PMSing about Testing (Pt4 – Fast Load Reduction Effectiveness)
Background:?Most modern dynamic positioning (DP) vessels cannot perform their industrial mission or maintain position without their main electrical power.?While some vessels escape this requirement with direct diesel drives or dedicated battery systems, most vessels need a power management system (PMS) to limit load and ensure sufficient power.?Split systems don’t have to be as dependent, but usually are - in the interests of fuel efficiency.?Failures of the PMSs can unnecessarily limit power, or fail to make power available to vital loads.?Although widely used and type approved, PMSs are not always well designed or properly integrated with vessel function.?So, thoroughly testing power management systems is fundamental to ensuring redundancy on most vessels.
Part 4:?This is the fourth article on testing power management systems.?The previous articles looked at testing main PMS, DP PMS, and mission PMS fail safe, load limitation logic, individual diesel generator (DG) overload protection (avoid), coordination between the three types of PMSs, load limitation mechanisms, low load DG stop (avoid & split system), power sufficiency logic and mechanisms, and blackout restart when these inevitably fail.?This article looks at testing the effectiveness of fast load reduction.
Review:?The last article looked at the mismatch between PMS efficiency assumptions and what was required to achieve power redundancy.?Efficiency requires running the minimum possible number of generators, but vessel specific or situation specific redundancy often requires more.?With the PMS in control, the power system has minimum generators online and they must survive larger load steps than modern engines are built for.?So, it is up to the fast load reduction function to ensure system survival by reducing load quickly enough that the reduced load step cannot overload the engines.?This article looks at testing the effectiveness of that protection.
Example:?Let’s look at the pictured example system to make this clearer.?If each switchboard has enough load to fully load one generator, then the split (open bus tie) system requires four generators online – two on each side.?If the bus ties are closed, then the PMS only requires three generators, but three generators aren’t redundant, as two can be lost to common faults.?If we temporarily ignore load reduction, each bus requires two generators to limit the load steps to 50% and maximum load to 100%.?Many modern engines are limited to 25% load steps, and that would require four generators on each bus, but the vessel only has three.?Is this a design error??Four would be terrible for efficiency, capital expenses, and maintenance costs, and running at low load is bad for the engines, so the generator engines are typically run at much heavier loads.?In fact, minimum engines are usually used by the PMS, and the modern PMS depends on the fast load reduction function to limit the size of load steps to something that the engines can survive.?The load reduction needs to be faster than the load step from failed equipment or an electrical fault, if the weak, modern engines are to survive.
Solutions over the Years:?Solutions to the efficiency vs. redundancy conflict have varied over the years.?Older vessels directly connected their major loads to the main switchboard power and many lacked fast enough or large enough load trips to sufficiently reduce load, but had robust generators and engines that could survive large overloads and take 100% load steps.?Split switchboard operation was used to increase efficiency and redundancy.?This was only effective if faults were limited to a single switchboard bus, if the equipment on the healthy bus was not dependent on power from the failed bus, and if load transferred from the failed bus was limited to avoid overloading the healthy bus.?One or all three of these assumptions were sometimes wrong, but they are still the base of electrical redundancy for most vessels today.?Some vessels had large enough and fast enough load trips to improve their chances or to risk operating closed bus.?Some vessels’ major loads were driven by DC drives that could be quickly phased back to reduce switchboard load.?After the fault was survived, the vital loads could be restored.?Being able to drop all the major loads and automatically recover them was a big improvement over tripping off limited non-essential load.?This improved system fault survival.?AC (and DC) drives improved over the years and rapidly reducing the power to the major loads’ AC drives is now the default protection.
Fast Load Reduction Requirements:?In order to be effective, the fast load reduction must be reliable, must be triggered in time to prevent the system from being lost, must not be triggered by normal operation, must drop enough load fast enough (MW/s), must work despite other failures, must not endanger the recovery, and must return to safe operation.?Otherwise, split the bus and start enough generators to survive.
Application:?In most modern vessels, independent, reliable, fast load reduction is best achieved with an independent, fast, load reduction located in each drive and based on low frequency and/or voltage.?This is independent of the PMS and network, and should have independent voltage and frequency measurement from the other drives.?For example, a typical scheme would perform load reduction when frequency hits 57Hz, wait until the frequency recovers (sometimes a set 3s and sometimes 1s after 1s of stable nominal frequency), and then gradually ramps up the drives to normal operation (perhaps over 3 to 5s).?The recovery ramp speed is normally conservative to avoid overloading the limited engine load acceptance capability.
Testing Introduction:?Having covered the background and importance of the function, we are ready to look at testing.?I’m going to show a conservative path to testing, as we don’t always know if the power system is stable or the load response acceptable for the given equipment.?I would perform simplified, focused, and more aggressive testing on a system that I have already tested or know the response of (e.g. after advanced generator protection system, or power system robustness and stability testing).?Similarly, the proposed testing assumes AC drives form the vast majority of the system load.?Some systems, with load shed trips or where drive unloading causes wear, will need more abbreviated, focused testing.?Similarly, the testing will need adapted for high, base, real or reactive load.?The first test performs increasingly severe load steps and culminates in a 50% load step.?Any decent system should be able to do this, but stop if there are problems.?The second test applies the 50% load step from switchboard to switchboard and should also be passed.?The third test is more realistic and works up from low load steps to large steps on a heavily loaded system.?Systems that expect to operate closed bus tie should be able to get to the last generator, but many cannot.?Stop testing at the first unintentional equipment trip or blackout, and evaluate the effectiveness shown by the results.?If the third test gets to a single generator, a fourth test could scale this final load step to the switchboard to switchboard level but most PMS/power systems fail before that.?The instructions reflect the example system shown in the picture but they can easily be generalized to 3 split, 4 split, etc.?Losing a third or a quarter of online power is a less severe fault than losing half.?The final tests look at possible failure modes that can affect fast load reduction.??
Testing Warning:?These tests assume oversight, adaption, and application by someone who knows what they are doing. They should have a qualified, experienced engineer with in-depth knowledge of the design, and the theoretical and possible practical limits of the system and equipment.?These are not tests for mindless, checklist execution.?Before starting, make sure the vessel is in a safe place to survive a blackout.?It shouldn’t happen in Test 1 or 2 but will probably happen in later tests.?Assume it will and be prepared to fail and recover safely.?These are not tests to destruction, so time must be given between major tests for equipment to cool off.?This is especially important for tests with multiple large load steps.?The given test descriptions normally show minimum samples, but increased test samples provide greater assurance at the cost or greater time and expense.
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Fast Load Reduction Effectiveness Test 1:?Close the bus tie, start and connect all generators, and increase switchboard load until all generators are loaded at 50%.?(Controlling the load will probably require partial joystick control of sway to provide sufficient thruster load or biasing of thrusters.?The DP PMS function will ramp down its commands when its power limits are exceeded but this is a relatively slow function that will not interfere with the testing of the fast load reduction.)?Pick and trip one DG.?Observe results to make sure all drives and the system responded properly.?Do not restore and adjust the load to stay at 50% on the remaining generators.?Repeat until only one generator remains. (DG tripping will eventually require use of a function that prevents automatic PMS reconnection, such as emergency stop, protection lockout, or lost breaker power, so be prepared.?Keeping the DGs running is preferred, as they can be recovered more quickly in the case of a black out.)
Effectiveness Test 2:?Having proven a general capability for one DG to survive a 50% load step, the ability to survive a group loss will be tested.?Restore all DGs to the switchboards and adjust load until they are at 50%.?All DGs on one switchboard will be simultaneously tripped, so they are locked out, and the reduced load step absorbed by the DGs on the other switchboard.?It is possible for a system that passed the last test to fail this one as the system needs to reduce more power as quickly.?In the example system, shown in the picture, that is three times as much power as a single DG load step, so the system needs more load reduction capacity and rate than in the first test.?Any decent system should pass this test.
Effectiveness Test 3:?If the first two tests have been passed, connect all generators and increase switchboard load as high as possible or until the generators are running near their normal, slower, PMS load limitations (usually 90 to 100% load).?Perform the test like Test 1, except generator load is to be maximized between each step.?The equipment is designed to take worse punishment than this, but don’t get pushy and stop testing as results deteriorate.?If you have to stop then you have found one of the limits of the fast load reduction’s usefulness.
Effectiveness Test 4:?If the system has passed the first three tests and full closed bus capability is required then repeat test 2 but with the generators loaded near 60%.?Restore, and if passed, repeat for 70%.?Repeat for 80%.?Repeat for 90%.?Repeat for 95% or 100%, if applicable.?Stop at the first unintended trips or blackout as the limits of the system have been discovered.?For example, if a brown out at 80% trips two drives, then 70% is the tested limit of the system, and generator load can be reasonably limited to 60% load in closed bus operation, to provide a safety margin for redundant operation.
Base Load Test 1:?We have seen that the ability of the system to absorb a load step is a function of the remaining capacity of the engine and the effectiveness of the fast load reduction function at reducing the load step – the smaller the remaining load absorption capacity of the engines, the smaller the acceptable reduced load step that the engines can absorb.?So far, testing has assumed that all engines are evenly loaded, but that assumption is false when the PMS base loading function is being used to heavily load one generator.?This reduces the remaining load absorption capacity of that generator and the mechanism used by the PMS to make the load imbalance may not be able to be quickly released.?For example, a bias voltage can be quickly set to zero, but could be slowly ramped, and contact speed adjustment up and down may take a while.?This vulnerability needs tested, so assuming Tests 1 & 2 were passed, repeat Test 2, but this time, apply base load to one of the DGs on the receiving side.?Most systems should immediately drop the base load and survive the test.?Systems that fail may be near their fast load reduction limit capacity, but often, the base load function is too slow to react and needs reworked.?This is made clear by the load shown on the formerly base loaded DG.?This is less important for open bus operation, if loss of a bus is acceptable, but is be a problem for closed bus systems, or open bus ones that cannot be allowed to blackout.
Base Load Test 2:?Assuming that the previous test was passed and that the vessel is going to operate closed bus tie, repeat the test at 60%, 70%, etc. up to the limit found in test 4 or the first failure.?Make sure you rest the equipment and allow it to cool between tests.?Our Test 4 example had a brown out at 80%, was OK at 70% and applied a limit of 60% as an operating safety margin.?Can it still achieve 70% with base load active, or will the redundant operating limit need lowered further?
Failure Tests:?So far testing has assumed healthy operation of the distributed fast load reduction, the PMS, and the power system, except for the test fault created.?Real failures often don’t occur in isolation, so the interaction of the fast load reduction with other causal, related, or hidden failures needs tested.
Conclusion:?This article has covered the function and importance of a proper, fast load reduction system, and has probably shown more testing than many readers are used to seeing, but if the redundancy of your vessel depends on the correct and reliable operation of the system, it is important to make sure that it will work.?Many vessels that blacked out were thought to have effective protection, but were not properly tested or maintained.?Apply these lessons to your own vessels.