14-12-2012, 06:16 PM
Automatic Voltage Regulator System
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Introduction
We are an engineering group in Business Area Power Systems – Substation Autmation of ABB Ltd, India a group company of the ABB Group of companies operates in around 100 countries. For Excitation Systems and Synchronizing devices, we are Local Engineering Centre (LEC) in India of our Product Responsible Unit (PRU), ABB Switzerland Ltd, Switzerland, who is the market leader in this business of excitation systems and voltage regulators.
The ABB group has more than 30 years of experience in the design & manufacturing of electronic voltage regulators and static excitation equipment and numerous customers all over the world are using our equipment and services to their full satisfaction.
We would like to emphasise the inherent characteristic of our products and services, such as,
proven quality; based on our experience,
high availability; the result of strict application of our quality assurance policy,
short commissioning time; due to extensive tests in our works and
little maintenance during operation; thanks to our reliable design and utilisation of solid state components.
The UNITROL®F system is a continuously acting, microprocessor based voltage regulator, utilising the most modern technology of printed circuit boards and the state of the art solid state components allowing easy maintenance. The basic module is a multipurpose power electronic controller, which is used in many applications and contains the voltage regulator function together with limiters and other control circuits. The voltage regulator together with the limiters controls the output voltage of the power converter. The resulting DC-voltage of the power converter is supplied through the field breaker to the field of the Exciter of the main alternator.
In line with our discussions, we herewith offer Dual Channel Voltage Regulator having Dual Auto and manual field current (i.e.) (AUTO -1 including its MANNUAL Mode / AUTO -2 including its MANNUAL Mode) controls AVR System. Thus AVR system provides 100% power & control redundancy.
AUTO Control
The basic functions, i.e. the regulation of the machine terminal voltage and the contribution to a stable reactive power flow in a network are covered by the AVR control. The actual value (measured terminal voltage) is compared with the set-point value. The difference between the two signals is amplified through PID-filter and controls the transistor or thyristor gate triggering. The behaviour of this control signal further depends on following functions.
• Adjustable Reactive current influence to ensure stable parallel operation with a network or/and other generators. For that reason, a signal I*sin(ø) which is formed in the signal processing unit UNS 1860 and digitalized at the control board CON-1 also applied to the AVR summing point. If a unit transformer is provided between machine and network, the polarity signal can be inverted to compensate a part of the unit transformer's voltage drop.
• Adjustable Active current influence to allow compensation of voltage drop over resistive part of lines or transformers connected to the stator of the machine.
• Soft-start facility for a smooth, gradual voltage increase and to prevent terminal voltage overshoots during excitation build-up.
• Adjustable PID controller to optimise the dynamic behaviour of the generator.
• Lower signal and Higher signal priority gates for limitation inputs for under- and over-excitation limiters
• Over-excitation limiters to avoid unnecessary disconnection of the machine in case of over-excitation. The following over-excitation limiters are included :
* Excitation current limiter with thermal and ceiling limitation
* V/Hz limiter to avoid over-fluxing
* Stator current limiter overexcited
MAN Control
There is a MAN control provided to be operated in case of an AUTO-channel failure (Actual value sensing failure) or for servicing purpose. In the MAN mode, the field current of the generator is controlled by means of a field current regulator. In this part of the program, the actually measured current is compared with the set point of excitation current. The difference between these two signals is applied to a PI-controller.
The changeover from the AUTO channel to the MAN control takes place in case of external command or in case of the actual value PT fault is detected. In this case the active control signal of the AVR sub-routine will be replaced by the resulting signal of the field current regulator sub-routine.
In order to ensure a smooth changeover from the AUTO to the MAN and vice-versa, an automatic follow-up control is also configured in the standard software. The non-active channel is always following the active one. This results in smooth changeover from auto to manual modes.