue improving its communication networks, according to Weller. "We are using the fiber backbone for all our SCADA system communications and will soon be using it for transmission relaying.
[Figure 2 ILLUSTRATION OMITTED]
Circle 262 for more information from TC Communications, Inc.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||Technology Information|
|Comment:||The Springfield, OR Utility Board had problems with network reliability caused by squirrels eating cable and other environmental issues which it resolved by implementing a self-healing fiber optic ring network.|
|Date:||Mar 1, 2000|
|Previous Article:||Networks thrive on attention.|
|Next Article:||Capitalize on cables, connectors, and certification.|