The twenty disturbances

IDV(1..20) is a bare integer array in the original, and the header at teprob.f:172-191 names each entry in prose. Nothing there connects a name to the line that implements it, and five of the twenty are called only "Unknown". This page makes the connection: every fault names the teprob.f line it acts on, where it is injected, and what shape it has.

There are twenty, not the twenty-one of the later literature. teprob.f:340 is DO 500 I=1,20. The extra one comes from later versions of the model, and this port follows the vendored source.

Three shapes, and the third is not a plant disturbance at all

Step faults change a feed condition the moment they are switched on and hold it. Seven of the twenty, at teprob.f:407-414, 567 and 568.

Random faults enable a walk channel through IDVWLK (teprob.f:347-358). Ten of the twenty, of which the last three drive spike trains rather than walks.

Sticking faults do not touch the model. They set IVST (teprob.f:793-798), which widens the dead band a valve command must cross before the valve follows it. Three of the twenty.

That third kind matters more than its size. A sticking fault is not a disturbance to the plant; it is a disturbance to the controller's authority over the plant. In an open-loop run, where the command never moves, it does nothing whatever, and a scenario engine that treated it as a plant fault would report an injected disturbance with no effect and look broken. Tier 4 confirmed this from a direction that could not have been arranged: over a four-hour run, IDV(14), IDV(15) and IDV(19) report worst errors identical to the nominal case to every digit, because with the command held still their trajectory is the nominal trajectory.

The table

The channel column is not decoration. A test asserts that this table agrees with the code that maps IDV flags to channel flags, that every one of the twelve channels is driven by exactly one fault, that the spiking flag is set exactly for channels 10 and above, and that exactly IDV(14), IDV(15) and IDV(19) fail to reach the plant. Two statements of one fact, which is the point.

The five "Unknown" faults are not unknown

The header calls IDV(16) through IDV(20) unknown, and every paper on TEP repeats it. The source is perfectly explicit about what they do; only their physical interpretation was withheld. They enter the model at these points:

So IDV(19) is a sticking fault and the other four are not, which the shared label hides. Three of the four are the spike channels, which is why they are reported in the literature as the hardest to detect: they are intermittent rather than sustained.

Nine walks and three spike trains

The twelve channels are not the same kind of object, which the shared array names hide (teprob.f:340-406).

Channels 1 to 9 are random walks. When one runs out, teprob.f:359-371 evaluates the old segment at its endpoint, takes the value and the slope there, and builds a segment that continues smoothly from them. TESUB5 (teprob.f:1506-1537) chooses the next knot value and slope from the uniform generator and fits a Hermite cubic; TESUB8 (teprob.f:1300-1359) evaluates the cubic at the current time.

Channels 10 to 12 are spike trains, and teprob.f:372-396 gives them their own rule. They alternate between two states. Dwelling: the channel sits at zero for a randomly drawn interval, its segment being a parabola rising from zero, and the dwell ends when the value reaches 0.1. Spiking: once the value exceeds 0.1, the channel is given a cubic through the current value and slope that lasts exactly 0.1 hours, with coefficients that drive it hard and then back down. So a spike channel is off, off, off, then briefly on.

The flag scales the dwell, not the schedule. CDIST(I) = IDVWLK(I) / h^2 at teprob.f:391 is the only place the flag enters a spike channel. With the disturbance off it is zero, the parabola is flat at zero, the channel never reaches 0.1, and it dwells forever, drawing a fresh interval each time. With it on, the parabola climbs and the channel eventually spikes. Either way it keeps drawing at the same rate.

That last property is load-bearing and is easy to optimise away. IDVWLK multiplies the two endpoint draws at teprob.f:1529-1530 but not the duration draw at teprob.f:1528, so an inactive walk channel still consumes all three draws. Skipping the two endpoint draws when the flag is zero produces identical segment values, because an inactive channel lands on SZERO with zero slope either way, and leaves the generator two steps behind. Every subsequent draw in the run then differs: the noise, the other channels, everything. That mutation was implemented deliberately to check the tests have teeth, and it was caught on the stream position rather than on any value. It is the entire argument for Tier 3 demonstrated at Tier 1.

Using them

$ tep faults                       # the table above, from the source
$ tep run --fault 4 --hours 24 --labels

or, from Rust, Scenario::fault(4) and Scenario::baseline().with_fault(4). Faults can be combined; the flags are independent.

One caveat, though it is no longer the default. The original driver switches IDV(12) on at eight hours whatever the scenario asked for (temain_mod.f:366-368). That is delta D-011, and it is off here: Tier 7 established that the published files were generated with that line replaced rather than kept, so a request for IDV(4) gets IDV(4) and nothing else. Scenario::faithful() and the driver_forces_idv12 field turn it back on, and the ground-truth labels record it either way, so a run carrying it is visibly fault-free for eight hours and then not.