INTRODUCTION
Various transmission systems or grids are interconnected to form an integrated or distributed system. Though power can be exchanged between the grids, each grid operates independently. This is to ensure rational distribution. Currently, every industrial customers prefer uninterrupted and high quality power supply. In case any power system in the grid is interrupted all units will be affected. Hence is-landing is considered.
Is-landing can occur when maintenance and scheduled interruptions are necessary or when unplanned disturbances such as lightning occur. In these cases, protective devices separate the power system from the grid. This ensures that the base load of the power station is satisfied and keeps the system in normal operation.
ABSTRACT
Islanding is a condition in which a part of the utility system which contains load and generation is isolated from rest of the utility system and continues to operate. When severe disturbances occur, the interconnected system is split into islands to preserve generation and base load. This scheme is implemented for providing rational distribution. Excess load results in frequency drop. This indicates that the load-generation is unbalanced. In the conventional or existing scheme when the system frequency falls below the set frequency, the under-frequency relay provides a trip signal to the circuit breaker. Our project is aimed at replacing this relay with a frequency transducer and also automating the system using fuzzy logic and computers. Self generating power, power import from grid, consumption of generating station, load at each feeder and frequency are connected to the computer through suitable transducers. This PC based system runs continuously even when the grid collapses by safe islanding.
PROBLEM (CONVENTIONAL SCHEME)
When a grid fault occurs, the frequency drops below the required level which may cause tripping of generators or failure of other connected machines. It may take many hours to restore the operation of generating units and bring them to normal status. The frequency level is sensed by an under frequency relay. Mostly the scheme adopted is to observe manually the own generation and to provide suitable trip links by anticipating the grid fault. The base load is to be calculated by the operator to provide suitable trip links whenever any change arises in generation.
DISADVANTAGES
1. Generation to be monitored continuously.
2. Under frequency trip links for connected loads have to be cut-in or cut-out suitably by the operator.
3. Trip links have to be changed manually if generation changes.
PROPOSED SYSTEM
A suitable C program is to be written with graphical presentation to achieve safe islanding of power system taking into account the own generation available. During islanding condition the system is connected to base load only. This is decided with the help of Fuzzy Logic aided C program. Fuzzy theory based grading such as very critical, critical and normal is given to each active feeder depending upon its application and importance. This software is loaded to a separate computer called ISLANDING COMPUTER.
In this case the frequency level is sensed using a frequency transducer for islanding condition. Under frequency relay is replaced by frequency transducer. This is highly advantageous especially to small generation stations and substations.
SCOPE
This project is aimed at connecting the self generating power, power import from grid, auxiliary consumption of generating station and frequency to computer through transducers. For project purpose analog simulation containing potentiometers are used. A separate binary input is given through micro switches used to switch ON/OFF the feeders. Drive circuits are employed to switch ON/OFF the feeders through computer programs on receiving inputs from binary input board with reference to desired islanding frequency set in the software.
OBJECTIVE
To provide fast and reliable islanding of feeders thereby facilitating uninterrupted supply to the base load.This project is also aimed at protecting the generating station due to the occurrence of the faults.It also exhibits balance between available generation and the load requirement.
PRESENT ISLANDING SCHEME USING UNDER FREQUENCY RELAY AND FUSE LINKS
The present circuit comprising of under frequency relay, fuse links and master relay etc. In this scheme the operator adjusts the base load under Islanding condition with the aid of above fuse links. If the fuse link is provided by the operator, the feeder will be tripped under grid fault condition. Alternately if the fuse link is not provided the feeder is continued in service even under Islanding condition after grid fault occurs.
However the above method is required manual decisions and more hardware circuits. Hence it is decided to provide suitable Islanding Scheme with the aid of computers which is implemented as in this project.
COMPUTER BASED ISLANDING SCHEME
Following scheme includes Islanding computers one in area1 and other one in area2 substation. Islanding computer available at area1 is provided with input such as frequency, generation available at area1 and power consumed by the base load in area1 and area2. This computer calculates the Islanding power which is to be used by the area2 substation and transmit this data to the Islanding computer provided at the area2 substation. The computer located at area2 substation receives this data continuously.
The Islanding computer at area2 substation is also provided with input such as frequency, power consumed by the 110KV feeders connected at area2 substation. The feeders which are connected to the area2 substation namely a1, a2, a3, a4, a5 and a6. The feeder a6 is isolated from the area2 substation at the time of Islanding at all occasions. The remaining feeders are being continued or tripped by the hardware connected to the Islanding computers depending upon the Islanding power data given by the computer located at area1. These feeders are provided with fuzzy logic based grading to achieve Islanding in case of grid fault as a fuzzy term.
LITERATURE REVIEW
S. Shahnawaz Ahmed Azhar B Khairuddin, MohdRuddin B AbdGhani,presented “A scheme for controlled islanding to prevent subsequent blackout” illustrated that the way a blackout can be prevented in real time through controlled segregation of a system The main advantage is that splitting the system into two or more islands or none at all depends on the severity and location of the fault. This threshold can be reviewed from time to time when there are changes in the system topology and generation/demand growth.Frequent monitoring of the threshold value involves large amount of work and wide differences between the peak and off-peak operating conditions of the system arises.
Xuedong L1, YuqinXu, Li Zhang, and Shan Gao, presented“On-line Islanding Operation Based on CSP” was proposed to solve the on-line islanding operation problem in the event of a blackout in a power distribution network with DGs. To enhance the searching speed, some calculating works which is not related to fault mode is finished off-line. In the process of searching for the best solution, backtracking algorithm is applied to solve the CSP.
JayeshJoglekar and Yogesh P. Nerkar,presented “Design and Development of Power Grid Restoration Scheme for Maharashtra State with Generating Plant Islanding”paper deals with the modeling and identification of small isolated power systems as an emergency reserve for the Maharashtra State power system.
DESIGN
POWER SUPPLY
Power supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU. The term is most commonly applied to electrical energy supplies, less often to mechanical ones, and rarely to others. Most electronic circuits require a direct voltage supply that is usually derived from the standard industrial or domestic AC supply by transformation, rectification and filtering. The resultant raw DC voltage is sufficiently stable for most purposes, and it normally contains an unacceptably large AC ripple waveform. To render the voltage more constant, and to attenuate the ripple, voltage regulator circuits are employed.
COMPONENTS
1. Step down Transformer (230/15V).
2. Diode (IN4007).
3. Capacitors (2200µf, 10µf)
4. Regulator (7805, 7812).
5. Resistor (330K).
6. LED (Red)
DESCRIPTION OF POWER BOARD
A 230v, 50Hz Single phase AC power supply is given to a step down transformer to get 15v supply. This voltage is converted to DC voltage using a Bridge Rectifier. The converted pulsating DC voltage is filtered by a 2200uf capacitor and fixed voltage regulator IC’s 7805 and 7812 to convert the 15V ac voltage to 5V and 12V voltage respectively as shown in the figure and then given to 7805 voltage regulator to obtain constant 5v supply. A RC time constant circuit is added to discharge all the capacitors quickly. To ensure the power supply a LED is connected for indication purpose. Heat sinks are provided to protect the IC in case of any overload currents. A 10µF capacitor is connected is employed to reduce the ac ripples.
ADC AND INTERFACING BUFFERS
The functional components involved in analog to digital conversion circuit: ADC 0809, 555 Timer, 74HC240 and 74HC245. The interfacing circuit consists of two buffers IC’s 74HC244, which is similar to 74HC240; but non-inverting.
COMPONENTS
1. ADC 0809.
2. Buffer (74HC240, 74HC244, 74HC245).
3. Optocoupler.
4. 555 timer.
5. Resistor (330K, 560K, 2.2K)
6. LED (White).
ANALOG SIMULATION BOARD
The analog simulation board receives supply from power supply board. There are four potentiometers which are used to feed the input value of generating power, base load 1, base load 2 and frequency. This input value is directly fed to the ADC board.
BINARY INPUT SIMULATION BOARD
BLOCK DESCRIPTION
The essentials of the binary input simulation circuit: micro-switches, two color LED’s and 330O resistors. The binary signals are realized practically by employing micro-switches for each binary signal.
The normally open state of the micro-switch is indicated by the green color displayed by the LED, which is considered as the normal condition of the binary input signal. In the contrary, under normally closed state of the micro-switch the LED displays red color, which specifies the trip condition. The output lines are soldered to an eight pin terminal which is directly connected to one of the two interfacing buffer IC’s 774HC244.
The power supply to the micro-switches is provided from the 5V output terminal of the 7805 regulator. The negative terminals are properly grounded to avoid stray currents. 330O resistors are connected to the output terminals of the micro-switches for overload current protection and avoid any back flow of currents.
FREQUENCY TRANSDUCER
LM2917 is frequency to voltage converter with a high output amp/comparator. The tachometer uses a charge pump technique and offers frequency doubling for low ripple. The op amp/comparator with tachometer has a floating transistor as its output. The collector may take above VCE up to maximum VCE of 28V. It is 8 pin device with a ground referenced tachometer input and internal connection between the tachometer output and the op amp non –inverting input.
DRIVE CIRCUIT
A Driver is an electrical circuit used to control another circuit, such as a high power transistor. They are used to regulate current flowing through a circuit. It is also used to control the other factors such as other components in the circuit.
CONCLUSION:
In the present system many of the times grid fault occurs leading to Over Islanding or Under Islanding or Un-Successful Islanding. After implementing the computer based islanding scheme if grid fault occurs, definitely it leads to perfect islanding and frequency after islanding will be in the range of desired frequency.
Pc based islanding will provide a better solution to the power generating stations to run continuously even when grid collapses by safe islanding and thereby feeding the base load with a required optimum frequency and the vital areas will be fed power continuously without interruption.
FUTURE WORK:
In this case the frequency level is sensed using a frequency transducer for islanding condition. Under frequency relay is replaced by frequency transducer. This is highly advantageous especially to small generation stations and substations.
Two separate islanding systems loaded with islanding programs for area1 and substation (area2) is to be installed to have complete safe islanding scheme.
REFERENCE
1. Adibi.M.M, Kafka R.J and Milanicz D.P “ Expert System Requirements for Power System Restoration”.
2. DING Lei, PAN Zhencun, CONG Wei,(2008)” Searching for Intentional Islanding Strategies of Distributed Generation based rooted tree. Proceedings of CSEE”
3. LIU Dong, CHEN Yunping, SHEN Guang, et al (2006)” CSP-based Model and Algorithm of Service Restoration for Large Area Blackout of Distribution System. Automation of Electric Power Systems”.
4. Jayesh Joglekar and Yogesh P. Nerkar.” Design and Development of Power Grid Restoration Scheme for Maharashtra State with Generating Plant Islanding”.
This project is done with the guidance and reference from ETPS, Chennai.
