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ABSTRACT
The project is designed to shut down the power supply when it is overloaded. Conventional circuit breaker like MCB based is on thermal bimetal lever trip mechanism. It is very slow and the trip time is dependent upon the percentage of overload. This project senses the current passing through a series element and the corresponding voltage drop is compared against the preset voltage proportional to the current by a level comparator to generate an output for the load to trip.
The concept of electronic circuit breaker came into focus realizing that conventional circuit breakers such as MCBs take longer time to trip. Therefore, for sensitive loads it is very important to activate the tripping mechanism at the shortest possible time, preferably instantaneously. This project is demonstrates fast tripping mechanism as against the slow one like MCB. Electronic circuit breaker is based on the voltage drop across a series element proportional to the load current, typically a low value resistor. This voltage is sensed and rectified to DC which is and then compared with a preset voltage by a level comparator to generate an output that drives a relay through a MOSFET to trip the load. The unit is extremely fast and overcomes the drawback of the thermal type. It uses a microcontroller from PIC family.
1.1 INTRODUCTION
This report presents work done on Automated Circuit Breaker Monitoring project during the year of 2006. Previous work is described in annual reports for 2003 and 2005. During year 2006, we have developed two CBM devices. Both devices are installed in CenterPoint Energy 345kV substation Obrien in south Houston. The first CBM device - CBM1 is installed a year ago. The second one, CBM 2, has implemented new features: New signal conditioning board, communication protocol and time synchronization. Detailed Functional Requirement Specification for Automated Circuit Breaker Monitoring system is given in Appendix. Final reports for year 2003 and 2005 are also provided in the Appendix.
This protection function prevents damage to the Power Supply itself due to overcurrent (including output short-circuits). The protection function is activated and the output current is limited when the load current is greater than the overcurrent detection value (this value depends on the model).
The output voltage will also drop according to the overload (load impedance).The drop level depends on the overload conditions and load line impedance.The following table shows the six types of output voltage drop characteristics for main models when the overcurrent protection function is operating.
These drop characteristics can be seen as indicating the limit on the output current that can be supplied to the load effectively in the process in which the output voltage starts when the AC input turns ON. When connecting a load (with built-in DC-DC converter) that starts operating from a low voltage or a capacitive load in which inrush current can flow easily, consider the trend in overcurrent protection drop characteristics and the startup characteristics on the load side when selecting the Power Supply.
The project is designed to shut down the power supply when it is overloaded. Conventional circuit breaker like MCB based is on thermal bimetal lever trip mechanism. It is very slow and the trip time is dependent upon the percentage of overload. This project senses the current passing through a series element and the corresponding voltage drop is compared against the preset voltage proportional to the current by a level comparator to generate an output for the load to trip.
The concept of electronic circuit breaker came into focus realizing that conventional circuit breakers such as MCBs take longer time to trip. Therefore, for sensitive loads it is very important to activate the tripping mechanism at the shortest possible time, preferably instantaneously. This project is demonstrates fast tripping mechanism as against the slow one like MCB. Electronic circuit breaker is based on the voltage drop across a series element proportional to the load current, typically a low value resistor. This voltage is sensed and rectified to DC which is and then compared with a preset voltage by a level comparator to generate an output that drives a relay through a MOSFET to trip the load. The unit is extremely fast and overcomes the drawback of the thermal type. It uses a microcontroller from PIC family
BASICS OF CIRCUIT BREKAR
2.1 Background
Electrical Power System protection is required for protection of both user and the system equipment itself from fault, hence electrical power system is not allowed to operate without any protection devices installed. Power System fault is defined as undesirable condition that occurs in the power system. These undesirable conditions such as short circuit, current leakage, ground short, over current and over voltage.
With the increasing loads, voltages and short-circuit duty in distribution system, over current protection has become more important today. The ability of protection system is demanded not only for economic reason but also consumers just expect ‘reliable’ service. In a Power System Protection, the system engineer would need to a device that can monitor current, voltage, frequency and in some case over power in the system. Thus a device called Protective Relay is created to serve the purpose. The protective relay is most often relay coupled with Circuit Breaker such that it can isolate the abnormal condition in the system. In the interest of reliable and effective protection, some designers of power distribution/power controllers select relay as opposed to electro-magnetic circuit breakers as a method of circuit protection.
2.2 Overview of Over Current Relay Project
An "Over Current Relay" is a type of protective relay which operates when the load current exceeds a preset value. In a typical application the over current relay is used for over current protection, connected to a current transformer and calibrated to operate at or above a specific current level.
This project will attempt to design and fabricate over current protection relay using PIC micro controller. The PIC micro controller will cause the circuit breaker to trip when the current from load current reaches the setting value in the PIC micro controller.
In order to design it, first the load current need to measure in order to monitor it using current sensor including testing the fault (over current) and when such condition arise, it will isolate in the shortest time possible without harming the any other electrical devices. It will also including in developing the algorithm for instantaneous over current relay and IDMT (Inverse Definite Minimum Time) relay for the circuit breaker to trip. In this project, PIC microcontroller will be used to control and operate the tripping coil in circuit breaker.
2.3 Objective
The objectives of this project are;
I. To design and fabricate over current protection relay using PIC micro controller which can operate on the permissible conditions by setting the over current value.
II. To test unwanted conditions (over current) and when such conditions arise to isolate the fault condition in the shortest time possible.
III. To investigate IDMT curve characteristic.
2.4 Scope of Project
The scopes of the project are;
i. To measure and analyze load current from current sensor.
- The load current (energizing current) will be measured by using current sensor and converted from analog voltage to digital using PIC16F877A. Then the load current will display on the LCD.
ii. Trip circuit breaker using PIC microcontroller.
- The over current value is set in the PIC and when faults (over current) occur, PIC will energize the circuit breaker tripping coil which will cause the circuit breaker to trip.
iii. Develop algorithm for instantaneous over current relay and IDMT relay.
- The over current setting may be given by definite time or inverse definite minimum time (IDMT) characteristic. There are four curves for over current complying with the IEC 255 and are named ‘Normal Inverse’, ‘Very Inverse’, ‘Extremely Inverse’ and ‘Long Time Inverse’. This project is to develop the ‘Long time Inverse’ characteristic of IDMT.
3.1 Introduction
This chapter will discuss the study about significant parts of protection system such as the important of protection system, protection devices, types of protection system and protection relay. It also includes the PIC Microcontroller which is ‘the brain’ for this over current protection relay.
3.2 What Is Over Current
The National Electrical Code defines over current as any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault. Current flow in a conductor always generates heat. The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Over current protection devices are used to protect conductors from excessive current flow. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating [4].
3.3 Why Protection System Is Important
Fault impose hazard to both user and the system itself and when it comes to user, life is the concern and when it concern the system it is merely to provide stable electrical power system on top of that prevent damage to the expensive equipment used. In summary, the needs of power protection are [1]: