Power cables are cables used to transmit and distribute electrical energy.
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Power cables are commonly used in urban underground power grids, power station outlet lines, internal power supply for industrial and mining enterprises, and power lines under river water.
In power lines, the proportion of cables is gradually increasing.
Power cables are cable products that transmit and distribute high-power electrical energy in the backbone of power systems, including various voltage levels of 1-500KV and above, and different insulated power cables.
1. Due to the influence of external factors (such as lightning, wind damage, bird damage, etc.), its power supply reliability is high.
2. The power cable is buried underground, and the project is concealed, so it has little impact on the city's appearance and environment. Even if an accident occurs, it will generally not affect personal safety.
3. The cable capacitance is large, which can improve the line power factor.
1. High cost, large investment in one-time construction, and investment in cable lines are about 10 times that of overhead lines of the same voltage level.
2. It isn't easy to branch the line.
3. The fault point is difficult to find, and it is inconvenient to deal with the accident in time.
4. The construction process of cable joints is complicated.
The following devices and equipment are used for power factor improvement in an electrical system.
We will discus the common methods used for power factor correction as follows:
We know that most industries and power system loads are inductive, which causes a decrease in the system power factor due to lagging current (see disadvantages of low power factor). To improve the power factor, static capacitors are connected in parallel with these devices operated on low power factor.
These static capacitors supply leading current, which balances out the lagging inductive component of the load current. This effectively eliminates or neutralizes the lagging component of the load current and corrects the power factor of the load circuit to enhance the overall efficiency.
To enhance system or device efficiency, these capacitors are installed near large inductive loads, like induction motors and transformers, to improve the load circuit power factor.
For example, let’s consider a single-phase inductive load shown in Figure 1, which is drawing lagging current (I), and the load power factor is Cosθ.
Figure 2 shows the load with a capacitor (C) connected in parallel. As a result, a current (IC) flows through the capacitor and leads 90° from the supply voltage. In other words, the capacitor provides leading current, and in a purely capacitive circuit, the current leads the supply voltage by 90°, which means the voltage lags 90° behind the current. The load current remains (I), and the vector sum of (I) and (IC) is (I’) which lags behind the voltage at θ2, as shown in Figure 3.
Figure 3 demonstrates that the angle of θ2 < θ1, implying that Cosθ2 is less than Cosθ1 (Cosθ2 > Cosθ1). Therefore, the capacitor improves the load power factor.
It is important to note that after power factor improvement, the circuit current is lower than the low power factor circuit current. Additionally, the active component of current remains the same before and after power factor improvement because the capacitor eliminates only the reactive component of current. Finally, the Active power (in Watts) remains the same before and after power factor correction.
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Advantages:
A capacitor bank offers several advantages over other methods of power factor improvement, including:
Disadvantages:
However, there are some drawbacks to using a capacitor bank, which include:
When a synchronous motor operates at no-load and is over-excited, it is called a synchronous condenser. When a synchronous motor is over-excited, it provides leading current and works like a capacitor.
In a synchronous motor, a separate DC source is used to excite the field winding. Therefore, the input supply only provides current to energize the stator, i.e., the current provided is in-phase with the supply voltage. So the power factor remains unity.
The power factor can be adjusted by varying the DC excitation. By increasing the DC excitation, the power factor varies from lagging to unity and leading power factor. When the DC excitation increases, the field windings are over-magnetized. The input supply provides a current component to the stator to compensate for this over-magnetization. This current leads to the supply voltage, causing a leading power factor or generating reactive power.
An inductive load consumes reactive power, causing a lagging power factor, while a capacitive load generates reactive power, causing a leading power factor. A synchronous motor can be used to improve the overall power factor of an electrical system by adjusting the DC excitation. The synchronous motor used specifically for power factor improvement without any mechanical load is called a synchronous condenser.
The synchronous condenser is used in parallel with the load to improve the power factor. Improving the power factor reduces the extra current drawn from the source that is wasted in the power lines. Consequently, it helps in the reduction of electricity bills and saves energy.
When a synchronous condenser is connected across the supply voltage (in parallel), it draws leading current and partially eliminates the reactive component. This way, the power factor is improved. Generally, synchronous condensers are used to improve the power factor in large industries.
Advantages:
Disadvantages:
The Phase Advancer is a simple AC exciter that connects to the main shaft of a motor and operates with the motor’s rotor circuit to improve power factor. It is commonly used in industries to improve the power factor of induction motors.
Since the stator windings of an induction motor take lagging current 90° out of phase with voltage, the power factor of the motor is low. By supplying exciting ampere-turns from an external AC source, the current does not affect the stator windings, and the power factor of the induction motor improves. This process is done by the Phase Advancer.
Advantages:
Disadvantage:
The capacitor bank (published as separate & descriptive article) is connected in parallel to the load, and when the inductive load draws current from the system, the capacitor bank supplies capacitive reactive power to offset the inductive reactive power. The amount of capacitive reactive power needed to improve the power factor depends on the characteristics of the load, such as the magnitude of the inductance and the phase angle between the voltage and current.
We have covered this topic in a separate article describing Static VAR Compensator (SVC) including a circuit diagram, construction , working principles, and applications. You can read the article to learn how SVCs are used for power factor improvement.
The following figure shows power factor improvement in a three-phase system by connecting a capacitor bank in:
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