Iacdrive|Automation Solution

Avoid generator overload

Two buses of 11kv, 750MVA, 3000A each fed by a transformer of 40MVA and connected through a tie breaker, now connect a generator of 18MW,11kv, 0.8 PF. How to avoid overloading?

The generator is being used as a backup power source in case utility power is lost, based on such info presented, you are going to have a hard time getting this to work with only ONE 18MW gen. In order to connect the 18MW gen to both buses, the total demand should not be more than 80% of 18MW or 14.4MW at .8 power factor. For short run times (10 or 15 minutes), you can load the gen up to 90% for continuous load, but for long run times, you need to keep it at 80%.

Demand is the diversified connected load. Not all 54.22MVA of connected load will be on at the same time, so this is why you “diversify” the load to get your actual demand load. You can look at your power bill or call your utility to find out your total demand. Or you can install a power quality monitor for a couple of weeks to get it.

A general rule of thumb is to assume that 67% of the connected load will be your demand load. But this depends on your operation. Based on this, one generator will not be sufficient for BOTH buses. However, if you are supplying each bus with its own generator, you may be ok.

Another issue is motor starting flicker. Make sure your generator can start your largest motor and that your disconnect breaker or fuses can handle the inrush. I have seen this as an issue, especially when soft starters are used. Soft starters lower the inrush by exploiting the time characteristic. If the soft starter settings do not bring the motor up to speed quickly enough, the overload trip setting on your generator may trip.

The bottom line is, you are going to have to look at this installation much closer in order to make this work with one 18MW gen. You may even have to disconnect some load when you are running on generator.

Creepage in thermal substations

The term creepage distance is specifically associated with porcelain insulators used in the Air Insulated substations. Insulator surface attracts dust, pollution (in industrial areas) and salt (along the sea coast) and these form a conducting layer on the surface of the insulator body when the surface is wet. As long as this surface is dry, there is not much problem. But when it becomes wet during early morning or during winter season the outer surface forms a conducting path along the surface from high voltage terminal to earthed metal fitting at the end of metallic structure and may lead to surface conduction and finally external flash-over. The insulators are provided with Sheds to limit the direct exposer to mist or dew. The protected area of the sheds will not allow formation of continuous conducting layer along the surface of insulator as the part of surface which is under the sheds may not become wet due to mist or dew and this part (length along the bottom surface) of the insulator surface is called protected creepage.

Measurement of corona inception and extinction voltages give a fair idea about the possible flashover even with protected creepage. But these will change under different levels of pollution.
This problem is not present with Composite insulators as the Silicone rubber sheds surface does not allow formation of continuous wet conducting layer as the surface of these insulators is Hydrophobic. Hence higher creepage is not considered for composite insulators.

However air density is also a limiting factor for deciding the creepage of insulators, necessitating higher creepage in case of higher altitudes.
You may have to assess the level of pollution and altitude of substation and select the creepage accordingly.
Medium pollution levels may be 25mm/kV
Very high pollution areas like on the sea coast and chemical and pharmaceutical industrial areas 31mm/kV where the insulators may become expensive alternatively periodic hot line washing is also another solution for cleaning of pollution on insulators.
In case of very high pollution levels GIS may be safe solution (if cost is not an issue)

Thermal substations where there are no electrostatic precipitators may also experience equipment failures due to pollution. Pressurized equipments like SF6 gas circuit breaker experienced external flash-overs during winter months in Northern India The utility was not accepting the theory of insulation failure due to pollution initially but they had to accept the cause of failure as pollution when they had similar failure in the consecutive year during the same winter months and they have resorted to hot line washing since then and there are no more such failures. Sometimes these deposits may not be seen glaringly but failure may happen.

High current intensity harmonics [%THD (A)] in several motors?

Most electric motors that suffer variations in Load already have variable frequency drives, we have capacitors installed in general switchboard to correct the reactive energy and so on. I did a discretization of the electrical consumption by product type, during this energy survey I noticed that in most motors Amperage THD was high, above 40%. I would like to know what effect does it have on efficiency and possible causes and solutions.

One more thing, when is it profitable to substitute motors by high efficiency motors? Because in the transport system I have about 60 electric motors below 10HP with a power factor of 0,6 , I was thinking in installing a capacitor in the switchboard of the transport system.

Variable frequency drives and other power electronic loads will draw harmonic currents from the power source. More VFDs, UPSs, rectifiers, etc means more harmonic current. When harmonic current flows through system impedance, it causes harmonic voltage to be present on the power system. That means there are essentially harmonic voltage sources at each harmonic frequency and therefore loads will draw current (harmonics) at each one of those frequencies. PF Capacitors offer a low impedance path to harmonics (attracting them) and may be damaged when connected to a system with harmonic producing loads. It is also possible for capacitors to cause a resonance condition whereby the harmonics can be amplified. Consider detuned capacitors (with harmonic blocking reactors) or addition of harmonic filters. There are several alternative methods of filtering the harmonics.

Automation solution

Automation is a solution:
1. To reduce the manpower & the CTC due to them.

2. Few skilled technicians can run the automated machines smoothly, with much lesser number of errors & faults (as human is not directly controlling every thing & is not burdened with multitasking challenge for extended duration which causes fatigue and hence errors/faults)

3. The power consumption & time to market can be estimated & reduced as machines will be operated on time & can work for longer durations than human beings & they don’t ask for tea/coffee/lunch breaks nor they ask for incentives. (Care for peoples who are maintaining them as well as care for machines which are earning profits for you, by regular maintenance & regular proper inspection of their conditions). Now days very good automatic power mgmt processors/controllers are available which can maintain the power as per the defined conditions as per the load & real time necessity.

4. Train the operators / technicians regularly to keep them up to date with tricks / methods / operations / principals to handle most situations by their own (will reduce the cost of a nonsense manager who is kept to yell, threat & discriminate subordinates and know only one slogan: “do it properly, otherwise, i will ….”). A training department which actually hold the capability to technically train employees from labor to talented engineers is a necessity of this age, as things are not remains just a lifting boulder & digging holes. We are living in an advance age in which we are having many expectations, competition & external pressures.

5. Finance bugs cries for expenses on NRE costs, salaries & treats this investments like invested in a share/equity/debt fund, but, earning from a business & financial mgmt capability must be inline with level & operations performed by the company. Instead of keeping low minds in tech industries, hire the engineers who has reached to an expertise level in automation industry & know the in depth issues occurring in between & underneath to estimate & expect correct values & timelines. Qualified project managers are much more realistic in their approaches, thoughts, assumptions & mentality.

Question about start a 450kW pump

Can I start a 450KW pump from the grid using star-delta and then use a bypass contactor to switch to an already running generator of 500kVA in order to avoid the starting current?

In my opinion, this operation is very dangerous. 500kVA is usually Diesel generator and interaction between load and source is very high.

Although maybe reduced starting current by means of your proposed figure but following comment shall be take in to account:
• The distance between load and generator is important
• Difference phase angle between grid and 500kVA generator possible to generate torsional effect and it is harmful for rotor in transfer moment
• Reacceleration is very important situation and maybe stall the motor
• Voltage dip due to starting another motor can make disturbance and this network is very weak respect to transient phenomena
• De-rating of generator maybe cause to have 70% or less then nominal rating of name plate (based on site elevation, ambient temperature and humidity)
• Meanwhile power absorption by electrical motor (450 kW) is more than generator normal capacity (500kVA).
As wrap up it is not safe and operational case

Actually I think it won’t work:
1). At 450 kw of a load is already bigger that the capacity of the Generator which is 500kva. (considering the pf of 20% the genset capacity is 400 kw which is way below even the maximum continuous power consumption of the load -450kw).

If your client had say 550KW GENSET, then I would definitely give him a solution which is sustainable. He just doesn’t even have to start the pump with the grid power then cross to Genset. We can propose an equipment that can give a smooth start of the motor and ration supply of power to the motor depending on the load requirement (the energy required to do a certain activity)

Soft Stop – When starting, an AC Induction motor develops more torque than is required at full speed. This stress is transferred to the mechanical transmission system resulting in excessive wear and premature failure of chains, belts, gears, mechanical seals, etc.

Additionally, rapid acceleration also has a massive impact on electricity supply charges with high inrush currents drawing +600% of the normal run current. The use of Star Delta only provides a partial solution to the problem. Should the motor slow down during the transition period the high peaks are repeated and can even exceed direct on line current. THE EQUIPMENT WE CAN PROPOSE provides a reliable and economical solution to these problems by delivering a controlled release of power to the motor, thereby providing smooth, stepless acceleration and deceleration. Motor life will be extended as damage to windings and bearings is reduced.

-Less mechanical stress.
-Improved power factor.
-Lower maximum demand.
-Less mechanical maintenance.

Soft Start and Soft Stop is especially useful with pumping fluids where torque transients often cause water hammer effects, and in some instances, failure to gradually slow the fluid down before stopping, can cause the kinetic energy to rupture pipes and couplings.

Cross regulation for multiple outputs

Cross regulation is a very important component of multiple outputs. This can be done in several ways: transformer coupling, mutually coupled output filter chokes (forward-mode) and/or shared output sensing voltages/currents. All, of which, are impossible to model. I have tried them all.

I have sort of written of the first two off, since it is under the control of external vendors, which make their own decisions as to their most cost effective solutions. At best, transformer solutions yield a +/- 5 percent regulation, and can be many times much worse. .Coupled inductors yield a much better cross regulation, but the turns ratio is critically important. If you are off by one turn, you loose a percentage of efficiency.

Shared current/voltage cross sensing is so much more common sense. First, choose the respective weighing of the percentage of sense currents from each outputs approximately in proportion to their respective output powers. Keep in mind that, without cross-sensing, the unsensed outputs can be as much +/- 12 % out of regulation. Decide your sense current through you lower sense resistor. Then multiply your percentages by this sense current from your positive outputs. Calculate each output’s resistance to provide that respective current. Try it, you will be amazed. The negative outputs will also improve immensely.

One can visualize this by, if one senses only one output, only the load of that output influences the feedback loop, which, for example, increases the pulsewidth for each increase in load of the heavily loaded output. The lighter, unsensed loads go crazy. By cross sensing, the lighter loads are more under control and the percent of regulation of the primary load is loosened somewhat.
By sharing the current through the lower sense resistor, you can improve the regulation of every output voltage in a multiple output power supply.

Motor starting time to reach full speed

It is not easily answered since there are many variables at play which will affect the starting time. For a large medium voltage motor, it is recommended that a motor starting analysis be performed so that proper control and protection of the motor can be set. The motor manufacturer is a good place to start to find a motor data sheet and torque curve responses; that should give you some good starting point data. Such an analysis can provide inrush current, voltage dip, and starting time.

The time that any motor to run up will depend on the actual load on the shaft. In broad terms the larger the load (related to the rated output) the longer it will take to run up. I would have expected 2 – 2.5MW motors to be manufactured to run on 10-11Kv and DoL. The startup times of these motors would typically be between 45 seconds (No Load) and 3 or 4 Minutes (dependent on the type and magnitude of the load).
I also tend to agree if the feed value is shut the motor will not initially see a significant load and should run up quite quickly.

I would start with Te time constant of the motor as the starting time in the worst case. If you intend let your motor live for long, you should design its protection to avoid starting times longer than Te and nor even close to it. As for specific application, it’s always try and error, but the guiding line should be: start at minimum load and increase it gently (some motor protection relays guard load increase rate).

Popularization of SPICE

I am currently writing a bullet point history of the popularization of SPICE in the engineering community. The emphasis is on the path SPICE has taken to arrive on the most engineering desktops. Because of this emphasis, my history begins with the original Berkeley SPICE variants, continues onto PSpice (its limited, but free student version made SPICE ubiquitous) and culminates with LTspice (because, at over three million downloads, it has reached many more users than all other SPICE variants combined).

I have contacted Dr. Laurence Nagel (the father of Berkeley SPICE) and Mike Engelhardt (LTspice) in order to verify the accuracy of the historical account (haven’t had a chance to fold in Dr. Nagel’s corrections yet), but I am lacking solid information about the beginnings of PSpice (I don’t even know who the technical founders of MicroSim were). Ian Wilson was an early technical V.P. Also, I am not sure what the PSpice acronym means. (Seems to me that it started out as uPspice?)

Here is what I have recently found about PSpice (more info appreciated):

User’s Guide to PSpice, Version 4.05, January 1991
From Chapter 1: INTRODUCTION, Section 1.1 Overview, starting with paragraph 2 (page 3):

“PSpice is a member of the SPICE family of circuit simulators. The programs in this family come from the SPICE2 circuit simulation program developed at the University of California at Berkeley during the early 1970’s. The algorithms of PSICE2 were considerably more powerful and faster than their predecessors. The generality and speed of SPICE2 led to its becoming the de facto standard for analog circuit simulation. PSpice uses the same numeric algorithms as SPICE2 and also conforms to the SPICE2 format for input and output files. For more information on SPICE2, see the references listed in section 13.2.1.4 (page 427, especially the thesis by Laurence Nagel.

“PSpice, the first SPICE-based simulator available on the IBM-PC, started being delivered in January of 1984.

“Convergence and performance is what sets PSpice apart from all the other ‘alphabet’ SPICEs. Many SPICE programs became available on the IBM-PC around mid-1985, after Microsoft released their FORTRAN complier version 3.0. For the most part, these SPICEs are little modified from the U.C. Berkeley code. Using benchmark circuits, we find that PSpice runs anywhere from 1.3 to 30 times faster than our imitators. In the area of convergence, PSpice has a two-year lead in improving convergence and a customer base that is larger than all of the other SPICE vendors combined (including those SPICEs offered for workstations and mainframes). This larger customer base provides more feedback, sooner, than any other SPICE program is likely to receive.”

From Chapter 1: INTRODUCTION, Section 1.4 Standard Features, last paragraph (page 7):

“PSpice, version 3.00 (Dec. 1986) and later, is a complete re-write of the simulator into the ‘C’ pro-gramming language. It is not a version of SPICE3, from U.C. Berkeley, which is also written in ‘C’. MicroSim has overhauled the data structures and code, however the analog simulation algorithms are similar and the numeric results are consistent with SPICE2 and SPICE3. Having the simulator re-written in ‘C’ allows faster development, allowing our team to reliably modify and extend the simulator in sev-eral directions at once.”

From the January 1987 Newsletter: PSpice went from version 2.06 (Fortran) to version 3.00 (C). Speed increased by 20%. PSpice 3.01 (Dec 86) introduced the non-linear Jiles and Atherton core model.

From the April 1987 Newsletter: PSpice 3.03 (Apr 87) introduced ideal switches.

From the July 1991 Newsletter: PSpice announced Schematics at the June 1991 Design Automation Conference. (Became available when PSpice 5.0 shipped in July 91?)

Solving Differential Equations with Mic

High starting torque, synchronous motor, induction motor or DC motor?

It depends on so much more than the simple requirements listed of high starting torque and variable speed. What kind of application are you using it for? Is it on an automobile (where you have DC already), a factory, and do you have the budget and/or space for a variable frequency drive. A synchronous servo motor gives great dynamic control and great starting torque per volume, but its speed range is limited (unless you’re field weakening by the back EMF). Servo-motors are also the most expensive due to their position sensors and more intelligent drives.

With a proper soft drive you can go with an induction motor, but it depends. if power is small you can go to step motor also. But dc series motor’s starting torque is high as expressed others.
DC Series motors have high starting torque but induction motors have wide range of speed control. So, If DC motor is used, then DC drives you can use, although it will be expensive and DC motors are tough to maintain than ac motors due to commutation Problem.

DC series motor would provide both the high starting torque and adjustable speed BUT beware that DC motors have high maintenance cost and also require AC-DC conversion. You could use other available options e.g. double wound induction motors etc, depending upon your requirements.

But today, there is no application where you cannot apply AC motors, asynchronous or synchronous. If the motor and the associated power electronics are correctly rated, you can have any starting torque you want.

The typical application of DC series motors was in locomotives. This technology has been replaced by AC motors since 20 years. The latest generation of high speed trains use synchronous, permanent magnet motors.

Simulator history

Power electronics has always provided a special challenge for simulation. As Hamish mentioned above, one of the problems encountered is inductor cutsets, and capacitor loops that lead to numerical instability in the simulation matrices.

In the 80s, Spice ran so slowly that is was not an option unless you wanted to wait hours or days for results, and frequently it failed to converge anyway. It was never intended to handle the large swings of power circuits, and coupled with the numerical problems above, was just not a feasible approach.

Ideal-switch simulations were used with other software to get rid of many of the nonlinearities of devices that slowed simulation down, but Spice really hated ideal switches as it would try to converge on the infinite slope edges.

Three universities started writing specialized software for converter simulation to address this shortcomings of Spice. Virginia Tech had COSMIR, which I helped write with a grad student, Duke University had the program which later became Simplis, and the University of Lowell had their program, the name of which I don’t recall (anyone remember?).

All of these programs started before Windows came along, and they were fast and efficient. With windows, the programming overhead to maintain programs like these moved beyond the scope of what university research groups in power electronics could handle. Only the Duke program survived, with Ron Wong leading the effort at a private company. The achievements of Simplis are remarkable, but it is a massive effort to keep this program going for a relatively small marketplace (power supply companies are notoriously cheap, so the potential market does not get realized), and that keeps the price quite high. If you can afford it, you should have this program.

Spice now runs at a reasonable pace on the latest PCs, so it is back in the game. LT Spice is leading the charge because it is free, and the models are relatively rugged. Now that speed is less of a factor, you can put real switches in, and Spice can handle them in a reasonable amount of time. (Depending on your definition of “reasonable”.)

PSIM was another ideal switch model, and they eliminated the convergence headaches that plagued all the other programs by not having convergence at all. You just cut the step size down to get the accuracy you needed, and this worked fine for exploring power stages and waveforms, but was not good for fast transient feedback loops. As the digital controller people quickly realized, the resolution on the PWM output needed to avoid numerical oscillations is very fine, and PSIM couldn’t handle that without slowing down too much.

When I left Virginia Tech, I felt the bulk of the industry needed a fast simulation and design solution so engineers did not have to add to their burdens with worrying about convergence and other problems. This is a hardware-driven field, and we all have our hands full dealing with real life blowups that simulation just doesn’t begin to predict.

I have observed in teaching over the years that engineers in a hurry to get to the hardware have very little tolerance for waiting for simulation. If you are building a well-known topology, about 2 seconds is as long as they will wait before they become impatient.

This is the gap that POWER 4-5-6 plugs. The simulation is practically instantaneous, and the program has no convergence issues so you design and simulate rapidly before moving to a breadboard. It is intended for the working engineer who is under severe time pressure, but would like some simulation to verify design integrity.