Thursday, March 16, 2017

Case study report of Kulekhani II hydropower plant


Acknowledgement

                                                                                
We are very grateful to Mrs. Smita Adhikari for her valuable suggestions, inspiration and full support for making it possible to perform a brief case study on microprocessor based instrumentation system according to the syllabus of Instrumentation-II and present this report within a short period of time.
We are also grateful to the Electrical Department, PaschimanchalCampus for providing us the recommendation letter subjected for visit to the industry.
We would like to thank Er. Ghanshyam Poudel from Kulekhani II for managing his valuable time to show around the plant and providing information as per our requirements.
We solely take the responsibility of any possible mistakes that may have occurred in preparing this report and we would like to welcome comments and queries during the presentation of this report.





Abstract



This report is prepared as per the visit for Case Study based on microprocessor based instrumentation system included in the syllabus of Instrumentation-II. This report includes brief introduction of Khulekhani 2 Hydropower plant, detailed study of the instrumentation system available at present in the Plant, and its characteristics. This report also presents suitable alternatives and advancement to the existing instrumentation system.
Like all production industries, Hydropower plant uses a large number of instrumentation devices such as sensors, Mechanical equipment and other electronic devices. These devices are synchronized very precisely so as to meet the required flow of process. A single disturbance or error in a system affects all the systems and devices connected together as well as other instruments connected through that transmission line. That is why necessary beforehand mechanisms and backup process are maintained to get through these errors and maintain the stability and functioning of the whole power system.
We studied the detail process and observe the possible changes that can enhance the functioning and efficiency of the system. We also observed the process followed in the global world and the improvement going in the Hydropower plant around the globe. This report enrolls all the possible features that can be implemented to this current system as per its hands on resources and other technical difficulties.




Table of contents

Topics                                                                                                            page no.
1.                 Introduction                                                                                     1
1.1            Powerhouse background                                                       1
1.2            Salient  features                                                                     2
2.                 Current process control system                                                       2         
2.1            Block diagram                                                                       2
2.2            Features                                                                                 3
2.2.1    Governor cabinet                                                         3
2.2.2    Relay and recording Panel                                           6
2.2.3    Resistance Temperature Detector (RTD)                     6
2.2.4    Control room                                                                7
2.3            Limitation                                                                              10
3.                 Recommended process control system                                            10
3.1            Block Diagram                                                                       11                 
3.2            Features                                                                                  11
3.3            Advantages over existing system                                          14
4.                 Recommendations                                                                           15
5.                 Conclusion                                                                                       15
6.                 References                                                                                       16


1.    INTRODUCTION
There is great potential of hydroelectricity in Nepal. According to statistical record Nepal has potential of 83,000 MW hydroelectricity power production. However, at present Nepal has produced only about 600 MW of hydropower, which is not sufficient to meet the demand of whole country. Thus, Nepal is facing serious attack of nearly 12 hours daily load shedding. This directly affects the industrial field and overall development of the country.
The hydropower projects in Nepal are directly dependent on the amount of water flowing in the river stream, so during dry season the power production decreases significantly which results increase in duration of load shedding. Among different hydropower models established in Nepal, Kulekhani is only the hydropower project with reservoir. This type of hydropower can be the permanent solution of the deep rooted load shedding.
The purpose of this research is to study about the technology that are used in this hydropower for generation of the electricity, for controlling the hardware equipment and manipulating the generated output electricity and think about the new technology and equipment as well as the new manipulating methods so that we can increase the production rate or to increase the efficiency of the power house.
1.1 POWERHOUSE BACKGROUND
Kulekhani -2 Hydropower station, located at Nibuwatar, Makwanpur, is a cascade of Kulekhani -1. Its installed capacity is 32 MW having two units each of 16 MW. This powerhouse was designed as peaking power station but it has been supporting as emergency stand by station also. The annual expected energy generation capacity as primary energy is 104.6 GWH. This project was constructed under the financial assistance of Overseas Economic Cooperation Fund (OECF) of Japan and Government of Nepal at a cost of NRs.124 million.
The main purpose for the construction of this power station was to take the peak load only but the unavailability of the sufficient power with respect to demand, the power station was forced to operate as and when required.
1.2 Salient Features
Type :Cascade of  Kulekhani -1
Rated net head : 103.17 meter
Design discharge : 16.65 m3/s
Headrace tunnel : 4294 meter
Penstock pipe : 843 m long, 2.1 – 1.5m diameter steel pipe
Installed capacity : 32 MW
Turbine type and numbers : Pelton, 2 sets
Rated speed : 750 rpm
Type of generator : Vertical shaft, Synchronous
Capacity : 18.8 MVA
Rated voltage : 6.6 KV
Power transformer : 6.6/132KV, 1 phase, 12.6 MVA, 3 Nos
Average annual generation : 104.6 GWh
Commissioning date : 1986
Construction cost : NRs 124 million
Financed by :OECF and OPEC Fund

1.       Current Process Control System
The water from reservoir is carried through penstock to the turbine which is controlled by hydraulic control (valves). There are two different turbines each of 16 MW capacities. The generated power at 6.6 KV is stepped up through step up transformer and is transmitted to Control Room. Control Room monitors and controls the power generation, power distribution and control of turbine through governor cabinet.

2.1 Block Diagram

2.2 FEATURES
2.2.1 GOVERNOR CABINET
A governor is a combination of devices that monitor speed deviations in a hydraulic turbine and converts that speed variation into a change of wicket gate servomotor position which changes the wicket gate opening. This assembly of devices would be known as a “governing system”. In a hydro plant this system is simply called the “governor” or “governor equipment”. This wicket gate opening which is responsible for the flow of water to the turbine thus controls:
·         Flow of water through the Penstock
·         Alignment of the generator’s shaft unit (vertical in the case of Kulekhani 2)
·         Power Production
GOVERNOR COMPONENTS
The main parts of the governor are
·         A speed sensing device, usually a ball head
·         An oil pressure system, hydraulic valves to control oil flow, and
·         One or more hydraulic servomotors to move the wicket gates
·         Auxiliary Control
Speed Sensor: The ball head is the component that responds to speed changes of the unit- the turbine. There are various designs of ball heads, but generally, they consist of two flyweights attached to arms that pivot near the axis of rotation. The arms are attached to a collar on a shaft. As the ball head rotational speed increases, the flyballs move out because of centrifugal force pushing a rod down. The rod, usually termed the speeder rod, acts on the pilot valve to route oil to the main valve and the servomotors. A decrease in speed will cause the valve to move upward, allowing the servomotor to drain and move in the opening direction. As the servomotor moves open, the valve is moved down by the speed droop lever, centering it over the port and stopping the servomotor. The unit is now operating at a slightly slower speed, but the servomotor will not overshoot because for a given speed the servomotor must move to a specific position.
Oil Pressure System: The oil pressure system consists of oil pump/s, oil accumulator tank/s, oil sump, and the necessary valves, piping, and filtering required.
Flow Distributing Valves: The hydraulic system consists of an oil sump, one or two oil pumps, an air over oil accumulator tank, and piping to the servomotors. Typically, there are two pumps with lead and lag controls so that there is always a backup pump. Some systems will share two pumps between two units so that in an emergency one pump could be used for both units. The accumulator tank is usually sized so that in the event the pumps fail, the gates can still be closed.
The size of the valve required to control the large amount of oil flowing to the servomotors is too large to be controlled by the ball head. Therefore, a hydraulic amplifier system is used. Oil is routed to a servo on the larger valve by a small pilot valve. The pilot valve is very small so that it is sensitive to the small forces that result from small changes in speed. The larger valve may be called the main valve, regulating valve, control valve, relay valve, or distributing valve. The pilot valve usually is designed with a moveable bushing. The plunger of the pilot valve is connected, through a floating lever, to the ball head, and the bushing is connected to main valve. Whenever the pilot valve moves off center, oil is routed to the main valve servo, causing the main valve to move. The pilot valve bushing is moved off center by the main valve movement, blocking the port of the pilot valve, stopping further main valve movement. The restoring lever between the main valve and the pilot valve bushing is usually adjustable so that the ratio of pilot valve movement to main valve movement is adjustable.
Auxiliary Control: Most governor cabinets also have auxiliary controls that control the auxiliary valves to control the gate position. Because of the relatively small ports of the auxiliary valve, the gates are moved slowly and can be positioned precisely. The auxiliary valve has no connection to the ball head, and therefore, no speed control. Some of these auxiliary controls also react on the alignment of the generator, that are based on the readings of the generator guide bearings (upper and lower) and generator thrust bearings (that carry the rotating weight of the rotor axis in the generator).
APPLICATION
It is an important thing to understand that the government cabinet, as shown in the figure, shows meters of various sensors such as speed meter, wattmeter, etc. in analog calibration. These values will be seen by the person at site and certain operations such as opening of valves, nozzle flows will be controlled.

2.2.2 RELAY AND RECORDING PANEL
This unit is the statistical unit of the power house which takes electrical input and gives mechanical change in flag as output. A protective relay is an electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected. Protection relays use arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay contacts, and phase-shifting networks. Protection relays respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency. Distance relays trip for faults up to a certain distance away from a substation but not beyond that point. An important transmission line or generator unit will have cubicles dedicated to protection, with many individual electromechanical devices.  Relay panel includes relays such as overvoltage relay, over current relay, over speed relay, percentage differential relay, time limiting relay, etc. These are emf coils current signal trigger coils and signal coils that have directly linked with recording panel. Recording panel includes over voltage for generator circuit, over current for generator circuit, emergency stop, over speed, etc. Flags trough which the operator manages the performances of power house. The timely records of every hour are recorded through this panel. It avoids malfunctioning and the performance of the power house elements.                                                                                                                                                 
2.2.3 RESISTANCE TEMPERATURE DETECTOR (RTD)
An RTD is a device which contains an electrical resistance source which changes resistance value depending on its temperature. This change of resistance with temperature can be measured and used to determine the temperature of a process or of a material. This principle based temperature measuring system was found to measure the temperature of power house elements. As the temperature of power house elements as stator, transformer, etc. changes the corresponding resistance changes and can be seen on display.
2.2.4 CONTROL ROOM

 
 Frequency control



VOLTAGE CONTROL

The exciter supplies the field winding with direct current and thus comprises the “power part” of the excitation system.

The controller treats and amplifies the input signals to a level and form that is suited for the control of the exciter. Input signals are pure control signals as well as functions for stabilizing the exciter system.

The voltage measurement and load compensation unit measures the terminal voltage of the generator and rectifies and filters it. Further, load compensation can be implemented if the voltage in a point apart from the generator terminals, such as in a fictional point inside the generator’s transformer, should be kept constant.
The power system stabilizer, PSS, gives a signal that increases the damping to the controller. Usual input signals for the PSS are deviations in rotor speed, accelerating power, or voltage frequency.

The limiter and protection can contain a large number of functions that ensure that different physical and thermal limits, which generator and exciter have, are not exceeded. Usual functions are current limiters, over–excitation protection, and under–excitation protection. Many of these ensure that the synchronous machine does not produce or absorb reactive power outside of the limits it is designed for.

2.3 LIMITATIONS
This hydropower project was established in 1982, and there has not been improvements in technology since then, so the limitations are basically due to the not implementing the modern technology. The limitations that we found during our visit are listed below:
Ø  The instrumentation system implemented in the powerhouse was open loop system i.e. observation are made in control room and the manual commands are made accordingly. The nozzle valve controls are controlled according to the load power demand manually. The recording of powerhouse performance is noted manually. Error handling and status signals are addressed by human operator. This made the operation of power house dependent on manual work; we found this as the main limitation of the power house.
Ø  Large use of mechanical switch instead of electronic switch.
Ø  In hydropower it uses all mechanical relays and flags and these are not replaced by microprocessor based digital relays which have great precision and maintenance capacity.
Ø  The commands form control room to power house was connected manually (through phone), that should be connected electrically.
Ø  The machines used in the powerhouse are too old to use and are not updated according to time.

3.                RECOMMENDED PROCESS CONTROL SYSTEM
3.1 BLOCK DIAGRAM
3.2 FEATURES
Programmable Logic Controller Concepts
Programmable logic controller is basically a digital electronic apparatus; it has a programmable memory for strong instructions to implement specific tasks for control. Advanced PLCs are microprocessor based and can perform complex mathematical calculation and function as well as logic, sequencing, timing and counting.



The major components of a PLC are:
1.      Processor
2.      Input Modules
3.      Output Modules
4.      Programming and Other Facilities
Component Description:
Programmable logic controllers typically contain a variable number of input/output (I/O) ports and usually employ reduced instruction set computing (RISC), which consists of simplified instructions that are intended to allow for faster execution.
Input Interface provides connection to the machine or process being controlled. The principal function of the interface is to receive and convert field signals into a form that can be used by the CPU.
CPU provides the main intelligence in the PLC as is evident in every other system. The output Interface takes signals from the processor and translates them into forms that are appropriate to produce control actions by external devices.

General Concept:
A hydropower operation will typically involve many operations and steps. Some of these steps would occur in series and some would occur in parallel. Some events may involve discrete setting of states in the plant like valves open or closed, accessories on or off, and so on. With help of PLC, use of hardwired relays is minimized. A large PLC has enough number of relays to do all the operations. The advantages of these relays are that these are
Digital, so minimum damage to the system, cost reduction and less maintenance is involved in their use. If there is any need to change the control system, only the program is to be changed and it can be done easily without any cost involvement.
In continuous processes we may need to convert the analog signal to the acceptable value to the PLC and then with A/D converter it is converted to digital input to processor. A control algorithm is to be developed to get a control signal to control the variable. There is always a set value to which the variable is taken. By calculating the error, algorithm can be applied to get a control signal. This control signal is then converted to analog signal and then amplified to control the variable.
·         Automatic Start Sequence
·         Automatic Shutdown (Normal, Emergency).
·         Digital Governing using PLC
·         Speed Governing
·         Position Control
·         Protection System with PLC
·         Alarm and Annunciation using PLC
User Interfacing:
SCADA (Supervisory Control and Data acquisition)/HMI concepts
Even though PLCs are responsible for automated control one needs to keep an eye on all the processes taking place in the station and the house. SCADA is a control system that is used for centralized monitoring and recording of pumps, tank levels switches, etc. That is SCADA is there for supervision of activities. Its components are:
·         A Human Machine Interface (HMI): Monitor most commonly.
·         A computer system: used to gather information and send control commands
·         RTU (Remote Terminal Units)/ PLC: connected to the physical equipments and convert actions to digital signals. All major PLC manufacturers have offered integrated HMI/SCADA system thus PLC are chosen above special purpose RTUs.

Visualization of plant status and condition, logging of events and metering, and annunciation of alarm conditions are generally managed by the station through HMI (Human Machine Interface). It serves as the local operator’s portal into the process. The feedback control loop passes through the RTU or PLC, while the SCADA system monitors the overall performance of the loop.
In this way, the operator can sit in his room and watch what goes on through the plant and control the actions from there if required. But this isn’t the only advantage the automation provides.
Some of the pros are:
·         Increased system availability and visibility
·         Decreased downtime requirements to recover from a failure
·         Decreased cost in materials and man hours for installation
·         Ease of control
1.3  ADVANTAGES OVER EXISTING SYSTEM
Our purposed system consists of digital electronic devices which are microprocessor based (PLC, SCADA).Thus this system is highly advantageous over existing system.

Ø One of the most important improvements of our purposed system is that it provides automation.
Ø It possesses centralized monitoring and recording of pumps, tank levels switches +etc.
Ø It is capable of performing complex mathematical calculation and function as well as logic, sequencing, timing and counting.
Ø With help of PLC, use of hardwired relays is minimized.
Ø  Increased system availability and visibility.
Ø  Decreased downtime requirements to recover from a failure.
Ø  Decreased cost in materials and man hours for installation.
Ø  Ease of control.

4.    RECOMMENDATIONS
v    Implementation of electronic PLC over the mechanical governor cabinet components.
v    HMI over analogue meters i.e. a computerized user interface and monitoring system over the analogue meters in the control room.
v    Proper installation of a system as per the outlines mentioned in our recommended system that uses PLC for controlled output and HMIs for supervision.

   5.    CONCLUSION
Thus, the visit to Kulekhani 2, provided us with the information on power generation and system control. Though system is robust and reliable the presence of local operator within the powerhouse is a necessity. This manual supervision throughout the powerhouse requires numbers. This can be improvised with the use of an automatic control system where supervision will be done from the control room and the device itself will make necessary changes to any unusual processing. Further these changes can be monitored and any interruption can be performed from the control room. Lastly we gained lots of information on hydro power generation and controlled instrumentation implementation.
    6.    REFERENCES

Ø  ASME, The Guide to Hydropower Mechanical Design,1996
Ø  Nepal Electricity Authority

1 comment:

  1. Really informative report—appreciate how clearly it breaks down both the current instrumentation setup and potential upgrades for improved system reliability and efficiency. It’s fascinating to see how precision and backup systems play such a critical role in hydropower. On a related note, for those interested in exploring natural energy landscapes firsthand, Darjeeling offers a great blend of scenic beauty and hydropower relevance: https://northbengaltourism.com/darjeeling-tour-packages/









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