Author: ABBdriveX

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AQ: How to find the KA rating of Circuit breakers?

Before breaker’s selecting for your electrical system, you need to calculate value of expected short circuit current at the place of breaker’s installation. Then you need to calculate value of heat pulse and 1s current (expected value of current during one second). After that you need to calculate power of breaker and finally, after all, you can select appropriate breaker. Values of characteristics of selected breaker need to be higher from calculated values of characteristics of your power system.

1. The fault level of the upstream NW (Source) to be known, normally 500MVA or 250MVA.
2. Upstream impedance (reactance and resistor, capacitor to be ignored for Short cct calculation) can be determined accordingly.
3. The LV System starting from the secondary of the distribution transformer.
4. Short circuit percentage voltage for Transformer is known (normally 4% for 1000kVA and 6% for 1500 kVA) and hence reactance and impedance can also determined.
5. Impedance of Cables also can be determined from manufacturer TD sheet.
6. Subtotal impedance to be determined by conventional way (Submit if are in series/ (Z1+Z2+….Zn)/(Z1XZ2X…Zn) if are in parallel.
7. divide voltage by the Impedance up to the required location, will give you the fault current at that location.
8. Determine Maximum and minimum fault current. By the former you can decide the breaking capacity of CB and by the later the setting can be achieved.
9. verify the thermal constraints of the conductors(cables). ie

I²t ≤ S²K² , I short cct current, t time( < 5 s valid), K cable material Factor and S cable section area.
I²t Known as let through energy. accordingly breaking capacity of CB should be > than Circuit Maximum fault.

The MCB, MCCB, & ACB are all Low Voltage Circuit Breakers, where SF6 is a Non-active gaze used in Medium Voltage Circuit Breakers.

Now, to determine the value of Breaking Capacity of any circuit breaker, we should, by calculation, the Maximum Short Circuit Current Value ” Isc3max ” at the installation point of that circuit breaker, where we can calculate it by assuming a ” Short Circuit between 3 phases at that point “, then after knowing ” Isc3max ” we can determine the Breaking Capacity value that should be ” equal or bigger than Isc3max “.

Further:
1- The value ” 250 … 500MVA ” is the short circuit power at Medium voltage side for up to 36kV.
2- About the Short Circuit Voltage percentage value:  we called ” Ucc or Usc ” and the value is ” 4& for up to 630kVA transformers “, and ” 6% for up to 2500kVA transformers “, but in all case, we can read it at the transformer’s name plate.
3- Sorry Mr. Omar, we can’t do, you mentioned, the sum of all Z, as these values aren’t on the same vector, so, we should first calculate ” R & X ” for each component, then do the sum of all R ” R total ” and all X ” X total “, then calculate the ” Z total “.
4- By knowing the Minimum Short Circuit Current value ” Iscmin “, we use it to determine the value of “Setting Value” of “Magnetic Protection or Short-time Protection”.

AQ: VFD replace mechanical gearbox to drive the load

Can an AC drive to replace the mechanical gearbox that used to decrease motor speed in conveyor application i.e to use a motor that will drive the load directly throw a coupling, belt or chain, without gearbox, motor rated up to 18.5 kw.

Theoretical is true as far the speed variation is concerned. Practically is not recommended for your application if the conveyor is required to be used with constant speed, on the other hand the gearbox also used for Torque purposes.

For light conveyors used on packing lines on which rate of production varies in accordance to some industrial parameters (Automation & PID control), direct coupled motor controlled by variable frequency drive may be feasible.

VFD is expensive (capital & running cost) its selectivity should be done carefully among the other available options.

By using a variable frequency drive we can change the speed of an AC Motor, and working for any time on any choosing speed, even in some case we can exceed the speed more than the normal one if the motor can withstand it. Noting that:
1- We should be careful when choosing the type of AC drive that should ne normally done according on the application “Conveyor, Fan, Pump, Compressor, ext ” to determine the torque’s level at running time.
2- In some special case when the motor runs at too low speed comparing by his normal one, maybe we need a forcing cooling for that motor.
3- Each VFD has a value of the Short Circuit’s level that can be withstanded, so, we should be careful of that point.

AQ: What need to be concerned to start a motor?

First, you need to know power (rated power and rated current) of your power source with whom you will supply your motor. For example, if you want to supply your motor by using low voltage synchronous generator (through high voltage power transformer), you need to know rated power and rated current of synchronous generator and rated power and rated currents of high voltage power transformer. This information is very important because if you don’t have powerful source for supplying your motor, there is possibility that you’ll never reach rated rotational speed during rated time which means that you’ll not start your motor.

Second, you need to know kind of your motor. Is that motor asynchronous motor with cage rotor or is that asynchronous motor with sliding rings? This information is very important because these kinds of asynchronous motors have different values of starting current: for asynchronous motors with cage rotor starting current is 6-8 times higher than rated current of mentioned kind of motor while for asynchronous motors with sliding rings starting current is 3-5 times higher than rated current of mentioned kind of motor. Also, too much higher starting current of your motor could be a reason for unallowed warming of windings of stator what it could lead to dangerous consequences, first all, for people in surrounding of motor and then also for equipment in surrounding of motor.

In relation with start of your motor with lower voltage because you will, on that way, reduce starting current 2 times and starting torque will be 4 times lower than rated torque of your motor. On that way, you will easily start your motor.

AQ: Earthing conductors size calculation

1. As per IEE/BS7671, The Minimum cross-sectional area of protective conductor in relation to the cross-sectional area of associated line conductor (Say =S mm2) by taking into consideration that are of same material, as follows:

If S ≤ 16 then the Minimum cross-sectional area of the corresponding earth conductor = S mm2

If 16 < S ≤ 35 then the Minimum cross-sectional area of the corresponding earth conductor = 16 mm2

if S > 16 , then the Minimum cross-sectional area of the corresponding earth conductor = S/2 mm2

2. It may be necessary to verify the same by using the following equation
I²t ≤ S²K

I earth fault current and t tripping time.

while the following equation is applicable for bonding conductor

Zs < 50/Ia where Zs = Earth loop impedance and Ia is protective device operating current

AQ: Reactive consumptions in AC power system

There are two types of reactive consumptions in AC power system, inductive and capacitive reactances. We can not call them losses. The loss of a transmission line is the active power consumed by the line resistance which is determined by the current on the line. Reactive power can adjust the power factor and control the apparent power, then the current and losses on the line.

The minus reactive power means capacitive load is higher than the inductive load, which happens when the transmission line has no load or with pure resistive load because the capacitive load along the TL dominates the reactive load. In this situation the voltage at the end of the line should be higher than the one at the beginning (you should get it when you get the negative reactive power).

When the load (80% of the industry load is inductive) increases, the reactive power will be positive as the inductive load will dominate the reactive power consumption, and then voltage will lower than that at the beginning. So the optimized choice for the reactive load is that in power plant generating less reactive power (reducing the losses on the line) and generating the compensating reactive power (negative reactive power) at consumer side by using capacitor banks or synchronizing motor, which can increase the power factor of the consumption and regulate the voltage (if the transformer has no taps), and then efficiency (save money) as well.

AQ: What happen if we put a magnet near digital energy meter?

In the “olden” days when there were only moving disk meters, I heard that people drilled small holes into the Bakelite cases and tried to get spiders to make a web inside the meter and slow the meter down. It probably wasn’t true, but there have always been people trying to get something for nothing.
I also heard that some people were using a welder and found that their moving disk meter went backwards, but it depended where they positioned the welder, and how strong the welding current was.

Back to electronic meters, if there are transformers inside the electronic meter, placement of a magnet as close to this transformer as possible could cause over fluxing every half a cycle, this could cause a diode like affect in the meter electronics, and if the electronics are designed to eliminate harmonics for calculating energy usage, then the magnet has let this person pay less for electricity, i.e. steal electricity.

Of course the meter may also have a detection circuit for high harmonics and send a message back to the utility to say the harmonic level is too high and a serviceman may then discover this magnet.
I do know that some electronic meter IC manufacturers have added a bump circuit into their ICs so I am sure they have thought about this sort of trickery too.

I like everyone paying full dollar for their electricity, otherwise most of us are carrying the small number of people doing these sorts of things.

“Meters should offer compliance to requirements of CBIP-304 and its amendments for tampering using external magnets. The meter should be immune to tamper using external magnets. The meters should be immune to 0.2T of A.C. magnetic fields and 0.5 T of D.C. magnetic fields, beyond which it should record as tamper if not immune.”
The above statement is a requirement during the manufacturing of digital energy meter. Hence we shall assume that digital meters are tamper proof using Magnets.

AQ: Parallel operation of autotransformers

Q:
We have 2 no 160MVA 220/132/11 kV transformers with short circuit impedance 46.06 ohm, and one 160MVA transformer, 220/132/11 kV with % impedance 15.02%. Can we parallel these three transformers?

A:
Indeed, the vector group is an important (mandatory) consideration when connecting transformers in parallel. And also important if a transformer is going to close a loop in either the HV or LV sides.

But it is perfectly OK to parallel transformers with different impedances. All it is going to happen is an uneven distribution of the power flow, among the parallel transformers. The unit with the lowest impedance would carry a larger share of the load.

Regarding “same ratio”: are you talking about the transformer ratio, such as 138/230 kV? Or are you talking about tap positions? Within certain constraints, it is possible to parallel transformers with different ratios (let’s think, for a second, of identical transformers at different tap positions). This is not recommended, though, because of reactive power circulation.

So, without disagreeing with the factors that you have listed, I would like to re-order, if you will, the conditions you have described:
1) Mandatory: making sure that the vector group and nominal voltages of transformers being considered for parallel operation are indeed adequate and compatible with the intended parallel operation
2) Desirable: ability to operate parallel transformers at the same tap positions or as close as possible, to minimize reactive current circulation
3) Almost indifferent: identical impedances on the parallel transformers simplify things a bit, but this is not a “show stopper” for parallel operation of these transformers. Actually, it is more realistic to expect some differences in impedances, even for otherwise “identical” transformers (same manufacturer, same nameplate ratings, etc.)

For the “Y-Y- Delta” transformers operated in parallel, there exist two kinds of the circulating currents between the tanks and between the banks of the delta side. As the circulating current between the tanks is 90 degree out of phase of the load current, it is estimated by decomposing the line current into the component 90 degree out of phase of the load current. The circulating current between the banks in the delta side is estimated from the delta winding current and the line currents.

The estimated circulating current depends on the power factor of the system even with the same tank currents. This characteristic is derived from the view point of the active and reactive power. Also, it needs the voltage as well as the tank and the load currents.

AQ: The effect of power failure on VFD

Q: I am planning to put some variable frequency drives on non-critical section of factory where there will be planned interruption of 30 seconds but 2 times a day.

A: Why are you going to use VFD, variable frequency drives are expensive. What is the application?
If the application require fix speed / rate Soft Start device is required.

However, if there might be frequent start stop (Power OFF/ON) AC Contractor Duty AC3 are recommended to be used to bypass the Soft Starter or Static device once the required motor speed is reached and then Start/stop have no impact on the installation.

The technique of using AC3 Contactors, is not applicable for VFD if the VFD is necessary for application. In this case the other advantage associated with VFD no longer will be valid (Protection, control and monitoring).

AQ: What causes cables to fault during weather seasonal changes?

I currently work for a small utility with a small amount of underground installations but a lot of it is aging and failing during weather changes. I am curious as to why it happens during weather changes and if there are scientific facts that can support it? Is there a way to predict when a cable will fail based on weather patterns? I’ve heard of different opinions on the matter, but is there a proven reason why? It is my goal as a young engineer and Gonzaga T&D engineering graduate student to learn more about these phenomena’s and what better way than to hear it from industry professionals in a technical discussion?

Scenario #1: Lightning strikes during summer on power cable installations can cause voltage spikes on the line, which in turn doubles back when it finds the open point on an underground cable installation. The initial voltage spike can cause the insulation of the cable to deteriorate or fail, and the reflection of the surge can cause the voltage to spike which can then finish off the already deteriorating insulation if it hadn’t faulted from the initial surge. Side note: This is why it is good practice to have transformers with load on them at the end of a cable run, or lightning arrestor at the termination points of an underground run and not just an open switch. Faults, recloser operations and other switching events can also cause a voltage spike on underground installations which can break down the cable insulation, thus making it more susceptible to failing after future events.

Scenario #2: Cables could fail more during the weather change due to the stresses that are inserted in to the cables during heavy irrigation motor start-ups and operation. Today you will see more soft-starts on your pump motors. With older cable supplying energy to older pumps you may find an across the line motor starter at the end of it. Cross-line starters rapidly heat the cables that have laid dormant over the cold winter months. If there was any sort of treeing, insulation deterioration, rodents chewing on the dormant cable, dig-ins, or any other common cable damaging scenarios during the winter, the startup will speed the deterioration process up in these locations, which in turn lead to cable faults.

Scenario #3: In areas where older open concentric cable has been installed, you are most likely experiencing many faults if it hasn’t already been changed out for newer jacketed solid-dielectric cable. As the ground dries out in the spring/summer, you will see higher resistances on the return path of the old and deteriorating open concentric neutrals. Without the cable being in wet conditions as it was throughout the winter, the electric field around the cable is no longer uniform and in some cases is a complete loss of your neutral.

Regardless of the insulation you use on cables, you most likely have faults. Maybe you’ve been “lucky” and it’s only in your joints and terminations? Regardless of which type of cable insulation, temperature has a significant impact on the dielectric withstand of the cable (i.e. higher temperatures will result in lower dielectric strength properties). Drying conditions also equates to higher insulation temperature due to poor heat transfer characteristics of your cable.

There are some common points in each of the above three scenarios but there really isn’t any scientific proof, just observations. Does it depend on your system load factor, your power factor, your installation practices, or even your cable design? Is it all of the above or is it much more simple than that? Is it different between different manufacturers of cable? Are there different scenarios that you’ve seen or heard of?

AQ: Cable faults/fails

There are only 2 distinct types of Cable fault
1. Due to system Parameter or operating conditions – which you have to take care at the initial design stage for selection of the cable. There is a long list of checks – Design Engineers know.
For example, if 50 sq.mm cable is adequate for a particular Load, you may have to choose higher size in relation to the Fault Level. In urban Distribution system, you will find large size of cable is connected to a small size distribution Transformer.

2. Failure of Cable does not normally occur in the run of the cable unless it is damaged for external reasons. Damaged cable may not fail unless there is ingress of moisture / water through the damage. You will find cable failing during rainy season, high tide in the coastal areas.

Cable fails at the joints mainly because the (construction) characteristics of Cable are changed at the joints – joints become weak links in the run of the cable.

It is a well documented phenomenon that underground cables fail a week or so after lightning activity. Some of the can be attributed to lightning surges that enter the primary conductor and reflect off an open as you indicate. I believe the majority of the failures come from lightning strikes on adjacent structures or trees that reach the cable through ground and cause slight damages to the cable insulation. The maddening part related to customer service is the cables end up failing on a sunny day. Most customers understand outages during inclement weather but are not so understanding of outages on clear days.