In-Situ Measuring Campaign at the Hydropower Plant ‘‘Perućica’’, Montenegro - Part 1: Open Channel System - page 5

 

Discharge

Continuous measuring of discharges was planned by using current meters at the bridge profile upstream of the compensation basin (Figure 1, point 8.2) during the entire measuring period. Nevertheless, a large amount of vegetation and deposition completely prevented the functioning of current meters (Figure 8). As an alternative solution, the Acoustic Doppler Current Profiler - TELEDYNE RD INSTRUMENTS StreamPro ADCP was used for discharge measurements (Figure 9).

 

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Figure 8: Current meter blocked by vegetation

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Figure 9: ADCP measuring point

 

 

An overall error for discharge measurements is determined by the velocity measurement error (ΔV) and the error of defining the cross section area (ΔA). The error of defining the cross section area (ΔA) is influenced by errors in measuring the depth (Δh) and the horizontal distance (ΔB).

ΔA= Δh+ ΔB= 1% + 0.5%= 1.5%

The accuracy of measurement was not constant across the entire cross section – there were some areas which couldn't be reached by the device, and some extrapolation had to be done (AE). For a trapezoidal cross section, it is estimated that the ratio of such areas is about 2%.

Therefore, the accuracy of measurement for the cross section area is:

AM = 0.98 A, AE = 0.02 A

ΔA = ΔAM + ΔAE = 0.98 x 1.5% + 0.02 x 5% = 1.57%.

The accuracy of measurement for velocity, of the used device, is ΔV = 1%.

The overall error estimation for discharge measurements is: ΔQ = ΔA+ ΔV = 2.57%.

 

In-Situ Measurement Overview

In order to define the values of the parameters for the mathematical model calibration, it is neccessary to determine the following:

  1. Hydraulic characteristics of the existing gates in the system,
  2. Hydraulic resistance in the channels,
  3. Unsteady flow along the channels.

Therefore, the entire measuring campaign was divided into three groups of measurements, based on the boundary conditions:

  • Measurements in unsteady boundary conditions
  • Measurements in steady boundary conditions
  • Measurements in unsteady downstream boundary conditions.

Measurements in unsteady boundary conditions

In order to measure the hydraulic characteristics of the gates, steady flow conditions are needed. Since the upstream system of open channels is inert, all changes of hydraulic variables (such as discharge, water level etc.) are slow, and a relatively long period of time is required in order to achieve a steady state. In order to obtain flow conditions as close to a steady flow as possible in the shortest period of time, the following principle was applied: the sum of discharge values of the upstream boundary conditions was always equal to the discharge of the downstream boundary conditions (at the power intake). Some of the upstream boundary conditions were not controllable (such as the discharge of the Zeta River or the gate opening of the Krupac dam), so the operating was limited accordingly. This principle was used for testing two scenarios.

Each scenario consists of three phases. The respective hydrograph at the power intake is presented in Figure 10.

In the first phase, small boundary discharges (20-30%) were kept constant for 6 hours. In the second phase, discharges were increased (40-60%) and kept constant for the next 6 hours.

The last phase comprises of reducing the discharge to the first level and keeping these conditions constant for the following 3 hours. Boundary discharges were changed according to the performances of gates and turbines, in normal operating conditions, which means that the change was conducted in less than 5 minutes.

 

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Figure 10: Hydrograph at the Power Intake for UP2 Scenario