Blog – Page 27 – AC DRIVES SOLUTION.

AQ: Special Protection System Advantages and Disadvantages

Quite a few yrs. ago around 1988, I was a Protection and Control Engineer at a large utility in the SE. We were doing our planning to bring the final unit of a large 4 unit plant on line, when it was discovered that we could encounter some unusual instability scenarios. The funny thing was that with all units on-line and above a certain MW output, all that would need to occur would be opening of a remote 500kV breaker on one of the particular lines and the event could trigger, eventually bringing ALL 4 units at the plant out-of-step and tripping off all of the generation in just a few minutes (3600 MW).

The studies were performed numerous times by internal and external experts but the results were always the same. The key problem seemed to be the existing network configuration of 4 units and only 3 transmission lines. Adding a 4th 500kV line from the plant seemed to cure the problem under all conditions, including close in 3 phase faults with breaker failure. Unfortunately, the cost and timeline to build a new t-line was a real challenge!

In order to proceed with commissioning the 4th unit and remediating any scenarios for tripping all generation, a Special Protection System (SPS) was developed. A transfer trip channel was installed at the remote substation, keying on the breaker contact opening. At the plant, a Unit Trip scheme was installed that had a MW meter supervising tripping of any one unit selected by the plant operator (U1-U2-U3-U4). If all units were on line and generation was above 2500 MW (margin of safety added), then a receipt of remote breaker opening would trip the selected unit to avoid having all units cascade into out-of-step condition.

Advantages: Clearly, this Special Protection System saved the day, and bought time until an additional line was added 4 years later.

Disadvantages: The downside was the challenge of installing and testing such a complicated scheme with the potential for mis-operation. I don’t recall any mis-operations occurring, but it was still a bit “dicey”. I have been at that same plant during a full load unit trip (Generator differential) and it was an “exciting” experience to say the least! While I did recommend that we conduct a “live test” to see what would really happen and perhaps test our system BLACK START procedures, this suggestion was not well received by management (LOL).

This was my only encounter with such a special protection system scheme in my 35 years of utility work, but it was very interesting to be involved with this project.

AQ: Resistance to ground

Resistance to ground is greatly influenced by the ambient conditions and the state of the motor when tested.

Factors Affecting Insulation Measurement:
First, it is important to understand that we are measuring a motor circuit. We are connecting our test instrument at a point where we can measure the majority of the de-energized circuit. As such, we do not necessarily know where an insulation anomaly is located when identified. We also have the motor circuit potentially exposed to differing environments. Ambient temperature and humidity can have a significant effect on any insulation measurements. When a motor circuit’s insulation is tested is also a major variable. Testing a motor circuit immediately after shut down will most likely yield good results. This is because the motor is warm and dry. Testing a motor after it has been shut down for a while may indicate insulation problems, but if the motor is allowed to reach ambient temperature, the insulation integrity may appear normal. This is because while cooling, particularly in somewhat humid conditions, moisture (condensation) will accumulate within the motor and lessen ground resistance. Is this a problem? Yes, particularly if starting from a partially cooled state. Most motor failures occur during starting. This is when the insulation is exposed to the most stresses. If your motors are only down for a few hours at a time, then this is when insulation testing should be conducted.

When conducting insulation testing, the most important consideration is consistency. Always test at the same location, use the same test voltage, perform the test for the same amount of time, and use the same test instrument. It is also important to note the motor temperature, ambient temperature, and relative humidity. It is also helpful to compare like motors and the motors that are operating within the same environment.

Insulation testing is somewhat ambiguous. Although there are reference standards, they cannot be rigidly followed because they do not factor in all of the potential variables that may be encountered. Temperature is the biggest variable. Temperature of the motor and the ambient temperature are of primary concern. One method to help negate the influence of temperature is performance of a “Timed Resistance Test.” This testing is comprised of “Dielectric Absorption,” “Polarization Index,” and “Step Voltage” testing. Dielectric Absorption is a 1 minute test. The resultant values at 30 seconds and 1 minute are logged and the ratio of the 30 second value divided into the 1 minute value, is a relative indicator of insulation integrity. A polarization index is a 10 minute test with the resultant ratio derived from the 1 minute value divided into the 10 minute value.

So, if ground resistance is low after prolonged shutdown and it is at ambient conditions, then you probably have an insulation issue. Conditioning of the insulation may be required. A motor shop can perform a “Clean, Dip and Bake.” process which will prolong the motor longevity. If the motor is several years old you may want to HiPot the insulation but if you are not using one of the newer units that automatically shut down upon a jump in current, you may cause insulation failure and that would necessitate a rewind.

AQ: Negative sequence

Negative sequence will not cause a physical rotation. This component creates a field which, though not strong enough, tries to counter the primary field, An increase in this component will cause the motor to overheat due to the opposition. a physical rotation is not likely to occur.

Negative sequence currents are produced because of the unbalanced currents in the power system. Flow of negative sequence currents in electrical machines (generators and motors) are undesirable as these currents generates high temperatures in very short time. The negative sequence component has a phase sequence opposite to that of the motor and represents the amount of unbalance in the feeder. Unbalanced currents will generate negative sequence components which in turn produces a reverse rotating filed (opposite to the synchronous rotating filed normally induces emf in to the rotor windings) in the air gap between the stator and rotor of the machines. This reverse rotating magnetic field rotates at synchronous speeds but in opposite direction to the rotor of the machine. This component does not produce useful power, however by being present it contributes to the losses and causes temperature rise. This heating effect in turn results in the loss of mechanical integrity or insulation failures in electrical machines within seconds. Therefore it is undeniable to operate the machine during unbalanced condition when negative sequence currents flows in the rotor and motor to be protected. Phase reversal will make the motor run in the opposite direction and can be very dangerous, resulting in severe damage to gear boxes and hazard to operating personnel.

AQ: Synchronous generator operating frequency

When synchronous generators (alternators) are connected in parallel with each other on an AC grid, they are all operating at a speed that is directly proportional to the frequency of the AC grid. No generator can go faster or slower than the speed which is proportional to the frequency.

That is, when a synchronous generator and its prime mover is operated in parallel with other synchronous generators and their prime movers, the speed of all of the generator rotors (and hence their prime movers if directly coupled to the generator rotors) is fixed by the frequency of the grid. If the grid frequency goes up, the speed of all the generator rotors goes up at the same time. Conversely, if the grid frequency goes down, the speed of all the generator rotors goes down at the same time. It is the job of the grid/system operators to control the amount of generation so that it exactly matches the load on the system so that the frequency remains relatively constant.

Isolated or is landed generators that are not in parallel with other generators have an added limitation in that keeping exactly 50Hz is somewhat difficult, or puts too much demand on controlling/governing systems. In such environments it is normal to accept some small deviation from the nominal frequency.

The vast majority of power for industry is supplied by large rotating AC generators turning in synch with the frequency of the grid. The frequency of all these generators will be identical and is tied directly to the RPM of the generators themselves. If there is sufficient power in the generators then the frequency can be maintained at the desired rate (i.e. 50Hz or 60Hz depending on the locale).

An increase in the power load is accompanied by a concurrent increase in the power supplied to the generators, generally by the governors automatically opening a steam or gas inlet valve to supply more power to the turbine. However, if there is not sufficient power, even for a brief period of time, then generator RPM and the frequency drops.

By operating transformers at higher frequencies, they can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers may need to be equipped with ‘volts per hertz’ over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

AQ: Solar power

On a purely theoretical level and ignoring interrelated economics and energy usage, it makes sense to charge EVs during the day – though never in non-distributed environments, IMO.

In reality, and the reality for likely the rest of my life, it makes more economic and particulate emissions sense to distribute solar power during the day to decrease, and ultimately decommission, fossil fuel sources used for peak demand supply that occurs during the day.

Thus, using solar output distributed to offset the dirtiest, most expensive and most distribution grid loading power enhances and optimizes the value and worth of that solar generated power – both economically and ecologically. Attempting, therefore, to do all of ones’ EV charging off peak is the optimal solution until the mix of energy sources changes dramatically – likely a 20 plus year process even in the most environmentally friendly “energy generation mix” regions of the world. Even if one charges during “peak”, it is better to simply charge from the grid as the distributed energy is allowed to go to areas of peak demand. Again, for at least my lifetime, I don’t project a more optimal use of that generation even assuming the archaic state of most “grids” persist.

Right now, even for a 1 story commercial building, solar cannot supply the energy needs used in the office, much less a manufacturing facility. In fact, it can normally only supply 1/3 or less for the most energy and resource intensive commercial environment in a UV intense region (and that is quite an optimistic calculation, more likely 1/5th). Once you get to two or more stories on the building, one is not even close. On a modest tower with a tower parking garage, the footprint is likely to small to even generate the needs on a theoretical basis. Distributing the energy to location of greatest needs will allow us to dial down and decommission peak sources, which again are the dirtiest and most wasteful.

At some point, we will hit a new equilibrium where the energy generation mix is much cleaner, solar generation specifically is much more efficient, and peak power generation is handled more efficiently and ecologically cleaner. I still believe, however, that distributed power is better than “off grid” type of scenarios as it allows the energy to go where it is being demanded at the moment, decreasing the need for redundant sourcing. And, even in the cleanest energy generation mix, redundancy means building more of something and is by definition more energy wasteful and ecologically wasteful than a scenario where the redundancy buffer that is required is lesser.

Much of this type of debate reminds me of the consumer sort recycle versus the destination sort recycle debate. Even with the advances in trash collection and recycling processes, 20 years later we are suboptimizing the recycling process. Much of the reason for that is the “style” statement, making people feel like they are contributing by sorting themselves. It may make some people “feel” better by imagining “independent” off grid or semi off grid solutions. In reality, however, we live in an interconnected world where “sharing” or distributing solutions to leverage scale and minimize redundancies is far more advantageous, economic, and a faster route to a solution to both particulate emissions issues and energy independence for groups of people.

AQ: Variable frequency drive saves energy on fans

Like pumps, fans consume significant electrical energy while serving several applications. In many plants, the VFDs (variable frequency drives) of fans together account for 50% to 60% of the total electricity used. Centrifugal fans are the most common but some applications also use axial fans and positive-displacement blowers. The following steps help identify optimization opportunities in systems that consume substantial energy running the fan with VFDs.

Step 1: Install variable frequency drive on partially loaded fans, where applicable. Any fan that is throttled at the inlet or outlet may offer an opportunity to save energy. Most combustion-air-supply fans for boilers and furnaces are operated at partial loads compared to their design capacities. Some boilers and furnaces also rely on an induced-draft fan near their stack; it must be dampened to maintain the balanced draft during normal operation. Installing VFDs on these fans is worthy of consideration.

Similar to centrifugal pump operation, the affinity law applies here. Because constant-speed motors consume the same amount of energy regardless of damper position, using dampers to maintain the pressure or flow is an inefficient way to control fan operation.

Step 2: Switch to inlet vane dampers. These dampers are slightly more efficient than discharge dampers. When a VFD can’t be installed to control fan operation, shifting to inlet vane control could provide marginal energy savings.

Step 3: Replace the motor on heavily throttled fans with a lower speed one, if applicable. Smaller capacity fans with high-speed motor VFDs operate between 25% and 50% of their design capacity. Installing a low-speed motor VFD could save considerable energy.

For example, a 2,900-rpm motor drove a plant’s primary combustion air fan with the discharge side damper throttled to about 75–80%. Installing a VFD on this motor would save considerable energy, but we recommended switching to a standard 1,450-rpm motor. This was implemented immediately, as 1,450-rpm motors are readily available. With the lower-speed motor, the damper can be left at near 90% open; the fan’s power consumption dropped to less than 50% of the previous level.

Step 4: Control the speed when multiple fans operate together. Fans consume a significant amount of energy in industrial cooling and ventilation systems. Supply fans of HVAC systems are good candidates for speed control by variable frequency drives, if not already present.

Step 5: Switch off ventilation fans when requirements drop. Ventilation systems usually run a single large centrifugal fan or several axial exhaust fans. A close look at their operation may indicate these fans could be optimized depending upon the actual ventilation needs of the building they serve.

Recently, we surveyed a medium-sized industrial facility where 26 axial-type exhaust fans were installed on the roof of one building. All fans were operating continuously, even though the building had many side wall openings and not much heat generated inside. To better conserve energy, we suggested the 26 fans be divided into four groups with variable frequency drives controlled for each group. As a result, energy consumption for the fans dropped by about 50%, as only the required fan groups now are switched on.

At another industrial site, the exhaust fan of a paint booth ran continuously but paint spraying was scheduled only about 50% of the time. Modifying fan operation with variable frequency drive and delayed sequencing saved energy.

Pumps and fans are the most common energy-consuming devices

AQ: Variable Frequency Drive Basics (Working Principle)

Variable Frequency Drive (VFD) Basic Configuration
The basic configuration of a variable frequency drive is as follows.
VFD Basic Configuration
Fig. 1 Basic configuration of variable frequency drive

Each part of a variable frequency drive has the following function.

Converter: Circuit to change the commercial AC power supply to the DC
Smoothing circuit: Circuit to smooth the pulsation included in the DC
Inverter: Circuit to change the DC to the AC with variable frequency
Control circuit: Circuit to mainly control the inverter part

Principle of Converter Operation
The converter part consists of the following parts as following figure shows:

Converter
Inrush current control circuit
Smoothing circuit

Fig. 2 Converter part

Method to create DC from AC (commercial) power supply
A converter is a device to create the DC from the AC power supply. See the basic principle with the single-phase AC as the simplest example. Fig. 3 shows the example of the method to convert the AC to the DC by utilizing a resistor for the load in place of a smoothing capacitor.

Fig. 3 Rectifying circuit

Diodes are used for the elements. These diodes let the current flow or not flow depending on the direction to which the voltage is applied as Fig. 4 shows.
Diode
Fig. 4 Diode

This diode nature allows the following: When the AC voltage is applied between A and B of the circuit shown in Fig. 3, the voltage is always applied to the load in the same direction shown in Table 1.

Table 1 Voltage applied to the load

That is to say, the AC is converted to the DC. (To convert the AC to the DC is generally called rectification.)

Fig. 5 (Continuous waveforms of the ones in Table 1)

For the three-phase AC input, combining six diodes to rectify all the waves of the AC power supply allows the output voltage as shown in Fig. 6.

Fig. 6 Converter part waveform

Input current waveform when capacitor is used as load
The principle of rectification is explained with a resistor. However, a smoothing capacity or is actually used for the load. If a smoothing capacitor is used, the input current waveforms become not sine waveforms but distorted waveforms shown in Fig. 7 since the AC voltage flows only when it surpasses the DC voltage.

Fig. 7 Principle of converter

Inrush current control circuit
The basic principle of rectification is explained with a resistor. However, a smoothing capacitor is actually used for the load. A capacitor has a nature to store electricity. At the moment when the voltage is applie

AQ: Negative Impact of Accelerated Depreciation on the Indian Economy

For argument sake or as an illustration, if we assume that 1 MW solar will generate 1.6 Mkwh and rs. 1.2/kwh is rebate for AD taken by the investor = 16 x 1.2 = Rs. 19.2 lakhs/year

[Now, Adani and Tata Power have been negotiating the firm Contract PPA to get more, like wise biomass people who based their PPA on LCOE, but, are asking more money from Government, hence, Solar PV developers may also follow the same route after few years, wherein this rebate of AD given will not have any meaning!!]

Total rebate given = 19.2/year x 25 years = Rs. 480 lakhs = Rs. 4.8 Crore (that too year wise depreciated / devaluated rupee value, which has no meaning !)

But, the tax saved is = 80% of investment = 0.8 x 10 cr = 8 Crore, upfront, right in the first year, which is great value, which government would have used as Equity to develop many more MWs.

Is this POLICY of providing 80% Accelerated Depreciation correct by any standards and why Finance Secretaries or policy makers can’t take note and issue corrective measure for INDIA FIRST Culture??

MNRE, in its Draft policy has proposed 20 to 40% Viability Gap Funding, which will further worsen the LOSS to the government !!

If Mahagenco (with 50% subsidy) goes ahead with the proposed business model, then, how and why State and hence Central government has to take the burden due to such errant policies??
We must put an end to the Scrupulous Project Development, which avails the Capital Subsidy (or Viability Gap Funding) and the Accelerated Depreciation and then the Promoters Sell the Project to a prospective buyer, who in turn approaches the Government for the Tariff hike in the 25 years tenure (please note the Politics dynamics or change of administrative set up will hamper the sustainability), thus, the nation is a great loser

Policies and the enabling tax advantages to few promoters (who claimed Capital Subsidy without creating good quality asset or with NON functional biomass power plants) have made a big dent on Indian Economy without any good results esp in Renewable energy sector.

Government or its administration through such policy (without checks or being accountable) transferred the Public Property to the Private Companies in the Form of Renewable Energy Generation through Capital Subsidy (or Viability Gap Funding) coupled with Accelerated Depreciation along with Low cost Debt fund to these Corporate companies (like EXIM etc) / Project Developers – entrepreneurs, which are not paid back as few of these projects are not functioning and still no action taken to recover the Capital Subsidy paid or Tax recovery which was availed through Accelerated Depreciation (AD).

If Government would have established all these projects from the Tax collections (which are doled out as free through AD), it would have needed only a fraction i.e only Rs. 51,504 Crores, which could have been managed from the taxes of Rs.137,344 Crores while retaining the land and property in Government’s name and could have generated lot of employment.

But, by giving an opportunity to Private sector, many have failed to deliver and no Action to recover the Capital Subsidy or the Debt (due to Tribunals etc…. Please be informed that Indian Parliament had to pass an act in Dec 2012 to recover debt (through wrong business cases of Project Promoters, approved by many banks which were certified by National and International Advisors or Consultants) which is around a whopping 40 Billion USD!!)

Total estimated Renewable energy project capacity = 12% of total installed 220GW = 26000 MW
Cost/ MW Investment Equity Debt Cap Sub AD
Source MW installed Total 30% 70% Rs(Cr) 80%adj
Biomass 6 4,500 27,000 8,100 18,900 6,750 21,600

Wind 7 20,160 131,040 39,312 91,728 104,832

Solar PV 10 1,300 13,000 3,900 9,100 VGF? 10,400
(Ground)

AQ: Power factor of a generator connected to national grid

Q: What should be the power factor of a generator connected to national grid in order to have maximum stability? Whether it should be high or low?

Steady State Stability:
1. National grid is like a infinite bus for an average size Generator. We can observe stable operation of generator within its capability limit for all ranges of power factor for infinite time , irrespective of power factor.
2. Observe the load cycle, The generators operate in overexcitation mode (lagging pf) during the day & during night ,when transmission lines generate enough reactive the same generators operate stable in underexcitation mode (leading pf).
3. Therefore as long as there is no instance of large disturbance, we can observe stable operation of generator within its capability limit for all ranges of power factor.

Transient Stability:
1. Depends upon the initial condition of the generator operation (see on Power vs Sin-delta plot)
2 The level of power thrown-off causing the disturbance & Equal area criterion of the energy balance & Inertia.
3 During transient/disturbance, the stability is ensured better if the angle delta (rotor angle or power angle) is small, meaning the amount of store energy in the rotating system is high. Theoretically this means delta angle =0 to have robust stability, but it is practically impossible to have power generation at that value.
4 In order to have maximum stability & power generation simultaneously , the value of rotor angle has to be non zero , on positive side. (negative means motor operation).
To Conclude : It means over-excited mode.(lagging pf ). Many colleges in discussion chain above have written near about 0.9 – 0.94 lagging . They are correct.

AQ: The cause of harmonics in variable frequency drive

Before you attempt to dissipate causative factors of harmonics verbally, you take a look at several studies done by NEMA regarding such, and look into variable frequency drive (VFD) a bit better. You can view articles and studies by subscribing to the NEMA newsletter, and find other sources quite readily through NEMA. It’s an easily accessible place for many current dissertations on this and other electrical topics, with excellent subject matter.

Categorizing all VFDs into the same bucket doesn’t get it. You can also look at EPRI reports done better than 15 years ago on this and other VFD oriented subjects. Of course, all VFDs use Pulse Width Modulation to create the AC type wave form output (AKA ‘Sinusoidal Flows) and of course all have rectifiers at the top end, as do all computers, PLCs, and many solid state control components. The differences of transient creation on the outputs of variable frequency drives depend upon the quality of the wave form output. The more transients or ‘spikes’ in the wave form, the more disruption potential. The quality of outputs of variable frequency drives can clearly be seen in testing with oscilloscopes. Several VFDs on the market significantly reduce this effect with chokes up front, and on the output. It really is a garbage in/garbage out situation that lesser drives don’t bother to address.

Anytime AC is rectified to DC a field is created, and this is at best an elementary statement. The solution is good grounding to bleed it off. It isn’t a problem to do so as long as the grounding pathway is adequate, a simple and proven fix. All drives employ capacitors. Motor field generation, field collapse of any wound coil has the potential of creating conductive/inductive reactance, and capacitors create capacitive reactance. To claim otherwise flies in the face of electrical fact. Phase balancing capacitor banks serve to bring about the same effect. As far as ‘putting drives on a pedestal’, you seem far more inclined to pursue a defensive posture than to take a better look at the correlation between capacitive and inductive/conductive reactance. Again, when these two factors meet the same frequency is when the distortion issue is brought to a peak, with these harmonics becoming the face of disruption.

I successfully remedied these situations by working with engineers in DOD and DOE facilities, as well as with a host of different independent companies, Iacdrive, General Electric, Shaw Nuclear, being a few among them.

AC DRIVES SOLUTION.

Category: Blog