DGC Brushless Excitation - Emerson Process Management

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DGC Brushless Excitation System Description 4.4.b High Initial Response Proportional control is linear. To achieve High Initial Response (HIR) the controller needs to produce a large response from a large error. Proportional control cannot provide a High Initial Response for large errors without over-reacting to small errors. To achieve High Initial Response the DGC implements a non-linear gain function. This function supports a linear region around zero, a higher order response beyond that, and a high-value linear region at the high end. 4.4.c Integral Action The Integral Action section of the controller generates a signal proportional to the sum of the errors over time. Integral Action can zero a steady state controller error and when setpoint matching is needed integral action should be applied. The gain of the integral action is tunable. The Integral Action section may be disabled, reverting the controller to proportional only action. The base adjuster follower function of the Base Adjuster (PAR) provides an integrator-like action to the control loop. The base follower function of the PAR is tuned with a long delay time and a slow ramp rate to minimize its interference with the transient response of the voltage controller. Base adjuster following is disabled if controller action moves the regulator into limit conditions. 4.4.d Derivative Action The Derivative Action section calculates a damping factor based on the rate of change of exciter field current acting as a transient gain reduction function. Transient gain reduction is implemented with a tunable washout filter which has field current as its input. A washout filter "washes out" constant signals, but responds to transient signals, producing a response related to the rate of change of its input. The output signal then decays towards zero according to a time constant. The value of the Damping Factor opposes changes in the controller output. The tunable characteristics are time constant and gain. 4.5 Limiters Three excitation limiter functions are incorporated into the DGC Control software. Each limiter is normally enabled but may be disabled at the customer’s discretion. The limiters act by modifying the controller’s error signal. The Maximum Excitation (MXL) and the Volts/Hz (VHL) Limiters subtract from the error signal, causing excitation and terminal voltage to be reduced. The Minimum Excitation Limiter acts by adding to the error signal, causing excitation and terminal voltage to be increased. The limiters (those enabled) are active in both AC and DC control. 4.5.a Maximum Excitation Limiter The purpose of the Maximum Excitation Limiter (MXL) is to protect the generator rotor from damage due to overheating caused by excessive current flow in the field windings. The temperature limit of the rotor winding is fixed by OEM design. The rate at which the rotor heats up is a function of the magnitude of the excess current flow. Since moderately excessive current flow will not immediately cause damage to the rotor windings, the limiting action is time delayed, dependent on severity of the transient. Instantaneous action is taken on a severe transient. The MXL integrator is a software function that emulates the heating characteristics of the rotor. The integrator is started when the Exciter Field Current exceeds the setpoint, typically 105% of rated field current. The output value of the integrator increases with time, proportional to the difference between the measured field current and the setpoint. During this period, the MXL is said to be “timing”, and the output of the integrator is analogous to the heat buildup in the rotor winding. © Emerson Process Management Power & Water Solutions. - 20 - PWS_005075 [3]
DGC Brushless Excitation System Description When the integrator output value reaches the maximum allowable value, the MXL is said to have “timed out”. This is the equivalent of the rotor temperature having reached its thermal limit. The MXL is a proportional plus integral controller whose output is from the main controller error signal. The setpoint to the MXL controller is the calculated maximum allowable field current. The feedback is the measured exciter field current. The output of the controller is a function of the difference between actual measured field current and the setpoint, and the duration of the transient. The MXL controller setpoint is modified during the thermal integration phase. It starts out set to the ceiling current and stays there until reaching a thermal integration setpoint, which is configurable from 100% of the maximum allowable value down to 0%. Upon reaching the corner, the controller setpoint ramps from the instantaneous current setpoint towards the maximum allowable sustained setpoint. As the setpoint drops below the actual value of field current, the controller output will begin to act. As the setpoint continues to ramp lower, the control action will increase gradually, ensuring a smooth transition into the limiting condition. If the field current rises above the ceiling current, the time delay action of the integrator is bypassed and the limiter is activated instantly. The MXL calculation generates two alarms. The MXL TIMING alarm is generated when the integrator starts. The MXL LIMITING is generated when the limiter is actively limiting excitation. Both alarms have hysteresis to prevent relay chatter. 4.5.b Minimum Excitation Limiter The purpose of the Minimum Excitation Limiter (MEL) is twofold. It protects the generator end-iron from overheating when operating extremely under-excited and it prevents loss of synchronization which may result from insufficient excitation. These conditions occur when the generator is operating with excessive "VARs In". Both of these conditions are avoided by preventing the regulator output from going too low. The MEL is a proportional plus integral controller. Its output is added to the main controller's error signal calculation. The output of the MEL controller is clamped at 0 until VARs fall below the minimum allowable value. As VARs fall below the setpoint, the MEL controller output adds to the main controller error signal. The MEL setpoint is expressed in VARs and is calculated from a setpoint curve as a function of MW, described below. The measured variables are VARs and MW from the Machine PTs and CT. The setpoint is a value of minimum allowable VARs for a given megawatt load, which becomes more restrictive as the megawatt load increases. The setpoint is defined in a four-segment "XY" curve, where X is in Megawatts and Y is in VARs. The data points for the curve are selected such that the curve approximates the bottom segment of the generator capability curve. The values for the curve are unit specific and are extracted from the particular unit's capability curve. There are no default values. An additional limit can be generated by enabling the steady state stability limit calculation. This is calculated continuously when enabled. The MEL limit will be the most conservative of the two limit sources. The setpoint is modified by the Damping Factor such that the faster the excitation current decreases; the more the setpoint value is raised to account for the lag of the generator response to a change in excitation. © Emerson Process Management Power & Water Solutions. - 21 - PWS_005075 [3]
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DGC Brushless Excitation - Emerson Process Management

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