25-09-2013, 03:58 PM
BHARATH HEAVY ELECTRICALS LIMITED
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OBJECTIVES OF INDUSTRIAL TRAINING
The primary objective of Industrial Training is to gain through practical experience, a sound appreciation and understanding of the theoretical principles learnt as a postgraduate at the University. Industrial Training is oriented towards developing the skills, knowledge and attitudes needed to make an effective start as a member of the engineering profession.
ABOUT BHARATH HEAVY ELECTRICALS LIMITED
Bharat Heavy Electricals Limited is one of the oldest and largest state-owned engineering and manufacturing enterprise in India in the energy-related and infrastructure sector which includes Power, Railways, Transmission and Distribution, Oil and Gas sectors and many more. It is one of the largest power equipment manufacturer in the world. BHEL was established more than 50 years ago, ushering in the indigenous Heavy Electrical Equipment industry in India. The company has been earning profits continuously since 1971-72 and paying dividends since 1976, 77.74% of the total power generated in India is produced by equipment manufactured by BHEL.
It is one of India's nine largest Public Sector Undertakings or PSUs, known as the Navratnas or 'the nine jewels'.
BHEL's operations are organized around three business sectors, namely Power, Industry - including Transmission, Transportation and Renewable Energy - and Overseas Business. This enables BHEL to have a strong customer orientation, to be sensitive to the customer needs and respond quickly to the changes in the market.
BHEL – ELECTRONICS DIVISION
The Electronics Division (EDN) of BHEL was formed in 1976, mainly to establish a strong base in the areas of power and industrial electronics and supplement the company’s pioneering efforts in power generation, transmission, and industry and transportation sectors.
A leader in the power sector market, it has already supplied and commissioned over 200 sets of DCS for thermal, combined cycle and hydro sets all over the country and overseas. BHEL offers a variety of solutions for power plants ranging from simple control systems to complex unified automation for power plants of any size. The synergy of BHEL’s expertise in power plant controls and cutting-edge technology of MAXDNA provides for DCS solution for entire power plant comprising steam generator etc.
BASICS OF SYNCHRONOUS GENERATORS
Synchronous generators are used because they offer precise control of voltage, frequency, VARs and Watts. This control is achieved through the use of voltage regulators and governors.
A synchronous machine consists of a stationary armature winding (stator) with many wires connected in series or parallel to obtain the desired terminal voltage. The armature winding is placed into a slotted laminated steel core. A synchronous machine also consists of a revolving DC field – the rotor.
A mutual flux developed across the air gap between the rotor and stator causes the interaction necessary to produce an EMF. As the magnetic flux developed by the DC field poles crosses the air gap of the stator windings, a sinusoidal voltage is developed at the generator output terminals. This process is called electromagnetic induction. The magnitude of the AC voltage generated is controlled by the amount of DC exciting current supplied to the field. If “FIXED” excitation were applied, the voltage magnitude would be controlled by the speed of the rotor , however, this would necessitate a changing frequency.
PRINCIPLES OF AUTOMATIC VOLTAGE CONTROL
Voltage transformers provide signals proportional to line voltage to the Automatic voltage regulator where it is compared to a stable reference voltage. The difference (error) signal is used to control the output of the exciter field.
For example, if load on the generator increases, the reduction in output voltage produces an error signal which increases the exciter field current resulting in a corresponding increase in rotor current and thus generator output voltage.
Due to the high inductance of the generator field windings, it is difficult to make rapid changes in field current.
This introduces a considerable “lag” in the control system which makes it necessary to include a stabilizing control to prevent instability and optimize the generator voltage response to load changes.
Without stabilizing control, the regulator would keep increasing and reducing excitation and the line voltage would continually fluctuate above and below the required value.
Modern voltage regulators are designed to maintain the generator line voltage within better than +/-1%of the nominal voltage for wide variations of machine load.
EXCITATION SYSTEM
INTRODUCTION
Excitation systems have a powerful impact on generator dynamic performance and availability; it ensures quality of generator voltage and reactive power, i.e. quality of delivered energy to consumers.
Main functions of excitation system are to provide variable DC current with short time overload capability, controlling terminal voltage with suitable accuracy, ensure stable operation with network and/ or other machines, contribution to transient stability subsequent to a fault, communicate with the power plant control system and to keep machine within permissible operating range
EXCITATION TRANSFORMER
The excitation transformer, a step down transformer derives its excitation power from the generator output. The transformer insulation is of class H and it steps down the voltage from generator system voltage (11kv, 13.5kv, 21 kv) to the desired voltage. The secondary of this transformer is fed to the thyristor convertor bridge input.
A current transformer is used for the over current of excitation transformer. It is further connected to an over current relay whish has facilities for both instantaneous and delayed tripping. Screen provided between HV and LV windings protects excitation transformer from voltage surges.
An over temperature protection for Excitation transformer is made possible through RTD’s embedded in the secondary of Excitation transformer.
CONCLUSION
Static excitation equipment is the device used to maintain the terminal voltage to a required value. Any variation in the load results in change in the terminal voltage. Variation in terminal voltage is measured by the components of Static excitation equipment and it adjusts the field current to such that the terminal voltage remains constant.
Different types of protection schemes are used for the protection of components of Static excitation equipment. It includes excitation transformer protection, thyristor bridge protection and field breaker protection.
Prior to the installation of the Static excitation equipment various tests are conducted to check the performance and durability of equipment.
Replacing the excitation system in older plants with Static excitation system increases/ improves the life span of power plant.