HYDROELECTRIC POWER PLANTS CONTROL SUPERVISION TYPES BASIC INFORMATION

After the location and mode of control is defined, the manner in which the plant is supervised or staffed should be recognized. Though this aspect is not commonly addressed in industry recognized labels for plant control, the equipment selection and its degree of automation is related to the plant supervision.

The greater the degree of sophistication in the controls, the greater the distance between the equipment being controlled and the control location.

As the main point of control moves from the equipment to an off-site locale, the need for staffing/supervision at the lower levels of control diminishes. Supervision can, therefore, be described relative to the plant as either attended or unattended.

Attended

- The plant is staffed 24 hours a day. The operator is available to perform control actions either locally or at a centralized area.

Unattended

- The plant is not staffed for the full 24 hours a day. An operator may be present for a single shift or make a routine visit to the project. With the exception of small hydro, the policy throughout the industry is to have some form of supervision or monitoring at a given plant. If the plant’s on-site control is defined as unattended, then it is implied that the supervision/monitoring is performed off-site.

Unattended operation is represented by two predominant examples:

1) Off-Site Supervisory Control

- Here, control of the remote plant exists for all essential operations and a full complement of indications for the remote plant are brought to the off-site control location. Occasional visits by operation and maintenance people are made to ensure plant security.

2) Off-Site Monitored Control

- All of the controls for the plant are local. A minimum representation of plant indication is brought to an off-site location where full attendance exists. The capability exists at the off-site location to dispatch an operator to the plant if conditions warrant. Routine maintenance visitations can also be made to the plant.

The practice is to describe a control system by location and then mode. It can then be modified by defining the type of supervision. Since the staffing of a plant varies within the industry, it is difficult to define this aspect generically.

All combinations of location and mode are legitimate and more than one combination can exist at a plant. For example, a multiple unit plant can have local manual controls at the unit auxiliaries and the unit switchboard.

It can have a control room that would have both centralized manual and centralized automatic controls. It may even have equipment at the plant that would allow the capability of off-site automatic controls.

To complete the scenario, the plant may be unattended except for routine maintenance visits. A hierarchy of control can, therefore, be developed, going from those controls closest to the equipment and the least complex to the controls located off-site and the most sophisticated. Starting from the lowest (most basic) echelon in the hierarchy and working to the highest (most sophisticated), we find

1) Local manual

2) Local automatic

3) Centralized manual

4) Centralized automatic

5) Off-site manual

6) Off-site automatic

It should be kept in mind that the above combinations can further be modified/described by appending either attended or unattended to them.

HYDROELECTRIC POWER GENERATION BASIC INFORMATION AND TUTORIALS

Hydroelectric power generation involves the storage of a hydraulic fluid, normally water, conversion of the hydraulic energy of the fluid into mechanical energy in a hydraulic turbine, and conversion of the mechanical energy to electrical energy in an electric generator.

The first hydroelectric power plants came into service in the 1880s and now comprise approximately 22% (660 GW) of the world’s installed generation capacity of 3000 GW (Electric Power Research Institute, 1999).

Hydroelectricity is an important source of renewable energy and provides significant flexibility in base loading, peaking, and energy storage applications.

While initial capital costs are high, the inherent simplicity of hydroelectric plants, coupled with their low operating and maintenance costs, long service life, and high reliability, make them a very cost effective and flexible source of electricity generation.

Especially valuable is their operating characteristic of fast response for start-up, loading, unloading, and following of system load variations. Other useful features include their ability to start without the availability of power system voltage (“black start capability”), ability to transfer rapidly from generation mode to synchronous condenser mode, and pumped storage application.

Hydroelectric units have been installed in capacities ranging from a few kilowatts to nearly 1 GW. Multi-unit plant sizes range from a few kilowatts to a maximum of 18 GW.

Planning of Hydroelectric Facilities

Siting

Hydroelectric plants are located in geographic areas where they will make economic use of hydraulic energy sources. Hydraulic energy is available wherever there is a flow of liquid and head. Head represents potential energy and is the vertical distance through which the fluid falls in the energy conversion process.

The majority of sites utilize the head developed by fresh water; however, other liquids such as salt water and treated sewage have been utilized. The siting of a prospective hydroelectric plant requires careful evaluation of technical, economic, environmental, and social factors.

A significant portion of the project cost may be required for mitigation of environmental effects on fish and wildlife and re location of infrastructure and population from flood plains.

Hydroelectric Plant Schemes

There are three main types of hydroelectric plant arrangements, classified according to the method of controlling the hydraulic flow at the site:

1. Run-of-the-river plants, having small amounts of water storage and thus little control of the flow through the plant.

2. Storage plants, having the ability to store water and thus control the flow through the plant on a daily or seasonal basis.

3. Pumped storage plants, in which the direction of rotation of the turbines is reversed during offpeak hours, pumping water from a lower reservoir to an upper reservoir, thus “storing energy” for later production of electricity during peak hours.

EXCITATION SYSTEM OF HYDRO POWER GENERATOR BASIC INFORMATION

The excitation system fulfills two main functions:

1. It produces DC voltage (and power) to force current to flow in the field windings of the generator. There is a direct relationship between the generator terminal voltage and the quantity of current flowing in the field windings.

2. It provides a means for regulating the terminal voltage of the generator to match a desired set point and to provide damping for power system oscillations.

Prior to the 1960s, generators were generally provided with rotating exciters that fed the generator field through a slip ring arrangement, a rotating pilot exciter feeding the main exciter field, and a regulator controlling the pilot exciter output.

Since the 1960s, the most common arrangement is thyristor bridge rectifiers fed from a transformer connected to the generator terminals, referred to as a “potential source controlled rectifier high initial response exciter” or “bus-fed static exciter” (IEEE, 421.1-1986; 421.2-1990; 421.4-1990; 421.5-1992).

Another system used for smaller high-speed units is a brushless exciter with a rotating AC generator and rotating rectifiers.

Modern static exciters have the advantage of providing extremely fast response times and high field ceiling voltages for forcing rapid changes in the generator terminal voltage during system faults. This is necessary to overcome the inherent large time constant in the response between terminal voltage and field voltage (referred to as T do , typically in the range of 5 to 10 sec).

Rapid terminal voltage forcing is necessary to maintain transient stability of the power system during and immediately after system faults.

Power system stabilizers are also applied to static exciters to cause the generator terminal voltage to vary in phase with the speed deviations of the machine, for damping power system dynamic oscillations.

Various auxiliary devices are applied to the static exciter to allow remote setting of the generator voltage and to limit the field current within rotor thermal and under excited limits. Field flashing equipment is provided to build up generator terminal voltage during starting to the point at which the thyristors can begin gating. Power  for field flashing is provided either from the station battery or alternating current station service.

GENERATOR TYPE SELECTION FOR HYDROELECTRIC POWER PLANTS BASIC AND TUTORIALS

Synchronous generators and induction generators are used to convert the mechanical energy output of the turbine to electrical energy. Induction generators are used in small hydroelectric applications (less than 5 MVA) due to their lower cost which results from elimination of the exciter, voltage regulator, and synchronizer associated with synchronous generators.

The induction generator draws its excitation current from the electrical system and thus cannot be used in an isolated power system. Also, it cannot provide controllable reactive power or voltage control and thus its application is relatively limited.

The majority of hydroelectric installations utilize salient pole synchronous generators. Salient pole machines are used because the hydraulic turbine operates at low speeds, requiring a relatively large number of field poles to produce the rated frequency.

A rotor with salient poles is mechanically better suited for low-speed operation, compared to round rotor machines which are applied in horizontal axis high-speed turbo-generators.

Generally, hydroelectric generators are rated on a continuous-duty basis to deliver net kVA output at a rated speed, frequency, voltage, and power factor and under specified service conditions including the temperature of the cooling medium (air or direct water). Industry standards specify the allowable temperature rise of generator components (above the coolant temperature) that are dependent on the voltage rating and class of insulation of the windings (ANSI, C50.12-1982; IEC, 60034-1).

The generator capability curve, describes the maximum real and reactive power output limits at rated voltage within which the generator rating will not be exceeded with respect to stator and rotor heating and other limits. Standards also provide guidance on short circuit capabilities and continuous and short-time current unbalance requirements (ANSI, C50.12-1982; IEEE, 492-1999).

Synchronous generators require direct current field excitation to the rotor, provided by the excitation system described in Section entitled “Excitation System”. The generator saturation curve, describes the relationship of terminal voltage, stator current, and field current.

While the generator may be vertical or horizontal, the majority of new installations are vertical. The basic components of a vertical generator are the stator (frame, magnetic core, and windings), rotor (shaft, thrust block, spider, rim, and field poles with windings), thrust bearing, one or two guide bearings, upper and lower brackets for the support of bearings and other components, and sole plates which are bolted to the foundation.

Other components may include a direct connected exciter, speed signal generator, rotor brakes, rotor jacks, and ventilation systems with surface air coolers (IEEE, 1095-1989).

The stator core is composed of stacked steel laminations attached to the stator frame. The stator winding may consist of single turn or multi-turn coils or half-turn bars, connected in series to form a three phase circuit.

Double layer windings, consisting of two coils per slot, are most common. One or more circuits are connected in parallel to form a complete phase winding. The stator winding is normally connected in wye configuration, with the neutral grounded through one of a number of alternative methods which depend on the amount of phase-to-ground fault current that is permitted to flow (IEEE, C62.92.2-1989; C37.101-1993).

Generator output voltages range from approximately 480 VAC to 22 kVAC line-to-line, depending on the MVA rating of the unit. Temperature detectors are installed between coils in a number of stator slots.

The rotor is normally comprised of a spider attached to the shaft, a rim constructed of solid steel or laminated rings, and field poles attached to the rim. The rotor construction will vary significantly depending on the shaft and bearing system, unit speed, ventilation type, rotor dimensions, and characteristics of the driving hydraulic turbine.

Damper windings or amortisseurs in the form of copper or brass rods are embedded in the pole faces, for damping rotor speed oscillations. The thrust bearing supports the mass of both the generator and turbine plus the hydraulic thrust imposed on the turbine runner and is located either above the rotor (“suspended unit”) or below the rotor (“umbrella unit”).

Thrust bearings are constructed of oil-lubricated, segmented, babbit-lined shoes. One or two oil lubricated generator guide bearings are used to restrain the radial movement of the shaft.

Fire protection systems are normally installed to detect combustion products in the generator enclosure, initiate rapid de-energization of the generator and release extinguishing material. Carbon dioxide and water are commonly used as the fire quenching medium.

Excessive unit vibrations may result from mechanical or magnetic unbalance. Vibration monitoring devices such as proximity probes to detect shaft run-out are provided to initiate alarms and unit shutdown.

The choice of generator inertia is an important consideration in the design of a hydroelectric plant. The speed rise of the turbine-generator unit under load rejection conditions, caused by the instantaneous disconnection of electrical load, is inversely proportional to the combined inertia of the generator and turbine.

Turbine inertia is normally about 5% of the generator inertia. During design of the plant, unit inertia, effective wicket gate or nozzle closing and opening times, and penstock dimensions are optimized to control the pressure fluctuations in the penstock and speed variations of the turbine-generator during load rejection and load acceptance.

Speed variations may be reduced by increasing the generator inertia at added cost. Inertia can be added by increasing the mass of the generator, adjusting the rotor diameter, or by adding a flywheel. The unit inertia also has a significant effect on the transient stability of the electrical system, as this factor influences the rate at which energy can be moved in or out of the generator to control the rotor angle acceleration during system fault conditions.