From Montreal Engineering Co. Ltd.
Reprinted from Water Power September 1970
By C. Humphreys, B.Sc., M.E.I.C., Eng.
This is the first of three articles on the control of hydro stations, and it describes how the Island Falls plant in Saskatchewan was converted to remote-control from the load centre 60 miles away.
THE ISLAND FALLS power plant in Canada is situated on the Churchill River 14 miles west of the Manitoba - Saskatchewan boundary, and some 370 miles from the port of Churchill where the river enters Hudson Bay. The site was developed initially between the years 1928 and 1930 to provide power for the Flin Flon mine and smelter which lies about 60 miles to the south just east of the Provincial border. During the next 27 years the plant was expanded to include seven units with an installed capacity of 110MVA.
The Hudson Bay Mining and Smelting Company Ltd has developed the mining and smelting operation at Flin Flon, and zinc is one of the main products. An electrolytic process is used and a substantial part of the power plant output is used for that purpose. The mine hoists are also electrically powered as is much of the equipment used in the smelter itself. Power is also provided to the city of Flin Flon (population 12000).
From its inception, the Island Falls plant has been a difficult site to reach. Although Flin Flon is accessible both by rail and road from Winnipeg, 400 miles to the south, there are no such facilities to the power plant itself. It is reached by air or by water over numerous lakes and involving several portages.
During the winter time, it is possible to operate snowmobiles and tractor trains over the ice, and that is the method used to transport heavy equipment to the plant. From the beginning it was, therefore, necessary to develop a townsite for the operators and their families, and by the mid fifties it had become a community of some 200 people. The Churchill River Power Co Ltd, a subsidiary of the Hudson Bay Mining and Smelting Co Ltd (HBM and S), operates the plant and power system.
Following a feasibility study carried out by Montreal Engineering Co Ltd in 1964, the HBM and S management decided to automate the plant and to install a supervisory control system to enable it to be controlled from a control centre to be built in Flin Flon. This would relieve the mounting economic burden of maintaining the townsite and its facilities and reduce the number of operators required to operate the plant. From this study, the following paragraph is quoted to illustrate the type of system adopted.
"The scheme proposed will provide means of controlling the generator units from the control centre in Flin Flon, in a manner similar to that how employed by the operators within the Island Falls plant. Indication of equipment positions, station alarms, shutdowns and abnormal conditions will be transmitted automatically to the Flin Flon control centre. The protective equipment within the generating plant will maintain operation of the plant equipment within predetermined safe limits, and will automatically take out of service equipment developing abnormal conditions. Indications of operating data will be given at Flin Flon so that under this arrangement the station operating procedures would be identical to those now in use."
|Downstream view showing the housing of unit No. 7 in the foreground.|
The power plant comprises three 13 MVA generators driven by 16500hp propeller-type turbines, and four 18MVA generators driven by 19000hp propeller-type turbines. There are also two 1 MVA units used for station service and local supplies. The turbines are rated at 56ft head.
Engineering for the project was started in July, 1965, and proceeded in two fields simultaneously.
(1) Modifications to the plant to make it automatic in operation, capable of being controlled either locally or remotely, and completely self-protecting.
(2) The design of a control centre at Flin Flon and the preparation of specifications for the supervisory control and telemetering equipment, and the communications equipment.
Automatic generator controls
The process of converting the generators to automatic control presented several problems, due to the necessity of maintaining plant output at the level required for production, throughout the entire period of the conversion. It was only possible to remove one unit from service at a time and to ensure its proper functioning after modifications, before restoring it to service.
In order to reduce the down time to a minimum, it was also decided to mount much of the additional control relays and other new equipment on new control panels and to incorporate the existing control panels when the automatic features were ready for wiring.
With the exception of the static exciters, which were bought as packaged units, all control and relay panels and cubicles were constructed on site by the power plant staff. The more usual practice in projects of this sort is to have such items fabricated by an electrical contractor, with the only work on site being physical installation and intercabling.
The panels and cubicles at Island Falls began as sheet steel, boxes of relays, switches, terminal strips, etc, and reels of wire. The panels were fabricated in the plant machine shop, and were then installed, with component installation and wiring done on site. The decision to operate in this manner was made mainly from consideration of economy and expediency, but it also made possible the maximum use of existing components before their transfer to the new circuitry.
A factor which had to be considered in the choice of components and design of circuitry was the previous record of the plant for continuity of power. From 1930 to 1965, complete outage time totalled approximately 8h, a truly astounding availability record of 99.997% to meet scheduled essential loads. This was undoubtedly the result of initial care in plant design, conservative component ratings, and an extremely well planned and executed programme of routine maintenance throughout the years. The philosophy of conservative component ratings 'was continued in the planning stages of the automation programme and care was taken to incorporate fail-safe features wherever possible.
Also, two station batteries are used in a switch over arrangement, each battery floated across its own charger. The transfer circuitry is fully adjustable for pick-up voltage, drop-out voltage, and time delay, allowing ample adjustment of its operating characteristics. The automatic controls may be divided into five major areas; turbine' and governor, generator, transformer, breaker and disconnector switches.
In the area of the turbine and governor, the following devices were installed:
- pneumatic actuators, relays and limit switches to control the head
- complete and partial run and shutdown solenoids.
- wicket-gate and gate-limit telemetery potentiometers.
- 30, 85, 95 and 105% speed switches.
- wicket gate and gate limit control motors and cam switches.
- permanent-magnet generator (PMG) ball heads on three of the units.
- monitors for accumulator and sump levels and oil pressure.
- air make-up pumps.
- monitors of pressure and flow of coolants and lubricants.
- brake-control solenoids and brake-limit switches.
- self-cleaning screens in cooling-water system.
- automatic greasing system.
- creep detectors.
For the generators, the following items were installed:
- static exciters for three of the units.
- bearing-lubricant control valves and pressure and flow sensors.
- automatic speedmatching and synchronising equipment.
- complete local annunciation.
- generator thermal relay.
- over-voltage protection.
- stator ground protection.
- bearing-coolant flow switches.
- lockout shutdown relays.
- split-phase protection.
- motorised voltage-adjusting rheostats.
The transformers and oil circuit breakers were essentially unmodified, although the protection was altered in line with the new devices installed elsewhere. The disconnectors were converted from manual to motorized operation and suitable limit switches installed. The' automatic sequencing and protection circuitry is fairly straightforward, incorporating the following features:
Five independent positive buses are used in conjunction with a common negative bus. The buses are called P, PA, PM, PS and P4, and are used to provide protection against undesirable simultaneous relay operation and sneak circuits.
In the REMOTE mode, the PM, or manual bus, is de-energised, preventing unwanted manual operation.
In the LOCAL-AUTOMATIC mode, both PM and PS buses are de-energised, preventing manual and remote operation.
In the MANUAL mode the PA, PS and P4 buses are de-energised, preventing any form of automatic operation. The P4 bus is used only during automatic synchronizing, and is only energised after a successful start in either of the automatic modes,
The automatic start sequence, whether remotely or locally initiated checks the following items--no lockout or non-lockout shutdown conditions present; both of the two-unit headgates raised, unit breaker tripped; generator upper and lower guide-bearing oil flow established; excitation set at nominal open-circuit voltage; brake shoes released. The signal is then given to the governor control solenoids to start the machine.
Once the unit starts and runs up to speed-no-load (SNL), it indicates that this speed has been achieved. The operator then calls for a breaker closure, which initiates speed matching and the automatic synchronisation sequence. This circuit incorporates dead-bus monitoring, anti-pump, and sequencing which permits several units to be synchronised at once without the possibility of two or more unsynchronised units closing simultaneously on to a dead bus.
Dead bus monitoring uses both time and delay and instantaneous relays, to guard against breaker closure during brief under-voltage condition on the bus, and to block closure should a dead bus suddenly become energised before the breaker seal-in circuitry operates, thus providing a trip-free form of operation.
An indication is provided of synchroniser operation both locally and remotely.
Once the unit is running on the line, it is loaded incrementally under-remote -control or- continuously under local control. Full data on governor gate limit and gate position, megawatts, megavars and field current are available to both the local and remote operators at all times.
When the machine is under local control, the remote operator has full alarm and telemetry information, but he can not perform any unit control functions, such as stopping or starting or adjusting load. He is, of course, informed of the fact that the machine is under local control. When the 'machine is under remote control, the local operator has full alarm and metering information displayed, but he can not perform unit control functions. He does have control of the mode switch itself and can place a unit in local control if necessary.
There are several shutdown modes. A lockout results in an immediate breaker trip and unit shutdown. A non-lockout will cause load rejection, breaker trip at SNL, and complete shutdown if the fault condition persists below 85% speed. If the fault resets between breaker trip and 85% speed, the unit will return to SNL and may be re-synchronised.
|Upstream view of the powerhouse.|
For non-protective shutdowns, the operator has two choices. He may call for a trip, which results in an immediate breaker trip and shutdown in the shortest possible time, or he may request a unit stop, which energises a pumping circuit which will reject load gradually, the breaker tripping at SNL and subsequent shutdown. The 'load rejection circuit requires about 2min to reject full load.
The turbine brakes are energised at 30% speed providing the unit breaker has tripped, the turbine gates are closed, and, the master start relay reset. The brakes are held on for about 15min to ensure that the bearing oil films disperse so as to provide friction against rotation due to any possible leakage of the turbine gates. The unit may be re-started at any time during the braking sequence, but a start will not be permitted unless the brake shoes have been fully released.
A mechanical interlock is provided between the solenoid and the manual brake lever to prevent any conflicts.
A special shutdown application exists in conjunction with the static exciters. Initially, these had been arranged to provide a lockout for firing suppression but this was later revised when it was found that suppression could result from causes external to the exciter.
Suppression is presently arranged to directly reset the master start relay, trip the field breaker. and trip the generator breaker, without operating through any other protective devices. Thus, this is a complete and immediate electrical fault shutdown, but does not require local reset should the cause be external. Should it be internal, of course, a restart will not be permitted, Non-lockout shutdowns, apart from the special-firing suppression shutdowns described above, are initiated for over-voltage, over-speed, ballhead failure, and low governor oil pressure. Lockout shutdowns are initiated for electrical fault, comprising generator and transformer differential, generator split phase and stator ground, and static excitation faults (except firing suppression), by low accumulator oil level, field failure, bearing over-temperature and transformer or oil circuit breaker fire. This latter was obtained by modifying an existing fire protection system.
The automation modifications described were carried out on the six main units at Island Falls. The seventh unit was already automatic in operation, but its circuitry was slightly modified to bring it in line with the other six units. Additionally, the two 1 MW house units, used for providing station service, were also automated along similar lines although their circuitry is more complex, due to the fact that they share a common synchronizer and may be run coupled or isolated, and also the requirements for continuity of service which are, if anything, more stringent than those for the main units.
In addition to the modifications to the generators, some modifications were performed on the substation controls, but these were limited for the most part to the installation of motorised disconnectors and the provision of remote controls and indications. Apart from local and expanded annunciation, the protection was left virtually untouched.
The modification programme is continuing with the conversion of further units to static excitation and PM G ballheads and the installation of new accumulators for the house units.
Supervisory control system
The control centre building is located close to the Flin Flon terminal of the 110kV transmission lines within the main smelter area, and adjacent to the HV substation. It comprises two floors and a partial basement and is utilised as follows. The basement contains the air conditioning and heating equipment, the ground floor holds the superintendent's office and equipment room, and the second floor, the general office and the control room. Records and office staff originally at the Island Falls plant were transferred to the new building.
Solid state, discrete component supervisory and telemetry systems were selected. each system being essentially independent. The only electro-mechanical devices employed are at the plant where relays provide isolation at the interface between the supervisory system logic and the plant automatic controls. This was considered necessary in order to prevent transient voltage on the station wiring from entering the solid state logic circuits and causing damage or false operation.
On both systems, information is exchanged between remote and master stations in 4 out of 8 code. This is a modified 11 bit ASCIE (American Standard Code for Information Exchange) code (8 level) in which a high degree of security is obtained by using bit inversion and 100% redundancy within each character. Start-stop operation provides for character synchronisation, and 16 characters are used in the systems. The speed of transmission is not the same for both systems; the supervisory operates at 150 bauds and the telemetry at 330 bauds.
The supervisory control system operates in two modes; one is the continuous alarm scan and the other the operation mode. When no operations are being carried out, the master station is receiving status indications from remote terminal continuously. In this mode of operation, the master station automatically interrogates the remote station repeatedly.
Each interrogation consists of a three-character coded message comprising a start of message character, function-code character requesting the status report, and the end of message character. The remote station, on receiving the message, reports the status in a fixed system format.
The master station receives the report serially in bit form and converts it to parallel form, character by character. Each character in parallel form is checked for code security and then channelled to the appropriate annunciator window or indicator light on the control desk. Changes in status which have not been called for by a control function, are interpreted as alarms. After receiving the status report. the master station re-interrogates the remote station with the same message and the process of reporting is repeated. The cycle time is approximately 65s.
When the operator at Flin Flon master station wishes to carry out an operation, he pushes a point select button and follows with an operation of the appropriate OPERATE pushbutton. The latter has no effect unless a point has first been selected.
|Powerhouse interior looking from No. 5 towards No. 1 machine|
These operations suspend the continuous scan mode and the master station transmits its five-character operate message, comprising a start of message, a function (eg. whether trip or close), a point address (two characters) and an end of message.
At the remote station, the message is checked for security and the operation carried out. It then replies to the master station with a message which confirms that the operation has been carried out and gives the new status. Upon receipt of this message, the system resets and reverts to the continuous scam mode.
The telemetry system is simple in operation, and continuously scans the 45 quantities to be transmitted. These quantities are available as DC analogs produced by suitable transducers from primary measuring devices. For example, watts and vars are produced from thermal converters, water levels and governor gate position readings are provided by mechanically driven potentiometers.
The remote station telemetry system uses a common analog-to-digital converter and scans each quantity in turn. Information is transmitted in binary coded decimal form, each quantity requiring three digits and a decimal point.
At the master station, a digital-to-analog converter translates each reading to DC analog form in which it is then passed to a holding amplifier. The output of this amplifier drives the display meter continuously. Each reading is measured and the reading updated where necessary, once per cycle, which corresponds to approximately once every 8s.
Upon receipt of a message from the master station via the supervisory system, the remote telemeter terminal goes into a logging mode. The message may, be sent automatically at pre-set intervals, ego hourly, and it may be sent at will by the operator's use of a pushbutton.
In the logging mode, certain selected quantities are transmitted to the master station to form part of the system log. During the logging operation, the speed of transmission remains at 330 bauds, but each character or digit is sent three times in succession thus enabling the print-out at the master station to proceed at the standard 110 baud rate (100 words/min).
The control desk at Flin Flon consists of 10 desk sections arranged in the arc of a circle, and in positioning it allowance has been made for future expansion at either end. Each desk section consists of wired and equipped control escutcheons, mimic bus, annunciators and display meters all easily accessible by the control operator.
A power line carrier system was installed to transmit control and telemeter signals between the plant and the control centre in Flin Flon. The power transmission line is a double circuit 110kV line carried on steel towers, and having a single spur line with switching arrangement three-quarters of the way from the plant to Flin Flon. This spur line supplies mining operations at Chisel-bake.
Power line carrier equipment is coupled between ground and the C phases of each line in a push-pull symmetrical manner. This provides the most reliable coupling under normal conditions, with only slightly impaired transmission when one of the lines is out of service. Wave traps rated at 850A continuous are installed at each terminal and at the spur line.
The carrier equipment was provided in duplicate at each terminal with automatic changeover circuits controlled by a reduced carrier pilot frequency. Single side-band sets provide audio frequency channels of band width from 300 to 3400kHz in each direction of transmission and these channels occupy the carrier bands 136-140k Hz and 148-152 kHz. These bands were dedicated to the transmission of control and telemetering signals, since other means of voice communication with the plant already existed. The data interchange takes place via frequency shift, voltage-keyed transmitters and receivers, frequency multiplexed on the carrier base band.
As might be expected in a project of this size and complexity, there were a number of problems experienced in both the automatic and supervisory control systems. The majority of them were discovered and corrected during the commissioning period and did not cause any serious delays. For example, the release time of certain control relays had to be increased in order to prevent unit shut-down during the operation of the master mode control switches. There were high electrical noise levels on some of the telemeter leads in the plant and these caused serious errors in the readings. This problem was solved by the use of transducers with higher level outputs or by re-arranging some of the longer wiring runs.
The problem still remaining is the unreliable operation of the headgate lower controls when initiated remotely. An electro-pneumatic system of brake release was installed, but due to the dampness of the locale the release does not always occur when called for. Local operation has always been satisfactory since the brakes were manually released. Plans are being drawn up to modify the whole mechanism of headgate control and eventual reliable operation is expected.
The automation programme described here was completed in October, 1967, and since then the plant has been operated as planned from the control centre in Flin Flon. The total population of the Island Falls townsite has been reduced from 200 to about 8. Those remaining are mainly occupied with routine maintenance and servicing.
This article courtesy of Keith Olson