Electrification Through Isolated Hydropower Stations

Introduction to micro hydropower system

Micro hydro is a small isolated power system that generally produces power up to the range of 100 KW. Installations below 5 KW are generally known as pico hydro. These types of installations are best suited to supply electric power to rural households or communities that are situated at distant locations from the national grid. Micro-hydropower is generally produced from the natural flow of water and with no direct emissions resulting from this conversion process, there are no significant harmful effects to the environment. However, the source of water can vary and in some cases, lakes or some form of water reservoirs are also used to produce electricity.

The cost of installing a micro hydro power plant usually ranges from 1,000 to 20,000 US dollars. With a small investment, a micro hydro power plant can be operated to supply rural households and even electric machines in small business ventures such as rice mills. Owing to its simple technology, numerous streams and fairly high rainfall in the hilly parts, it is best suited for the rural communities in the hilly regions where the supply of electricity through the national grid becomes difficult.

Theoretical power calculation of electrical power from a micro hydropower system

Let us state that,

Q = Discharge of the stream

H = Net head available to the micro hydropower plant

η = Efficiency of the plant

Then, theoretical power output of the plant is given as,

P (Watts) = 9.81 × Q (liters/sec) × H (meter)

The practical power output after deducting the losses is obtained as,

P (Watts) = 9.81 × Q (liters/sec) × H (meter) × η

Numerical example

Calculate the efficiency of a micro hydropower plant having a discharge of 120 liters per second, a net head of 132 meters and power output of 65 KW.

Solution:

According to the problem,

Q (Discharge of the stream) = 120 liters per second

H (Net head available to the micro hydro power plant) = 132 meters

P (Power output) = 65 KW = 65,000 W

We have the relationship,

P (Watts) = 9.81 × Q (liters/sec) × H (meter) × η

or, 65,000 = 9.81 × 120 × 132 × η

On solving, we obtain,

η = 0.4183 = 41.83 %

NOTE: If the stream discharge is given in m³/s, the power obtained will be in the range of kilowatts instead of watts.

Components of a micro hydropower plant

The schematic diagram of a micro hydropower plant system is shown in the diagram below.

Civil components

Weir and intake

In order to operate a hydropower system smoothly, it is necessary to ensure that a constant and controllable supply of water is provided. The water flowing in the channel must be regulated during the high and low flow conditions in the stream. This can be achieved by using a weir which maintains a constant flow of water in the channel. If a naturally formed pool is present in the stream, the construction of weir can be avoided.

The intake features as the diversion structure which is responsible for letting some portion of water flow into the canal or channel.

Canal

A canal is responsible for carrying the required flow of water from the intake into the forebay tank. The length of the channel can be considerable and in most cases in Nepal, the canals may be constructed with a length of few kilometers to create a head of 10 to 30 meters. The construction of canals may vary according to the location and its length may also vary in accordance with the length of the penstock pipe to suit the needs.

Trash rack

A trash rack is a filtering structure which is used to prevent the flow of coarse materials and logs and tree debris into the canal.

Spillways

Spillways act as flow regulators for the canal. They are designed so as to permit controlled overflow of water at certain points along the canal. Water flow through the intake during floods can be twice the normal channel flow, so a spillway must be large enough for diverting this excess flow.

In addition, spillways can be combined with control gates to provide a means of emptying the canal for repair and maintenance purpose.

Settling basin

The water is drawn from the intake usually consists of sand and suspended particles which if allowed to flow into the turbines can corrode and damage the turbine blades. Therefore, it is necessary to allow these suspended particles to settle down before the water is carried out to the penstock pipe. A settling or desilting basin is designed to slow down the water so as to allow the silt to settle down.

Forebay tank

A forebay tank acts as a connection between the penstock pipe and the canal. Its purpose is to allow the last particles to settle down before the water enters the penstock pipe. It can also serve as a temporary reservoir to store water depending upon its size.

Powerhouse

A powerhouse is a structure constructed to house all the electrical and mechanical components so as to protect them from rain and harsh weather. It also serves to house the backup power supplies such as battery banks and communication equipment.

Penstock pipe

A penstock pipe is a structure that carries water under pressure from the forebay tank to the turbine. It accounts for the major installation cost of a micro hydropower system. To reduce the head loss due to friction in the pipe, the pipe diameter can be increased but it increases the total cost in return. So a compromise between the pipe diameter and cost has to be made.

Mechanical components

Turbine

A turbine is a mechanical structure which is coupled to the generator. It consists of rotating blades which rotate when struck by water flowing out of the penstock pipe. For mountainous regions, aPelton wheel can be used if waterfalls of height more than 50 m are available. Francisorpropeller-type turbines are used for low heads. Propeller-type turbines in a pit may be used for very low heads of only a few meters.

Valve

A valve is used to control the flow of water from the penstock pipe to the turbine.

Drive system

A drive is responsible for the transmission of mechanical power of the turbine to the generator which produces electrical power.

Expansion joints

Expansion joints allow the penstock pipe to expand as the temperature rises which reduces the stress during high-temperature conditions thereby preventing it from damage.

Sluice gate

A sluice gate is responsible for controlling the flow of water which is diverted from the stream to the headrace.

Electrical components

Generator

A generator utilizes the mechanical power of the turbine to produce electrical energy. Self-excitation induction generators can be used to produce electrical power owing to their ability to produce useful power at varying rotor speeds. The reader is assumed to be familiar with the operation of generators and hence, it is not discussed here.

Electronic load controller (ELC)

An electronic load controller is used whenever the plant does not have a control system to control the amount of water fed to the turbine. It consists of a voltage controlling circuit typically using thyristors which connect or disconnect the ballast load according to the load supplied by the system. The students will study about ELC in detail in their course of Power Electronics (Chapter - AC voltage Controller) so it is not described in detail here.

Ballast load

It is the dummy load which is connected to the generator in parallel to the consumer load so as to dissipate the excess power during the light load conditions. The generator thus produces a constant output at a constant frequency.

Transmission and distribution system

The power generated by the plant is transmitted to the load centers by the transmission system. It may include a transformer which is used to increase the voltage level for economical reasons. Poles are used to hold ACSR cables and service wires. The distribution system is responsible for the distribution of electric power to the consumers.

Protective devices

Protective devices are responsible for the protection of the electrical equipment from damage due to excessive voltage or current. Load or power limiting devices are used to cut off electric power supply in case the load exceeds the limit. Lightning arrestors are also used to protect the electrical equipment from the dangerous voltage levels of lightning. The main switch is also used to cut off the load from the supply for maintenance and repair works.

Capacitor banks

Capacitor banks are used to provide excitation to the induction generators. In induction machines, the rotor field always lags the stator field due to which it always consumes reactive power regardless of its operation mode (either motoring or generating). Induction generators connected to a national grid draw the excitation power from the grid but since a micro hydropower system is a stand-alone system, capacitor banks are required to provide the generator with the necessary excitation.

References

1. Wikipedia. (2016). Micro hydro.Retrieved from https://en.wikipedia.org/wiki/Micro_hydro
2. Jonker Klune, Wim. (2016).Hydropower basics: Civil Work Components. Retrieved fromhttp://www.microhydropower.net

The images were taken from

1. http://energy-access.gnesd.org/cases/15-micro-hydro-village-electrification-in-nepal.html
2. http://www.worldwatch.org/hydropower-central-america-renewable-sustainable-and-without-alternatives