Efficiency of hydro powerplant turbines defined.
The efficiency guaranteed by turbine manufacturers is that which may be verified in accordance with the “International Code for the field acceptance tests of hydraulic turbines” (publication IEC-141) or, if applied, in accordance with the “International Code for model acceptance tests” (publication IEC-193).
It is defined as the ratio of power supplied by the turbine (mechanical power transmitted by the turbine shaft) to the absorbed power (hydraulic power equivalent to the measured discharge under the net head).
It is to be noted that for impulse turbines (Pelton, Turgo and Cross-Flow), the head is measured at the point of impact of the jet, which is always above the downstream water level. This effectively amounts to a reduction of the head.
The difference is not negligible for low-head schemes, when comparing the performance of impulse turbines with those of reaction turbines that use the entire available head.
Due to the head losses generated in reaction turbines the runner only uses a head Hu lower than the net head Hn, These losses are essentially friction losses in the spiral case, guide-vanes and runner blades plus velocity head losses in the draft tube.
The draft–tube or diffuser is designed to recover the biggest possible fraction of the velocity head generated by the velocity of the water leaving the blades. This loss is particularly critical in the high specific speed runners, where it may reach up to 50% of the net head (whereas in the slow Francis runner it rarely exceeds 3%-4%).
The head used by the runner is in fact the equivalent to the net head diminished by the kinetic energy dissipated in the draft-tube, quantified by the expression Ve / 2g, where Ve is the average velocity of the water leaving the draft-tube.
To reduce the velocity the draft tube is commonly designed with a conical section. Small divergence angles require long, and consequently costly, diffusers, but otherwise the angle cannot exceed about 7º without danger of flow separation.
Trying to find equilibrium between flow separation and cost some designers increase the angle up to about 15º. The draft-tube design has such implications on the turbine operation that it is strongly recommended to leave it to the turbine manufacturer or at least fabricate it under his advice and drawings.
At present no IEC code defines the net head across a cross-flow turbine or its efficiency. Care must be taken in comparing reaction turbine efficiencies with cross-flow efficiencies 11. Anyhow cross-flow peak efficiencies calculated from the net head definition given by the IEC code for impulse turbines, reach a ceiling slightly over 80%, but retain this efficiency value under discharges as low as a sixth of the maximum.
To estimate the overall efficiency the turbine efficiency must be multiplied by the efficiencies of the speed increaser (if used) and the alternator. A turbine is designed to operate at or near its best efficiency point, usually at 80 per cent of the maximum flow rate, and as flow deviates from that particular discharge so does the turbine’s hydraulic efficiency.
Double regulated Kaplan and Pelton turbines can operate satisfactorily over a wide range of flow -upwards from about one fifth of rated discharge. Single regulated Kaplans have acceptable efficiency upward from one third and Francis turbines from one half of rated discharge. Below 40% of the rated discharge,
Francis turbines may show instability resulting in vibration or mechanical shock. Propeller turbines with fixed guide vanes and blades can operate satisfactorily only over a very limited range close to their rated discharge. It should be noted that with single-regulated propeller turbines the efficiency is generally better when it is the runner that is adjustable.
