DGC Brushless Excitation - Emerson Process Management

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The contingency conditions are: • Any Alarm that enforces the NO AUTO restriction i.e., Bad PT’s • Customer input requests a Regulator Trip • OXP requests a Regulator Trip • VHP requests a Regulator Trip DGC Brushless Excitation System Description In the FORCED Mode, the OFF lamp blinks at approximately 2 Hz until the fault condition is cleared and the Operator returns the control switch to TEST position. Figure 3: Simplified Voltage Regulator Control Loop 4.2 Basic Operation of the Voltage Regulator The generator output voltage is directly proportional to the magnitude of the excitation current. Excitation current to the generator field is supplied by the exciter output, which in turn receives its stimulus from the power amplifiers. The power amplifiers and exciter are typically capable of producing more than 130% of the rated excitation current. The excess capacity is provided to support rapid transient response. The magnitude of the excitation current is directly proportional to the excitation demand. Excitation demand is the sum of two signals: the Base and the Regulator Output. The Base signal is controlled by the operator using the Base Adjuster control switch. It is capable of providing an excitation demand from 0% to 100%. The Base signal is also adjusted automatically when it receives the follower command. © Emerson Process Management Power & Water Solutions. - 16 - PWS_005075 [3]
DGC Brushless Excitation System Description When the DGC is in OFF or TEST modes, the regulator output signal is held at 0%, thus the excitation demand is entirely controlled by the Base signal. The Regulator Output signal is produced by the DGC. The DGC is usually configured as a proportional only controller, receiving a positive setpoint from the Voltage Adjuster (+) and a negative feedback signal from the generator. The two signals are added together to produce an “error signal” which can be positive or negative. Integral control can be utilized if desired, usually seen when a numeric setpoint is provided. In lieu of DGC integral control, the Base following action acts as an integrator. If the generator voltage is too low, then the error signal is positive, calling for more excitation. If the generator voltage is too high, then the error signal is negative, calling for less excitation. If the generator voltage is equal to the setpoint, then the regulator output is zero. The Regulator Output is capable of providing an excitation demand from –100% to +100%. When the DGC operational mode implements the AC or DC control type, the Regulator Output is permitted to pass. If the regulator output signal is zero, then the excitation demand is equal to the Base signal. If the regulator output goes positive, then the excitation demand increases above the base. If the regulator output goes negative, then the excitation demand decreases to a value below the base. If the operator wishes to raise the output voltage of the generator, then he raises the voltage adjuster. The setpoint signal is greater than the feedback signal so the error signal is positive. The regulator output signal goes positive, adding to the base signal, causing the excitation demand to be increased. Excitation current increases and generator voltage goes up. If the operator wishes to lower the output voltage of the generator, then he lowers the voltage adjuster. The setpoint signal becomes less than the feedback signal so the error is negative. The regulator output signal goes negative, subtracting from the base signal, causing the excitation demand to be reduced. Excitation current decreases and generator voltage goes down. Due to the inherent voltage drop of the generator, as the generator is loaded, the output voltage will drop correspondingly. If the generator voltage drops below the setpoint, then the error signal is positive. If the generator is unloading the output voltage will rise correspondingly. If the generator voltage rises above the setpoint, then the error signal is negative. While in automatic, the Base Adjuster is not able to change terminal voltage because the regulator will fight against it. For example, if the operator raises the Base Adjuster, the excitation demand will increase. Excitation current increases and generator voltage goes up. The feedback goes up which makes the regulator output signal move in the negative direction. The reduction in regulator output causes the excitation demand to return to its initial starting point. Excitation current will drop back to its starting point and the generator output voltage will return to its starting point. 4.3 Automatic AC Voltage Control Software The term AC control is applied when the controlled variable is measured from the generator terminals and is thus an AC quantity. This is most often the terminal voltage, but reactive power and power factor angle can also be the measured variable. The following paragraphs provide greater detail for each of the features of the software. © Emerson Process Management Power & Water Solutions. - 17 - PWS_005075 [3]
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DGC Brushless Excitation - Emerson Process Management

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