Grid Connection of Gotikhel Micro Hydropower Plant without Interrupting Isolated Load

Table of Contents

LIST OF PHOTOS AND FIGURES

LIST OF TABLES

LIST OF SPREADSHEETS

LIST OF ABBREVIATIONS AND SYMBOLS USED

1. INTRODUCTION
1.1 GENERAL
1.2 GOTIKHEL MICRO HYDROPOWER PLANT
1.3 OBJECTIVES OF GRID CONNECTION OF GHP

2. GHP IN GENERAL
2.1 GENERAL LAYOUT
2.2 SALIENT FEATURES OF GHP

3. PROBLEM ANALYSIS OF GHP
3.1 PROBLEM ANALYSIS OF EXISTING GHP
3.1.1 Forebay
3.1.2 Penstock
3.1.3 Pelton Turbine
3.1.4 Induction/ Asynchronous Generator
3.1.5 Electrical Cabinet
3.2 PROBLEM ANALYSIS FOR GRID CONNECTION

4. TECHNICAL ASPECTS OF GRID CONNECTION
4.1 TURBINE AND TURBINE SELECTION
4.1.1 Pelton Turbine
4.1.2 Cross Flow Turbine
4.1.3 Turbine Selection
4.2 TURBINE CONTROL SYSTEM
4.3 GENERATOR
4.3.1 Synchronous Generator
4.3.2 Sizing of Synchronous Generator for MHP in Nepal
4.4 DISTRIBUTION TRANSFORMER
4.5 SINGLE LINE DIAGRAM OF GHP
4.6 SHORT CIRCUIT ANALYSIS OF GHP
4.7 SWITCHGEAR EQUIPMENT
4.7.1 Low Voltage Circuit Breaker
4.7.2 Surge Arrester
4.7.3 Fuses
4.8 PROTECTION SYSTEM
4.9 CONNECTION BETWEEN GHP AND NEA
4.10 INSTRUMENTATION
4.10.1 Current Transformer
4.10.2 Meter
4.11 SYNCHRONIZATION

5. LOAD FLOW ANALYSIS OF GHP

6. FINANCIAL ASPECTS OF GRID CONNECTION
6.1 METHODS USED FOR FINANCIAL ANALYSIS OF GHP
6.1.1 Internal Rate of Return
6.1.2 Net Present Value
6.1.3 Benefit/ Cost Ratio
6.1.4 Payback Period
6.2 FINANCIAL ANALYSIS OF GHP

7. POSSIBLE IMPACTS OF GRID CONNECTION
7.1 TECHNICAL IMPACTS OF GRID CONNECTION
7.2 FINANCIAL IMPACTS OF GRID CONNECTION

8. CONCLUSION

9. ANNEXES
9.1 SUBSIDY POLICY OF MHPS IN NEPAL
9.2 PEA
9.3 GROUND CLEARANCE
9.4 DOMESTIC CONSUMERS OF NEPAL
9.5 ROYALTY ARRANGEMENTS FOR HYDROPOWER OF NEPAL
9.6 CALCULATION

REFERENCES

List of Photos and Figures

List of Photos

3.1 Forebay and leakage water from splitting gate of GHP
3.2 Penstock of GHP
3.3 Two jet Pelton turbine and its runner
3.4 Induction generator directly coupled with Pelton turbine at GHP
3.5 Induction generator controller and capacitor banks of GHP

List of Figures

2.1 General layout of the existing Gotikhel Micro Hydropower Plant

4.1 Pelton wheel turbine
4.2 Axes of the nozzle of Pelton turbine
4.3 Cross flow turbine
4.4 Ranges of application of different types of turbines
4.5 Digital Turbine Controller
4.6 Modern DTC-Vario showing Vario processor at left side and PLC processor at right side
4.7 Principle diagram of a synchronous generator
4.8 Schematic diagram of a synchronous generator
4.9 Single line diagram of synchronous generator of GHP
4.10 Single line diagram of distribution transformer of GHP
4.11 Block diagram of GHP for grid connection without interrupting isolated load
4.12 Single line diagram of GHP for grid connection without interrupting isolated load
4.13 Short-circuit current of a far-from-generator short circuit with constant AC component
4.14 Short-circuit current of a near-to-generator short circuit with decaying AC component
4.15 Single line diagram for short circuit current calculation
4.16 Equivalent circuit diagram for short circuit calculation of GHP at fault location A
4.17 Factor for series circuit as a function of ratio R/X according to VDE 0102
4.18 MCCBs used in GHP
4.19 TN- S system
4.20 A part of transmission line of GHP
4.21 Current transformer

5.1 Load flow at no village load
5.2 Load flow at 15.61 kW to national grid and 4 kW to village load
5.3 Load flow at 5.11 kW to national grid and 14.5 kW to village load
5.4 Load flow when GHP under no operation and 4 kW of demand of village load
5.5 Load flow when GHP under no operation and 14.5 kW of demand of village load
5.6 Load flow when GHP under no operation and village load is 25 kW
5.7 Load flow when GHP under operation and village load is 25 kW

6.1 A day load demand of HHs and surplus power injection to NEA grid
6.2 Annual cash flow analysis of GHP
6.3 Cumulative cash flow of GHP

7.1 System load curve of January 20, 2009

List of Tables

1.1 Classification of SHPs

2.1 Salient features of GHP

4.1 Pelton and Cross flow turbine specifications of Nepal
4.2 Generator de-rating factors
4.3 Important specifications of synchronous generator
4.4 Features of distribution transformer Dyn11 of Nepal
4.5 Values of different parameters for short circuit calculation of GHP at fault location A
4.6 Different parameters for fault location A, B, C and D
4.7 Different rated and stress parameters of the selected MCCBs
4.8 Specifications of ACSR for Micro/ Mini Hydropower Plants

5.1 Important values of load flow calculation for different possible cases of GHP

6.1 Approximate investment cost for grid connection
6.2 Total energy, losses and income of GHP for first year
6.3 Cash flow analysis of GHP in Euro

List of Spreadsheets

4.1 Selection of turbine 1
4.2 Selection of turbine 2

6.1 Financial analysis of GHP

List of Abbreviations and Symbols used

illustration not visible in this excerpt

1. Introduction

1.1 General

Hydropower is the most matured, reliable and largest renewable source of power generation. At present, about 20% of the world’s total electricity supply is from hydropower. The world’s hydropower installed capacity increased from 695.8 GW in 2001 to 888.8 GW in 2009[1]. Though fossil fuels dominate generating electricity, more than 60 countries use hydropower for meeting more than half of their electricity needs. Due to the adverse impacts of large hydropower plants in environmental sectors (deforestation, rehabilitation and others) as well as in economy (huge investment, high risk); Small Hydropower Plants (SHPs) has turned into an excellent and abundant source for power generation especially in developing countries. SHPs are easier to construct and commission due to uncomplicated designs. SHPs facilitate community participation and capitalize on local skills for plant construction.

Mini/ Micro Hydropower Plants (MMHPs) and Pico Hydropower Plants (PHPs) are used in developing countries to provide electricity to isolated communities where the electricity grid is not available. China, Nepal, Vietnam and many South American countries have developed a large number of MMHPs and PHPs that are providing electricity to many households in the last 30 years[2]. According to European Small Hydropower Association (ESHA), the classification of SHPs with respect to capacity is as shown in table 1.1.

Table 1.1: Classification of SHPs from[2]

illustration not visible in this excerpt

The capacity of SHPs rises for large countries like India (25 MW) and China (50 MW)[2]. The upper limit of SHPs in Nepal has also risen recently from 10 MW to 25 MW[3].

1.2 Gotikhel Micro Hydropower Plant

MMHPs play a vital role in rural electrification of a mountainous country like Nepal. According to Mini Grid Year Book of Nepal 2007[4], Nepal has altogether 1194 Pico Hydropower Plants with installed capacity of 2.22 MW, 651 Micro Hydropower Plants with installed capacity of 9.684 MW and 40 Mini Hydropower Plants with installed capacity of 14.948 MW. Total installed capacity of these hydropower plants is 26.853 MW till 2007. Except Mini Hydropower Plants, all Micro and Pico Hydropower Plants in Nepal are in isolated mode. These Pico and Micro Hydropower Plants (MHPs) are owned by the isolated communities/ private in the rural areas of Nepal. MHPs are very much successful for village electrification in Nepal as compared to other countries in the world. Electrification in rural areas by grid extension seems particularly unfeasible because of high initial installation costs, low consumption per household and less number of consumers. Difficult geographical terrain and low settlement density with large distances between houses make it hard to carry out installation work. The length of the lines required is high and long transmission lines have other implications such as high costs and high line losses resulting in low voltages in such areas. Those are the reasons behind the popularity of MHPs in Nepal as a decentralized energy. In addition, all electro-mechanical equipments except generators for MHPs are manufactured itself in Nepal. So, MHPs are matured technology in Nepal. Agriculture Development Bank, Alternative Energy Promotion Centre (AEPC¹ )) and Small Hydropower Promotion Projects (SHPP² )) are some organizations for the development of MHPs in Nepal. AEPC provides certain subsidy (see Annex 9.1 for further detail) to isolated MHPs in rural areas of Nepal[5].

Gotikhel Micro Hydropower Plant (GHP, 20 kW) is constructed in 1993 which is located 60 km away from the capital city Kathmandu, Nepal. It is one of the nearest isolated private MHP from the main city out of 651 isolated MHPs available in Nepal which still supplies electrical power to 145 households (173 initial households)[6]. Recent extension of 11 kV national grid in this area made the existence of GHP much harder as some households shifted to national grid for more reliable power and this trend will increase more in future. So, grid connection of GHP will be one of the best solutions for sustainability of this power plant as it can sell surplus power to the grid during off peak period as well as can also supply power to the households during load shedding in the grid.

1.3 Objectives of Grid Connection of GHP

The extension of national grid has made life of MHPs insecure as consumers want the energy from more reliable source i.e. from national grid. In the context of Nepal, especially in rural areas, construction of MHPs are very costly and because of unplanned extension of national grid, some of MHPs are in closing conditions and same cases will continue more in future. So, there is a huge risk in big investments and valuable efforts of villagers. Synchronization of MHPs to the national grid will be the ultimate solution for the existence of MHPs in Nepal without interrupting existing isolated load (village load). The need of the hour is to ensure that extant MHPs are operated even after intrusion of the grid in the area served by such MHPs. This can be achieved, technically by synchronizing MHP with the grid, and by executing Power Exchange Agreement (PEA) (see Annex 9.2) to take care of important non-technical issues (commercial, legal, financial, etc.). Under this arrangement, the local Rural Electrification Entity (REE) will generate power from the MHP and distribute to its members and sell the excess energy to the grid. Besides, the REE will also be able to purchase power from the grid when the demand of its members surpass the generation by MHP under it.

Taking GHP as a private/ community pilot project for grid connection in Nepal, the following objectives of grid-connected MHPs has been generalized:

i. To ensure optimum use of national resource and fulfill the possible new demand of energy in rural areas since grid connection and PEA allow the REE to sell their excess energy to Nepal Electricity Authority (NEA³ ))[7] grid and the REE can purchase the required energy from the grid when the demand of its members surpass the generation by MHP(s) under it.
ii. To synchronize MHPs with the grid which are now operated in isolated mode.
iii. To encourage Individual Power Producers (IPPs), communities and others to invest in MHPs.
iv. To facilitate development of new MHPs by local communities, IPPs as they can profiteer by selling the excess energy to the grid.
v. To ensure market for spill energy of MHPs.
vi. To ensure sustainability of MHPs.

2. GHP in General

This is an old MHP of Nepal which is running smoothly till today. However, the electrical cabinet is poorly maintained, some parts of 713 m long penstock pipe (made of polyethylene) is not buried perfectly under the ground, poor maintenance in the forebay, one jet of Pelton turbine is out of operation and under capacity of induction generator (only 20 kVA) have made the output power around 12 kW although it is said to be 20 kW MHP[8].

2.1 General Layout

The existing GHP consists of different mechanical machines as shown in figure 2.1 below. This power plant provides power to villages in the morning and in the evening. All the machines mentioned in figure 2.1 like milling machine, oil expeller, wheat grinding machine and others run mechanically from the shaft of the turbine during the day time.

illustration not visible in this excerpt

Figure 2.1: General layout of the existing Gotikhel Micro Hydropower Plant from[6]

2.2 Salient Features of GHP

This power plant has been successfully electrifying 145 households till today. But with proper inspection and maintenance of this plant, the efficiency of this plant could have increased significantly. An example has been illustrated here.

The power of water available at the turbine in general is given as:

illustration not visible in this excerpt

The net head of this plant is 80 m and have total head loss of 7 m .The average flow (Qavg ) of the river is 0.0275 m[3] /s and maximum flow (Qmax ) is 0.4 m[3] /s. That means the power of water available at the turbine according to equation (2-1) is:

illustration not visible in this excerpt⁴

The wet season of Nepal according to Nepal Electricity Authority (NEA) starts from 16th April to 15th December and from 16th December to 15th April is dry season[7].

The electrical output power of the plant (Pel) in general is given as:

illustration not visible in this excerpt

However, this power plant currently generates only 12 kW which shows that around 40% of power has been reduced. One of the reasons behind this is only one jet of Pelton turbine is functioning. Some of the key features of this plant are as shown in table 2.1 below.

Table 2.1: Salient features of GHP from[6]

illustration not visible in this excerpt

According to table 2.1, GHP has an installed capacity of 20 kW with a net head of 80 m using induction type of generator with IGC load controller. The length of the penstock is 713 m with its outer diameter of 200 mm and is made of polyethylene substance. This power plant initially supplied the power to 173 households in that village.

3. Problem Analysis of GHP

This section has been divided into two parts and they are problem analysis of existing GHP and problem analysis for grid connection.

3.1 Problem Analysis of existing GHP

Different problems of existing GHP have been identified during the field visit on 04.12.2009[8]. The field visit members were Edwin Thuerig (Entec AG, Switzerland), Dhruba Raj Mishra (SHPP, Nepal), Suman Budhathoki (BTU, Germany) and Tilak Kandangwa Limbu (AEPC, Nepal). The following are the existing problems followed by possible solutions of this plant.

3.1.1 Forebay

The forebay is only temporary covered so many leaves falling from the trees into forebay as shown in photo 3.1 can block the trash rack and will not allow water to enter into the penstock. Only a small trash rack is fixed at the penstock in feed. A spilling gate is leaking in photo 3.1.

illustration not visible in this excerpt

Photo 3.1: Forebay and leakage water from spilling gate of GHP from[8]

The solution is to cover the forebay and to enlarge the surface of the trash rack. If reasonable, the leaking spilling gate should be repaired as well.

3.1.2 Penstock

The length of the penstock is 713 m and the diameter is 200 mm. Some of the parts of penstock are exposed to the ground which might be dangerous from stone and tree fall. One section of penstock is even use as a bridge to go to forebay of this plant and can be seen in photo 3.2.

illustration not visible in this excerpt

Photo 3.2: Penstock of GHP from[8]

All the parts of PE penstock should be buried minimum 1 m depth according to the guidelines of AEPC of Nepal[9].

3.1.3 Pelton Turbine

Pelton turbine, 20 kW installed capacity, is made in Nepal. It has two nozzles or jets with deflector as shown in photo 3.3. This deflector is manually operated for start-up and stop. One jet is out of function which has reduced the electrical output power of this plant.

illustration not visible in this excerpt

Photo 3.3: Two jet Pelton turbine and its runner from[8]

The runner of the Pelton turbine is still in good condition. Deflector has to be made automatic by a self activation mechanism, so turbine and generator can be prevented from over speed in case of faults (see section 4.2). Both of the nozzles of the turbine shall be brought into operation to get optimum power output.

3.1.4 Induction/ Asynchronous Generator

The installed capacity of asynchronous generator is 20 kVA which is equal to the plant capacity 20 kW. But according to AEPC guidelines[9], safety margin of generator should be 20% more. Voltage regulation in island mode is more difficult by induction generator than by synchronous generator. In addition to that, reactive power should be provided by grid or capacitor bank. So, this generator as shown in photo 3.4 should be replaced by synchronous generator.

illustration not visible in this excerpt

Photo 3.4: Induction generator directly coupled with Pelton turbine at GHP from[8]

3.1.5 Electrical Cabinet

The electrical cabinet is in worst condition as shown in photo 3.5. The installed ballast controller is not suitable for grid connection, as there is no analog interface to adapt the frequency during synchronization. Without this frequency adaption the synchronization cannot take place. Required synchronization switches, protection relays and separate cabinet for grid

illustration not visible in this excerpt

Photo 3.5: Induction generator controller and capacitor banks of GHP from[8]

are missing. In order to get this site synchronized to the grid, the electrical cabinet has to be re- designed.

3.2 Problem Analysis for Grid Connection

MHPs are always located in remote areas. If grid exists, only the far end of distribution lines of NEA will be near to MHPs. That means there will be huge power losses, if MHPs are connected at 11 kV grid. Once NEA makes Power Purchase Agreement (PPA) or PEA with MHPs, then NEA should be able to take all the power from MHPs as stated in the agreement, otherwise NEA has to pay penalty for that. There is a huge risk in 11-kV-lines as frequent faults occur in these 11-kV-lines in Nepal. Hence, there is a high probability of financial losses for NEA in grid connection of MHPs. The other reason is load shedding. Nepal faces long load shedding during dry seasons[7]. So, it is not possible for NEA to buy power during load shedding. However, NEA is still positive in grid connection of MHPs for their sustainability.

GHP also faces same problems. But GHP is a pilot project; this will get benefit from NEA to get connected to national grid. GHP itself has some problems like replacing of induction generator with synchronous generator, electrical cabinet, and maintenance of turbine, penstock and forebay. The mechanical machines as shown in figure 2.1 cannot be run parallel with grid connection as it reduces the power of GHP drastically and this will not be financially feasible to connect GHP with very low power to grid.

4. Technical Aspects of Grid Connection

In this section, turbine and turbine selection, generator, protection system, turbine control system, distribution transformer, transmission and distribution system, single line diagram and short circuit current calculation of GHP will be discussed.

4.1 Turbine and Turbine Selection

A turbine converts potential energy of water to rotational mechanical energy. Two types of turbines namely Pelton turbines and Cross flow turbines for Mini/ Micro Hydropower Plants of Nepal as well as of some developing countries are locally manufactured in Nepal. Both Pelton and Cross flow turbines are impulse turbines in which kinetic energy is exchanged with the runner of the turbine; whereas in reaction turbines like in Francis and Propeller turbines, the kinetic and potential energy is exchanged with the runner[10]. But in this thesis, only Pelton and Cross flow turbine are discussed because other turbines are not manufactured in Nepal and subsidy is only provided by Nepalese government (AEPC) to locally manufactured turbines for Mini/ Micro Hydropower Plants in Nepal.

4.1.1 Pelton Turbine

Pelton turbines are impulse turbines where one or more jets impinge on a wheel carrying on its periphery a large number of buckets. Each jet issues through a nozzle with a needle (or spear) valve to control the flow (see figure 4.1). They are only used for relatively high heads. The axes of the nozzles are in the plane of the runner (see figure 4.2). Pelton turbine is essentially a wheel with a set of double cups or ‘buckets’ mounted around the rim. A high speed jet of water, formed under the pressure of the high head, hits the splitting edge between each pair of cups in turn as the wheel spins. The water passes round the curved bowls, and under optimum conditions gives up almost all its kinetic energy[11]. The power can be varied by adjusting the jet size to change the volume flow rate, or by deflecting the entire jet away from the wheel. To stop the turbine for instance when the turbine approaches the runaway speed due to load rejection, the jet (see figure 4.1) may be deflected by a plate so that it does not impinge on the buckets. In this way the needle valve can be closed very slowly, so that overpressure surge in the pipeline is kept to an acceptable minimum. Any kinetic energy leaving the runner is lost and so the buckets are designed to keep exit velocities to a minimum. The turbine casing only needs to protect the surroundings against water splashing and therefore can be very light.

illustration not visible in this excerpt

Figure 4.1: Pelton wheel turbine from[10]

illustration not visible in this excerpt

Figure 4.2: Axes of the nozzle of Pelton turbine from[11]

The hydraulic efficiency of Pelton turbine is as follows[11]:

illustration not visible in this excerpt

For modern turbines:

illustration not visible in this excerpt

For older turbines:

illustration not visible in this excerpt

4.1.2 Cross Flow Turbine

This is also an impulse turbine which is also known as Banki-Michell. It can operate with discharges between 20 l/s and 10 m[3] /s; heads between 5 m and 200 m[10]. Water (see figure 4.3) enters the turbine, directed by one or more guide-vanes located in a transition piece upstream of the runner, and through the first stage of the runner which runs full with a small degree of reaction. Flow leaving the first stage attempts to crosses the open centre of the turbine. As the flow enters the second stage, a compromise direction is achieved which causes significant shock losses. It is used for wide range of heads overlapping those of Francis, Pelton and Kaplan turbines.

illustration not visible in this excerpt

Figure 4.3: Cross flow turbine from[10]

4.1.3 Turbine Selection

The selection criteria of a turbine can be determined by net head, range of discharges through the turbine, rotational speed, cavitations and cost. Net head, efficiency, specific speed, runaway speed of Pelton and Cross flow turbines are mentioned in table 4.1[12]. The first criterion to take into account in the turbine’s selection is the net head. The gross head is the vertical distance, between the water surface level at the intake and at the tailrace for reaction turbines and the nozzle level for impulse turbines. Once the gross head is known, the net head can be computed by simply subtracting the losses along its path.

Table 4.1: Pelton and Cross flow turbine specifications of Nepal from[12]

illustration not visible in this excerpt

*10 to 30 is 1 Jet Pelton, 30 to 40 is 2 Jet Pelton and 40 to 50 is 3 Jet Pelton

According to the net head, turbine of GHP could be chosen as Pelton or Cross Flow T15 as stated in the table 4.1.

illustration not visible in this excerpt

Figure 4.4: Ranges of application of different types of turbines from[10]

The rated flow and net head determine the set of turbine types applicable to the site and the flow environment as shown in figure 4.4. All of those turbines are appropriate for the job, and it will be necessary to compute installed power and electricity output against costs before taking a decision.

From above conventional enveloping curves it is clear that Turgo and Pelton turbines will be appropriate for GHP. However, Turgo turbine is not locally manufactured, so only option for GHP is now Pelton turbine.

The specific speed comprises a reliable criterion for the selection of the turbine and it is more precise than the conventional enveloping curve as shown in figure 4.4.

Specific speed of an imaginary turbine is that generates 1 kW at net head of 1 m and is given as[10]:

illustration not visible in this excerpt

Specific speed of an imaginary turbine is also with a discharge of 1 m[3] /s at net head of 1 m and is given as[10]:

illustration not visible in this excerpt

The equation (4-8) states that the specific speed of the turbine with respect to power output of the turbine is approximately equals to the specific speed of the turbine with respect to the discharge/ flow rate of the power plant[13]. This statement can be verified from equations (4-9) and (4-10) which has been shown below.

In the context of GHP, the maximum turbine power output of the plant is 23.55 kW from equation (2-5) and turbine is directly coupled to the generator with the nominal speed of 1500 rpm. So, the specific speed of the turbine with respect to the power output of the turbine of GHP is calculated with the help of equation (4-6) as:

illustration not visible in this excerpt

Similarly, the specific speed of the turbine with respect to the discharge of the turbine of GHP is calculated with the help of equation (4-7) as:

illustration not visible in this excerpt

Now from the table 4.1, it seems that both Cross flow and Pelton turbines can be used. With the help of the software of MHP Design Aids 2006.05[12], the suggested turbine are Turgo and Cross flow if the number of jet of Pelton turbine is given as 1 as shown in spreadsheet 4.1. But, in spreadsheet 4.2, it can also be seen that there is an existence of Pelton turbine when there is a use of 2 jet Pelton turbine. In both cases, the specific speed of the turbine is 36 rpm which is given by this MHP Design Aids in spreadsheet 1 and spreadsheet 2 for 40 l/s discharge, 80.057 m net head, 1500 turbine nominal rpm and 23.55 kW of turbine power output.

From MHP Design Aids and from table 4.1, it is clear that GHP could choose 2 jet Pelton, Cross Flow T15 or Turgo turbines. However, Turgo turbine is not locally manufactured. So, this type of turbine is not suggested to use as subsidy will not be provided for such turbines by the government. In other hand, the efficiency of Cross flow turbine is relatively lower than Pelton turbine and addition to that, the net head of this plant lies exactly on the range of Cross flow turbine.

Hence, this plant has chosen 2 jet Pelton turbine which has been proved to be correct by table 4.1, figure 4.4, spreadsheet 4.1 and spreadsheet 4.2.

Spreadsheet 4.1: Selection of Turbine 1 from[12]

illustration not visible in this excerpt

Spreadsheet 4.2: Selection of Turbine 2 from[12]

illustration not visible in this excerpt

4.2 Turbine Control System

Turbines are designed for a certain net head and discharge. Any deviation from these parameters should be compensated by mainly two methods; either by controlling the water flow to the turbine known as flow control method or by keeping the water flow constant and adjusting the electric load by an electric ballast load connected to the generator terminals known as Electric Load Controller (ELC) for synchronous generator or Induction Generator Controller (IGC) for induction generator. Flow control in MHPs will be more expensive than ELC. But ELC or IGC are not recommended for hydropower plants greater than 100 kW[9].

In the context of GHP, it is now supposed to be connected to the grid by using synchronous generator, so this plant is suggested to use Digital Turbine Controller (DTC)[13] although the plant capacity is only 20 kW. This is because the synchronosation of the grid will be more easier and reliable with DTC.

Digital Turbine Controller

A DTC controls, protects, monitors and performs data acquisition in a single device as shown in figure 4.5. It provides a safe supervision and a reliable fault diagnosis. If the plant uses a synchronous generator then it facilitates with automatic synchronization. DTC implements full automatic start and stop as well as manual operation for troubleshooting.

illustration not visible in this excerpt

Figure 4.5: Digital Turbine Controller from[13]

Voltage, current, power, speed and frequency parameters are used for data acquisition. Water level and power can be controlled in real time; cosφ as well as reactive power can be controlled using AVR interface. This turbine control system is suggested to use in GHP as GHP will be connected to the grid; and DTC is more reliable than ELC although DTC is more expensive. One type of DTC that is suggested to use at GHP is DTC-Vario (see figure 4.6); a product of Entec AG[14]. The only reason behind choosing this device is that branch office of Entec AG is also located in Nepal. It will be more convenient and cheaper to install this system in GHP.

illustration not visible in this excerpt

Figure 4.6: Modern DTC-Vario showing Vario processor at left side and PLC processor at right side from[14]

In figure 4.6, DTC-Vario has a two parts in which Vario processor measures, monitor as well as process the electrical system and on the other hand Programmable Logic Controller (PLC) processor controls turbine, plant monitoring and remote control communication. The turbine of the GHP will be fully automatic controlled (speed, water level, frequency and power) with this device in order to ease the synchronization process with the national grid. This device also supports for automatic start, re-start and stop functions of the plant. The cost of this device has been shown in table 6.1 under section 6.2.

4.3 Generator

Generators convert mechanical power into electrical power. Only three phase AC synchronous generator is discussed here.

4.3.1 Synchronous Generator

Generator which generates electrical power with a frequency propotional to the speed of the rotor so the electrical frequency and mechanical speed are synchronous is known as synchronous generator. In the context of Nepal, MMHPs should use synchronous generator in order to get connected with the national grid[7]. So, GHP should use synchronous generator (preferably self exicted, self regulated brushless synchronous generator) in order to synchronize with the grid.

[...]

---

¹ ) AEPC is a semi autonomous organization under the Ministry of Environment of Nepal.

² ) SHPP is a project of Ministry of Water Resources of Nepal and German Technical Cooperation (GTZ) of Germany.

³ )NEA is an organization under Ministry of Energy of Nepal (Previously Ministry of Water Resources) which is responsible for generation, transmission and distribution of electricity in Nepal.

⁴ )The efficiency of the turbine is only 75% because this power plant has used the homemade Pelton turbine as there is a rule to use homemade turbine in order to get subsidy from the government.