|Installing a new monitoring system that utilizes radar to measure water levels in the reservoir significantly increased the accuracy of the measurement.|
Measurements of the level of water in the reservoir behind Machadinho Dam have been problematic since the hydropower plant for which it impounds water began operating in 2002. At this plant, called Machadinho Carlos Ermirio de Moraes, the level measuring system that was used until 2011 consisted of submersible pressure-resistant hydrostatic probes: one each for reservoir level, upstream level, the generating units after the trashracks and the tailrace level. Constant accumulation of residues, mostly wood, in the reservoir interfered with the measurements and resulted in errors and constant corrective interventions, such as removing the probe for cleaning, resetting the sensor positioning, or replacing the probe. To correct this ongoing problem, radar level transmitters were installed in 2011. Use of these transmitters has corrected the previously stated problems, increasing reliability of the measurements and eliminating corrective interventions.
Understanding the situation at Machadinho
This facility is on the Uruguai River between Piratuba (Santa Catarina) and Maximiliano de Almeida (Rio Grande do Sul) in Brazil. The plant, which is owned by Consorcio Machadinho and operated by Tractebel Energia S.A., contains three 380 MW turbine-generating units, for total installed capacity of 1,140 MW. The project was commissioned in 2002 is controlled via a digital system operated from the central control room.
The level measuring system was one of the first systems to become operational at the plant, which enabled the reservoir to be filled August 28, 2001.
Level monitoring was needed to allow the project owner to measure the tailrace, intake, and reservoir levels, as well as determining hydraulic losses at the trashracks covering the three turbine intakes, specifically those caused by accumulation of wood. Correct measurements are invaluable, as they are needed to detect the amount of waste accumulated on the trashracks. When preset levels of waste accumulation are reached, an alert is sent to the control center so that the operator can reduce the power dispatched to the generating units, reducing the flow and decreasing the head loss until the alert in the control center turns off. The operator also can convert the unit to synchronous compensating mode, which makes the waste trapped in the trashracks float away. The final action is to remove the wood using a giant claw mechanicsm. This is vital to avoid a system overload, which could cause severe damage to the power plant.
The original level measuring system made use of submersible pressure-resistant hydrostatic probes, manufacturered by Brazil-based Hytronic, as a primary sensor. This type of sensoring is regularly used at hydropower plants in Brazil.
However, several problems arose, the most critical one being inaccuracies in calculation of head loss at the trashracks. These racks, placed at the entry to the penstock for each unit, prevent the passage of solid materials, such as branches and wood trunks, through the turbines. Passing these materials could result in damage to the turbines and wicket gates that would be costly to repair and result in extensive downtime for the unit.
The continuous accumulation of wood and other waste material on the trashracks results in partial or complete obstruction, which increases the pressure over the rack and results in greater head loss. Head loss is calculated by comparing the reservoir water level with the level past the trashracks of each unit. The difference in elevation between these two measurements, minus the portion corresponding to the dynamic pressure, demonstrates the head loss.
Defining the problem
The more waste accumulates on the surface of the trashrack, the larger the head loss here. At Machadinho, the admissible maximum value for head loss based on the design of the trashracks is 3.5 mca (meters in water column). Once this limit is surpassed, the trashrack can suffer structural damage and may even by pulled into the penstock.
To prevent head loss, the water column between the trashrack and reservoir must be reduced, and this can only be accomplished by reducing power demands on the generating unit. This situation leads to a less-than-optimal utilization of the available hydraulic resource.
The operations and maintenance team evaluated the situation to determine why measured head loss values with the existing system were inconsistent. To find that, the maintenance team had to perform a simple procedure that consisted of determining with a measuring tape the difference in the level in the reservoir and the level past the trashracks of each unit. Comparing these measures with the value displayed in the control room revealed the errors.
They determined that several conditions provided the incorrect information, including level variation and water quality by mud impregnation. However, it was determined that the varying level in the reservoir as a result of the drawdown was the main contributing factor to measuring errors and the resulting need for system intervention.
In addition, water quality further worsened the measuring errors because the mud suspended in the water obstructed the holes in the submersible probes where the pressure measurements were taken. The structures to which the submersible probes were installed were also often obstructed by wood, which changed the water column's level by creating level variations.
Developing a solution
Because of the lack of system reliability and the constant need for corrective interventions that could vary depending the condition of the reservoir, occurring more than once per week in the worst situations, Consorcio Machadinho undertook a search for alternative technologies available for level measurement. Given the history of failures of the existing system, one of the requirements for the solution at Machadinho was a technology that does not have to stay in direct contact with the water.
Ultrasound, radar and guided radar options were all considered. Ultrasound sensors did not appear to be workable because most of the available options did not have the range needed (40 meters) for this application, and external temperatures are known to interfere with ultrasound signals, thus affecting the measurement accuracy.
To use a guided radar system at Machadinho, a wave guide, which is a metal guide that links the sensor to the water level, would have to be installed. In addition, this guide would be in constant contact with the water, a factor the operators were hoping to avoid.
Given the limitations of these methods, radar was chosen as the most suitable measurement option for the needs of the project. A radar level transmitter operates according to the microwave irradiation principle: by propagating electromagnetic waves. The instrument receives a portion of the energy reflected off the surface of the environment being measured. The time it takes the signal to travel from is the wave's reflection distance, defined as the distance from the radar sensor to the surface of the water.
The output is a miliampere signal that gives a distance in meters from the sensor to the water surface. In the control system used, we configure the lowest possible level (the largest distance in meters from the radar) and the highest possible level (the smallest distance in meters from the radar).
For example, the minimum level in cote (comparison to sea level, or zero meters) at the tail race is 372.90 meters. The radar sensor shows that there is a 40 meter distance between the sensor and the water surface.
We do the same thing with the maximum level, which is 397.15 meters. As the tailrace level cannot be increased to this, the sensor installation cote is set at the maximum level. With this information, the distance in meters from the radar to the surface level can be translated in the right cote.
Because these waves do not propagate mechanically, they are immune to temperature variations. In addition, the radar sensors allow an application over a wide range of distances, reaching more than 50 meters.
The hydrostatic sensor installed after each unit trashrack, upstream sensors and tailrace water level sensors were replaced with new radar transmitters. A number of infrastructure improvements were required to allow installation of the new sensors. According to operational requirements, the radar transmitter must be installed at least 60 cm from any walls, avoiding interference in the radar beam.
In total, five radar transmitters were installed: one to measure tailrace level, another to measure reservoir level, and one for each generating unit to measure the level past the trashracks. It took us one week to prepare the civil and supports structures, and after that, one afternoon to start up and configure the radar transmitter. The units were not taken offline for the installation.
The radar transmitter measuring system has been operating for two years at Machadinho. During this time, the plant operators have observed a significant improvement in the precision and reliability of measurements. The hydrostatic sensors presented a scale accuracy rate of ±0.25% and resolution of 0.015%; the new system provided a significant gain in full-scale accuracy of ±0.01% and resolution of 0.003%.
This improvement has added great value to the plant's operations, as it provides precise and reliable information for each unit and prevents unnecessary waste of resources otherwise available for power generation. The new system has also positively impacted the maintenance schedule for the plant. Before the improvements, 260 hours were dedicated to corrective maintenance and system adjustments in a five-year span. Once the system was adopted, corrective interventions dropped to zero.
— By Christiano Dalosto Pase, electrical engineer, and Edson Leandro Tomaselli, electrical engineer, Tractebel Energia S.A.