Category: Blog

Home  ›  Blog  ›  Page 16

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.

AQ: Excitation system in generator

The excitation system requires a very small fraction of the total power being generated. If we could simply increase the excitation (a very small amount of power) and increase the generator’s real power output, the world’s energy problems would be solved, because we would have a perpetual motion machine.

In the case of a generator connected to a large grid, the generator will inject any desired amount of power into the grid if its prime-mover is fed the desired power (plus a small additional amount of power to take care of losses). This is true, regardless of the total load on the grid, because the generator’s output is an extremely small fraction of the total grid power, and it alone cannot make drastic changes to the grid’s frequency.

Normally, the load varies by a very small fraction of the total grid power. If the load increases, the frequency of the entire grid (including the generator in question) lowers a very small amount, generally less than one-hundredth of one Hz. The frequency slew (that is, the rate-of-change of frequency) is very low, because there is a massive amount of energy that is stored as the kinetic energy of the rotors of all of the generators. At this point, nothing needs to be done; the system simply runs a little faster or slower.

Over time, as the load changes a greater amount, the frequency moves further from the nominal frequency (50 Hz or 60 Hz). When the difference between the actual frequency and the nominal frequency becomes greater than about 0.01 Hz, action is taken to make changes to the output of the grid’s generators.

The specific action may be determined by the regulating authority (for instance a power pool in the US) and it is usually based on economics, subject to other constraints. If the load has increased (and the frequency is less than the nominal frequency), the generators that have the lowest incremental cost of power will be asked to increase their output, or if all generators are near their limits, new generators (with the lowest incremental cost) are asked come on line. It’s important to note that a generator’s limit is usually 80% or 90% of its rating. The 10% or 20% of unused capacity is the system’s “spinning reserve”, which is used to maintain grid stability for sudden, large power variations.

The same thing happens with a generator connected only to its load or a weak grid with just a few other generators. However, because there is relatively little kinetic energy stored in the rotors of the one or few generators, the change in frequency associated with a load change is much greater, so frequency variations are much greater and corrective actions may not be implemented before the frequency varies by more than a few Hz.

AQ: 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.

AQ: EMI & EMC

EMI/EMC is rather a subjective topic than theoretic, but we shall look at it with start from noise prevention then noise suppression.

Prevention or design in the solution is needed to concentrate on noise making part/component or its mechanism play in the circuit. These are referring to those part and circuit that directly involve in switching, like PFC mosfet and its driver, PFC diode, DC/DC switching mosfet and its driver, and its output diode, do not left out the magnetic part and layout design, bad design will cause ugly switching then give you headache in EMC problem.

Part/component and topology selection is somehow important in which had some level predetermine your EMI/EMS need to take care, like what Stephen had explained; phase-shift is better than none phase shift.

Mosfet would have higher noise at high frequency but it can be somehow compensated or tolerated or trick by driving speed, by using snubber and may be shielding. The output Diode should be carefully selected so that its high frequency noise is within your output noise spec else is an issue, please make sure this noise is not able to be transmitted out as Radiated noise, or it is not couple into your Primary circuit, else it will all the way out to the input AC then transmitted as Radiated noise. Trr is the parameter to look at, sure the lower the best. Anyhow, some snubber (RC or feerite bead,..) shall be determined and add-in.

Noise suppression is what refer to Filter, energy dumping circuit,.. but somehow is basic need, one of it Input filter that give noise isolation between what generated internal in psu not pass to input supply system that could interfere other system/supply environment (EMI), or what noise environment that could enter into your power supply and interfere your power supply (EMS). Input filter is definitely a must for your switching frequency and its sub-harmonics, which is fall into the EMI standard range.

There are many technique to suppress the noise and is depend on what location, nature of the circuit, switch and diode, like what you means by RCD, is not mistaken is refer to RCD that add across the main transformer, DC/DC switcher and typically at the output Diode, you are on right direction with using this snubber around these component.

Shielding may be needed for your main transformer if you have some gap in it (but is not needed if your controller and your switcher is so call good part), but may be needed after you have made some study it the samples.

Good layout always give peaceful mind, whereby noise part have to be some distant away from noise sensitive controller or decision making circuit, decision making connection point have to wise at right termination point that prevent sense the high noise content signal, but if no choice some RC filter is unpreventable, anyhow and mostly RC is commonly located even is known clean in noise to those decision making circuit.