Sweet Success: Rehabilitating Governor Controls at a Century-Old Facility
Hydro Review Worldwide Magazine Article: CSE’s Unique Turbine Governor Upgrade on 100 Year Old Hydro Turbines located on Maui, Hawaii, USA
Upgrading from mechanical to digital controls on venerable hydroelectric generation units optimizes operation and reliability.
Read article here.
Michael Keehn is a control systems engineer for CSE Engineering Inc. Garret Hew is manager of water resources for Hawaiian Commercial & Sugar Co.
Hawaii’s last remaining sugar mill is powered by its in-house power grid, part of which receives energy from a duo of company-owned hydroelectric power plants located on its sugar plantation. The oldest of these locations has been generating power from its sole, originally installed unit for more than 100 years. A second of the three hydropower plants utilizes units that began generating power in 1924.
Increasing maintenance costs and the inability of the project owner to find replacement parts for the units’ governor control system at both sites necessitated a much-needed upgrade to a total of four generating units.
Concord, Calif.-based control system integrator CSE Engineering Inc. might have a smile of sweet satisfaction after completing rehabilitation of the hydroelectric governor control system for their project on the island of Maui in Hawaii. The new hydroelectric digital governor control system, and four new governors installed in hydroelectric units at two sites that send energy to the Hawaiian Commercial & Sugar Co.’s (HC&S) power grid, will hopefully aid the rehabilitated hydropower units provide an additional century of power for some sweet production.
CSE specializes in retrofitting controls for rotating machinery, balance of plant/distributed control systems integration and human/machine interface upgrades. CSE has been an HC&S product provider for a decade. In 2005, CSE provided engineering services to HC&S by helping to complete two, extraction steam turbine governor upgrade projects, according to CSE.
CSE provided design definitions and working drawings for the HC&S control system, while Woodward’s South America division, which specializes in hydroelectric turbine controls, produced the initial graphical application programmer software programs.For the HC&S hydroelectric turbine governor upgrade project, CSE collaborated with Woodward Inc. in Ft. Collins, Colo., which produces electronic control systems for the aerospace and energy markets.
The newly installed system includes custom-programmable 828 digital controls and automatic synchronizers manufactured by Woodward.
“The Woodward 828 digital control provides state-of-the art control for new and retrofit situations as it manages and controls reciprocating engines used in power generation…,” according to Woodward.
An additional 828 feature of great importance to HC&S is the ability to remotely control the digital system from within the plant’s main powerhouse.
In its retrofit package, CSE was tasked with providing pre-alarms and first-out indication (notice of first trip signal in sequence of events) for remote monitoring and troubleshooting. In addition, HC&S wanted the new governors to have the capability for remote startup and loading.
Understanding the situation
HC&S is Hawaii’s only raw and specialty sugar provider. The company annually produces 150,000 to 200,000 tons of raw sugar — roughly 5% of the sugar cane supply for the U.S. — and about 60,000 tons of molasses at its Pu’unene Mill on Maui.
HC&S generates enough electricity, primarily from renewable sources, to be 100% energy self-sufficient and sells its excess energy not used for internal purposes to the Maui Electric Co. Ltd., the local utility. Besides providing a firm renewable energy source, HC&S also provides ancillary services such as voltage regulation and emergency power to the local grid when needed.
Pu’unene Mill’s power grid supplies energy to its 36,000-acre plantation and all manufacturing needs.
The plant’s main powerhouse at the mill supplies the majority of energy with steam produced in high-pressure boilers burning bagasse — the fibrous residue left over from the sugarcane milling process.
HC&S also operates two hydroelectric plants located along its irrigation ditches: 4.5-MW Kaheka and 1.1-MW Paia. The plants use water collected from the East Maui Irrigation ditch system, built in the 1800s, which collects water from the windward slopes of the Haleakala volcano. After generating energy for the plantation’s power grid, the water continues on its way in the ditches to be used for irrigation.
In 1912, the Paia hydroelectric power plant began operating at 800 kW of capacity using the plant’s only unit, a Francis-Kaplan turbine. In 1923, the Paia penstock was extended to a higher elevation and according to the Maui Economic Development, Energy, Agriculture and Recreation Committee; Paia’s capacity is now 1.1 MW.
The Kaheka power plant operates three dual Pelton units, each with a capacity of 1.5 MW.
Neither the Paia nor the Kaheka sites are manned and they are accessed by a steep dirt road on the mountain. Operations personnel monitor the units from within the plant’s main powerhouse about 8 miles away. In the past, when a problem was detected at one of the units, an operator was dispatched to the effected unit to troubleshoot an antiquated system that gave scant information on what caused the unit to malfunction.
Garret Hew, HC&S Water Resources manager, said, “Even though the original mechanical governors have worked well for the last century, we realized how much daily attention the governors needed and the effort they required in troubleshooting and restarting a tripped unit.”
Additionally, it was becoming increasingly difficult to find replacement parts and people who had technical skill and experience with these types of governors to support the aged mechanical devices.
Given all of the shortcomings, HC&S wanted to upgrade to a digital governor system that would easily interface to the existing communication network, which would allow operations to manage governor controls from within the main hydroelectric powerhouse.
The Francis-Pelton turbine functions differently than a Pelton turbine, so each hydro site needed custom governor functions and custom graphical application programmers. Neither application needed much input/output communication, but robust hardware needed to be capable of serial communications.
The old, mechanical governors used low hydraulic pressure generated by a gear pump (originally driven by leather belt wrapped around the turbine shaft) to drive the fly-ball and servos. CSE’s retrofit replaced the low-pressure system with a 1,600-psi high-pressure hydraulics package that included a power unit, actuators, hydraulic motors and servo valves compatible with the 828’s driver current range.
In addition to governor functions, the 828s also performed the hydraulic actuator positioning (as a servo position controller, or SPC, does on other applications). Linear variable differential transformers were selected because of their high reliability and signal converters, which produce the 4-20 mA signal needed by the 828 for position feedback. For safety and protection of the unit, an independently controlled trip manifold removed the water power from the turbine runner when tripped.
One novel aspect of controlling hydro units is that maintaining water delivery is much more important than power generation. In both the Pelton and Francis-Pelton designs, if power is interrupted, irrigation water must still be delivered. Thus, if a turbine power trip occurred, the system was designed so that water flow into the turbine would remain constant.
Each of the three dual Pelton units has two water nozzles with independently controlled deflectors. The nozzles and deflectors work together with complementary tasks. During startup, deflector action provides the sensitive speed control for synchronizing, while the nozzles are held in one position. Once the generator goes online, the deflectors ramp out of the way so that load can be controlled with nozzle flow. If a large load rejection or trip occurs, the deflector gates act quickly to divert the nozzle stream away from the turbine (runner) so water power can be removed quickly.
Nozzle flow-rate changes must be adjusted slowly to guard against sudden changes in water velocity — water-hammer effect — that could damage the turbine or penstock. In this case, the penstock, built in the early 1900s using riveted steel plate, is 1 mile long and pressure in the penstock at the turbine inlet can be as high as 287 pounds per square inch.
Single-phase, reversible AC motors operated the original governors controlling each unit’s nozzles. Often, the large starting torque needed to overcome the nozzle hysteresis “burned-up” the motors. Additionally, the simple “on-off” only control, did not allow for fine water flow control.
CSE replaced the AC motors and upgraded each hydro unit with variable-speed hydraulic motors. Driven by hydraulic fluid at 1,600 psi, these hydraulic motors have a high starting torque to easily address any reasonable hysteresis. As a result, the new motors slow down as nozzle position approaches set point, allowing precise flow control of the water delivery system. The same servos that were used on the deflector actuator were also used on the nozzle motors.
The Francis-Pelton power generation design naturally differs from the Pelton.
The Francis-Pelton unit generates power using water volume that flows through an impeller under low pressure. To control speed and load, the Francis-Pelton employs wicket gates that vary the volume and angle of water hitting the turbine impeller. The digital governor in each unit controls the wicket gate position and, similar to the Pelton, the wickets must move slowly to guard against water-hammer.
CSE completed governor upgrades on the three Pelton units in November 2012 and the Francis-Pelton unit in January 2013. HC&S immediately saw positive results from the work.
“Our units can start-up and get online much quicker now,” Hew said. “A typical start-up used to take 10 minutes. Now, with our electronic auto ramp and [SPMD-10] auto-synchronizers in place, start-ups typically take 3 minutes for the unit to go from stopped to online and loaded.”
Another positive outcome is that the units can run at significantly lower power levels and remain stable.
“Our mechanical governors used to become unstable when we tried to run them at lower power levels,” he said. “Now, with 828s, they can run down to 200 kW and remain stable. This allows us to keep the units operating in rolling-reserve even when water flow is curtailed. That means fewer starts and stops.”
“Being able to keep the unit lightly loaded not only means fewer trips up the mountain, but it frees up our people to attend to other duties,” Hew said, “and that’s a real benefit to us and the entire island.”
Chris Klein, former vice president of engineering at CSE, contributed heavily to this article. Klein has upgraded more than 200 steam, gas and hydro turbines using Woodward Governor products.