DGC Brushless Excitation - Emerson Process Management

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DGC Brushless Excitation System Description The MEL function generates two alarms. The MEL WARNING is generated when the VARs In is close to the allowable limit. The MEL LIMITING is generated when the limiter is actively limiting excitation. Both alarms have hysteresis to prevent relay chatter. 4.5.c Volts/Hz Limiter The purpose of the Volts/Hz Limiter (VHL) is to protect the generator armature cores and the Main Transformer cores from damage due to overheating. Overheating results from iron saturation of the cores that occurs when operating above the rated voltage or below the rated frequency. In general, the main transformer has a higher tolerance for core saturation than the generator. However, in some cases, the transformer ratings may be more limiting than the generator. The V/Hz value is the ratio of voltage to frequency. Since the output voltage of generators, transformers and transmission-lines are monitored with PT’s; the nominal value of voltage is 120 VAC. The nominal operating frequency of the North American power grid is 60 Hz, thus the nominal Volts/Hz ratio is 2.0. Operation above the ratio of 2.0 will result if the voltage goes too high or if the frequency goes too low. The VHL is a proportional plus integral controller. Its output is subtracted from the controller error signal. The output of the VHL controller is clamped at 0 until the ratio of Volts/Hz exceeds the setpoint. As the Volts/Hz ratio exceeds the setpoint, the VHL controller output adjusts the main controller error signal as a function of the amount of excess Volts/Hz. The measured variables are Machine Voltage and Generator Frequency from the Machine PT. The setpoint calculation takes into account the ratings of the main generator and the main transformer. The calculated setpoint is based on which of the two devices is most limiting. The setpoint is modified by the damping factor such that on increasing excitation current, the setpoint value is reduced to account for the lag of the generator response to a change in excitation. The VHL function generates two alarms. The VHL WARNING is generated when the Volts/Hz ratio is close to the limit. The VHL LIMITING is generated when the limiter is actively limiting excitation. Both alarms have hysteresis to prevent relay chatter. 4.6 Protection Software The DGC is provided with three protection features: Over Excitation Protection (OXP), Volts/Hz Protection (VHP) and Minimum Excitation Protection (MEP). Each of the functions is capable of tripping the generator to protect it from damage. All three features are available but may they be individually disabled if not required or desired. 4.6.a Over Excitation Protection The purpose of the Over Excitation Protection (OXP) 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 Protective action is time delayed, dependent on severity of the transient. The OXP function operates similarly to the MXL function. The actions of the OXP are coordinated with the MXL to ensure that the MXL has an opportunity to work. © Emerson Process Management Power & Water Solutions. - 22 - PWS_005075 [3]
The design OXP sequence of actions is as follows: • Field Current exceeds the OXP setpoint: o Start OXP integrator o Disable the Base Adjuster Follower o Initiate a preposition request to the Base Adjuster (optional) • Wait for a while as: o Integrator integrates o Base Adjuster moves to the Full Load value • Integrator times out: DGC Brushless Excitation System Description o If the DGC is in AC control or DC control, OXP forces the DGC into Forced mode (Regulator Trip) and then waits for a timeout period (oxp_timeout1) o Base Adjuster is at the Full Load value of current • Final action: o If taking the DGC out of AC or DC control lowered the current below the OXP setpoint; nothing is done, the unit is safe o If not, OXP initiates Generator Trip contact output. Note that if the DGC was not implementing AC or DC control that OXP will initiate the Generator Trip contact output immediately. If at any time during the sequence the field current drops below the OXP setpoint, the sequence is terminated and the OXP functions are reset. The OXP 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 110% of rated field current. The thermal integrator increases with time, proportional to the difference between the measured field current and the setpoint. During this period, the OXP is said to be “timing” and the output of the integrator is analogous to the heat buildup in the rotor winding. Upon starting to integrate, the OXP software disables the Base Adjuster Follower and initiates a digital output to move the Base Adjuster to a known safe value in preparation for the regulator trip. The recommended known safe value is the equivalent of 100% rated field current. When the integrator output value reaches the maximum allowable value, the OXP is said to have “timed out”. This is the equivalent of the rotor temperature having reached its thermal limit. Upon timeout OXP takes one of two paths: 1. If the regulator was in AC or DC control, it initiates a regulator trip changing the DGC mode to “Forced”, leaving the DGC in manual operation with the Base Adjuster in control of excitation. This is followed by a configurable time delay (oxp_timeout1) (usually in the range of 3 to 5 seconds). The reasoning is that the automatic controller may be contributing to the problem and by forcing manual control the problem situation would then be alleviated. The time delay provides a small time period for the correction to take place. © Emerson Process Management Power & Water Solutions. - 23 - PWS_005075 [3]
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DGC Brushless Excitation - Emerson Process Management

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