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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: Variable frequency drive Constant Torque/Variable Torque

A typical variable torque application would be a centrifugal pump. A typical constant torque application would be a conveyor, and there are positive displacement pumps that are also constant torque. Have a talk with a mechanical engineer, get them to show you curves and explain.

DBR stands for Dynamic braking resistor. Regeneration will happen when the motor rotates a speed higher than the speed which corresponds to the frequency setpoint ie.. the rotor speed is more than the speed of the rotating magnetic field.
Regeneration feeds back energy to the drive which results in DC bus overvoltage. To prevent the drive from tripping due to DC bus overvoltage the DBRs are used. The regenerative energy is discharged in the resistor as heat.

Regenerative Breaking – we used to have VFD on a vehicle rolling road. So when the car is travelling faster than the VFD, the VFD generate back into the power supply – causing a break effect. If you had a large mass- large inertia that you want to stop quickly, you need to break the load- you can do that with regenerative breaking. Otherwise, disconnecting the variable frequency drive, will mean your load just freely rotates, and that can mean it will take 30 minute to come to a stop for a large inertia.

Active Front end- I first came across this term with ABB. It is all to do with how to mitigate harmonics from VFDs. You can use phase shift transformers, but with modern electronics, you can use a opposite phase current to counter act the harmonics generated from the VFD. So the overall impact on the network is small.
In active front end technology the rectifier is basically an inverter with IGBTs.
The main advantage are:
1) Low current THD <5 %
2) It is basically a four quadrant rectifier .Referring my last post please note that you will not require a DBR with AFE. The increase in voltage of DC Bus due to regeneration can be fed back to the input AC supply in the form of energy. So you don’t require a DBR.
3) AFE drives have very good immunity to input voltage fluctuations.

Just an advice. Please go through variable frequency drive literatures (available in plenty) to have a good understanding of the different VFD technologies.
Selection of VFD requires proper understanding of the VFDs and the overall electrical system. There are lots of marketing gimmicks in the world of VFD. Always be careful before selecting a VFD specially higher KW drives.

For large drives, you need to speak with supplier to configure your machine correctly. There are many options, but yes active front ends are available. But there are other solutions; ASI Robicon use a current driven VFD, so harmonics are lessened in the first place, so an active front end is not the right terminology. It is a different solution. I used a 10MW version of that type of ac drive. I think Siemens have bought the company since.

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