In a hydropower generation plant, it is essential to predict the transient pressures that could occur and to implement an adequate surge control strategy to ensure the safety and reliability of the unit. The impact of gradual or diurnal load variations on the turbine-generator may be of interest during normal operations but an electric or mechanical governor can control moderate transients.

The primary purpose of hydraulic transient simulations is therefore to protect the system against rapid changes in the electrical and/or hydraulic components of the hydroelectric system. In each case, hydraulic transients result from changes in the variables controlled by the governor.

Electrical Load or Torque on the turbine-generator system varies with the electrical load in the distribution grid. In steady-state operation, the electrical torque and the hydraulic torque are in dynamic equilibrium. From a hydraulic perspective, electrical torque is an external load on the turbine-generator unit.

Speed is another possible control variable for numerical simulations. For turbines, however, the governor strives to keep the turbine at synchronous speed by varying the wicket gate position during load variation and acceptance (assuming a perfect governor). If field data were available, the speed could be used to determine whether the model simulates the correct flow and pressures.

Once the time-varying electrical torque and wicket gate positions are known, the turbine equations (Numerical Representation of Hydroelectric Turbines), HAMMER solves flow, Q, and rotational speed, N, in conjunction with the characteristic curves for the turbine unit(s). This yields the transient pressures for the load rejection, load acceptance, emergency shutdown, operator error or equipment failure. The possible emergency or transient conditions are discussed separately in the sections that follow.

Load Rejection

Load rejection occurs when the distribution grid fails to accept electrical load from the turbine-generator system. After the load is rejected by the grid, there is no external load on the turbine-generator unit and the speed of the runner increases rapidly. This can be catastrophic if immediate steps are not taken to slow and stop the system. To keep the speed rise within an acceptable limit, the wicket gates must close quickly and this may result in high (followed by low) hydraulic transient pressures in the penstock. Since load rejection usually results in the most severe transient pressures, it typically governs the design of surge control equipment.

During load rejection, the generation of electrical power by the turbine-generator unit should decrease to zero as quickly as possible to limit the speed rise of the unit. To accomplish this, the wicket gates close gradually in order to reduce flow. The table below shows an example of electrical load and wicket gate position versus time to simulate load rejection. In a real turbine a governor would control the wicket gate closure rate, however the turbine governor is not modeled explicitly in HAMMER and the user controls the rate of wicket gate closure.

If the power generated by the water flowing through the turbine is greater than the electrical load, then the turbine will speed up; if the electrical load is greater, the turbine will slow down.

Load and Wicket Gate Changes for Load Rejection

Instant Load Rejection

Instant Load Rejection is similar to the Load Rejection case, except the electrical load on the turbine drops instantaneously to zero (i.e. the turbine is disconnected from the generator).

During instant load rejection, the generation of electrical power by the turbine-generator unit should decrease to zero as quickly as possible to limit the speed rise of the unit. To accomplish this, the wicket gates close gradually in order to reduce flow. The table below shows an example of wicket gate position versus time to simulate Instant Load Rejection. In a real turbine a governor would control the wicket gate closure rate, however the turbine governor is not modeled explicitly in HAMMER and the user controls the rate of wicket gate closure..

Wicket Gate Changes for Instant Load Rejection

Load Acceptance

Full load acceptance occurs when the turbine-generator unit is connected to the electrical grid. Transient pressures generated during full load acceptance can be significant but they are usually less severe than those resulting from full load rejection.

HAMMER assumes the turbine initially operates at no-load speed (NLS), and the turbine generates no electrical power. When the transient simulation begins, HAMMER assumes the electrical grid is connected to the output terminal of the generator and wicket gates have to be open as quickly as possible to meet the power demand - all without causing excessive pressure in the penstock.

Note that in this case, HAMMER assumes the turbine governor is 'perfect' - in other words the power produced by the turbine always equals the electrical load. Therefore the user doesn't need to enter an electrical load; just a curve of wicket gate position versus time, and the turbine's rated flow and head. Under the Load Acceptance case the turbine will always operate at its rated (or synchronous) speed.

Wicket Gate Changes for Full Load Acceptance

Load Variation

Load variation on the turbine-generator unit can occur due to the diurnal changes in electricity demand in the distribution grid. During load variation, the governor controls the wicket gate opening to adjust flow through the turbine so that the unit can match the electrical demand. The water column in the penstock and conduit system accelerates or decelerates, resulting in pressure fluctuations.

The transient pressures that occur during general load variation may not be significant from a hydraulic design perspective since they are often lower than the pressure generated during a full load rejection or emergency shutdown.

At steady-state, the turbine-generator system usually runs at full load with the wicket gates 100% open. The amount of electricity produced by the system depends on the flow through the wicket gates. A decrease in electrical load requires a reduction in the wicket gate opening to adjust the flow.the table below shows an example of typical user input to simulate transient pressures for load variation.

Note that in this case, HAMMER assumes the turbine governor is 'perfect' - in other words the power produced by the turbine always equals the electrical load. Therefore the user doesn't need to enter an electrical load; just a curve of wicket gate position versus time. Under the Load Variation case the turbine will always operates at its rated (or synchronous) speed.

Wicket Gate Changes for General Load Variation