AQ: Frequent tripping in Unit Station Transformer
We are facing problem of frequent tripping in Unit Station Transformer -2 during heavy rain. During checking, not found any abnormalities
1. Fault UST2 OTI Trip Observation- * Direct OTI trip initiated without Alarm
* Oil temperature is normal
* Tripping contacts are not physically operated
* OTI trip contacts are operated frequently as per in Disturbance record Action taken-Spare Core of another cable used for OTI Trip contact
2. UST2 PRD Trip
* PRV contacts are not physically operated
* PRV trip contacts are operated frequently as per in Disturbance record New cable used for all contacts
Make sure for rainy conditions the marshalling box is properly closed and weather protected as moist condition also leads to tracking and may simulate tripping inadvertently. Please also make sure the tripping impulse from OTT/WTT/BT/MOG/PRD etc are driving a mechanical hand reset type VAJH type relay and the output contacts of this relays are used for trip ckt initiation.
If these are substituted by numerical relay binary inputs then also there will be a problem of spurious tripping.
Make sure the grounding of the multicore control cable if sheathed the grounding should be at TRPP panel end only, if armoured cable is used the armour grounding at the panel end only.’
I am assuming that the trip circuit is floating DC (ungrounded). If so, the moisture could be causing a “sneak” circuit, otherwise known as a “hot short” in the tripping circuit, which essentially bypasses the sensing relay contacts and actuates the tripping relay coil. I would check the cabling between the sensing relay contacts and the trip relay coil and the cabling on the hot side of the sensing relay contacts for insulation problems.
AQ: Pressure switch on three phase motor
Q: Is there a way of connecting a three phase pressure control switch on a three phase motor. Also is there a three phase float switch for a three phase submersible pump i know of a single phase switch.
A: The switch only needs to have a single contact since you use a three-phase motor controller to operate the motor. The switch is wired into the low voltage contactor coil circuit to turn the motor on and off.
You can connect a pressure switch for the purpose of motor control on its low level pressure or high level pressure. It is advisable to utilize pressure switch on the control cct, and connect the contactor coils through the auxiliary NC/NO contacts depending on whether you are interested on low pressure or high pressure control. It is not good practice to allow power cct through control ccts. You don’t need a 3 phase float switch to achieve controls of a 3phase submersible pump. You should be interested in the auxiliary terminals which will allow control flexibility for low level and upper level control. A single phase float switch will give you desired result. If you are controlling more than 1 no. 3phase motors located at different places from the same pressure signal, then your 3 phase pressure switch can be employed to control the different motors separately. In addition, some terminals could be used for indication/annunciation purposes. However, a single phase pressure switch can give you all the controls you need for a 3phase motor.
AQ: “Hissing sound” in SF 6 Gas insulated HV Switch Gear
This could be internal corona discharge. The switchgear should be de-energized and closely examined. That means pump out the SF6 and take it apart. Examine all insulating components.
Especially if the sound can be localized to portions of the switchgear which do not have bushings for connection to overhead lines. Even if the sound is in the area of air bushings, deenergizing will allow more in-depth inspection and addressing any sharp edges or cracked insulators, etc.
Take this pieces of equipment out of service immediately, perform a “hi-pot” or high potential test on the various elements of the switchgear and attempt to locate the area that is “leaking” to ground (or between phases). Inspect closely for indications of tracking on insulators from corona discharge and replace any compromised components. After component replacement, installation of new SF6 gas and other repairs, re-run the Hi-Pot test to confirm that the switchgear is able to withstand voltages at least 50% greater than the name-plate rating. Of course all of this advice is worthless if the unit has already failed.
Remember that a Hi-pot is actually a destructive test. It challenges the insulation to the point of breakdown. Check the vendor recommendations before you Hi-pot equipment that has been in-service.
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.