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AQ: Starter of SAG Mills with rotor resistance

Q: For now I am working on a mining project which involves starting two SAG mills, the method of starting these mills is by rotor resistance and likewise we are using an energy recovery system (SER), could someone tell me how this system works SER? Each mills have two motors of 8000 kW at 13.8 kV.

A: For large mills requiring variable speed, the wound rotor motor and SER drive are economical for a total rating of approximately 2MW to 16MW. Above 16MW, the gearless drive (cyclo-converter) is typically used because gearboxes and pinion gears reach their present limit in size. Around 2MW and below, the squirrel cage/VVVF drive is simple and cost effective.

Advantages of the wound rotor/SER drive are:
1. If the SER converter drive fails, the drive can be switched to fixed speed bypass – starting the usual way with the LRS.
2. The converter only needs to be sized for 15-20% of the total motor rating with associated reduction in floor space, air-conditioning etc. The converter is only sized for the feedback energy which is proportional to the speed difference from synchronous speed. The drives are typically set up to run between about 85% to 110% of synchronous speed for an optimized arrangement.
3. Relatively low capital cost when all things considered – including spare motor cost etc.

Brush/slip ring maintenance is one issue. However, when the brushes are specified correctly for the load, the wear is manageable. Once the maintenance program is set up for shutdowns, it is not a major issue.
I expect that this type of drive would be the most common large mill variable speed drive in the world’s minerals processing industry for the range mentioned above for the last 15 years (approximately).

The SER drive converter controls the voltage in the rotor. Motor speed is proportional to rotor voltage. Resistance in the rotor indirectly achieves the same thing (with a different torque curve shape), but energy is lost in the resistors which is very inefficient. The SER drive via a feedback transformer feeds energy back into the power supply. This returned energy is proportional to the speed difference from synchronous speed. So at say 85% speed, 15% of the motor rated power is returned from the rotor to the supply. At a hypersynchronous speed, the SER drive feeds power into the motor rotor allowing it to run faster than synchronous speed. So for a fixed torque and higher speed, the power obtained from the motor is higher than the motor nameplate rating.

Gearbox ratio is best set up to allow the speed range to be covered using the SER drive’s hyper-synchronous capability.

AQ: Are variable speed drives harmful to motor?

Variable speed drive switches very fast which brings high dv/dt on motor. How often do we face with problems coming with VSD? How harmful is the common mode currents in windings and other parts of motor due to high dv/dt. Do we see winding isolation failure? How much does the life of motor reduce? Also, is the filtering of voltage at the output of inverter common or applicable practice in the field?

The waveforms for the INVERTER are not good to the motor…. Makes the motor run hot and less efficient….. and all the above….
In-line filters to reduce harmonics is a must in many cases…
Depending on power levels you can have in line reactors for CM and DM or balanced bridge methods for CM… There is methods of harmonic canceling with reactors called harmonic blockers, where you arrange the 3 phase windings in such a manner to cancel certain harmonics….not all harmonics will be blocked, usually in grouping intervals…you need to be aware of what harmonics are your worst offenders…

Mostly in medium and high voltage motor drives the very fast change of the voltage can induce high capacitive currents inside the motor with harmful results.
A way to reduce this negative effect is to increase the number of voltage steps (levels) such that the dV/dT will decrease proportionally (dT=turn on switching time, dV=one voltage step). The most popular method used is SVPWM (space vector sine PWM) NPC (neutral point clamped) multilevel frequency converter. Line L-C filters are also used for EMC.

The first step in any filter analysis is knowing what harmonic vectors your dealing with.
Mathcad is a great tool for modeling the PWM modulation with the sub carrier and generating the harmonic matrices..vectors…I usually go above the 100th harmonic in some analysis, then doing this over the operating ranges of the motor….you then pick your Worse Case operating point and now you have a matrices to work with…. Summing the harmonic magnitudes will give you an idea of how much garbage your feeding your motor windings.

They could be harmful for high frequency current and voltages which are not economical to be eliminated.
But this weakness is so neglect able to the benefits providing. These benefits are very comprehensive. The harmful harmonics are controlled by the standards, so in order to improve harmonic characteristics, we need an improved standard.

AQ: What is the best laptop for field work?

Dell D630 – it is the best laptop for field use I have used. And for some applications standard RS232 port is a must. We have Freja 300 test set which totally refuses to communicate with PC via widely available cheap USB-to-serial adapters. The only usable adapter I have found is semi-industrial type, costing about 50 Euro. Not that a price is so much concern, but it is not very convenient to deal with additional boxes, power supply units for them, etc. when commissioning at field.

But I do not expect you will have problems connecting Omicron via converters. We have been used CPC256 via various USB-RS232 converters without serious problems.
For communication with relay protections from Siemens and AREVA never had problems too. Cannot remember how it was with older ABB relays (last case we used them was 4 years ago), but newer ABB series are all with Ethernet communications.

So my advice will be – by special laptop for field work, not mix it with that for everyday office use. Load it with the minimal necessary software – MS Word, Excel, Adobe Reader, Omicron’s Test Universe and software for relays which will test.
For all these needs most older type laptops (4-5 years older) would be sufficient and you can buy for 200-300 Euro solid business class laptop. And also very important: look for non-glossy displays only!

AQ: Read control wiring diagram of relays in substation

There is a ANSI/IEEE standard that defines the standard number identification for electrical devices. You will find that some of the more common ones are 50 over current, 51 short terms over current, 27 under voltage, 59 over voltage, and 50G ground over current detection relay.

Older installations may have the older electro-mechanical relays. Mst installations have converted to using sensing devices that transmit to PLC, DCS or protective relays. Today’s protective relays are essentially PCs that monitor a number of power system parameters for metering or protection purposes. They also have programmable outputs and settings.

As stated previously, search the web for some examples or guidelines on electrical schematics. There is also a reference standard develop by IEEE and ISA to define the symbols used to represent the hardware or software functions that input to PLCs or are the functions within the PLC (or DCS). The older form of schematics was drawn horizontally, but the same ladder logic used today is drawn vertically with each line numbered in sequential order. The line numbers are used in the device identification and as a reference in the PLC programming.

It can be confusing, but think of items in series as AND logic versus items in parallel being OR logic. Understanding the And / OR logic will enable you understand how the respective logic components function on an electronic card used in a PLC.
I recommend that you join IEEE and ISA. They usually have local chapters that meet to network and share information. It is also a good way to network and meet persons at other companies and of course meet vendors who are eager to meet persons who work for engineering firms that they market to.

AQ: Designing Gate drivers for IGBT

Q:
When designing gate drivers for IGBT’s, how reliable are the gate driver IC’s ? Now there are a lot of gate driver IC’s available in the market. For example i am using the Hybrid IC M57962L for driving IGBT’s for 3 phase inverter application. The peak output current of this Hybrid IC is 5A and it’s written in data sheet that it can be used for driving IGBT’s up to 200A, 1200V and many features in it.

For an initial design and for lower power rating the configuration is working fine. But, before going for higher power rating, i want to make sure about the reliability of Gate driver IC’s in general.
Is it advisable to design gate drivers using commercially available IC’s or go for a design which includes a gate drive transformer . What are the issues that may arise when using driver IC’s.

A:
I’ve seen and developed designs using these hybrid gate drives quite successfully with long term field reliability in applications requiring from 800 V to 1.25 kV voltage isolation in power conversion products for the semiconductor capital equipment market. Powerex offers various different isolated drivers like the M57962L – my personal favorite is the VLA-502 which also contains the isolated DC/DC converter used to power the isolated gate drive electronics.

There are only two problems that I remember in the last 10 years with these types of commercial drivers – and both problems, if I remember correctly were with the stand alone DC/DC converter intended to be used with the stand alone isolated driver. One problem was a voltage isolation issue from primary to secondary inside the DC/DC switcher. Powerex acknowledged the problem, and upgraded the design. I simply do not recall the part numbers involved. The second problem was with regard to how the isolated VEE rail was established – the monopolar output of the DC/DC converter was offset negative, and ground referenced with a zener diode – and when the IGBT gate would become active at high frequency (25 kHz for that particular application), the gate charge was high enough to sag the negative supply rail against the zener shunt.

Bottom line: Use a good isolated DC/DC converter, with solid VCC and VEE regulated outputs. The isolated drivers themselves are solid in my experience – a nice, simple solution with typically better rise and fall times than gate drive transformers. They also have the added benefit of being capable of holding positive or negative DC bias if the application requires it.

AQ: Overcurrent protection of generators

Overcurrent protection uses as back-up protection for protection generators from faults between two windings of stator (two phases of stator). Setting of overcurrent protection depends from two settings: current setting of relay protection and time setting of relay protection.

Current setting of relay protection represents minimal value of current under which relay protection will send signal to breaker to act and this value is higher from value of rated current in generator (higher from maximum allowed value of current in generator).

Time setting of relay protection represents time after that relay need to send signal to breaker to break fault. Of course, when we talk about time setting of relay protection, we need to have on mind time delay. Time delay represents time during other protections need to act before overcurrent protection acts in case where is overcurrent back-up protection for protection of generator.

Then there is voltage restrained time overcurrent protection (ANSI 51V) which is commonly applied on generators. The pickup setting of these relays reduces (becomes more sensitive) when the applied voltage reduces. It is supposed to aid in sensing faults that are electrically close to the generator terminals as there is insufficient fault impedance to maintain the voltage at the generator. It is especially useful in tripping out faults that have persisted long enough for the generator fault decrement curve to get to the portion where the synchronous reactance is the characteristic impedance. When this happens the fault current will be at the same levels as normal load currents and increased sensitivity is needed.

AQ: Harmonic current

I hate to call them harmonic currents. The do submit to Fourier analysis, but you are probably dealing with AC to DC power supplies. If you look at the current pulses, you will see that each pulse is about 1-2 milliseconds in duration in alternating directions. If you sum these all in the neutral there is the appearance of what looks like 180 Hertz in the neutral. If you use different sized power supplies on each phase, you can see that it is just the addition of the three phases. So the neutral current when you have non power factor corrected power supplies is the sum of the three phases. Unless the current waveforms overlap, there is no cancellation of current in the neutral, hence the neutral current is the sum of the phase currents. The reasoning behind this is the rectifier diodes in the front of the power supply and the DC storage capacitors size relative to the DC load on the capacitor. The general rule of thumb is that the capacitor is about 800 to 1000 microfarads per amp of current in the capacitor.

Realize that the extra heating in the three phase delta-wye transformers is due to the extra circulating current in the primary delta causing excessive heating of the primary conductor. The world calls transformers designed to deal with this “K” factor transformers. Let the world of electrical engineers bury all this simple stuff behind the maze of Fourier analysis. Change the incoming voltage slightly and your Fourier analysis is garbage. The issue here is switches and storage caps— not some magical mathematical garbage.

By the way if someone wanted to use the wire sizing guidelines of the National Electrical Code in the US to size wire for 100% power supply load, the neutral wire would be 8 gauge sizes larger than the phase conductors. People need to start demanding PFC power supplies. Fix a switching problem with switches.

AQ: Bushing insulation testing

In bushing insulation test there are three major current elements which any of those could affect the test result. These current elements are Capacitive current

AQ: Neutral current is less than phase current?

In a balanced 3-phase system with pure sine waves, the neutral current is zero, ideally.
If there is phase imbalance, it shows up in the neutral, so check for imbalance.

The other major cause of high neutral currents is full wave rectification, where the current of each phase is flowing only at its peak voltage. In this case, the neutral current can be as high as three times the phase currents, theoretically.

If you can see the frequency of the neutral current, line frequency currents indicate imbalance. Current due to full wave rectification is high in third harmonics, so it may show mostly 3 x line frequency, or be a ratty square wave at 3 x line frequency.

High neutral currents, and some resulting fires, are largely responsible for the adoption of power factor correction requirements. If your loads are balanced and pfc corrected, you should not have neutral currents.

The neutral current (In) is summation of the phase currents. And obviously, the three phases are decoupled now; and not loading Y makes Iy=0.
So In = Ir + Ib (vectorial sum). Now depending on the amount of loading, nature of loads and their respective power factors, a variety of possibilities (for neutral current magnitude and phase) arise; which may include the case of In being higher.
The statement “neutral current is usually less than phase currents” is naive and not universal.

Nonlinear loads (i.e. rectifiers as Ed mentioned above) draw significant harmonic current. In many cases the current Total Harmonic Distortion (THD) is >100%. In a 3-phase, 4-wire system, the triplen harmonic currents (3, 9, 15, 21…) sum in the neutral wire because they are all in-phase. This is why the neutral current can be much higher than the phase currents even on an otherwise balanced load application. If you can put a current probe on the neutral and look at the waveform – you can see how much fundamental vs. harmonic current there is.

AQ: Torque ripple information from low resolution speed signal

Q:
I am trying to develop a controller for switched reluctance motor which minimizes torque ripple. My design is acquiring torque ripple information from speed signal. In simulation a high pass filter for speed gives me good ripple information. But in experiments I am using a 500 PPR optical absolute encoder to get the position and then calculate the speed using microcontroller (dspace) capture module. But the filtered speed signal does not provide much ripple information. Can you suggest any method to extract ripple information from low resolution speed signal.

A:
1. In simulation, do you consider motor inertia? Inertia filters out torque ripple’s impact on speed, resulting in a smooth speed signal. 2. Generally speaking, a low resolution position sensor produces speed signal of more noise, especially at low speed. I would expect more noise out of your high pass filter.

An encoder generally does not specify an accuracy for the A to A! channel or B to B! channel or it is so broad a spec that it is useless. If you have the ability to trigger a clock on A and B to determine the period between A and B channels the difference between successive reads will give you a good indication of your ripple.

In some cases of motor – encoder installations the mechanical alignment of the encoder to the exact center of motor shaft can cause misalignment noise to occur in the resolved speed signal. In theory the ripple signal could provide useful information however in practice there are too many other influences. Even the shaftless encoder mounting has some of these difficulties.