AC Power (back to basics)

[Steven Mill]

AC power in any electric circuit is the rate of flow of energy at a specific part of that electric circuit. As we all know, the electric energy flow from the generation side toward the consumer load side. But some part of this energy is bounce back to generation side and some part is completely utilized by the load.

Electric power energy that oscillates between source and load is known as reactive power while completely utilized electrical power energy is known as real power. Both the reactive power and real or active or true power are related with the apparent or total power through the following figure.

From the figure, it can be deduced that;

(Total Power)² = (True Power)² + (Reactive Power)²

Or (kVA)² = (kW)² + (kVAr)²

The power factor terminology also rises due to interrelation of active and reactive power components. As in active power, both the current and voltage are in phase with each other, hence power factor, which is the phasor angle between voltage and current phasor is zero. Thus power factor (cosine of angle zero) is one, so total power in such a circuit is completely utilized and is only possible for a pure resistive load.

But if the load is not purely resistive (i.e. it is either inductive or capacitive or both inductive and capacitive load along with some resistance), then such load will store part of electric power energy in the form of electric field in capacitor and some part of electric power energy in the form of magnetic field in the inductor. Hence this storage of energy and is swinging back and forth between source and load causes a time lag in the voltage and current phasors, thus rising to a non unity power factor load. Furthermore this power factor is categorized as leading and lagging due to capacitive load and inductive load respectively. It is shown in the following figure.

In usual practice, all the loads are inductive, thus the power factor is always lagging, hence in grid station, a capacitor bank is installed, which is commonly known for the purpose of power factor improvement. This capacitor bank actually store the leading reactive power, and thus when it is made a part of the transmission lines coming in or going out of the grid station, then this capacitor bank transfer the stored leading reactive power, and consequently the phasor angle between voltage phasor and current phasor decreases, thus the active power component is increased and hence power factor is improved.

Furthermore it is common observation that why transformers are rated in kVA and electric motor are rated in kW? It is because that transformer actually transfers the total apparent power from its primary winding to secondary winding while electric motor need some real power consumption for its normal operation, that’s why it is rated in kW.

When a practical transformer is on a no load condition, then still it draw a very small no load current. This no load current is divided into two rectangular components, i.e. active current component and magnetizing current component. The active current component is in phase with the applied voltage, and thus both together contribute for the iron loses and very small primary copper loss. While the magnetizing current component lags the applied voltage by 900 and is responsible for the production of mutual flux in the core. Here again the real power utilization is due to the active current component, because it is only component that is responsible for supplying both iron and copper loses.

This was a basic explanation of Active Power. Did this help you?

[Spir Georges GHALI]

Dear Mr. Mill ;

As I remember, that I mentioned before in other Topic to the same remarks that are :

1- The Reactive Energy doesn’t oscillate between “ Source & Load ” ( as mentioned in this Topic ).

2- The reactive Energy is a “ Consumed Energy ” by the installed loads either Inductive or Capacitive, and for that the Utilities install the Reactive Energy Counters “ kVAR meters ”, because if it’s oscillate that means it doesn’t be consumed and no need for these counters.

3- The Capacitors and Inductive Loads consume Reactive Energy, and they don’t store this energy and swinging back and forth between source and load ( as mentioned in this Topic ).

4- The Transformers’ Power are rated in “ Apparent Power – kVA ” because we don’t know the type of loads that will be supplied by them, as the Apparent power will be divided into “ Active Power  &  Reactive Power ” according to the “ Power Factor value ” of the supplied loads.

[zulfadhli]

@Spir Georges GHALI said:
Dear Mr. Mill ;

As I remember, that I mentioned before in other Topic to the same remarks that are :

1- The Reactive Energy doesn’t oscillate between “ Source & Load ” ( as mentioned in this Topic ).

2- The reactive Energy is a “ Consumed Energy ” by the installed loads either Inductive or Capacitive, and for that the Utilities install the Reactive Energy Counters “ kVAR meters ”, because if it’s oscillate that means it doesn’t be consumed and no need for these counters.

3- The Capacitors and Inductive Loads consume Reactive Energy, and they don’t store this energy and swinging back and forth between source and load ( as mentioned in this Topic ).

4- The Transformers’ Power are rated in “ Apparent Power – kVA ” because we don’t know the type of loads that will be supplied by them, as the Apparent power will be divided into “ Active Power  &  Reactive Power ” according to the “ Power Factor value ” of the supplied loads.

I agree with your explanation.

1. Reactive Power is indeed consume by inductive load and produced by Capacitive load. 

2. Utility Company does install kVAR meter as in located in Industry Area, as they install / use a lot of inductive machinery.

[Spir Georges GHALI]

@zulfadhli said:

@Spir Georges GHALI said:
Dear Mr. Mill ;

As I remember, that I mentioned before in other Topic to the same remarks that are :

1- The Reactive Energy doesn’t oscillate between “ Source & Load ” ( as mentioned in this Topic ).

2- The reactive Energy is a “ Consumed Energy ” by the installed loads either Inductive or Capacitive, and for that the Utilities install the Reactive Energy Counters “ kVAR meters ”, because if it’s oscillate that means it doesn’t be consumed and no need for these counters.

3- The Capacitors and Inductive Loads consume Reactive Energy, and they don’t store this energy and swinging back and forth between source and load ( as mentioned in this Topic ).

4- The Transformers’ Power are rated in “ Apparent Power – kVA ” because we don’t know the type of loads that will be supplied by them, as the Apparent power will be divided into “ Active Power  &  Reactive Power ” according to the “ Power Factor value ” of the supplied loads.

I agree with your explanation.

1. Reactive Power is indeed consume by inductive load and produced by Capacitive load. 

2. Utility Company does install kVAR meter as in located in Industry Area, as they install / use a lot of inductive machinery.

 

Dear Mr. Zulfadhli

For remark No. 1 : OK for the 1st part that Inductive Loads consume Active Energy & Reactive Energy, but please note that :

1- The Capacitors doesn’t produce any power.

1- The Capacitors are ” Loads “ consume a lot of Reactive Energy and too small of Active Energy.

2- The Current’s Vector of Capacitive Loads is ” Leading “, and the Current’s Vector of Inductive Loads is ” Lagging “, therefore the Sum of these 2 currents is ” Vector Adding “.

[Anonymous]

Thanks for the great explanations and detail, I have been using this blog to show to an apprentice and explain this to him.
He is struggling with the concept in his final year. This has been a great help. Very in depth.
http://www.jacksonelectrical-services..com

[Jacques Schonek]

Back to the initial post on this topic, I have a few remarks:

– reactive power is a purely mathematical concept. It comes from the vector representation of voltage and current. The current component which is in phase with the voltage is the active current. The current component which is in quadrature is called “reactive” current. When multiplied by the voltage, it gives the reactive power. This is consistent with the definitions given by the International Electrotechnical Commission IEC. This is illustrated in one paper on this blog: “6/8 – Electrical equipment: improve power factor”

-reactive power does not oscillate between generator and load. In fact, when reactive power is present, the power transmitted from generator to load is positive during a part of the power line period, and negative during another part. This is illustrated in one paper on this blog: “4/8 – Electrical equipment: power harmonics, power factor”. This is why oscillation is often mentionned.

-in order to avoid confusion, let’s use the definitions used by IEC: apparent power (and not total power), active power (and not true power).

– the symbol for reactive power is “var” (with small letters). See the paper ” KVAR, kVAr or kvar? on this blog.

– transformers are rated in VA, kVA or MVA because the size of a transformer depends on the current AND voltage ratings, whatever the phase angle between voltage and current.

– motors are rated in kW because this is the mechanical power they can continuously deliver on the shaft. The number of kW of a motor is a mechanical unit, and not electrical unit. The electrical power absorbed by a motor can be calculated by taking account of efficiency.

[Spir Georges GHALI]

Dear Mr. Schonek

First, you’re right, the symbol of Reactive Power according of “ IEC 60027-1 ” is “ var ”.

It’s also right that the Reactive Power is mathematical concept, but as it’s measured that means it’s a consumption of a kind of Power.

[Anonymous]

i like this information electrical engineering . please tell me calculated to motor running load & inveteor saving percent load mathed .
thanking
regard
riaz bukhari

[zulfadhli]

Sir Georges,

 

Let talk about power generation,

I had done some visit to Power Generation Control Room during my university year. I donno if this is concept is wrong or correct. The Technician said that, the generator produce active and reactive power. But, production of reactive power is limited to provide extra life span to the generator. In addition, he said that, therefore on the transmission line system, utility company design reactor to supply reactive power into the transmission line to recap the usage of reactive power in the transmission and improve power quality in term of power correction.

[Spir Georges GHALI]

@zulfadhli said:
Sir Georges,

 

Let talk about power generation,

I had done some visit to Power Generation Control Room during my university year. I donno if this is concept is wrong or correct. The Technician said that, the generator produce active and reactive power. But, production of reactive power is limited to provide extra life span to the generator. In addition, he said that, therefore on the transmission line system, utility company design reactor to supply reactive power into the transmission line to recap the usage of reactive power in the transmission and improve power quality in term of power correction.

dear Mr. zulfahdli ;

As we know all, any Electrical Power Generation Center generates ” Apparent Power ” in ” kva “, that will be divided into ” Active  &  Reactive Powers ” depending on the Power Factor’s value of the loads, where the resistive loads consume only ” Active Power “, and the ” Inductive & Capacitive Loads ” consume both kind of power ” Active & Reactive ” depending on the Power Factor’s value.

Beside , in most of countries, the Utilities oblige the end users of important values of Electric Energy to install the convenient Capacitors to improve the Power Factor’s Value of there sites till a predefined value ( ≥0.93 ). Also, the utilities use ” Capacitors and/or Synchronous Motors ” to improve the Power Factor value in the Transformation & Generation Centrals.

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