Applying Standardized Industrial Automation to the Small Hydro Plant

Applying Standardized Industrial Automation to the Small Hydro Plant. By David de Montmorency, P. Eng, Rapid-Eau Technologies Inc, and Christopher de ...
Author: Alice Mathews
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Applying Standardized Industrial Automation to the Small Hydro Plant. By David de Montmorency, P. Eng, Rapid-Eau Technologies Inc, and Christopher de Montmorency, Rapid-Eau Technologies Inc.

Rapid-Eau Technologies has been in the small hydro business for the past 26 years. We design water turbines and electrical control systems for the small hydro industry. We currently operate three sites from our offices in Cambridge Ontario varying in capacity from 5 megawatts to 700 kilowatts. Each site is remote and has a dedicated internet connection via satellite which allows us to control, monitor and gather data for preventative maintenance, financial and management decisions. All three sites are controlled by a PLC. PLC Based Control Rapid-Eau’s control systems are based on programmable logic controllers (PLC), a standard industrial control device which is used universally in industrial automation. PLC’s are based on solid-state technology with no moving parts and are designed to have an exceptionally long life under severe industrial conditions. We chose to base our control systems on PLC technology to take advantage of their inherent properties: • • • • • • • • • •

Extremely reliable, industrially hardened with no moving parts Flexible, a single PLC can control multiple machines Modular, components can be easily added to operate new equipment “Off the shelf” components are widely stocked Simple to program using traditional ladder-logic Program is easily modified, changes can be made in the field Service can be performed by local technicians Visual program operation makes troubleshooting quick and simple Compatible with a wide range of communication networks Allows remote access for operation, information transfer, troubleshooting, and program modification

PLC’s can be the ideal tool for the automation of a small hydro plant and to build a SCADA system on.

The High Falls Project Bracebridge Generations High Falls upgrade and turbine addition is a good example of what can be done with PLC technology. Bracebridge Generation operates three small hydro stations on the North Muskoka River close to the town of Bracebridge Ontario.

High Falls at Bracebridge Ontario

At the High Falls plant the river drops some 14 metres. The plant was built in 1947 with a single Barber pressure case turbine rated at 800 kilowatts. The generator was 6900 volts at 360 rpm. In 2005 Bracebridge Generation decided to expand their High Falls plant by installing a 1800 kilowatt double regulated “S” turbine. At the same time they decided to upgrade the existing turbine and generating system by rewinding the generator, decreasing its voltage to 4160, while increasing its capacity to 1000 kW, rebuilding the turbine, and replacing the controls and switchgear.

The mechanical/electrical work for the renovation and expansion was undertaken by Norcan Hydraulic Turbine under a design build contract. Eaton Corporation and RapidEau Technologies acted as design build subcontractors to Norcan. Eaton supplied and installed the switchgear and protection. Rapid-Eau supplied the instruments, controls and SCADA system. Construction commenced in the summer of 2005 and commissioning was completed 8 months later.

The new penstock and draft tube for the 1800 kW expansion

The original plant is shown on the left the 1800 kW addition to the right

Original 800 kW pressure case turbine and switchgear

1800 kW double regulated “S” turbine manufactured by Norcan

Direct connected 327 rpm 1800 kW generator

The PLC Control System Requirements for the design of the new control system included: • • • • • • • • • • • • • • • • • •

Manual control over both turbines Govern the turbines during synchronization Automatically adjust the runner pitch on the new turbine based on a programmable cam curve Control the turbine brakes and detect creep with the new turbine Operate a redundant head pond level sensing system Calculate the spillage over the weir and through the sluice gate Automatically and efficiently control both turbines to ensure the head pond level and spillage requirements are met Automatically adjust the head pond level and flow requirements based on a flow distribution plan Record hourly and daily average levels and flows to meet regulatory requirements Integrate the switchgear protection relays and transducers to the PLC controls via a Modbus network Record alarm and trip events and annunciate them both locally and remotely Trigger the station alarm system on various alarm and trip conditions Monitor the turbine mechanical state and provide protection for abnormal conditions Operate the sluice gate, howler, and strobe Manual and automatic control of the building ventilation systems Record and trend station, electrical, and mechanical data Allow a remote operator to have full access to the control and information systems Provide redundant local control in the case of an HMI system failure

Inside the High Falls PLC cabinet

The PLC was provided with the following information to carry out the required functions: • • • • • • • • • • • •

Water elevation of the head pond from three independent pressure sensors Water elevation behind each of the three trash racks and the tailrace Head and sluice gate position from the gate actuators Guide vane pitch from an absolute optical rotary encoder Runner pitch from a laser distance sensor Shaft speed from an inductive proximity sensor Bearing vibrations from velocity sensors Bearing and building temperatures Discrete control signals from the auto-sync relay Electrical parameters from the metering transducers and multi-function protection relays Discrete alarm and trip signals from the protection relays Status signals from the switchgear, battery bank, hydraulic units and physical plant

Turbine Synchronization One of the most interesting and challenging aspects of the control system was using the PLC to govern the turbines during synchronization. With the PLC driven control system it is possible to synchronize a number of turbines each with its own synchronizing regime. Thus in a large plant the PLC could replace any number of individual governors. The Woodward governor on the Barber turbine had been upgraded from fly-balls to a 723H electronic governor in 1999. The PLC system replaced the Woodward electronics but utilized the hydraulic valves that controlled the oil flowing to the cylinder controlling the guide vane pitch. The valves included an industrial proportional valve with position feedback.

Woodward governor with electronic conversion

Modified governor showing the electric servo valves

The Barber turbine’s acceleration curve was not linear. As the turbine gates were opened from a cold start, the speed would rise initially and then after about 30 seconds begin to drop as much as 25% and hold this speed for approximately 30 seconds while the draft tube built up suction. This idiosyncrasy would cause motoring if synchronization occurred too quickly. The synchronization cycle was broken into three stages. The first stage of the cycle, controlled by the PLC, would open the guide vanes to obtain approximately 80% synchronous speed. A programmable timer held the gates at this position until turbine speed began to drop at which point the second stage began. During the second stage the PLC would ramp the turbine speed up to the point where the auto-sync relay would activate and begin to issue speed commands. An adjustable time interval and gate increment controlled how fast the turbine would accelerate during this stage. The PLC would open the guide vanes by the increment amount at the end of each time interval until the auto-sync relay activated to begin stage three. During stage three the auto sync relay was in control of gate movement. The PLC would listen for raise/lower commands and move the gates accordingly. The operator could adjust the amount of gate movement, and the repeat interval for the auto-sync commands. In response to an auto-sync command the PLC would move the gates by the programmed amount immediately and at the end of each repeat interval until a new auto-sync command was received. This simple program resulted in a very effective synchronization cycle. The synchronization of the new turbine proved more complex. It was double regulated and its speed at no load was very sensitive to guide vane pitch. A movement of less than a degree in the guide vanes would result in significant speed swings. This caused excessive hunting during stage three when the PLC was controlling gate position under the command of the auto-sync relay. To correct the problem we considered reducing the gate movement to .35 degrees in response to each command. This was equal to the resolution of the encoder measuring the guide vane position. Instead we changed the configuration of the auto-sync relay such that it would send a pulse train for the speed commands. The PLC would move the guide vanes for the duration of each pulse and an adjustable maximum pulse width prevented large speed swings. This system worked well.

The new hydraulic pumping unit on turbine #2

Runner Control The PLC was programmed to control the pitch of the runner blades in respect to the guide vanes when the generator was connected to the grid. The commissioning cycle required that this “cam curve” be changed to maximize efficiency and minimize hydraulic noise. The simplicity of PLC programming allowed these changes to be made at site in a matter of minutes. The device that reads blade position on the new turbine is a Braumer Electric laser distance measurement sensor mounted in the chamber upstream of the runner. A carefully aligned disk attached to the end of the ram that controls the blade pitch is used as the target. This disk rotates and must be setup to eliminate any wobble. The laser distance sensor is stationary about 6 inches away from the target. With no mechanical linkages, and zeroing capabilities in the PLC program it is expected to have a long trouble free life.

HMI Systems The operators interact with the PLC through an HMI or human machine interface. The HMI allows the operators to adjust program set points and control regimes as well as view information such as trends, machine parameters, water levels and power production from each of the turbines. A solid state industrial HMI located at the switchgear acts as the primary interface to the control system and as a backup system for data logging. An industrial computer was installed to act as a backup HMI and to perform detailed trending and data logging as well as allow remote access. Spillage, water elevation, generation, gate pitch, vibration, and temperature are all continuously trended through both HMI’s to supply additional operational information.

Switchgear line-up with the HMI interface and PLC cabinet to the right

Turbine Control In the manual mode the HMI allows the operators to set the gate pitch for the turbine selected, and the permissible range of head levels that the turbine is allowed to operate within. This mode effectively allows the operators to set the power level they wish the turbine to operate at.

In the auto mode the HMI allows the operators to decide which turbines will operate, the head level that the PLC will attempt to maintain, the preference of operation of the turbines, and the gate limits that the turbines will operate within. The purpose of this mode of operation is to establish a head level that the control system will attempt to maintain by adjusting the output of the turbines. The normal method of operation in the auto mode is to set the lag turbine with a minimum gate opening in excess of 50%. When the lead turbine reaches its maximum gate opening and the head level is exceeded for a preset time, the lag unit is started by the PLC. The PLC then reduces the gate setting of the lead unit to maintain the head level. When maximum head level is again reached the PLC will increase the lag unit gate opening to maintain head level. On decreasing flow the lag turbine is shutdown when the lead turbine reaches a preset gate pitch adjustable by the operator. Flow Distribution The environmental approval for the plant addition required water to spill over the adjacent waterfall through the tourist season. The plan was complex with different spillage rates overnight, during business days, weekends and holidays.

Chart showing the spillage requirements for High Falls

The flow distribution plan was programmed as an additional feature of the automatic operating mode. With this feature the operator is able to enable the program for the summer season to ensure maximum generation while meeting the varying need for water over the falls. The program works by automatically adjusting the flow requirement to follow the plan. The flow requirement is converted to a head pond elevation that the PLC will attempt to maintain when one or both turbines are operating automatically. There are two flow requirements during each day, the higher is active for a 10 hour period commencing at 10 am, the lower for a 14 hour period commencing at 8 pm. Adjustable timing allows the lower flow requirement to be met for 10 am. Conclusion The PLC based controls have operated as anticipated over the past 6 months. They allowed for a smooth commissioning process through their ability to be readily reprogrammed for unanticipated problems. A single PLC in a plant can govern a number of turbines through the synchronization cycle with separate programs to match the characteristics of each turbine. The operational trending has been very useful tool for the operators. It allows one to see the rate of decline in the river flow and react according. Overall PLCs can be an economical solution to monitoring and controlling the small to large hydro station, eliminating the complexity of separate governing systems.