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Old 07-06-2011, 10:34 AM   #1
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Default Example of capacitive load?

Can someone give me examples of capacitive loads encountered in residential, commercial or industrial electric? (i.e. not looking for semiconductor examples)
The 3 I'm aware of are:

1. Florescent lighting I'm not sure about this. If true then can it have a significant effect on the power factor?

2. Synchronous motor.

3. Capacitor bank.

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Old 07-06-2011, 01:10 PM   #2
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Originally Posted by swimmer View Post
Can someone give me examples of capacitive loads encountered in residential, commercial or industrial electric? (i.e. not looking for semiconductor examples)
The 3 I'm aware of are:

1. Florescent lighting I'm not sure about this. If true then can it have a significant effect on the power factor?

2. Synchronous motor.

3. Capacitor bank.
http://www.wagesterlease.com/aboutus...werfactor.html
Power Factor Correction
by Ralph A. Frye III, P.E.
Power factor has recently become a hot topic of discussion. Demand is no longer the only consideration for modern utility power systems. Due to new technology and other changes in typical power system characteristics, more emphasis has been placed on total capacity, or apparent power. Utilities are contractually required to provide apparent power to each customer and typically only actively bill for demand or real power. Power factor is the difference between the apparent power and the demand. Therefore, it is in the best interest of the utility to ensure that each of their customers is maintaining a good power factor in order to maximize capacity. But what does this mean to a new or existing power system?
Principle of Power Factor Correction
The total or apparent power used in all power systems consists of real power and reactive power.
Apparent power is described as kilo volt amperes (KVA). Real power is the working power and makes up the demand of the power system. Real power is described as kilowatts (KW). Reactive power is the power necessary to create the magnetic fields to operate the inductive equipment. Reactive power is described as kilovolt amperes reactive (KVAR). The power factor is the quantity that describes the relationship between real and apparent power being used by the power system. Power factor is described as a number less than or equal to one.
Power system loads consist of resistive, inductive, and capacitive loads. Examples of resistive loads are incandescent lighting and electric heaters. Examples of inductive loads are induction motors, transformers, and reactors. Examples of capacitive loads are capacitors, variable or fixed capacitor banks, motor starting capacitors, generators, and synchronous motors.
Inductive and capacitive loads are opposite in nature. Equal amounts of inductive and capacitive loads within the same system will offset each other leaving only real power. This is defined as a power factor of 1 or unity. When a unity power factor is achieved the real power (KW) or demand is equal to the apparent power (KVA). Achieving a unity power factor will provide the most efficient power system.
The reactive load of an industrial power system typically consists of a large number of AC induction motors. This can cause the total load to be up to 50% inductive. The large inductive load causes the apparent power to be 25% to 41% higher than the real power. If the utility billing is based on real power (KW) only, the utility must provide up to 41% more capacity than they are billing for. If the utility only bills for 2/3 of the total usage, steps must be taken to provide and recover the costs for providing the additional power. This is the reason utilities use power factor penalties or have changed to billing for apparent power (KVA) rather than (KW) demand. The bottom line is that the customer pays for the necessary capacity.
Power Factor Correction in the Real World
In the real world, utilities normally only require a power factor of 0.9. Although a unity power factor will provide the most efficient power system, a unity power factor will leave the power system susceptible to harmonic problems. Harmonic problems cause excessive heating in motors, nuisance tripping, and premature failure of solid state components.
Power factor correction is usually achieved by adding capacitive load to offset the inductive load present in the power system. The power factor of the power system is constantly changing due to variations in the size and number of the motors being used at one time. This makes it difficult to balance the inductive and capacitive loads continuously.
In addition, harmonic problems can be introduced if the capacitors are not sized with the specific power system characteristics in mind. The utility company may also restrict or deny the introduction of KVAR's into their power system. These are all considerations that need to be addressed prior to making any decisions about the size or type of power factor correction.
The most inexpensive and widely used method of correcting the power factor is through the use of one fixed capacitor bank connected to the incoming transformer or switchgear bus. The fixed capacitor bank is sized to regulate a 0.9 power factor during maximum operational inductive loading. This means that during periods of operation where less than maximum inductive loading is utilized, extra KVAR capacity will be introduced into the utility power system. The only draw backs to this method are utility restrictions and future inductive loads that change the maximum operational inductive loading.
A variation of the above method can be used if only a discrete number of motors are causing the power factor problems. Individual capacitors can be connected in parallel with each motor. When the motor is energized, the capacitor bank is also energized to provide power factor correction while the motor is being used. The benefit of this method is that the amount of capacitive load is regulated with the amount of inductive load. The drawbacks to this method are that it may not be feasible physically or economically to have an individual capacitor for each motor, and maintenance of multiple units may be costly and difficult.
Another method of power factor correction is the use of a variable capacitor bank. This bank would be connected just like the fixed bank. The advantage of the variable capacitor bank is that the bank monitors the system power factor and automatically regulates the amount of capacitive load connected to the system to offset the inductive load. Since the capacitive load is regulated, there would be no conflict with the utility. The variable capacitor banks normally come with internal protection, provide space for additional banks, and provide a centrally located easily maintained unit. The draw backs to the variable capacitor bank are an increased chance of harmonic problems due to the variations in capacitance, initial cost, and maintenance costs of internal parts used for capacitor switching.
A combination of the above methods seems to be the normal configuration that is used once correction is decided upon. Normally, capacitors are connected to the largest motors to provide correction while they are running. In addition, a variable or fixed capacitor bank is connected to the main transformer or switchgear. The advantage of this is regulation of the capacitive load and a reduction in the size of the capacitor bank connected to the main transformer or switchgear.
Benefits of Power Factor Correction
The primary benefit of power factor correction is the elimination of charges related to reactive power-consumption. If the utility is adding a power factor penalty or billing for apparent power (KVA), reduction in reactive power will net savings. The amount of savings seen will depend on the size, configuration, and operation of the power system. Typically, the costs for correction are paid back inside of one year, and after that, the savings will reduce operating costs. In addition, power factor correction will improve the overall performance of the power system which can increase switchgear, starter, and motor life. The bottom line is protection, efficiency, and savings.
If you have concerns or questions about power factor correction, we would be happy to take a look at your specific system needs.
2003 Wagester & Lease, Inc., Port-Land Systems, Inc. All rights
http://en.wikipedia.org/wiki/Power_factor

http://pjm.com/training/~/media/trai...2011%2009.ashx


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Old 07-06-2011, 06:50 PM   #3
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Originally Posted by swimmer View Post
Can someone give me examples of capacitive loads encountered in residential, commercial or industrial electric? (i.e. not looking for semiconductor examples)
The 3 I'm aware of are:

1. Florescent lighting I'm not sure about this. If true then can it have a significant effect on the power factor?

2. Synchronous motor.

3. Capacitor bank.
While it may not amount to much, every circuit has a bit of capacitive coupling. any 2 conductors with a potential difference and insulation between is a capacitor. So the wire in the walls is a capacitive load.
More to the point of your question, the electronic loads have lots of internal capacitors. Radio installations have lots of capacitors but by and large the only big loads are power factor correction capacitors and filters. The majority of effective capacitance is only added to correct inductive reactance for power factor.
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Old 07-06-2011, 07:16 PM   #4
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Floro lights are typically an inductive load,
The capacitors are installed to correct the power factor.
And not all floros have caps any way.
In australia they are available with or with out caps.

Most motors too are inductive loads,
Caps are used for starting the motor.

Power factor correction is more important to the poco's
than any body else,
because power meters dont read the higher frequency
currants flowing, but they are there non the less
And the network has to carry them.

most residential customers dont have to worry too
much about it.
How ever larger industrial type users would.

Most modern electronic equipment with internal
power supplies would be an capacitive load
due mainly to the large filter capacitor in the
power supply.
But modern designs are tring to correct this.
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Old 09-23-2013, 01:31 PM   #5
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I stumbled across this thread and perhaps somebody here may be able to shed some light on my situation.

Background:

I work in a laboratory that has somewhere around 150 induction motors ranging in size from 5 HP to 300 HP. Several of these motors are controlled with VFDs. Others are connected across the line. Quite a few of them are two speed variable torque while about 6 of them are two speed constant torque. I would say the average size motor is 50 HP to 75 HP. We also have an abundance of fluorescent lighting and approximately 45 high bay 250W metal halide lamps.

We have three 1.5 MVA substations 13.8 kV delta primary 480Y/277 secondary.

We recently had digital power meters installed on each of the substations to track energy usage and to develop energy conservation strategies. What I am struggling to understand is the data I am getting from the meters related to power factor and reactive power.

We recently had a facility wide planned power shutdown for maintenance and this gave me a chance to get some data from the meters as we shut everything down.

With nothing energized in the building at all except for the 480/240 stepdown transformers and inductrol regulators, all with no load on them, and a few dozen 100W crankcase heaters for compressors, I read 224.1 kW and +55.1 kvar. (sum of all three substations) This I understand, the transformers are inductive loads and they are unloaded so we're "consuming" a good amount of vars. As I turn the building lighting on, the vars decrease on the substation that serves the lighting panels. This I can also understand because of the capacitors in the ballasts. What I'm failing to understand is that as I start turning motors on the vars continue to decrease. If I start a motor in low speed the vars might increase slightly then as I go to high speed they decrease quite a bit. On motors with compressor loads, if I start the compressor at minimum load the vars increase slightly then decrease drastically as I load the motor up to and beyond full load. Right now at this time a fair amount of equipment is running in the building. My power is reading 712.9 kW and my vars are reading -141.7 kvar. All meters are indicating a capacitive load and they say that I am "exporting" vars on the net energy page.

Could somebody explain how my load is so capacitive with primarily induction motors running? All VFDs above 5 HP have line reactors on them. Even as the VFD controlled motors increase load the vars go more negative.

We have no power factor correction capacitors in the building or at the substations.

Any insight would be greatly appreciated.

Thank you.
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Old 09-23-2013, 01:43 PM   #6
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Old 09-23-2013, 01:51 PM   #7
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Any insight would be greatly appreciated.

Thank you.
I'm not experienced with this stuff but is it possible that harmonics from VFDs and other digital controllers are influencing the VAR measurements?
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Old 09-23-2013, 02:37 PM   #8
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I'm not experienced with this stuff but is it possible that harmonics from VFDs and other digital controllers are influencing the VAR measurements?

I wouldn't rule it out but I would hope that the line reactors are mitigating most of the harmonics. These meters measure harmonics as well and the voltage THD is less than 1% while the current THD is between 3% and 5%.
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Old 09-23-2013, 08:23 PM   #9
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I'll take a stab at this:
Quote:
Originally Posted by allenc View Post
...I read 224.1 kW and +55.1 kvar (sum of all three substations) This I understand, the transformers are inductive loads and they are unloaded so we're "consuming" a good amount of vars.
I believe this is the other way around. You have positive VARs, you have a leading power factor of 0.97. I would guess this is because a large unloaded MV transformer has a very significant capacitance, but I'll admit I've never seen that.
Quote:
...My power is reading 712.9 kW and my vars are reading -141.7 kvar.
Here you have a lagging power factor of 0.98 because your VARs are negative.

It would make sense that as you tend to put more current on your transformer and increase the magnetic flux density, you are increasing the energy stored in the magnetic field and increasing the lagging reactive power. This would also be true to what you would "expect" to see of a heavy inductive load.

My gut is that the type of transformer secondary load doesn't affect primary PF, i.e., it wouldn't matter if your transformer was running inductive or capacitive loads, the primary PF would only follow the change in kVA.
Quote:
...All meters are indicating a capacitive load and they say that I am "exporting" vars on the net energy page...
I can't explain this. I'm tempted to ask if there's a CT backwards...? But if that was true, none of the other readings would be correct.
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Old 09-23-2013, 09:00 PM   #10
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Thanks very much for the response.

I think I follow what you are saying. Let me clarify that the metering is on the secondary side of the substation transformers.

My first reaction when we commissioned the meters was "there's a CT backwards." With these meters you can reverse the polarity through the software. I figured, as you said, that if everything else seems to be reading properly then the CTs must be installed and configured properly. Just for the hell of it I reversed the polarity. My watts went negative and my vars went positive so I knew that wasn't right. I'm certainly not generating power. Everything else is very believable except for the vars.

We have loads with known kW and kVA draws that have previously been measured with calibrated power meters that were tested before the utility meters were installed. When I energize these loads the utility meters agree with the calibrated meter's numbers very closely so I'm very confident that my kW and kVA measurements are real.

The meter records net energy and the kWh and kVAh increment upwards as I would absolutely expect them to. But when we're running balls to the wall the vars decrement. The "import" value of watts and kVA increment and the "export" value of vars increment. (negative vars increment the "export" value, decrement the "import" value) It also has an indicator of "load type." Represented by a symbol of an inductor, or a symbol of a capacitor. (neither if the load is purely resistive) In the middle of the day when we're running, it is indicating a capacitive load type which simply does not make sense to me given the primarily inductive nature of our load.

The meters are AcuvimII meters from Accuenergy Corp. with 2000:5 CTs.

Again, thanks very much for the response. This has got me and a lot of the more senior engineers stumped.
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Old 09-23-2013, 09:15 PM   #11
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...Let me clarify that the metering is on the secondary side of the substation transformers....
Dammit, well, ignore everything I wrote previously.

Give me a bit to look through that manual.
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Old 09-23-2013, 09:16 PM   #12
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Can someone give me examples of capacitive loads encountered in residential, commercial or industrial electric? (i.e. not looking for semiconductor examples)
The 3 I'm aware of are:

1. Florescent lighting I'm not sure about this. If true then can it have a significant effect on the power factor?

2. Synchronous motor.

3. Capacitor bank.
Underground single shielded cables by far.
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Old 09-23-2013, 10:49 PM   #13
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The only other thing I can think of is if your PT polarity is somehow goofed up causing the Vars to read backwards.
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Old 09-27-2013, 08:27 AM   #14
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The only other thing I can think of is if your PT polarity is somehow goofed up causing the Vars to read backwards.
I think this is the problem. I have checked everything that I can check in the software and also verified the configuration of the CTs. I spoke with the meter manufacturer on the phone and he is thinking that the PTs arent aligned with the phase to phase voltages that they should be and that's causing the vars to read backwards. I'm going to check this as soon as we have a window to shut down the substations. Hopefully during the next weekend or two.

Thanks.

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