전력품질개선설비

  ◑  APF Products Model Series   

l  Cabinet/Panel type APF

The stationary reactive power compensator (SVC) is a stationary reactive power compensator that actively controls reactive power with a function similar to that of a synchronous generator. It is used to maintain stable voltage by actively adjusting the power factor of the power system and to improve the overall power quality, such as increasing the capacity of transmission lines. It is also useful for limiting voltage fluctuations on the bus due to rapidly changing loads, such as suppressing over voltages and improving system voltage stability.

  ◑  SVC External Appearance   

  ◑  Main Function of SVC   

If the reactive power in the system is not compensated, the phase change of voltage (V) and current (I) according to the capacitive and inductive load characteristics is shown below.

    < Phase change of voltage (V) and current (I) according to load characteristics >

​

For the above reasons, the SVC system effectively compensates for the capacitive or inductive reactive power required for the system according to the load characteristics by controlling the line constant of the system by connecting it in parallel to the bus of the power system, thereby improving the power factor, minimizing the voltage fluctuation and harmonic influence caused by nonlinear load, and improving power quality such as three-phase load  balance, thereby reducing unnecessary power loss and contributing to improving productivity and product quality. The main functions are summarized 

as follows:

√  Resolving dynamic load imbalance with voltage stabilization and regulation function

√  Reducing voltage drop even in sudden load fluctuations to alleviate flickering

√  Improving power factor by compensating for inductive or capacitive reactive power

√  Removing harmful harmonics

  ◑  Economic Advantage of SVC   

√  Improve factory manufacturing productivity
√  Extend the life of electronic equipment by stabilizing voltage
√  Save money by reducing heat loss through improved energy efficiency
√  Reduce power costs for power utilities and other customers
√  Minimize disturbances to adjacent customers by mitigating flickering
√  Minimize malfunctions of protective devices, etc. by reducing harmonics
√  Improve real power transmission efficiency
√  Reduce facility maintenance costs

  ◑  Basic Circuit of SVC   

SVC is a device that controls the combination of reactors and capacitors that affect the line constant with a thyristor. It is connected in parallel with the bus and directly controls the line constant of the system, thereby also having a harmonic filter function.

√  TCR (Thyristor Controlled Reactor) method: A method that phase-controls the current flowing through the shunt reactor with a thyristor

√  TSC (Thyristor Switched Capacitor) method: A method that switches the capacitor bank with a thyristor

      < Basic Circuit of SVC with TCR & TSC >

< Basic Circuit of SVC with TCR & Controller >

  ◑  Application area of  SVC   

1)  Power Transmission system

     Real-time control of reactive power during long-distance transmission maintains voltage stability and improves transmission efficiency.

2)  Large Scale of industrial facilities

     Compensates for voltage instability caused by rapid changes in loads such as arc furnaces, blast furnaces, and large motors in steel mills, 

     smelters,  and cement plants.

3)  Electric railway system

     Stabilizes instantaneous voltage fluctuations that occur during the operation of electric trains to maintain power quality in the railway 

     power system.

4)  Renewable energy power generation complex

     Compensates for voltage fluctuations and reactive power in power sources with irregular outputs such as wind or solar power generation 

     to improve grid connection stability.

     Compensates for reactive power generated at the connection point with the high-voltage direct current (HVDC) system to help stabilize 

     the voltage of the AC system.

   ◎ SVC Application Cases (A large capacity of Arc furnace)    __________________

The following is an application example in an industrial field, explaining the application effect of the SVC system in an Arc Furnace in the steel 

making process.

1)  SVC System Configuration

Basically, TCR and TSC are configured together to enable real-time alternating control of inductive (L) and capacitive (C) loads depending on 

driving conditions.

2) SVC system application specifications

①  SVC compensation capacity: 0~400Mvar

②  Bus controllable voltage: 6~500kV

③  SVC rated voltage: 6~66kV

④  Total dynamic response time (reactive power output): <15ms

⑤  SVC availability: >99%

⑥  SVC maximum loss: <0.8%

3) SVC system application effects

①  Optimal operation for power factor improvement of large-capacity electric furnaces (arc furnaces, induction furnaces, DC furnaces) 

      with extreme load fluctuations

②  Suppress flickering and prevent malfunction of various protection devices by stabilizing voltage

③  Increased melting capacity (shortened melting time and improved productivity)

④  Reduced electrode replacement cost (extended replacement cycle)

⑤  Reduced wear of electric furnace lining by stable arc supply

⑥  Reduced power loss

SVG(Static Var Generator) is also called real-time reactive power generator or active power factor compensator (APFC). It is a representative technology in the field of reactive power compensation as the ultimate solution to improve low power factor and various power quality problems caused by increased reactive power. SVG is connected in parallel to the power grid bus and operates as a variable reactive current supply source. 

  ◑  Main Function of SVG   

SVG can quickly absorb or release all the required reactive power by adjusting the voltage amplitude and phase on the AC output side of the inverter, or by directly controlling the amplitude and phase by measuring the AC current, so it can achieve its purpose very quickly. By controlling the DC component by adjusting the reactive power in the system, it is possible to track and compensate for the inrush current of the impulse load as well as track and compensate for the harmonic urrent. Recently, by using power electronics inverter technology, it has become possible to control the power factor close to 1 by generating a reverse current of the opposite phase with the same size as the compensation target and canceling each other out. In addition, SVG is  high-performance electronic module that can respond immediately and effectively to power quality problems in the power supply system and also has a kind of active filter (APF, Active Power Filter) function. The main functions of SVG are summarized as follows.

√  Real-time reactive power compensation by using high-speed IGBT devices

√  Active harmonic filter function prevents various harmful effects due to harmonics such as overheating and malfunction of equipment

√  Improves equipment stability, reliability, and lifespan by tracking and controlling various abnormal voltages, inrush currents, etc.

√  Increases power system capacity by reducing energy loss

  ◑  Basic Circuit of SVG  

SVG is a power electronic circuit device that is connected in parallel with a load that requires power factor improvement or harmonic reduction, and when the load generates inductive or capacitive current, it generates a voltage corresponding to the load current. At this time, it detects the phase difference between the current and voltage on the load side, generates Lead or Lag current in real time, and injects it into the bus to make the phase angle of the load current almost identical to the phase angle of the voltage, and continuously maintains the basic power factor.

.

< Configuration of basic circuit of SVG >

  ◑  SVG System Operation Principle and Characteristics   

   ◎  Power factor compensation characteristics by conventional capacitor compensation method  

        (stepwise, intermittent control)  

< Power factor compensation characteristics by conventional capacitor compensation method (stepwise, intermittent control) >

   ◎  Power factor compensation characteristics by new technology SVG control method  

       (Stepless, continuously controled)  

√  PF maintenance: 0.99

√  Capacitive, inductive reactive power compensation: -1 to +1

√  Harmonic filtering: 2nd to 12th harmonic

√  3-phase unbalanced load compensation

√  Generates inductive or capacitive reactive power, injects it complementarily to improve power factor 

√  Operates as a voltage source with PWM control, responds so quickly to sudden load changes, stabilizing voltage and resolving load imbalance

√  Responds within a few ms with high-speed IGBT control, quickly eliminating voltage distortion caused by harmonics

    3)  Selection of LV - SVG installation spec.

   ◎  Basic Circuit of HV-SVG Series   

      ☞.  Note：27.5kV 와 33kV 시리즈는 리엑터를 포함하지 않는다.

Most industrial electrical equipment has inductive load characteristics. Therefore, in order to operate them efficiently, it is necessary to supply and offset the corresponding capacitive reactive power with the opposite characteristics. This reactive power can be supplied by installing a capacitor bank in parallel with the load. If the capacitor bank is used with a capacity corresponding to the inductive load, the power factor can be maintained at almost 1. This type of power factor improvement is ultimately a process of reducing the phase difference between the voltage and current on the load side. 

Basically, the capacitor bank performs the function of restoring the load current on the lagging side to normal by creating and injecting a leading current.

  ◑  Basic Circuit of  Capacitor Bank   

Capacitor banks are composed of multiple capacitors connected in series or parallel, and are mainly used to improve the power factor of power systems, stabilize voltage, and reduce harmonics. The main devices that make up capacitor banks are as follows, and protective devices such as circuit breakers, relays, and current transformers are also configured as needed.

√  Capacitor 

√  Serial Reactor 

√  Discharge Coil 

< Basic configuration of capacitor bank for power factor improvement >

   ◎  Capacitor   

Capacity determination of a capacitor for power factor correction is mainly carried out in the following order.

① System power factor analysis

System current power factor analysis

Load calculation (Check the power factor of the entire load)

Check the power factor of each system where the capacitor will be installed

② Target power factor setting

Set the desired target power factor (power factor required by each country is slightly different)

In Korea, KEPCO requires 0.9, but in reality, the power factor can vary depending on the load, so set the target value to at least 0.93

③ Calculate the required compensation capacity

The capacity of the capacitor for power factor compensation can be calculated using the following calculation formula.

Where,

P (kW): Active power used in the system (kW)

θ1: Current power factor value (cosθ1: Current power factor)

θ2: Target power factor value (cosθ2: Target power factor)

Required capacitor capacity: Pr (kVar)

Pr = P (kW) x tan (θ1-θ2)

④ Select capacitor capacity

Based on the calculated capacitor capacity, select a capacitor with an appropriate capacity that can be purchased on the market

If the capacity is very large or a power factor compensation device is used, calculate the number of steps and the capacitor capacity 

of each unit per step. Usually, three banks are installed and the capacity of each bank is different, so that the control resolution can 

be increased depending on the combination.

Ex. > If a total capacity of 3000 kVar is required,

Ans. > By configuring three banks with capacities of 500, 1000, and 1500 kVar,

            a total of six steps of 500, 1000, 1500, 2000, 2500, and 3000 kVar can be created with the combination.

   ◎   Series Reactor   

The series reactor in the capacitor bank has two main functions: inrush current limiting function and harmonic current limiting function.

① Inrush current limiting reactor

It is applied to reduce the starting current by suppressing the terminal voltage in a load with a large starting current such as an 

induction motor and to reduce the circuit breaker capacity.  If it is used only for the inrush current limiting function at the start, 

the reactance is 0.2%, and this value varies depending on the design conditions. In this case, the inrush current can be up to

1 + √(1.0 / 0.002) = 23.4 times.

< inrush starting current limiting reactor >

② Harmonic current limiting reactor

It is connected in series with a capacitor for power factor improvement to reduce harmonics and protect the capacitor. 

The harmonic reduction reactor usually applies a reactance of about 6%, and in this case, the inrush current can be up to 5.1 times.

    < Harmonic current limiting reactor >

   ☞. Types and insulation methods of REACTOR CORE

The core types of reactors are air core and iron core, and the insulation methods are dry type and oil type.

Air core: It can only be manufactured dry, and it is difficult to build a cubicle with only a coil, so it is mainly installed outdoors.

Iron core: It can be manufactured dry or inflow type, and it can be built into a cubicle.

In the case of inflow type, it cannot be installed indoors, and even if it is installed outdoors, measures against oil leakage are required.

   ◎  Discharge Coil   

The purpose of the discharge coil is to remove the residual voltage of the capacitor circuit and prevent overvoltage applied to the capacitor when

reinserted into the line. If the residual voltage can be lowered below the specified value by the discharge resistance built into the power capacitor 

alone, a separate discharge coil does not need to be installed. However, if it is absolutely necessary due to the characteristics of the equipment, 

such as high voltage and large capacity, a separate discharge coil must be installed.

  ☞. Discharge characteristics of power capacitors

KS C 4802 High voltage and extra high voltage power capacitors à Discharge within 5 minutes, 50 V or less

KS C IEC 60871-1 Shunt capacitors for AC systems with rated voltage exceeding 1000 V à Discharge within 10 minutes, 75 V or less

KS C 4804 Discharge coils for high voltage and extra high voltage phase-leading capacitors à 50 V or less after 5 seconds of starting

discharge

  ◑  Capacitor Bank Installation and Maintenance   

Power factor correction capacitors can greatly help improve the efficiency of the power system and reduce electricity bills, so power factor management

can be a very important issue.  Also, the capacity of capacitors can decrease over time, so regular inspection and maintenance must be performed after

installation in the system.

Reactor bank is configured in various ways depending on the purpose. For power, it is mainly used to suppress abnormal voltage by limiting surge

current or inrush current. When installed in combination with a capacitor, it has the function of eliminating harmonic waves generated by the capacitor

and also has the effect of improving the voltage waveform by suppressing inrush current and overvoltage.

  ◑  Reactor Functions and Features by Application   

The reactors applied to power circuits are classified according to their purpose and function as follows:

√  Serial reactor: Eliminates harmonics and improves voltage waveform

√  Shunt reactor: Prevents transmission line ferenty phenomenon

√  Current limiting reactor: Limits fault current, short-circuit current, and inrush current

√  Neutral point grounding reactor: Limits ground fault current and suppresses abnormal voltage in case of ground fault

√  Small arc reactor: Extinguishes arc in case of ground fault

   ◎  Series Reactor for Harmonic Suppresion   

①  Harmonic elimination and voltage waveform improvement

The biggest problem when connecting a capacitor to a transmission line for use as a voltage source is the distortion of the voltage waveform. 

This is because the harmonic voltage is induced due to the magnetic saturation phenomenon of the transformers on the line, and the harmonic 

voltage is amplified when the capacitor is connected. Among them, the third harmonic voltage is eliminated by the delta connection on the

low-voltage side of the transformer, and the fifth harmonic voltage remains without being eliminated.

②  Surge current limitation

A reactor is connected in series to an AC circuit to limit the inrush current or short-circuit current or to limit the current sharing of a paralle

l circuit. A reactor that limits the current during a short circuit is also called a current-limiting reactor, and it is common to require a relatively 

small reactor whose voltage drop is about a few percent of the phase voltage when the rated current is applied.

③  5th harmonic current limitation

The impedance for the 5th harmonic can be increased by connecting it in series with a capacitor for power factor improvement, thereby 

limiting the amount of harmonic inflow. For this purpose, the capacity of the reactor applied is about 6% of the capacitor capacity, and a 

gapped core reactor is mainly used. In cases where the 3rd harmonic is included in large quantities, a capacity of about 13 to 15% is used,

and the margin is appropriately about +20 to 30% of the guaranteed value.

④  Characteristics of air-core reactors

They are mainly used between the two ends of transmission lines because their reactance values are constant and stable regardless 

of the size of the current.

⑤  Precautions for using iron-core reactors

Since the impedance of series reactors is specified according to the maximum fault current of the system, when using iron-core reactors, 

which have a phenomenon in which reactance is distorted at the magnetic saturation point, care must be taken to ensure that they are 

operated only in sections where the iron core is not saturated.

   ◎  Shunt Reactor   

①  Transmission line leading current compensation

Shunt reactors are connected in parallel to the transmission line busbars to compensate for the line leading current (charging current of

cables under light load). Recently, as the scale of the power system has expanded and the number of long-distance high-power 

transmissions has increased due to remote power supply and load concentration, the capacitance of the cable system has increased, 

which has led to an increase in the capacity of the power system's power supply equipment. Typically, short-term capacities of about 30 

or 60 [MVA] are combined and used.

②  Shunt reactor installation locations

Shunt reactors are basically installed at substations at both ends of the transmission line, and can also be used as a power supply equipment 

function in combination with a condenser.

< Application of Transmission Line Shunt Reactor >

③  Types of shunt reactor applications

Shunt reactors are mainly applied to high-voltage systems, so the structure is usually an iron core type with an inlet gap or an air core

type with magnetic shielding iron pieces at the top and bottom.

< Air-Core Reactor Structure >

④  Reactance margin

Since the reactor is problematic due to eddy current loss of the core, vibration noise, etc., each manufacturer designs a structure with its own characteristics, and the reactance margin of the shunt reactor is generally applied as ±5% of the guaranteed value.

   ◎  Current Limitting Reactor   

   ① Short-circuit current limiting

The current limiting reactor is used to limit the inrush current or short-circuit current flowing on the primary side when the transformer is turned on to a specified value or less. This reduces the breaking capacity of the circuit breaker and also has the function of preventing damage to the series device.

   ② %Z (Impedance)

As shown in the previous short-circuit current equation, the short-circuit current decreases as the reactance value increases. Here, %Z (impedance) is calculated from the following equation.

   ◎  Neutral Grounding Reactor (NGR)   

① Grounding reactor installation location

It is connected between the neutral point of the transformer and the ground and is used to limit the ground fault current and suppress 

the abnormal voltage in the event of a ground fault in a three-phase power system.

      < Neutral Grounding Reactor (NGR) Installation Location >

  ②  Limitation of Grounding Current

        The neutral grounding current is proportional to the transformer capacity. If necessary, a supplementary resistor can be directly connected 

        and used.

  ③  Usage Time Rating

        The current rating for short-term use is usually based on 10 seconds unless otherwise specified.

  ④  Impedance Margin

        The impedance margin of the neutral grounding reactor is set to ±20~30%.

< Installation location of arc suppression reactor  > 

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