Fast load reduction systems

Fast load reduction system with communication with DP-System

Advanced power systems are equipped with fast load reduction system with various successes.

The existing DP and PMS systems sense the power system parameters, (frequency, kVA, kW, and kVar) in the medium voltage switchboards. These signals are hardwired to the PMS process station.

The PMS process station performs computations and transmits this data via Ethernet to the DP Controller (DPC). The DP controller calculates the revised thruster speed signals and transmits these via Ethernet to the thruster process stations. Multiple thruster process stations (6 to 8 on drilling rig) convert these signals to analogs. These analog signals are hardwired to the Thruster PLC. The time required for the DP and PMS to calculate a thruster load limit and transmit it to the Thruster PLC is 1 to 2 seconds. (See Figure 1 below.) The time it takes a thruster to reduce power from receipt of this decreased speed signal is 2-3 seconds. Testing has shown the systems response varied from 3 to 6 seconds [21]. Therefore, such system can not prevent blackout in case of engine trip.

Conventional fast load reduction

+ 2 to 3 sec

Conventional fast load reduction

+ 2 to 3 sec

Conventional fast load reduction

Conventional fast load reduction

+ 2 to 3 sec Figure 5.4. Conventional fast load reduction [21]

Fast load reduction system with event-based algorithm

Event-based load reduction algorithm can be fast enough to prevent blackout. If the thruster control is coordinated with fast load reduction, then ramp loading function, which normally must be used with large electric motors, can be overridden. That is very important and will save maybe 2-3 seconds, see figure 5.4.

The function is triggered by generator breaker on the switchboard, which is hardwired to the remote I/O unit located near the switchboard, figure 5.5. So, immediately when first generator trips the remote I/O will transmit the signal to the PMS controller (substation of the total distributed PMS system) via fieldbus and made available for the event based load reduction program. This time is negliable. The fast load reduction function is also initiated by I/O unit located near the generator-set. Both signal transmition lines are shown on figure 5.5. with red lines.

The PMS program initiating load reduction on the thrusters are dedicated to this task and its execution time is set to 50 milliseconds, which is five times faster than the rest of the PMS. Consequently, worst case scenario from signal is received to command is sent to converter is 100 milliseconds. The command to the frequency converter of the thruster is send via fieldbus. The converter will reduce the load to zero in less than 100 ms. With VSI this time can be reduced in less than 50 ms. Hence, the load should be reduced in less than 200 milliseconds, and thus well within the required 0.5 seconds.

After a configurable number of seconds the “reduce load signal” is reset and thrusters are allowed to use the available power. Extensive lab experiments from ABB have verified that the fast load reduction algorithm is fast enough to avoid blackout in case of generator trip, and it has been installed in recent ABB deliveries.

Figure 5.5. Fast load reduction with event-based algorithm [10]

ABB Marine

Fast load reduction with frequency sensitive - fast phase back system (FPBS)

With the frequency sensitive – fast thruster phase back modification, the system frequency is sensed directly by each of the thruster PLC controllers (6 to 8 on drilling rig), see figure 5.6. There is no lag time for computations or transmission of data, as for system on figure 5.4. The fast load reduction is initiated by frequency decreasing signal from the network and not by the switchboard breaker, as for the system represented on figure 5.5.

When the system frequency dips in the 58 to 55 hertz range, the FPBS system is initiated. Testing has shown the power reduction in the thruster motor occurs in 300 to 500 milliseconds after the frequency drops below the 58 hertz threshold [21].

Each thruster PLC is programmed to initiate phase back of the thruster over a range of frequencies of 58 to 55 hertz. The maximum 50% thruster current limit occurs only when the frequency is equal to or less than 55 hertz, see figure 5.7. If the initial frequency drops to 56.5 Hz, the thruster current limit would be 50 % · 50 % = 25 % of the initial current.

Figure 5.6. Fast load reduction with frequency sensitive - Fast Phase Back System [21]

Figure 5.7. Initial phase back current limit

The rate of initial current (power) phase back is set at 200% per second (100% per 0.5 sec) while the frequency is decreasing. After the frequency starts to increase, as detected by the df/dt being positive, the power to the thruster drive is allowed to increase at 5% per second, until it equals 100%, and the spillover turns off, see figure 5.8.

If the frequency starts to drop again, as detected by df/dt becoming negative and does not go below 55 Hz, the power in the drive will be reduced at the rate of 5% per second. Should the frequency drop below 55 hertz, a second major frequency spillover will occur.

The current limit will be recalculated based on an even lower value of the drive current, see figure 5.7.

The thruster drive manufacturer refers to the frequency sensitive – fast thruster phase back as

“frequency spillover”.

Figure 5.8. Diesel engine response to step load changes