PWM is shorted for Pulse Width Modulation, it’s a variable frequency drive (VFD) regulate way to change the pulse width according to certain rules to adjust the output volume and waveform.
PAM is shorted for Pulse Amplitude Modulation, it’s to change the pulse amplitude according to certain rules pulse amplitude pulse train to adjust the variable frequency drive output volume and waveform.
At zero speed the motor requires torque which is flux (voltage) and current (mostly reactive). Only a little bit of active current to compensate for the motor power losses.
Only the power losses need to be drawn from the grid at that time, which means a very small amount of current. It may produce 200% current on the motor and pull only 10% current from the grid.
Of course, as the motor is accelerating, the motor will require kW and the current pulled from the grid will increase accordingly, as the active power consumed by the motor is increasing.
Regarding the kick of torque on the motor, it is controlled by the maximum current ramp limit or through the speed reference as the ramp rate defines the current and the derivative of that rate is the current rate. For this reason, many large machines will be started using an S-Curve speed reference where the S part will adjust the torque (current) rate to avoid stressing the mechanical components, especially if there is mechanical backlash in the gears.
Actually the starting method depends on the type of motor itself SR or SQ type the voltage supply, the motor capacity and motor function, for the MV Motor a liquid or oil starter was the best solution used before.
In case the operation process required a change in the equipment speed the variable frequency drive (Air or water-cooling) based on the drive capacity is the optimum and reliable solution.
Definitely it reduces starting kick of the motor. Actually, the degree of starting kick of a motor is depending upon the starting speed of the motor. If you start your motor at low speed you will have a low starting kick but if started at high speed, you will have high starting kick. This is generally the condition for low and high kw motors. One factors of varying the speed of motors is by varying the frequency of the motors (from the formula N=120f/p) and VFD drive is use to vary the frequency, thus varying the speed of the motors. But if used for starting only, this is expensive as there is more cheaper way like using the Soft Starter or use a WRIM/Slip-ring motors with LRH/Resistor Starters or other. Normally, variable frequency drive is use on operation with speed reduction/varying requirements at required number of time or continuously.
Transformers are rated in {VA, kVA, MVA etc.} due to flows of active and reactive power through transformer. In case of transformer we have active power losses as consequence of existence inside resistance of windings (primary and secondary) and existence of active losses of ferromagnetic core and other side we have reactive power losses as consequence of existence losses of magnetic flux (primary and secondary) and existence of reactive power losses of ferromagnetic core.
[VA]=sqrt(sqr[W]+sqr[VAr])
Transformer is rated in kVA by the manufacturer to inform users about the maximum power (voltage and current) that support it, the reason for not rating it in KW is that the active power (kW) is depend on the loads (lighting, machines..)
The simple answer is: It is because the kVA (or MVA) rating is only rating that matters to express a transformer’s “capacity” to allow the “passage” of power. That capacity is the thermal capacity dictated by the current it can carry at a given ambient temperature, regardless of the power factor. So combined with its voltage ratings, kVA (or MVA) is the value that matters. kW rating does not matter as transformer can handle unity power factor or in other words, a transformer can handle kW equal to its kVA rating at any time.
Remember that a transformer, as the name suggests, is only a transformation device or a pass through device and not a power producing device like a generator or an UPS, where their capacity to produce real power (kW) is an independent limit from the thermal ( kVA) limit.
To take it a step further, if you have an ability to cool the transformer further, you can augment the kVA (or MVA) rating of a transformer. This would explain having multiple kVA/ MVA ratings on transformers with forced cooling aids installed on them.
If you think of it, this is not different from a cable or a conductor’s capacity expression. Except that a transformer can have more than one voltage levels and different ampacities on primary and secondary, but the kVA rating remains the same on either side. So that makes kVA a more convenient way to express its thermal capacity vs. the amperes alone.
Soft Starter, Auto Transformer, Electrolyte, series resistance – wound rotor- etc,). The starting factor of VFD drive is usually 1 up to 1.2 with respect to the rated load current while for Direct On line about 5-6.
Moreover and as you know the variable frequency drive can control the speed of the AC motors in accordance to the formula N=120f/P rpm
where f = the supply frequency and P = number of the Poles.
According to this formula, Motor Speed can be changed either by changing/control the frequency or by changing the number of Poles of the Motor by which step changed in the RPM will be given, while the former gives continuous variable speed as per application demand.
However, as per newly developed power Semi Conductor IGCT based on PWM VFD became the most smart, effective and efficient control device in Industries since is associated also with protective and monitoring means.
From my experience, I know that variable frequency drive plays around with the frequency which the motor operates. It starts at low speed and varies the frequency to attain maximum speed. This reduces the high starting torque usually experienced when motors are started on DOL, Star/Delta etc. When you are driving delicate materials through your conveyors or pumping liquid through pipes etc., VFD plays a useful role. It reduces hammering in pipes usually experienced when using DOL. In large hotel application, variable frequency drive could be used with pressure switches to regulate water flow and reduce hammering when guests are showing. The volume of water required will determine the speed at which the motor runs through VFD control. However, very large KW motors at high voltage level are usually started DOL due to the cost of ac drive but that is when one has enough (power) capacity otherwise it will impact on other users in the network.
This is a finite element analysis tool for various applications.
In power we get the voltage (stress) distribution in equipment like cables, bends in cables etc including stator winding of generators.
Once you go deep into it the applications become more apparent. In mechanical engineering using FEM you can identify the stresses in each member of the structure and so on.
I believe ANSYS, Abacus, Nashtran etcare extensively used for detailed analysis of stresses including electrical stresses. Some of the above offer introductory courses on line.
One needs extensive and considerable insight into partial differential equations and advanced mathematics.
Q: I want to know just what the surge impedance loading (SIL) is but its relevance towards the improvement of stability and reliability of a power network especially an already existing one with various degrees of low voltages and overload situations?
A: The surge impedance loading will provide you with an easy way of determining if your transmission line is operated as a net reactor (above SIL, so external sources of
(2) line-voltage-drop limitation
(3) steady-state-stability limitation
In contrast with the line voltage drop limitation, the steady state stability limitation has been discussed quite extensively in the technical literature.
However, one important point is rarely made or given proper emphasis; that is, the stability limitation should take the complete system into account, not just the line alone. This has been a common oversight which, for the lower voltage lines generally considered in the past, has not led to significant misinterpretations concerning line loadability
At higher voltage classes such as 765 kV and above, the typical levels of equivalent system reactance at the sending and receiving end of a line become a significant factor which cannot be ignored in determining line loadability as limited by stability considerations, so surge impedance loading plays a fundamental role in reliability and stability.
As for XLPE cable testing, if XLPE is used for insulation in the switchgear, the cross linking will be treed by HV DC and permanently destroyed. For this reason, HV DC is no longer used for XLPE cable testing. The switchgear should have a power frequency withstand test only and not HV DC. Refer to the relevant switchgear standard for the applied rms voltage. Any XLPE insulation will need to be replaced as it is most likely has been damaged by treeing of the cross linkages in the insulation. A maximum of say 2.5 kV DC is allowed for IR and PI only.
Humidity plays important part in flashover. We faced a problem of flashovers in Air insulated 11kV Switchgear busbar compartments in rainy seasons. Any sharp edge will ionize the surrounding air, which becomes conductive to high voltage discharge. Moisture will hasten the process of discharge. During HV test also this aspect should be kept in mind.
And make sure the following:
Clean all the supporting bus insulators and spouts with CRC spray.
Ensure the earth bus continuity and its connection with the earth grid.
all PTs are taken out.
all CT ckt output shorted at the panel.
All LAs are disconnected
Conduct a general cleaning of busbars through CRC-sprays.
Megger the bus bar with 5KV between phases, and between phase to earth for 1 mints before HV test.
Ensure the earth bus continuity and its connection with the earth grid.
Use AC high voltage test preferably
Connect HV test kit body ground to the SWGR body ground.
Apply 80% of the power frequency voltage applied at the FAT test.
If you are doing with AC hv kit then this may be a larger unit and leakage current is exceeding and tripping.
Try for smaller sections of busbars/increase the leakage current if options are available.
Rate of rise of voltage should be in steps of 2KV/s and gradual.
Check tripping function of the test kit.
Apply voltage betweenL1-(L2+L3)=G-1mints
apply voltage in the same way between other phases also.
If it withstands ok alternately you have to go for individual inspection of the insulators/spouts.
The surge impedance loading (SIL) of a line is the power load at which the net reactive power is zero. So, if your transmission line wants to “absorb” reactive power, the SIL is the amount of reactive power you would have to produce to balance it out to zero. You can calculate it by dividing the square of the line-to-line voltage by the line’s characteristic impedance.
Transmission lines can be considered as, a small inductance in series and a small capacitance to earth, – a very large number of this combinations, in series. Whatever voltage drop occurs due to inductance gets compensated by capacitance. If this compensation is exact, you have surge impedance loading and no voltage drop occurs for an infinite length or, a finite length terminated by impedance of this value (SIL load). (Loss-less line assumed!). Impedance of this line can be proved to be sqrt (L/C). If capacitive compensation is more than required, which may happen on an unloaded EHV line, then you have voltage rise at the other end, the ferranti effect. Although given in many books, it continues to remain an interesting discussion always.
The capacitive reactive power associated with a transmission line increases directly as the square of the voltage and is proportional to line capacitance and length.
Capacitance has two effects:
1 Ferranti effect
2 rise in the voltage resulting from capacitive current of the line flowing through the source impedances at the terminations of the line.
SIL is Surge Impedance Loading and is calculated as (KV x KV) / Zs their units are megawatts.
Where Zs is the surge impedance….be aware…one thing is the surge impedance and other very different is the surge impedance loading.
Many years ago I had an experience of 4nos 37kW fin-fan motors wrongly connected at site to a star. After running for almost 1 year, the operators reported these motors were very warm and felt unusual. We removed one of them to the workshop and opened for inspection. All windings were OK but the rotor lamination surface had turned to light blue colour which showed a sign of abnormal heating.
I asked different experts in the industries for advices. From the advices, we suspected the motor could be designed for a delta connection even though the nameplate indicated a Star connection for 415V. We contacted the motor manufacturer by quoting the motor serial no. The manufacturer confirmed that the motors were designed for delta connection at 415V. The manufacturer apologized for the error in nameplate and gave us a free spare motor.
One clear sign that could lead us to believe that the motor was in a wrong star connection instead of delta was, for a 2 or 4-pole motor the no load running current should be more or less around 30% of FLC. When we tested run the motor in the workshop, the no load current was less than 15%xFLC.
After the rectification of all the 4 motors to delta connection, we had no complaint anymore. It was a good lesson out of this solved problem.
