Iacdrive|Automation Solution

MCCB burn out in connection with 22kW motor

125A rated MCCB is connected with 22KW motor. The motor runs in normal condition, no overload. But the contacts of MCCB is burnt out. Why?

When the transition between wye and delta takes place, be very sure that that the mechanical interlocks on the contractors are properly adjusted. If one doesn’t completely open before the other closes, you have a line to line fault. That will eventually take out the upstream breaker. Be careful, this is a very dangerous starter. I have been done this road many times.

“Star/delta switching” is delicate maneuver.
* The engine has almost no force to push something more than itself at startup. When it’s time for mode switching start to delta, may not happen too quickly. The arcs that occurs when the star contactor switch off, needs a few milliseconds (minimum 20 ms). Typical of an MCCB to go off is 50 ms and on is 20 ms.
* If the load is too high during startup, the engine will get overloaded when switching from star to delta and arcs can become very powerful and devastate even the strongest Components. In case the engine does not start at idle, the start method is directly harmful to both motor and power grids. Instead of a soft and comfortable start, we get instead two powerful surges. (In that case it may be even better with only one MCCB).Should this be the case, there are two solutions: Make sure the engine starts without load or replace the Star/Delta switch to a soft starter.
3: Next step is a frequency converter but then it’s about something completely different.

Parallel connection transformers

Change autotransformer to VFD or soft starter

What the parameters needed if we change from autotransformer (6.6kv/3.3kv) to VFD or Soft starter?

Since you got Auto Transformer 6.6/3.3 kV mean you have inductive load with constant speed in that case you should go for Soft starter, best way is collect all the motor data and send it to Soft Starter manufacturer along with the motor starting curve if available also you may ask for capacitor bank for PF improvement.

If you’re planning to use a Soft Starter or VFD for motor starting, you need to specify the horsepower rating, voltage rating, rated frequency, the type of motor (induction wound rotor or induction squirrel cage), the type of motor load (compressor, pump), motor load starting torque requirement and motor loading cycle. This information is important in selecting the type or model of the Variable Frequency Drive (VFD) or Soft Starter unit.

Directional Numerical over current relay

If current will flow in positive direction then the relay will behave as a Normal over current relay and if current will flow in negative direction then the relay will behave as a Directional over current relay…..Why the angle between healthy line voltage and fault current is required for sensing the direction??

Suppose you have purely resistive circuit with a voltage source connected to it. Now take any arbitrary node X in the network Look at the current flow from the node. Now when the source voltage is positive the current flows from the say upper node of the source to the lower node of source. Now if you look at this current flow from the arbitrary node X mentioned above, the current will be moving towards the node X from one side and it is moving away on the other side of the arbitrary node X node in one half cycle.
Now in the next half cycle the same thing repeats but with one difference, that the direction of current flow changes.

Now you want to operate your relay when the current is moving away from the arbitrary node in the first half cycle. place a CT at the node with primary P1 towards the node X1 and p2 away and take Secondary S1 to the relay . Now when current flow is from P1 to P2 current flow in the secondary will be say S1 to S2 thru relay.

Now we can say that, when the arbitrary node voltage is positive current flows from P1 to P2 and we take this as our direction required for the relay to operate.

Again when you look back from the node you will see that when the arbitrary node is at higher potential current is flowing towards you. Now place another CT with P1 towards the arbitrary node P2 on the other side and connect another relay at s2 side (now you can visualise 2 CTs on either side of the node with P1 towards the node X in both CTs). The current will be flowing from P2 to P1 in this CT and hence S1 will be negative with respect to S2 and current flow in this relay will be in reverse direction as that of the first relay.

In the next half cycle the current direction reverses and first relay current will be s2 to s1 thru relay and second relay current is from S1 to S2 thru second relay.

Now we want the relay no 1 to operate and relay no 2 not to operate. how do you achieve it when you connect the current alone to the relay which is changing the direction in every half cycle .
To achieve this now you connect a PT at the same arbitrary node X and connect the voltage to both the relays. The same point of the PT secondary voltage is connected to both the relays. Now find the phase angle between the current in the first relay and the voltage. You will see that when node voltage is positive, the current flow in the first CT will be from P1- P2 in primary and S1 to S2 in the secondary. In this case let us say that the phase angle between voltage and current is Zero in the first relay.

What happens in the second relay and CT /PT. The voltage is same as first relay which is positive and the current flow in the CT is P2 to P1 and in relay it is S2 to S1
– meaning opposite direction to first relay . As the current is in reverse direction with respect to voltage we can say that they are out of phase (180 degree).
Now you set both relays to operate when the voltage and currents are in phase. observe the result in first half cycle . Relay 1 operates (phase angle between v and I zeo ) and relay 2 no operation ( phase angle 180 deg ).

In the second half cycle observe the phase angle of relay 1 . Voltage at node is negative. (voltage phasor reversed ) Current flows from P2 to P1 and S2 to s1 in the relay 1 (current phasor also reversed) still the phase angle is zero and hence relay 1 operates Similarly relay 2 restrains.

Hence we found that, the relay 1 operates in both half cycle and relay 2 restrains. This is the importance of Voltage for directional relay.

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.

Resistance Grounding System

Low Resistance Grounding:
1. Limits phase-to-ground currents to 200-400A.
2. Reduces arcing current and, to some extent, limits arc-flash hazards
associated with phase-to-ground arcing current conditions only.
3. May limit the mechanical damage and thermal damage to shorted
transformer and rotating machinery windings.
4. Does not prevent operation of overcurrent devices.
5. Does not require a ground fault detection system.
6. May be utilized on medium or high voltage systems. GE offers low
resistance grounding systems up to 72kV line-to-line.
7. Conductor insulation and surge arrestors must be rated based on the lineto-
line voltage. Phase-to-neutral loads must be served through an
isolation transformer.

High Resistance Grounding:
1. Limits phase-to-ground currents to 5-10A.
2. Reduces arcing current and essentially eliminates arc-flash hazards
associated with phase-to-ground arcing current conditions only.
3. Will eliminate the mechanical damage and may limit thermal damage to
shorted transformer and rotating machinery windings.
4. Prevents operation of overcurrent devices until the fault can be located
(when only one phase faults to ground).
5. Requires a ground fault detection system to notify the facility engineer that
a ground fault condition has occurred.
6. May be utilized on low voltage systems or medium voltage systems up to
5kV. IEEE Standard 141-1993 states that “high resistance grounding
should be restricted to 5kV class or lower systems with charging currents
of about 5.5A or less and should not be attempted on 15kV systems, unless
proper grounding relaying is employed”.
7. Conductor insulation and surge arrestors must be rated based on the lineto-
line voltage. Phase-to-neutral loads must be served through an
isolation transformer.

Conclusion:
Resistance Grounding Systems have many advantages over solidly grounded systems including arc-flash hazard reduction, limiting mechanical and thermal damage associated with faults, and controlling transient overvoltages. High resistance grounding systems may also be employed to maintain service continuity and assist with locating the source of a fault.
When designing a system with resistors, the design/consulting engineer must consider the specific requirements for conductor insulation ratings, surge arrestor ratings, breaker single-pole duty ratings, and method of serving phase-to-neutral loads.

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?

480volt Solidly grounded system versus HRG system

High Resistance Ground will limit the current to about 5 amps. The good news is that it no longer be necessary to trip on a ground fault. The bad news is that you may not connect any single phase loads to that substation. If the single phase loads are an issue, it may be possible to support all those loads with one or two feeders. In that situation, an isolation transformer is added to create a separately derived ground.

High resistance grounding is an excellent option in systems where continuity of service is important. However it is important to understand that if a ground fault occurs, it needs to located and repaired. This can be at a time convenient to facility operation, but it must be chased down and fixed.

This troubleshooting is accomplished by pulsing the ground fault current between 5 and 8 amps. Then a hand held clamp on ammeter is used to search out the offending feeder. Most HRG manufacturers include this feature into their design.

480volt solid ground system the fault current L-g is limited to the fault rating of the system may be 50KA,and usually have a disastrous consequences during faults and system has to be properly selected and protected accordingly.

Presently for some emergency power systems in power plants there is a 480voltb 3ph supply is made with neutral grounded through a NGT ie to limit the L-g fault current within 10Amp, and this may result in overvoltage of the other two phases too. Overall this becomes a more stable system.

I would be very hesitant to use HRG for a medium voltage system. While a fault is on the system the neutral will not be at earth potential. This is usually not so important because the neutral is not carried to any of the loads on a high resistance grounded system. But the effect of the neutral straying away from ground potential is the two unfaulted phases will have a higher than normal line-to-ground voltage. Low voltage systems have a lot of built in margin in their insulation so it is not a problem with most equipment connected to that system. However MV equipment does not have so much “spare” insulation so the effect of high resistance grounding of a MV system is that it significantly increases the chance of migrating the fault from a single-line-to-ground fault to a double-line-to-ground or line-to-line fault.

Also at the point of the fault, the energy of 5 amps flowing in a 480 volt or 400 volt system may be somewhat dangerous but in most cases will be dissipated easily. On a higher voltage system, whether it is 4.16kV, 6.6kV, or higher, the energy at the point of the fault is higher and is much more likely to damage the insulation on the adjacent conductors and quickly turn into a more severe fault.

So I would resist using a high resistance grounded design on a medium voltage system (over 2000 volts).

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).

AC induction motor constant power

An AC induction motor is supposed to be a constant power motor, which implies it draws more current on low voltage. Consider a motor running a constant torque load at a particular speed. Suppose now the voltage is reduced, which should cause it to settle down at a lower speed supplying the same torque as per the new torque speed characteristic. If we consider the electrical side, higher slip will cause more current to be drawn that too at higher pf, which should maintain the power justifying the above theory. But on the mechanical side the new output power Torque x speed is supposed to be lesser now as speed is less now. Is it this contradiction?

The following guidelines prove there is no contradiction since your question about Motor under running condition:

1. Torque / Slip characteristic for Induction Motor has three Zones.
a)- Starting Torque @ S=1, selection of this torque depends on the application. The starting should be greater the system torque at time of starting.
b)- Unstable Zone during which acceleration and torque development took place. This zone up to the Max. Torque can be developed.
In this regard, it may be necessary to mention that the seventh harmonics to be considered otherwise crawling / clogging may occur.
c)- Normal Operating Zone. NOZ about which your query raised. NOZ ranged as ” 0 < S< 1″ ie up to the Max. Torque. It is worth mentioning that Max. torque always remain the same regardless to its location of occurrence.

2. The torque is directly proportional to rotor resistance “r2” & varies with slip “S”. hence increase of rotor resistance is the most practical method of changing the torque (ie wound rotor Slip ring Motors). Moreover, the Max torque achieved when rotor Resistance “r2” = The Stator impedance, At starting S=1.

3. Accordingly, the ration r2/x2 gives the location of the max. Torque w.r.t Slip (if the max. torque is required at starting (S=!) then r2/x2 should equal “1”.

4. load being constant. Mechanical output = Electrical input – losses.

5. Tmax Propotional to Sq(v). decrease of 50% of the supply voltage generate a reduction of 20% in the max. running Torque (zone c) , increase in slip and also Full load current and temperature raise increase while the full load speed decrease. the status of the above parameters will be opposite if the voltage increases by 10%.

Based on the above, in all cases since the Motor is running within the operating range will be no issue unless the supply voltage falls behind the above limits (-50%, +110%). Accordingly, variable frequency drives provided by under/Over voltage protection relay to avoid damage to insulation due to Heat/temperature rise that will be generated due to excessive current intend to composite load.