Iacdrive|Automation Solution

Sub-transmission network

Q:
What factors determine current distribution between two 33kV feeders feeding a 33/11kV Substation within a sub-transmission network.

A:
Try the voltage divider rule. Take the R + X of each feeder (resistance and reactance) and find the Z. Remember that square root of R² + X² = Z. Now that you have the Z of each feeder, now find the Z of the two in parallel. To do this we have Z = 1/(1/Z1 +1/Z2). So, if Z1 = 2.16 ohms and Z2 = 1.67 ohms, then our Z of the two in parallel is 0.94 ohms. Now we pass the current of the entire substation between these two feeders. Let’s say that the current is 240 amps. Now we have 240Ax0.94ohms = 226 volts (IxZ=V). And since voltage divided by impedance gives us current (V/Z = I) we now take the voltage drop across the two feeders in parallel and divide each of the feeder impedances to get the separate feeder current. So we get 226V/2.16ohms = 105 amps (feeder 1) and 226V/1.67ohms = 135 amps (feeder 2). I have not tried this with your exact situation. Having different voltages from two different substations will change things, but at least this way you have a good start on the problem.

Since one end is tied together and the two other ends are from different substations, then you will have the classic voltage sending and receiving formula. Since the load is the one substation, then their will only be one power factor of the one load, so I would think this formula would apply: Es = Square Root of ((ErCosƟ + IR)² +(ErSinƟ +IX)²), which is square root of ((Receiving voltage times the cosine of the current phase angle plus current times resistance of the line)² + (Receiving voltage times Sine of the current phase angle plus current times reactance of the line)²). The voltage drop across each line would be VD=I(RcosƟ +XsinƟ) where R is the line resistance and X is the line reactance and the Ɵ is the phase angle of the load.

System with difference neutral

Q:
I have one system with two source. One from genset and the other from PLN (national power supply company) that each system has neutral.
The question is
1. Is there any problem if I connect both neutral directly?
2. Is there any spark when I connect both neutral?
3. How is the best solution to connect both neutral?

A:
1. I understand that the Genset is dedicated for essential load as an Emergency power supply which will be operated by hand (only in Manual Mode).

2. The Control Philosophy for a Generator that intended to be connected to PLN as emergency power source depends on the local service provider regulations.

3. Usually in your case there should be Electrical as well as Mechanical interlocks between the mains incomer & genset main breaker. ie both Sources will never be in Synchronism ( will not feeding the same load simultaneously).This measures will ensure that there will be only neutral point to the system.

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.

Soft start motor tripped in fuel oil suction and discharge

First of all check all the component i.e.CB, CT, Heat Element, and the O/L setting then megger the motor to be shore that there is no problem with the motor winding insulation.
After that let the mechanical check the vibration analyses during the start-up also measure the startup currant of the motor and diffidently you will find where is the problem.
It could be a relay setting; or problem in the insulation; or even a problem in the motor itself.

On the other hand, check the motor on No Load condition and tune it to the Soft starter before coupling it to the pump.
Auto Tuning feature is generally inbuilt to Advanced soft starters.
If the No load startup of the motor is perfect, 2 causes arise:
1) Improper design.
2) Viscosity _ this can be tackled if you can make some temporary arrangement for pre-heating to confirm if this is the culprit.

As using soft starter could result in reducing torque of the motor. Soft starter normally reduces starting current by reducing starting voltage. However, decreasing voltage will lead to starting toque reduction. Hence, the motor may take longer time, especially when driving high-inertia load, with somewhat high current until it reach its full speed. Using an inverter will help you get full starting torque or even boost up it to 150-200% while keeping starting current at 150-200% of full load. Installation of heat tracing might also help and economic.

Assuming it is an electrical problem. On a motor of this size it has separate overload protection from the ground fault and short circuit protection. There are tolerance levels for motor that you may not be within. However a megger will not answer all the possibilities with motors unless you are ready to perform polarization index test etc….A power analyzer will allow you to see the operation in real world application. Assuming you have confirmed this is an electrical problem your next step would be to use a power analyser. You should be able to confirm by the signature and different placements of the analyzer the problem. Analyzer should be around all three phases.

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.

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.

High AC current inductors

There are several issues at work here. For high AC current inductors, you want to have low core losses, low proximity loss in the windings, and low fringing effects.

At normal frequencies, ferrites are by far the lowest core loss, much better than MPP and other so called “low-loss” materials. So you would like to use them from this aspect.

A toroid gives the greatest winding surface for the magnetic material, letting you use the least number of layers and hence minimizing proximity loss. The toroid also has the advantage of putting all the windings on the outside of the structure, facilitating cooling. This is very important.

However, you can’t easily gap a toroid of ferrite, it’s very expensive.

Some aerospace applications actually cut the ferrite toroid into segments and reassemble them with several gaps to solve the problem. The multiple gaps keep fringing effects low. It might be nice if you could buy a set of toroidal segments so you don’t have to do the cutting because that is a big part of the cost. I don’t know if that is a reasonable thing to do, maybe someone can comment.

Once you go to MPP, the core loss goes up, but the distributed gap minimizes the fringing losses.

The MPP lets you run somewhat higher on current before saturation, but if you have high ac you can’t take full advantage of that due to the core losses.

All these tradeoffs (and quite a few more not mentioned for brevity) are the reason that so many different solutions exist.

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.

Experience: Design

I tell customers that at least 50% of the design effort is the layout and routing by someone who knows what they are doing. Layer stackup is very critical for multiple layer designs. Yes, a solid design is required. But the perfect design goes down in flames with a bad layout. Rudy Severns said it best in one of his early books that you have to “think RF” when doing a layout. I have followed this philosophy for years with great success. Problems with a layout person who wants to run the auto route or doesn’t understand analog layout? No problem, you, as the design engineer, do not have to release the design until it is to your satisfaction.

I have had Schottky diodes fail because the PIV was exceeded due to circuit inductance causing just enough of a very high frequency ring (very hard to see on a scope) to exceed the PIV. Know your circuit Z’s, keep your traces short and fat.

Fixed a number of problems associated with capacitor RMS ratings on AC to DC front ends. Along with this is the peak inrush current for a bridge rectifier at turn on and, in some cases, during steady state. Unit can be turned on at the 90 deg phase angle into a capacitive load. This must be analyzed with assumptions for input resistance and/or a current inrush circuit must be added.

A satellite power supply had 70 deg phase margin on the bench, resistive load, but oscillated on the real load. Measured the loop using the AP200 on the load and the phase margin was zero. Test the power supply on the real load before going to production and then a random sampling during the life of the product.

I used MathCAD for designs until the average models came out for SMPS. Yes, the equations are nice to see and work with but they are just models none the less. I would rather have PsPice to the math while I pay attention to the models used and the overall design effort. Creating large closed form equations is wrought with pitfalls, trapdoors, and landmines. Plus, hundreds of pages of MathCAD, which I have done, is hard to sell to the customer during a design review (most attendees drift off after page 1). The PsPice schematics are more easily sold and then modified as needed with better understanding all around.

Power supply prototypes is the best way to learn it

I have been designing power supplies for over 15 years now. We do mostly off line custom designs ranging from 50 to 500W. Often used in demanding environments such as offshore and shipping.
I think we are the lucky ones who got the chance to learn designing power supplies using the simple topologies like a flyback or a forward converter. If we wanted to make something fancy we used a push-pull or a half bridge.

Nowadays, straight out of school you get to work on a resonant converter, working with variable frequency control. Frequencies are driven up above 250kHz to make it fit in a matchbox, still delivering 100W or more. PCB layouts get almost impossible to make if you also have to think about costs and manufacturability.
Now the digital controllers are coming into fashion. These software designers know very little about power electronics and think they can solve every problem with a few lines of code.

But I still think the best way to learn is to start at the basics and do some through testing on the prototypes you make. In my department we have a standard test program to check if the prototype functions according to the specifications (Design Verification Tests), but also if all parts are used within their specifications (Engineering Verification Tests). These tests are done at the limits of input voltage range and output power. And be aware that the limit of the output power is not just maximum load, but also overload, short circuit and zero load! Start-up and stability are tested at low temperature and high temperature.

With today’s controllers the datasheets seem to get ever more limited in information, and the support you get from the FAE’s is often very disappointing. Sometime ago I even had one in the lab who sat next to me for half a day to solve a mysterious blow up of a high side driver. At the end of the day he thanked me, saying he had learned a lot!
Not the result I was hoping for.