Iacdrive|Automation Solution

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.

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.

Phase rotation errors

Phase rotation errors are not as rare as they ought to be. I’ve seen more than one building with a systematic phase rotation error. This can be prevented by carefully following the color coding system (Yellow Orange Brown and Red Blue Black for 480 volt and 208 volt systems in the US for example) and tagging feeders at both ends to assure proper connections.

To check for proper phase rotation sequencing (ABC and not ACB) you can use a phase rotation meter. Without that you can bump a three phase motor that should be correctly connected to see if it turns in the right direction. If it’s wrong, reverse any two phase wires from the source to the distribution equipment. However, if you have a tie breaker and intend to operate the secondaries of two transformers in parallel by closing it that is not good enough. Both transformer distribution networks have to be connected correctly on all three phases. You have to check the voltage across each corresponding pair of terminals on the tie breaker and be certain they are all about zero volts. If you don’t and there is an error, closing the tie breaker if that is possible at all (some electronic breakers may lock you out) will result in a phase to phase bolted fault that can result in severe damage to your distribution equipment. Phase rotation errors are invariably the result of incompetent installation, inadequate specifications for feeder identification, and inadequate inspection.

There are times when the phase rotation error is made on the primary side of the transformer. If this happens it can be compensated for by reversing the phase rotation error from the secondary side. This is less desirable but it will work. If you have multiple phase rotation errors in the same distribution network you have quite a mess to clean up. It will be time consuming and expensive tracking all of them down to be certain you have eliminated them. False economies by cutting corners on the initial installation of substations and distribution equipment will result in necessitating very expensive and inconvenient repairs. If it is not corrected you risk severe damage to three phase load equipment.

Moving data around within memory of an individual PLC

The first question would have to be – why do want to do it? If the data already exists in one location that is accessible by all parts of the program, why are you going to use up more PLC memory with exactly the same data?

Well, there are a couple of candidate reasons. One might be recipe data. You have an area of memory with a set of stored recipes for different products, and at an appropriate moment you want to copy a specific recipe from the storage area to the working area. The first thing to be said about that is that if your recipes are at all complex and you have a requirement to have a significant number of different recipes, then PLC memory is probably not the right place to be storing them. The ultimate, these days, of course, is that recipes are created by techies on PCs away from the production area, in nice quite, comfortable labs or whatever, and are stored on a SQL server. Only the recipe for today’s actual production run gets transferred to the PLC. But there are some applications where there is a limited number of different recipes required and the recipes themselves are quite simple, when it can be reasonable to store the recipes in PLC memory.

A second reason for copying memory areas within the same PLC is for procedures, sub-routines or whatever. But again, these days, all PLC languages have some sort of in-built facility for procedures – what Rockwell uniquely call Add On Instructions, what everyone else calls UDFBs – user defined function blocks. In any case, the point is that these facilities usually make all that memory management stuff transparent to the programmer. You just configure the UDFB and call it as required. The compiler takes care of all the memory data moves for you.

Another reason for copying memory, actually related to the previous, is a technique much used by PLC programmers where they use an area of memory as a ‘scratch-pad’. So they will copy some unprocessed data to the scratchpad area, all of the operations performed on the data take place using the scratchpad, and at the end, they copy the processed data back again. Again, it is questionable how much this technique is actually required these days, I would suggest that it most cases, there probably is a better way using a UDFB. But I have seen some programmers who routinely include a scratchpad area within any UDFBs they define.

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.