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AQ: Improve PF of pumping motor with soft starter controlled

I have 3 pumping motors of 1750 kw 6.6kv, with soft starter they are maintaining a pf of .96-.97. Now I want to install HT capacitors to use these motors in d.o.l, can I take the pf to .99 by using this?

If you are using soft starters now, do not take them out. These are really large motors and starting them across the line is not a good idea. The utility serving you should have designed their service based on you having soft starters for these motors. They probably also have a stipulation stating that you cannot start them all at the same time. Starting one or more them across the line may cause the utility’s transformer fuses to fail. Even if it doesn’t, the flicker may cause other processes in your facility to trip. Especially drives or undervoltage relays in MCC’s.

The only reason to install caps at this point would be to correct for power factor. Since your pf is .96 it will take years if not decades to get a return on your investment (ROI). My utility does not charge a pf penalty until you drop below .90. And even then, it is usually not worth installing a cap bank unless you are under .85 and correct to >.95. Most customers require a 3 to 5 year ROI and you will never get that. We always recommend designing for a .95 pf to leave some “headroom”. So, your existing design sounds like it is correct. Your company may also have a “kva rate” instead of a “kw rate” with the utility. Check with your utility marketing rep to verify what type rate you are own and to help you evaluate your ROI.

Also, when you install a capacitor bank you have to make sure that you do not hit a resonant harmonic frequency. You will have to get the utility involved to give you the short circuit data at the PCC (point of common coupling). If the calculated harmonic resonant point is near the 3,5,7,11 or 13 harmonic, you will need a harmonic filter installed in conjunction with the capacitor bank. That means more money and a longer ROI.

AQ: 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.

AQ: 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.

AQ: Power electronics design

If you are interested in power electronics design at the board or system level, I would recommend LTspice (note the correct spelling) by far above all the others. In addition to being superb for IC design (Linear Tech uses LTspice to design all their own ICs), it also has been specifically designed to run board level, switched mode simulations.

Because of its robust, excellent performance and because it is available at zero cost, LTspice has become the de facto standard SPICE with by far more engineers using it than any other flavor of SPICE. LTspice allows 100 percent transportability and work sharing, i.e., anyone, even those who have not been previous users, can open your files and run your simulations (the free download is well under 10Mb, installs very quickly and is very system friendly – not cookies, messy registry alterations, scattering of installation folders, etc. – removal, if you so choose, is easy and complete).

Like most versions of SPICE today, LTspice has a fine user interface, but that feature should be low on your list. Schematic entry is NOT where you will be spending most your time when doing serious design work. Beyond a point, desktop eye-candy does nothing to help you understand your design and see its flaws and weaknesses (in fact, too many layers of hand holding can just get in the way of that).

Personally, I never breadboard a design anymore until it has proven itself in LTspice (unlike with a breadboard, a simulated circuit’s internals are ALL easily viewable – a great boon for understanding tricky operation). For me, first hardware is always a complete layout (and matches the simulation every time). Of course, the old axiom “garbage-in, garbage-out” very much applies, which means I often spend a lot of upfront time verifying (and modifying and/or making) models to match their components’ data sheets. In fact, I would recommend doing that as a very worthwhile exercise and as something that should impress a potential employer.

When developing a design in SPICE, you will want to spend your time debugging your design, not your simulation or your simulator, therefor it is worthwhile to learn what a simulator needs to run smoothly (with LTspice, all that means is that the input has to be realistic). It was years of working with simulators and a lot of sweat and aggravation before the keys to problem-free simulations gradually crept into my understanding.

1. If possible, make all nonlinear circuit elements be functionally continuous with continuous derivatives (this is not possible for some component behaviors), and

2. *always* craft your simulations so that the nonlinear bits become linear at high frequencies (this is always possible). Non linear devices should never be strict voltage sources. They should be Nortonized and be shunted with small capacitances such that the capacitances (which are linear elements) dominate at small time steps.

3. Always verify that the building blocks of your simulation behave realistically (GIGO).

Follow these guidelines and you will never see the “time step too small” message (I have never met a simulation that couldn’t be made to run well). Note that many (if not most) vendor supplied models fail to meet these guidelines and will give you nothing but headaches if you try to use them “as is.”

AQ: 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.

AQ: Calculate current setting of overcurrent relay

You can calculate current setting of overcurrent relay by using next expression:

Isetting ≥ (ks*Imaxopam)/(a*pi)
Imaxopam=kam*Imaxoptr

where are:

Isetting-current setting of overcurrent relay
ks-safety coefficient
Imaxopam-maximum operational current under which overcurrent relay shouldn’t to act
a-coefficient of layoffs overcurrent relay (0,85-0,95)
pi-ratio of current transformer
kam-coefficient which describes influence of common starting of all asynchronous electrical motors in the appropriate power network after elimination of fault (1-6)
Imaxoptr-maximum operational current of power transformer

Besides I have already explained meanings of all appropriate sizes, I would like to underline differentiate between Imaxopam and Imaxoptr.
Imaxoptr is maximum operational current of power transformer in normal conditions while Imaxopam is maximum operational current of power transformer after interruption of fault and mentioned current includes influence of common starting of all asynchronous electrical motors in the appropriate power network after elimination of fault. It is very important to say that appearing of fault in power network leads to significant decreasing of voltage what has a consequence deceleration of all asynchronous electrical motors in appropriate power network. After interruption of fault, it comes to appearing of process during which all asynchronous electrical motors are starting in some parts of power network which are still turned on. This is situation which is significant different from situation where asynchronous electrical motors are starting one by one while in this case all asynchronous electrical motors are starting at the same time. Because of this fact, after elimination of fault, value of current isn’t same as value of current before appearing of fault. After elimination of fault, value of current, which I called Imaxopam, is higher than value of operational current of power transformer, which I called Imaxoptr, but under those conditions there is no fault, so current setting of overcurrent relay should to be set on that value of current and mentioned relay shouldn’t act under those conditions.

After calculation of current setting of overcurrent relay, you need to check coefficient of sensitivity of acting of overcurrent relay by using next expression:

ksens=Ifmin/(Isetting*pi)

where are:

ksens-coefficient of sensitivity of acting of overcurrent relay
Ifmin-minimum fault current (1 phase fault to the earth or 2 phases fault)
Isetting-current setting of overcurrent relay
pi-ratio of current transformer

Value of coefficient of sensitivity of acting of overcurrent relay should to be higher or equal with 1,5 in case when is fault at opposite busbars (busbars where isn’t overcurrent relay) or higher or equal with 1,2 in case when is fault at the end of the longest feeder which begins at those opposite busbars. Time setting should to be selected like that overcurrent relay needs to wait acting of another protection which is on the feeders (for example distance protection).

AQ: 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.

AQ: Insulating resistance measurement

Please remember that Insulating resistance (IR) measurement and associated polarization index tests is just one of the many tools used for insulation system integrity analysis. Its value and repeatability is dependent on the environmental condition at the time it is taken; as mentioned temperature, humidity contaminations all contribute/effect the reading.

The baseline figure should be obtained from either factory or during initial commissioning (as per factory condition). So performing commissioning in the rain, dirty surface, high humidity may result in low values for both dry type and oil filled equipment. Low reading in itself does not indicate bad insulation where the machine cannot be returned to service.

The bottom line is assessment lacking or other data would be:
1. The machine was running at it was running ok before the test.
2. The leakage value at operating voltage will be V/R; therefore the heat loss will be I^2R. Is that OK or warrant some corrective measure.
3. PI may approach 1, is that OK or not? Is this mtruly and indication of wet insulation or of resistive value but will still be OK when energized as per 3. above?

IR, PI measurement along with Cap bridge / dissipation tests, PF test and others are performed to ensure the insulation integrity for maintenance and commissioning.

If cable and equipment have gone through routine maintenance, it is good practice to perform these tests and making sure no ground are left before energizing.

Please read a “a stich in time” by Megger.

It by itself is just a test. The test is meaningful.

AQ: 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).

AQ: Simulation on EMI

As a mathematical tool eventually, simulation can help to quickly approach the results that we need. If everything is done in right way, simulation can give us reliable conductive EMI results at the low frequency range.

Differential mode conductive EMI can be simulated with good accuracy at the low frequency range. The accuracy of common mode conductive EMI depends on the accuracy of a few parasitic parameters that need to be measured.

Personally for research, I would like to use simulation as a validation tool for calculation, and test results of prototypes can be used as proof for simulation.

E.g. for EMI filter:

1. Do the calculation for the differential mode conductive EMI filter;
2. Do the calculation for common mode conductive EMI filter base upon the parasitic parameters in the hand or estimation;
3. Use the simulation to check and validate if the calculation is right or if something is wrong and needs to be corrected;
4. Use prototype test results to check and validate if the simulation results are right.

Some other issues that caused by EMI filter can be found during system level simulation before prototyping. E.g. audio susceptibility and EMI filter damping problems.