Iacdrive|Automation Solution

Transformer uprating

I once uprated a set of 3x 500KVA 11/.433kv ONAN transformers to 800KVA simply by fitting bigger radiators. This was with the manufacturers blessing. (not hermetically sealed – there were significant logistical difficulties in changing the transformers, so this was an easy option). Limiting factor was not the cooling but the magnetic saturation of the core at the higher rating. All the comments about uprating the associated equipment are relevant, particularly on the LV side. Increase in HV amps is minimal. Pragmatically, if you can keep the top oil temperature down you will survive for at least a few years. Best practice of course is to change the transformer!

It is true that you can overload your transformer say 125 %, 150 % or even greater on a certain length of time but every instance of that overloading condition reflects a degradation on the life of your transformer winding insulation. Overload your transformer and you also shorten the life of your winding insulation. The oil temperature indicated on the temperature gauge of the transformer is much lower than the hotspot temperature of the transformer winding which is a critical issue when considering the life of the winding insulation. Transformers having rating of 300 KVA most probably do not even have temperature indicating gauge. The main concern is how effectively can you lower the hotspot temperature in order that it does not significantly take away some of the useful life of your transformer winding insulation.

AM & FM radio

For AM & FM radio & some data communications adding the QP filter make sense.
Now that broadband, wifi, data communications of all sizes & flavours exist – any peak noise is very likely to cause interuptions & loss of integrity of data – all systems are being ‘cost reduced’ ensuring that they will be more susceptible to noise.
I can understand the reasons for the tightening of the regulations.
BUT, it links in to the other big topic of the moment – the non-linearity of managers.
William is obviously his own manager – I bet if his customer was to ask him to spend an indefinite amount of time fixing all the root causes to meet the spec perfectly without any additional cost it would be a different matter.

Unfortunately for most of us the realities of supervisors wanting projects closed & engineering costs minimized we have to be careful in the choice of phrasing.
Any suggestion that one prototype is ‘passing’ suddenly can be translated to job finished, & even in our case where the lab manager mostly understands, his boss rarely does & the accountant above him – not at all.

It gets worse than that – at the beginning of a project (RFQ) – the question is “how long will EMC take to fix?” with the expectation if a deterministic answer; the usual response of a snort of derision & how long is a piece of string generally translates to 2 weeks & once set in stone becomes a millstone (sorry mile-stone).

We already have a number of designs that while not intentionally using dithering, do use boundary mode PFC circuits which automatically force the switch frequency to vary over the mains cycle. These may become problematic at some future variation of the wording of the EMC specs.

While I have a great deal of sympathy for the design it right first time approach, the bottom line for any company is – it meets the requirement (today) – sell it!!

Simulation interpretation in automation industry

Related to “automation industry”, there are generally 3 different interpretations of what simulations is:
1) Mechanical Simulations – Via various solid modeling tools and cad programs; tooling, moving mechanisms, end-effectors… are designed with 3D visualizations, connecting the modules to prevent interference, check mass before actual machining…
2) Electronics Simulations – This type of simulations are either related to the manufacturers of “specific instrumentations” used in automation industry (ultrasonic welders, laser marking systems,…) or the designers of circuit boards.
3) Electrical & Controls Simulations.
A) Electrical Schematics, from main AC disconnect switch, down to 24VDC low amps for I/O interface.
Simulation tools allow easy determinations of system’s required amperage, fuse sizes, wire gauges, accordance with standards (CE, UL, cUL, TUV…)…
B) Logic Simulations, HMI interface, I/O exchange, motion controls…
a) If you want to have any kind of meaningful simulations, get in the habit of “modular ladder logic” circuit design. This means, don’t design your ladder like one continuous huge program that runs the whole thing; simulating this type of programs is almost impossible in every case. Break down the logic to sub-systems or maybe even down to stand alone mechanisms (pick & place, motor starter…), simulating and troubleshooting this scenario is fairly easy.
b) When possible, beside automated run mode of the machine or system, build “manual mode logic” for it as well. Then via physical push-buttons or HMI, you should have “step forward” & “step back” for every “physical movement or action”.

Simulating the integrity of the “ladder logic program” and all the components and interfaces will be a breeze if things are done meticulously upfront.

Spread spectrum of power supply

Having lead design efforts for very sensitive instrumentation with high frequency A/D converters with greater than 20-bits of resolution my viewpoint is mainly concerned about the noise in the regulated supply output. In these designs fairly typical 50-mV peak-to-peak noise is totally unacceptable and some customers cannot stand 1-uVrms noise at certain frequencies. While spread spectrum may help the power supply designer it may also raise havoc with the user of the regulated output. The amplitude of the switching spikes (input or output) as some have said, are not reduced by dithering the switching frequency. Sometimes locking the switching time, where in time, it does not interfere with the circuits using the output can help. Some may also think this is cheating but as was said it is very difficult getting rid of most 10megHz noise. This extremely difficulty applies for many of the harmonics above 100kHz. (For beginners who think that being 20 to 100 times higher than the LC filter will reduce the switching noise by 40 to 200 are sadly wrong as once you pass 100kHz many capacitors and inductors have parasitics making it very hard to get high attenuation in one LC stage and often there is not room for more. More inductors often introduce more losses as well.) We should be reducing all the noise we can and then use other techniques as necessary. With spread spectrum becoming more popular we may soon see regulation on its total noise output as well.

One form of troublesome noise is common mode noise coming out of the power inputs to the power supply. If this is present on the power input to the power supply it is very likely it is also present in the “regulated” output power if floating. Here careful design of the switching power magnetics and care in the layout can help minimize this noise enough, that filters may be able to keep the residual within acceptable limits. Ray discusses some of this in his class but many non-linear managers frequently do not think it is reasonable or necessary for the power supply design engineer to be involved in layout or location of copper traces. Why not, the companies that sell the multi-$100K+ software told their bosses the software automatically optimizes and routs the traces.

Spread spectrum is a tool that may be useful to some but not to all. I hope the sales pitch for those control chips do not lull unsuspecting new designers into complacency about their filter requirements.

Voltage transmission & distribution

If you look back over history you will find how things started out from the early engineers and scientists looking at materials and developing systems that would meet their transmission goals. I recall when drives (essentially ac/dc/ac converters) had an upper limit around 200 to 230 volts). In Edison and Tesla days there was a huge struggle to pick DC or AC and AC prevailed mainly because it was economical to make AC machines. Systems were built based on available materials and put in operation. Some worked great some failed. When they failed they were analyzed and better systems built. Higher and higher voltages lowered copper content and therefore cost as insulators improved. Eventually commitees formed and reviewed what worked and developed standards. Then by logical induction it was determined what advances could be made in a cost effective and reliable manner. A lot of “use this” practice crept in. By this I mean for example, I worked at a company and one customer bought 3,000 transformers over the course of ten years, They had a specific size enclosure they wanted.

Due to high volume purchase the cost of the enclosure was low. Other small jobs came thru and this low cost enclosure was used on them to expedite delivery and keep cost minimum. Guess what, that enclosure is now a standard enclosure there because it was used on hundreds of designs over ten years. Is it the most economical box, probably not in the pure engineering sense but changing something that works is seldom a good idea. Today, they are raising voltage levels to new high values. I read of a project in Germany to run HVDC linesover huge distance. They are working to overcome a problem they foresee. How do you break the circuit with HVDC economically. If you ever put DC thru a small contactor maybe 600VDC you find quickly that the arc opening the contactor melts the contacts. Now, what do you do at 800kVDC or 1.2MVDC. What will the cost of the control circuit be to control this voltage level. (Edison and Tesla all over again)And there you have it, my only push for the subject of history to be taught.

Signal processing and communications theory

Coming from a signal processing and communications theory background, but with some experience in power design, I can’t resist the urge to chime in with a few remarks.

There are many engineering methods to deal with sources of interference, including noise from switching converters, and spread spectrum techniques are simply one more tool that may be applied to achieve a desired level of performance.

Spread spectrum techniques will indeed allow a quasi-peak EMC test to be passed when it might otherwise be failed. Is this an appropriate application for this technique?

The quasi-peak detector was developed with the intention to provide a benchmark for determining the psycho-acoustic “annoyance” of an interference on analog communications systems (more specifically, predominantly narrow band AM type communication systems). Spread spectrum techniques resulting in a reduced QP detector reading will almost undoubtedly reduce the annoyance the interference would have otherwise presented to the listener. Thus the intent was to reduce the degree of objectionable interference and the application of spread spectrum meets that goal. This doesn’t seem at all like “cheating” to me; the proper intent of the regulatory limit is still being met.

On the other hand, as earlier posters have pointed out, the application of spectrum spreading does nothing to reduce the total power of the interference but simply spreads it over a wider bandwidth. Spreading the noise over a wider bandwidth provides two potential benefits. The most obvious benefit occurs if the victim of the interference is inherently narrowband. Spreading the spectrum of the interference beyond the victim bandwidth provides an inherent improvement in signal to noise ratio. A second, perhaps less obvious, benefit is that the interference becomes more noise like in its statistics. Noise like interference is less objectionable to the human ear than impulsive noise but it should also be recognized that it is less objectionable to many digital transmission systems too.

However, from an information theoretic perspective the nature of the interference doesn’t matter, but rather only the signal to noise ratio matters. Many modern communication systems employ wide bandwidths. Furthermore they employ powerful adaptive modulation and coding schemes that will effectively de-correlate interference sources (makes the effect noise like); these receivers don’t care whether the interference is narrow band or wide band in terms of bit error rate (BER) and they will be effected largely the same by a given amount of interference power (in theory identically the same, but implementation limitations still provide some gap to the theoretical limits).

It is worth noting however that while spectrum spreading techniques do not reduce the interference power they don’t make it any worse either. Thus these techniques may (I would argue legitimately as per above) help with passing a test which specified the CISPR Quasi-Peak detector and should not make the performance on a test specifying the newer CISPR RMS+Average test any worse.

It should always be an engineering goal to keep interference to a reasonable minimum and I would agree that it is aesthetically most satisfying (and often cheapest and most simple) to achieve this objective by somehow reducing the interference at source (this is a wide definition covering aspects of SMPS design from topology selection to PCB layout and beyond). However, the objective to control noise at the source shouldn’t eliminate alternative methods from consideration in any given application.

There will always be the question of how good is good enough and it is the job of various regulatory bodies to define these requirements and to do so robustly enough such that the compliance tests can’t be “gamed”.

1:1 ratio transformer

A 1:1 ratio transformer is primarily used to isolate the primary from the secondary. In small scale electronics it isolates the noise / interference collected from the primary from being transmitted to the secondary. In critical care facilities it can be used as an isolation transformer to isolate the primary grounding of the supply from the critical grounding system of the load (secondary). In large scale applications it is used as a 3-phase delta / delta transformer equipment to isolate the grounding of the source system (primary) from the ungrounded system of the load (secondary).

In a delta – delta system, the equipment grounding is achieved by installing grounding electrodes of grounding resistance not more 25 ohms (maximum or less) as required by the National electrical code. From the grounding electrodes, grounding conductors are distributed with the feeder circuit raceways and branch circuit raceways up to the equipment where the equipment enclosures and non-current carrying parts are grounded (bonded). This scheme is predominant on installations where most of the loads are motors like industrial plants, or on shipboard installations where the systems are mostly delta-delta (ungrounded). In ships, the hull becomes the grounding electrode. Electrical installations like these have ground fault monitoring sensors to determine if there are accidental line to ground connections to the grounding system.

Self Excited Induction Generator (SEIG)

The output voltage and frequency of a self excited induction generator (SEIG) are totally dependent on the system to which it is attached.

The fact that it is self-excited means that there is no field control and therefore no voltage control, instead the residual magnetism in the rotor is used in conjunction with carefully chosen capacitors at its terminal to form a resonant condition that mutually assists the buildup of voltage limited by the saturation characteristics of the stator. Once this balance point is reached any normal load will cause the terminal voltage to drop.

The frequency is totally reliant upon the speed of the rotor, so unless there is a fixed speed or governor controlled prime mover the load will see a frequency that changes with the prime mover and drops off as the load increases.

The above characteristics are what make SEIGs less than desirable for isolated/standalone operation IF steady well regulated AC power is required. On the other hand if the output is going to be rectified into DC then it can be used. Many of these undesirable “features” go away if the generator is attached to the grid which supplies steady voltage and frequency signals.

The way around all the disadvantages is to use a doubly fed induction generator (DFIG). In addition to the stator connection to the load, the wound rotor is provided with a varying AC field whose frequency is tightly controlled through smart electronics so that a relatively fixed controllable output voltage and frequency can be achieved despite the varying speed of the prime mover and the load, however the costs for the wound rotor induction motor plus the sophisticated control/power electronics are much higher than other forms of variable speed/voltage generation.

Differences of Grounding, Bonding and Ground Fault Protection?

Grounding (or Earthing) – intentionally connecting something to the ground. This is typically done to assist in dissipating static charge and lightning energy since the earth is a poor conductor of electricity unless you get a high voltage and high current.

Bonding is the intentional interconnection of conductive items in order to tie them to the same potential plane — and this is where folks get the confusion to grounding/earthing. The intent of the bonding is to ensure that if a power circuit faults to the enclosure or device, there will be a low-impedance path back to the source so that the upstream overcurrent device(s) will operate quickly and clear the fault before either a person is seriously injured/killed or a fire originates.

Ground Fault Protection is multi-purpose, and I will stay in the Low Voltage (<600 volts) arena. One version, that ends up being seen in most locations where there is low voltage (220 or 120 volts to ground) utilization, is a typically 5-7 mA device that’s looking to ensure that current flow out the hot line comes back on the neutral/grounded conductor; this is to again protect personnel from being electrocuted when in a compromised lower resistance condition. Another version is the Equipment Ground Fault Protection, and this is used for resistive heat tracing or items like irrigation equipment; the trip levels here are around 30 mA and are more for prevention of fires. The final version of Ground Fault Protection is on larger commercial/industrial power systems operating with over 150 volts to ground/neutral (so 380Y/220, 480Y/277 are a couple typical examples) and — at least in the US and Canada — where the incoming main circuit interrupting device is at least 1000 amps (though it’s not a bad idea at lower, it’s just not mandated); here it’s used to ensure that a downstream fault is cleared to avoid fire conditions or the event of ‘Burn Down’ since there’s sufficient residual voltage present that the arc can be kept going and does not just self-extinguish.

In the Medium and High Voltage areas, the Ground Fault Protection is really just protective relaying that’s monitoring the phase currents and operating for an imbalance over a certain level that’s normally up to the system designer to determine.

Hazardous area classification

Hazardous area classification has three basic components:
Class (1,2) : Type of combustible material (Gas or Dust)
Div (I, II) : Probability of combustible material being present
Gas Group (A,B,C,D): most combustible to least combustible (amount of energy required to ignite the gas)

Hazardous Area Protection Techniques: There are many, but most commonly used for Instrumentation are listed below:
1) Instrinsic Safety : Limits the amount of energy going to the field instrument (by use of Instrinsic Safety Barrier in the safe area). Live maintenance is possible. Limited for low energy instruments.
2) Explosion proof: Special enclosure of field instrument that contains the explosion (if it occurs). Used for relatively high energy instruments; Instrument should be powered off before opening the enclosure.
3) Pressurized or Purged: Isolates the instrument from combustible gas by pressurizing the enclosure with an inert gas.

Then there are encapsulation, increased safety, oil immersion, sand filling etc.

Weather protection: Every field instrument needs protection from dust and water.
IP-xy as per IEC 60529, where
x- protection against solids
y- protection against liquids
Usually IP-65 protection is specified for field instruments i onshore applications (which is equivalent of NEMA 4X); IP-66 for offshore application and IP-67 for submersible service.