What is kvar? What are kVA and kW - how to convert kW to kVA Kilovolt ampere reactive.
In this article we will look at what kVA, kW, kVAr are? What does each quantity mean and what is the physical meaning of these quantities.
What is KVA? KVA is the most mysterious word for the electricity consumer, as well as the most important. To be precise, we should discard the prefix kilo- (10 3) and get the original value (unit of measurement) VA, (VA), Volt-Amperes. This value characterizes Total electrical power, having an accepted letter designation according to the system - S. Total electrical power is the geometric sum of active and reactive power, found from the relation: S 2 =P 2 +Q 2, or from the following relations: S=P/
or S=Q/sin(φ). The physical meaning of Total power is to describe the total consumption of electrical energy to perform any action by an electrical apparatus.
The power ratio can be represented as a Power Triangle. On the triangle, the letters S(VA), P(W), Q(VAr) indicate Total, Active, Reactive power, respectively. φ is the phase shift angle between voltage U(V) and current I(A), which is essentially responsible for increasing the total power of an electrical installation. The maximum performance of the electrical installation will be at tending to 1.
What is kW? kW is no less a mysterious word than kVA. Again, we discard the prefix kilo- (10 3) and get the original value (unit of measurement) W, (W), Watt. This value characterizes the Active consumed electrical power, which has an accepted letter designation according to the system -P. Active consumed electrical power is the geometric difference between total and reactive power, found from the relation: P 2 =S 2 -Q 2 P=S*
.
Active power can be described as the part of the Total power expended to perform a useful action by an electrical apparatus. Those. to do "useful" work.
The least used designation remains - kVAR. Again, let's discard the prefix kilo- (10 3) and get the original value (unit of measurement) VAR, (VAR), Volt-ampere reactive. This value characterizes the Reactive electrical power, which has an accepted letter designation according to the system - Q. Reactive electrical power is the geometric difference between total and active power, found from the relation: Q 2 =S 2 -P 2, or from the following relation: Q =S* sin(φ).
Reactive power can have or character.
A typical example of the Reaction of an electrical installation: an overhead line relative to the “ground” is characterized by a capacitive component; it can be considered as a flat capacitor with an air gap between the “plates”; while the motor rotor has a pronounced inductive character, appearing to us as a wound inductor.
Reactive power can be described as the part of the Total power expended on transient processes containing . Unlike Active power, Reactive power does not perform “useful” work when an electrical apparatus is operating.
Let's summarize: Any electrical installation is characterized by two main indicators from the following: Power (Full (kVA), Active (kW)) and the cosine of the voltage shift angle relative to the current - . The value ratios are given in the article above. The physical meaning of Active power is the performance of “useful” work; Reactive - spending part of the energy on transient processes, most often these are losses due to magnetization reversal.
Examples of obtaining one quantity from another:
Electrical installation given with indicators: active power (P) - 15 kW, Cos(φ)=0.91. Thus, the total power (S) will be - P/Cos(φ)=15/0.91=16.48 kVA. The operating current of an electrical installation is always based on the total power (S) and is for a single-phase network - I=S/U=15/0.22=68.18A, for a three-phase network - I=S/(U*(3)^0, 5))=15/(0.38*1.73205)=22.81A.
Electrical installation given with indicators: total power (S) - 10 kVA, Cos(φ)=0.91. Thus, the active component of power (P) will be - S*Cos(φ)=10*0.91=9.1 kW.
Electrical installation given- TP 2x630 kVA with indicators: total power (S) - 2x630 kVA, active power must be allocated. For multi-apartment housing with electric stoves, we apply Cos(φ) = 0.92. Thus, the active component of power (P) will be - S*Cos(φ)=2*630*0.92=1159.2 kW.
The basic unit of power measurement for electrical equipment is kW (kilowatt). But there is another unit of power that not everyone knows about - kvar.
kvar (kilovar)– unit of measurement of reactive power (volt-ampere reactive – var, kilovolt-ampere reactive – kvar). In accordance with the requirements of the International Standard for Units of Measurement Systems SI, the unit of measurement of reactive power is written “var” (and, accordingly, “kvar”). However, the designation "kvar" is widely used. This designation is due to the fact that the SI unit of measurement for total power is VA. In foreign literature, the generally accepted designation for the unit of measurement of reactive power is " kvar". The unit of measurement of reactive power is equated to non-system units, acceptable for use on a par with SI units.
AC power receivers consume both active and reactive power. The power ratio of an AC circuit can be represented as a power triangle.
On the power triangle, the letters P, Q and S indicate active, reactive and apparent power, respectively, φ is the phase shift between current (I) and voltage (U).
The value of reactive power Q (kVAr) is used to determine the apparent power of the installation S (kVA), which in practice is required, for example, when calculating the apparent power of a transformer supplying equipment. If we consider the power triangle in more detail, it is obvious that by compensating for reactive power, we will also reduce the consumption of total power.
It is extremely unprofitable for enterprises to consume reactive power from the supply network, since this requires increasing the cross-sections of supply cables and increasing the power of generators and transformers. There are ways to receive (generate) it directly from the consumer. The most common and effective way is to use capacitor units. Since the main function performed by capacitor units is reactive power compensation, the generally accepted unit of their power is kVAR, and not kW as for all other electrical equipment.
Depending on the nature of the load, enterprises can use both non-regulated capacitor units and units with automatic regulation. In networks with sharply variable loads, thyristor-controlled installations are used, which allow capacitors to be connected and disconnected almost instantly.
The working element of any capacitor installation is a phase (cosine) capacitor. The main characteristic of such capacitors is power (kVAr), and not capacitance (μF), as for other types of capacitors. However, the functioning of both cosine and conventional capacitors is based on the same physical principles. Therefore, the power of cosine capacitors, expressed in kVAr, can be converted into capacitance, and vice versa, using correspondence tables or conversion formulas. Power in kVAr is directly proportional to the capacitance of the capacitor (μF), frequency (Hz) and square of the voltage (V) of the supply network. The standard range of capacitor power ratings for the 0.4 kV class ranges from 1.5 to 50 kVAr, and for the 6-10 kV class from 50 to 600 kVAr.
An important indicator of energy efficiency is the economic equivalent of reactive power kE (kW/kVAr). It is defined as a reduction in active power losses to a reduction in reactive power consumption.
Values of the economic equivalent of reactive power
| Characteristics of transformers and power supply systems | At maximum system load (kW/kVAr) | At minimum system load (kW/kVAr) |
|---|---|---|
| Transformers powered directly from station buses using generator voltage | 0,02 | 0,02 |
| Network transformers powered by a power plant using generator voltage (for example, industrial transformers powered by factory or city power plants) | 0,07 | 0,04 |
| Step-down transformers 110-35 kV, powered from district networks | 0,1 | 0,06 |
| Step-down transformers 6-10 kV, powered from district networks | 0,15 | 0,1 |
| Step-down transformers fed from district networks, the reactive load of which is covered by synchronous compensators | 0,05 | 0,03 |
There are also “larger” units of measurement of reactive power, for example megavar (Mvar). 1 Mvar is equal to 1000 kVAr. Megavars usually measure the power of special high-voltage reactive power compensation systems - static capacitor banks (SCB).
When talking about the power of electrical appliances, we usually mean active energy. But many devices also consume reactive energy. This article explains what kVA is and how kVA differs from kW.
Active and reactive energy
In an alternating current network, the magnitude of current and voltage varies in a sinusoidal manner with the frequency of the network. This can be seen on the oscilloscope screen. All types of consumers can be divided into three categories:
- Resistors, or active resistances, consume only active current. These are incandescent lamps, electric stoves and similar devices. The main difference is the phase coincidence of current and voltage;
- Chokes, inductors, transformers and asynchronous electric motors use reactive energy and convert it into magnetic fields and back EMF. In these devices, the current is 90 degrees out of phase with the voltage;
- Capacitors - convert voltage into electric fields. In alternating current networks they are used in reactive power compensators or as current-limiting resistors. In such devices, the current leads the voltage by 90 degrees.
Important! Capacitors and inductors shift the current relative to the voltage in opposite directions and, when connected to the same network, cancel each other out.
Active is the energy released at an active resistance, such as an incandescent lamp, electric heater and other similar electrical appliances. In them, the phases of current and voltage coincide, and all the energy is used by the electrical appliance. In this case, the differences between kilowatts and kilovolt-amperes disappear.
In addition to active energy, there is reactive energy. It is used by devices whose design contains capacitors or coils with inductive resistance, electric motors, transformers or chokes. Long cables also have it, but the difference with a device with purely active resistance is small and is taken into account only when designing long power lines or in high-frequency devices.
Full power
In real conditions, purely resistive, capacitive or inductive loads are very rare. Typically, all electrical appliances use active power (P) together with reactive power (Q). This is the total power, designated “S”.
To calculate these parameters, the following formulas are used, which you need to know in order to carry out, if necessary converting kVA to kW and vice versa:
- Active is useful energy converted into work, expressed in W or kW.
KVA can be converted to kW using the formula:
where “φ” is the angle between current and voltage.
These units measure the payload of electric motors and other devices;
- Capacitive or inductive:
Displays energy loss due to electric and magnetic fields. Unit of measurement – kVar (kilovolt-ampere reactive);
- Full:
- U – network voltage,
- I – current through the device.
Represents the total electrical energy consumption of a device and is expressed in VA or kVA (kilovolt-amperes). Transformer parameters are expressed in these units, for example, 1 kVA or 1000 kVA.
For your information. Such devices of 6000/0.4 kV and a power of 1000 kVA are among the most common for powering electrical equipment of enterprises and residential neighborhoods.
Kvar, kVA and kW are related by a formula similar to the famous Pythagorean theorem (Pythagorean pants):
Important! It should be noted that a 10 kW electric motor cannot be connected to a 10 kVA transformer, since the electricity consumed by this device, taking into account cosφ, will be about 14 kilovolt-amperes.
Bringing cosφ to 1
The reactive energy used by consumers creates an extra load on the cable and starting equipment. In addition, you have to pay for it, just like for an active one, and in portable generators the lack of compensation increases fuel consumption. But it can be compensated by using special devices.
Consumers needing cosφ compensation
One of the main consumers of reactive energy are asynchronous electric motors, consuming up to 40% of all electricity. The Cosφ of these devices is about 0.7-0.8 at rated load and drops to 0.2-0.4 at idle. This is due to the presence of windings in the design that create a magnetic field.
Another type of device is transformers, the cosφ of which falls, and the consumption of reactive energy increases in unloaded devices.
Compensating devices
Different types of devices are used for compensation:
- Synchronous motors. When a voltage higher than the rated voltage is supplied to the excitation winding, they compensate for the inductive energy. This allows you to improve network parameters without additional costs. When replacing some asynchronous motors with synchronous motors, the compensation capabilities will increase, but this will require additional costs for installation and operation. The power of such electric motors reaches several thousand kilovolt-amperes;
- Synchronous compensators. These synchronous electric motors have a simplified design and a power of up to 100 kilovolt-amperes, are not intended to drive any mechanisms and operate in the X.X. mode. Their purpose is to compensate for reactive energy. During operation, these devices use 2-4% of active energy from the amount of compensated energy. The process itself is automated in order to achieve a cosφ value as close as possible to 1;
- Capacitor batteries. In addition to electric motors, capacitor batteries are used as compensators. These are groups of capacitors connected in a “triangle”. The capacity of these devices can be changed by connecting and disconnecting individual elements. The advantage of such devices is their simplicity and low active power consumption - 0.3-0.4% of the compensated one. The disadvantage is the impossibility of smooth adjustment.
So how many kW is in 1 kVA? This question cannot be answered unambiguously. This depends on various factors, and, above all, on cosφ. To carry out calculations and interpret the results, you can use an online calculator.
Knowing all the components of power, what the differences are between them, and how to convert kVA to kW is essential when designing electrical networks.
Video
When purchasing a diesel power plant, the first thing a consumer faces is choosing the power of the diesel generator set. In the specifications, manufacturers always indicate two units of power measurement.
kVA – total power of the equipment;
kW – active power of the equipment;
When choosing a generator or voltage stabilizer, it is necessary to distinguish the total power consumption (kVA) from the active power (kW), which is expended to perform useful work.
Power is a physical quantity equal to the ratio of work performed over a certain period of time to this period of time.
Power can be apparent, reactive and active:
- S – total power is measured in kVA (kiloVolt Amperes)
Characterizes the total electrical power of alternating current. To obtain the total power, the values of reactive and active powers are summed up. At the same time, the ratio of total and active power may differ for different electricity consumers. Thus, to determine the total power of consumers, their total, rather than active powers, should be summed up.
kVA characterizes the total electrical power, which has the accepted letter designation according to the SI system - S: this is the geometric sum of active and reactive power, found from the ratio: S=P/cos(ph) or S=Q/sin(ph).
- Q – reactive power is measured in kVar (kiloVar)
Reactive power consumed in electrical networks causes additional active losses (to cover which energy is consumed at power plants) and voltage losses (worsening voltage regulation conditions).
- P – active power is measured in kW (kilowatts)
This is a physical and technical quantity that characterizes useful electrical power. With an arbitrary load, an active current component acts in the alternating current circuit. This part of the total power, which is determined by the power factor and is useful (used).
The unified power factor is denoted by Cos φ.
This is the power factor, which shows the ratio of (losses) kW to kVA when connecting inductive loads.
Common power factors and their interpretation (cos φ):
1 – best value
0.95 is an excellent indicator
0.90 – satisfactory value
0.80 – average most common indicator
0.70 is a bad indicator
0.60 – very low value
kW characterizes the active consumed electrical power, which has the accepted letter designation P: this is the geometric difference between the total and reactive power, found from the relationship: P=S*cos(f).
In consumer terms: kW is net (net power), and kVA is gross (total power).
1 kW = 1.25 kVA
1 kVA = 0.8 kW
How to convert kVA power to kW?
To quickly convert kVA to kW, you need to subtract 20% from kVA and you get kW with a small error that can be neglected. Or use the formula to convert kVA to kW:
P=S * Сos f
Where P is active power (kW), S is apparent power (kVA), Cos f is power factor.
For example, to convert a power of 400 kVA into kW, you need 400 kVA * 0.8 = 320 kW or 400 kVA-20% = 320 kW.
How to convert kW power to kVA?
To convert kW to kVA, the following formula is applicable:
Where S is apparent power (kVA), P is active power (kW), Cos f is power factor.
For example, to convert a power of 1000 kW into kVA, you should have 1000 kW / 0.8 = 1250 kVA.