Iacdrive|Automation Solution

Special Protection System Advantages and Disadvantages

Quite a few yrs. ago around 1988, I was a Protection and Control Engineer at a large utility in the SE. We were doing our planning to bring the final unit of a large 4 unit plant on line, when it was discovered that we could encounter some unusual instability scenarios. The funny thing was that with all units on-line and above a certain MW output, all that would need to occur would be opening of a remote 500kV breaker on one of the particular lines and the event could trigger, eventually bringing ALL 4 units at the plant out-of-step and tripping off all of the generation in just a few minutes (3600 MW).

The studies were performed numerous times by internal and external experts but the results were always the same. The key problem seemed to be the existing network configuration of 4 units and only 3 transmission lines. Adding a 4th 500kV line from the plant seemed to cure the problem under all conditions, including close in 3 phase faults with breaker failure. Unfortunately, the cost and timeline to build a new t-line was a real challenge!

In order to proceed with commissioning the 4th unit and remediating any scenarios for tripping all generation, a Special Protection System (SPS) was developed. A transfer trip channel was installed at the remote substation, keying on the breaker contact opening. At the plant, a Unit Trip scheme was installed that had a MW meter supervising tripping of any one unit selected by the plant operator (U1-U2-U3-U4). If all units were on line and generation was above 2500 MW (margin of safety added), then a receipt of remote breaker opening would trip the selected unit to avoid having all units cascade into out-of-step condition.

Advantages: Clearly, this Special Protection System saved the day, and bought time until an additional line was added 4 years later.

Disadvantages: The downside was the challenge of installing and testing such a complicated scheme with the potential for mis-operation. I don’t recall any mis-operations occurring, but it was still a bit “dicey”. I have been at that same plant during a full load unit trip (Generator differential) and it was an “exciting” experience to say the least! While I did recommend that we conduct a “live test” to see what would really happen and perhaps test our system BLACK START procedures, this suggestion was not well received by management (LOL).

This was my only encounter with such a special protection system scheme in my 35 years of utility work, but it was very interesting to be involved with this project.

TIA portal, a nightmare!

I have been using TIA since it was launched and it has come on leaps and bounds since it was first launched. Its a great tool and as already mentioned it has its bad aspects but it also has its good aspects. The biggest improvement (in my opinion) is the drag and drop functionality in the WinCC part and the code editor. Just need a field PG to be launched with screens to fold out so you can have multiple screen!!

They are moving in the right direction and it was always going to be resource hungry WinCC was bad enough for that in previous versions.

New improvements make a long list but one of the most recent is being able to switch a DB to and from optimised. How many times in previous versions did I forget to check the box then have to delete the DB and create it again. PID loop tuning function within TIA is useful and if you look on the Siemens Automation website (UK/Europe) the example files are growing all the time and they have some great examples that can be integrated easily in to application, I have used the ASi maintenance and monitoring example which was very well put together along with a few others. Even if you don’t use them but need some pointers on which way to go they are a good starting point.

I could list the gripes I have, but all in all its coming together nicely, just need a decent well priced Field PG to run it on £5K is a bit steep for an M4 which maybe no great improvement on the M3 which, in my opinion, wasn’t very good.

Few months ago I had a project with TIA Portal v11. Hardware targets: Simatic S7-300 and Simatic Comfort Panels.
Compared to RSLogix 5000 / FactoryTalk View for example, TIA Portal is a nightmare, especially on commissioning and start-up, when the pressure is huge and you have to work FAST.

The main problems:
1. Very slow on every operation (compiling, downloading, on-line editing, project printing/documenting).
2. Requires a very high resolution display (it is almost unusable on a 1366×768 laptop)

3. Weird behavior (HMI display alterations, crashes).

Synchronous generator operating frequency

When synchronous generators (alternators) are connected in parallel with each other on an AC grid, they are all operating at a speed that is directly proportional to the frequency of the AC grid. No generator can go faster or slower than the speed which is proportional to the frequency.

That is, when a synchronous generator and its prime mover is operated in parallel with other synchronous generators and their prime movers, the speed of all of the generator rotors (and hence their prime movers if directly coupled to the generator rotors) is fixed by the frequency of the grid. If the grid frequency goes up, the speed of all the generator rotors goes up at the same time. Conversely, if the grid frequency goes down, the speed of all the generator rotors goes down at the same time. It is the job of the grid/system operators to control the amount of generation so that it exactly matches the load on the system so that the frequency remains relatively constant.

Isolated or is landed generators that are not in parallel with other generators have an added limitation in that keeping exactly 50Hz is somewhat difficult, or puts too much demand on controlling/governing systems. In such environments it is normal to accept some small deviation from the nominal frequency.

The vast majority of power for industry is supplied by large rotating AC generators turning in synch with the frequency of the grid. The frequency of all these generators will be identical and is tied directly to the RPM of the generators themselves. If there is sufficient power in the generators then the frequency can be maintained at the desired rate (i.e. 50Hz or 60Hz depending on the locale).

An increase in the power load is accompanied by a concurrent increase in the power supplied to the generators, generally by the governors automatically opening a steam or gas inlet valve to supply more power to the turbine. However, if there is not sufficient power, even for a brief period of time, then generator RPM and the frequency drops.

By operating transformers at higher frequencies, they can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with ‘volts per hertz’ over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

Stiff voltage sources

Stiff voltage sources are not problematic as long as they don’t get in the way of the solver’s attempts to linearize the behavior of the circuit matrix via step size reduction. It is the highly nonlinear stiff sources that are heavily fed back into the rest of the circuitry that can cause the solver to hang. Linear sources that are ground referenced or nonlinear ones that don’t feed back anywhere are not likely to cause problems.

In the initial versions of SPICE there were a few elements that could not be simulated directly with nodal analysis in the circuit’s admittance matrix, ideal inductors and voltage sources being the most common among them. However, starting with some version of SPICE 2 this deficiency was removed when modified nodal analysis (MNA) was added to the simulation engine (requiring an additional computational enhancement sometimes called the auxiliary matrix, I believe).

Modified nodal analysis is an extension of nodal analysis which not only determines the circuit’s node voltages (as in classical nodal analysis), but also some branch currents. This permits the simulation engine to crunch ideal inductors and voltages sources (true Thevenin circuit elements) but at a cost of incrementally increasing the matrix size and difficultly about twice as much as for when “easy” Norton type elements (e.g., resistors, capacitors and current sources) are added.

In other words, adding one ideal inductor slows down the simulation about as much as adding two ideal capacitors. However, there is a small additional silver lining to this, as it also comes with the possible advantage of “free” (whether you use it or not) automatic sensing of instantaneous inductor current.

LTspice (my simulator of choice) treats inductors in a special way in that they are normally given a default series resistance of 1 m-ohm unless a value of zero is explicitly entered for that parameter. Having a non-zero series resistance allows LTspice to “Nortonize” the inductor such that it can be processed as a normal branch within the circuit matrix, thereby allowing the simulation to run marginally faster. This also makes the inductor “look” like any other of the “easy” elements so that it is not a numerical problem to parallel it with a stiff voltage source. If a series resistance parameter is entered for a voltage source, it also becomes Nortonized by LTspice.

Nortonizing an inductor or voltage source comes at the cost of giving up free sensing of the instantaneous branch current, which is not a cost at all if this current is not being used elsewhere. However, as soon as you call out the inductor current in *any way* in any b-source behavioral expression, LTspice changes the default series resistance for that inductor back to zero ohms and reverts back to the standard MNA way of processing it within the circuit matrix so that it can get access to the inductor’s instantaneous current.

Only true Thevenin type elements have the possibility of being used as the instantaneous current sense for a current controlled switch (or other similar current controlled devices). The SPICE standard is to only allow voltage sources for this purpose, but apparently LTspice accepts zero ohm inductors as well.

One last note, LTspice is indeed able to measure the current in any element, including Norton type devices, but for these devices the current measured will necessarily be a time delayed version that may not be suitable for tight feedback loops (there is a warning about this in the LTspice Help file section on b-sources).

Variable Frequency Drive Basics (Working Principle)

Variable Frequency Drive (VFD) Basic Configuration
The basic configuration of a variable frequency drive is as follows.
VFD Basic Configuration
Fig. 1 Basic configuration of variable frequency drive

Each part of a variable frequency drive has the following function.

Converter: Circuit to change the commercial AC power supply to the DC
Smoothing circuit: Circuit to smooth the pulsation included in the DC
Inverter: Circuit to change the DC to the AC with variable frequency
Control circuit: Circuit to mainly control the inverter part

Principle of Converter Operation
The converter part consists of the following parts as following figure shows:

Converter
Inrush current control circuit
Smoothing circuit

Fig. 2 Converter part

Method to create DC from AC (commercial) power supply
A converter is a device to create the DC from the AC power supply. See the basic principle with the single-phase AC as the simplest example. Fig. 3 shows the example of the method to convert the AC to the DC by utilizing a resistor for the load in place of a smoothing capacitor.

Fig. 3 Rectifying circuit

Diodes are used for the elements. These diodes let the current flow or not flow depending on the direction to which the voltage is applied as Fig. 4 shows.
Diode
Fig. 4 Diode

This diode nature allows the following: When the AC voltage is applied between A and B of the circuit shown in Fig. 3, the voltage is always applied to the load in the same direction shown in Table 1.

Table 1 Voltage applied to the load

That is to say, the AC is converted to the DC. (To convert the AC to the DC is generally called rectification.)

Fig. 5 (Continuous waveforms of the ones in Table 1)

For the three-phase AC input, combining six diodes to rectify all the waves of the AC power supply allows the output voltage as shown in Fig. 6.

Fig. 6 Converter part waveform

Input current waveform when capacitor is used as load
The principle of rectification is explained with a resistor. However, a smoothing capacity or is actually used for the load. If a smoothing capacitor is used, the input current waveforms become not sine waveforms but distorted waveforms shown in Fig. 7 since the AC voltage flows only when it surpasses the DC voltage.

Fig. 7 Principle of converter

Inrush current control circuit
The basic principle of rectification is explained with a resistor. However, a smoothing capacitor is actually used for the load. A capacitor has a nature to store electricity. At the moment when the voltage is applie

Different brushes at same ring

Recently I had to do a report explain why is impossible join brushes, at same time, from different companies, even with same characteristics.
I used the follow points:
1 – Even with same characteristics the final results is different because tue proportion of material and/or manufacturing process different lead to a different brushes;
2 – Guarantee, because our machine is new, and is a good practice use brushes recommended by Manufacturer;
3 – The film, that is formed on the rings by the brushes could change (but I don’t have any sure if chage for bad);

Unfortunately my report was based on experience for old engineer and recommendation of Manufacturer.

One
of the most important thing about brushes in high current density
environments is uniformity. If there are any variations in material
composition, manufacturing methods, dimensions, porosity, density,
surface hardness, friction coefficient, pig-tail attaching means, size
of pig-tail conductor, etc., there will be a variation in the current
division and/or wear.

Ultimately some brushes will carry more current than others and the increased current density in those brushes will lead to overheating, pitting, scoring, and ultimately costly repairs to the commutator/slip-rings. You might also accidentally mix brush grades when dealing with multiple vendors.

Although manufacturers publish data for brush materials which may prove to be very close to one another, mixing them on a collector surface is not a good practice. Any signs of undesirable performance would be difficult to identify the root cause for and small differences in electrical resistance can produce staggeringly varied performance from each brush.

While the materials used have good material data supplied with them, the manufacturing of the cable connection does not which can account for many times the resistivity differences of the material. Brush manufacturers do use a variety of materials here also and so some brushes, even of the same grade and from the same supplier but with different connection material, cannot be used together.

Mixing of grades is an uncontrolled practice which leads to variable surface conditions especially where the numbers of each grade used is not controlled.

Lower resistance brushes will “grab” the current possibly over filming the collector surface leaving the higher resistance brushes to run at lower than prescribed minimum current densities which results in higher coefficients of friction at the brush/collector interface. You would never know when your film is stable which endangers machine life.

Most machine manufacturers select a grade of carbon to use which is useful at the machines fully rated capacity. However, manufacturing tolerances, specifications etc can produce a machine vastly over rated for your application. Running the manufacturers supplied brushes at reduced load can be very damaging. Most Manufacturers will accept that you need another brush grade for your specific use and will maintain warranty provided they have been consulted regarding any changes.

Many overlook that by moving a machine from one position in their plant to another, that they well need to consider the brush grade at that time also. Sometimes a simple and cost effective reduction of brushes (of the same grade) within the machine can increase plant reliability and longevity dramatically. Other times a consultation with a brush expert can lead to an alternative grade to produce better performance.

Can I operate a 50Hz transformer at 60Hz power supply?

Well first let get one thing straight for transformers: the higher the line frequency, the lower the core (iron) losses! The core power loss are proportional to kf*B^2 approximately for any machine, dynamic or static. But transformers are self-excited static machines, meaning the flux density B is reverse proportional to the line frequency, therefore Pcoreloss = kB^2*f=k*(1/f)^2*f=k/f… so the higher f, the lower the losses. However, increasing the frequency also increases the magnetizing inductance – lowering the magnetizing current. For if you increase the frequency you may want to increase the voltage. But of course this is not usually practical, as line voltage of 60Hz systems is usually lower than those of 50Hz systems. So operating a 50Hz motor at 60Hz should be safe, but may result in higher voltage drop because of lower magnetizing current and because of higher leakage inductance (the series inductance).

It is true that the higher the frequency, the higher the hysteresis (and eddy current) losses will be. But is it a common misconception to assume higher power losses when frequency increases in a transformer. Simply because the hysteresis losses depends not only on frequency, but on the max magnetic flux density as well (Bmax^2). The flux density is reversely proportional to the line frequency, which eventually causes lower core losses as you raise the frequency. This holds true for low and mid frequency ranges. For higher frequencies, skin effect and eddy currents dominates, so the picture may be different. However, iron core transformers do not operate in such high frequencies. We use ferrite core instead. In a practical transformer model, the core losses are represented by a parallel resistor (Rc). The resistor’s value is linearly dependent of the line frequency (Rc=k*f), and the core losses are given by Pc=U^2/Rc… Of course this model is limited to mid-low frequencies…

Electrical drives for off-highway vehicles

I’ve seen some attempt of electrical driven prototypes in the field, but is still not an enough big sector that let you find specific literature. Excluding the large dumpers for mining, probably the only machine that is built in series is D7E from CAT.

One of largest engineering challenge that you will face on a similar application, is the cooling to the power electronic. You can consider that you will have to dissipate 3-5% of the power that your driver is processing and the max temperature of IGBT’s is not so far from the max temperature in that your vehicle can operate. A small temperature delta, mean a large heat exchanger and/or pretty high speed of air through it. (That with all the problems related to that). A possible solution is liquid cool the IGBT’s mounting them on the aluminum plate. You can’t use the engine cooling fluid because it too warm, but you may can use hydraulic oil (that should never get warmer of 55C).

If you are thinking to expand some gas from the AC, please take in account the possible condensation issues (your voltage on the DC bus can arrive around 800V when the vehicle is breaking, you do not want condensation around). Using SR motors is opening another challenge. For take max advantage of the technology, you want the motor spinning pretty fast (motor get smaller for same size of rotor and with that design, no problems retaining magnets). That means use high ratio gears. In off road vehicle are often used planetary gears because they are compact and cheap. As soon you rise the input speed, the efficiency of those kind of gears drop because you incur in hydrodynamic loss (for a series of problems that are connected to the level of oil that you need to keep in the gear housing). Probably if you are using an SR motor, you want consider to use an angular stage like first reduction after the motor.

I’m not too sure if I would use a battery like energy storage. Batteries take time for convert from electrical to chemical. Most of the braking will happen in a short time so you will end up burning most of the regenerated energy trough a braking resistor (the DC bus can’t go up to infinite about voltage). If you are driving a dozer that has a very low efficiency (most of the vehicle kinetic energy will be burnt in the tracks etc. and very little will arrive to the SR motor to be regenerate), probably the regeneration is not too important, on other vehicle is maybe more important so look to capacitors or flywheels for storage is probably more appropriate.