T & D - 38) Differences between Shunt Reactor and Power Transformer

Differences between Shunt Reactor and Power Transformer

Main Differences

Shunt Reactor and Transformer both appear similar in construction. Reactors are also often equipped with Fans for cooling similar to Power Transformers.

However, there are major differences between the two. While a Power Transformer is designed for efficient power transfer from one voltage system to another, a shunt reactor is intended only to consume reactive VArs (or in other words it can be stated as to produce lagging VArs).

Thus, there are more than one winding on a Power Transformer with magnetic core which carry the mutual flux between the two. In reactor there is just one winding. The core is not therefore meant only to provide a low reluctance path for flux of that winding to increase the Inductance.

In case of a Power Transformer, primary Ampere-Turns (AT) is sum of exciting AT and secondary AT. AT loss (in winding resistance, eddy loss and hysteric loss) is kept to as minimum as possible. Exciting AT is small compared with the secondary AT. Rated current is based on the load transfer requirement.

Detailed view of an iron core divided by air gaps

Magnetizing current is small and is negligible value when compared with the secondary rated current. Further, since mutual flux is main flux which results in transformation, leakage flux is kept small and will be based on fault current limitation.

In case of a Shunt Reactor due to absence of other windings, all primary AT is equal to the exciting AT. Similar to a Power Transformer, loss in AT (in winding resistance, eddy current and hysteresis) are also kept to minimum by design. Magnetizing ATis major component of a Shunt Reactor. Reactor magnetizing current is its rated current.

Since a Shunt Reactor magnetizing current is large, if it is designed with Iron alone as a Power Transformer, there will be large hysteresis loss. Air gaps in Iron core are provided in a Shunt Reactor to reduce this loss and to minimize the remanent flux in the core.

Thus a Shunt Reactor may also be constructed without iron (air-core).

By construction, a Shunt Reactor can be oil immersed or dry type for both with and without iron core.

Dry type Reactors are constructed as single phase units and are thus arranged in a fashion to minimize stray magnetic field on surrounding (in the absence of metallic shielding). When such an arrangement is difficult, some form of magnetic shielding is required and designed with care to minimize eddy current loss and arcing at any joints within the metallic loops. One of the advantages of dry type reactor is absence of inrush current.

Oil immersed reactors can be core-less or with gapped iron core. These are either single phase or three phase design with or without fan cooling. These are installed within tanks which hold oil & act as metallic magnetic shields.

In some cases, a Shunt Reactor may have additional small capacity winding which can provide power for small station power loads. Since Shunt Reactor rating is normally based on MVAr rating, this added station load VA shall be accounted for in designing the Reactor for such applications.

Types of shunt reactors

Shunt reactors are used in high voltage systems to compensate for the capacitive generation of long overhead lines or extended cable networks.

The reasons for using shunt reactors are mainly two

The first reason is to limit the overvoltages and the second reason is to limit the transfer of reactive power in the network. If the reactive power transfer is minimized i. e. the reactive power is balanced in the different part of the networks, a higher level of active power can be transferred in the network.

Reactors to limit overvoltages are most needed in weak power systems, i.e. when network short-circuit power is relatively low.

Voltage increase in a system due to the capacitive generation is:

ΔU(%) = Q_C x 100 / S_sh.c

where:

Qc is the capacitive input of reactive power to the network
S_sh.c is the short circuit power of the network

With increasing short circuit power of the network the voltage increase will be lower and the need of compensation to limit over-voltages will be less accentuated.

Reactors to achieve reactive power balance in the different part of the network are most needed in heavy loaded networks where new lines cannot be built because of environmental reasons. Reactors for this purpose mostly are thyristor controlled in order to adapt fast to the reactive power required.

Especially in industrial areas with arc furnaces the reactive power demand is fluctuating between each half cycle.

In such applications there are usually combinations of:

1. Thyristor controlled reactors (TCR) and
2. Thyristor switched capacitor banks (TSC).

These together makes it possible to both absorb, and generate reactive power according to the momentary demand.

Four leg reactors also can be used for extinction of the secondary are at single-phase reclosing in long transmission lines. Since there always is a capacitive coupling between phases, this capacitance will give a current keeping the are burning, a secondary arc.

By adding one single-phase reactor in the neutral the secondary arc can be extinguished and the single-phase auto-reclosing successful.

Resource: Shunt Reactors and Shunt Reactor Protection - S.R. Javed Ahmed

T & D - 22) Defining Size and Location of Capacitor in Electrical System (2)

Defining Size and Location of Capacitor in Electrical System (2)

Continued from part 1: Defining Size and Location of Capacitor in Electrical System (2)

Content

• Size of circuit breaker (CB), fuse and conductor of capacitor bank:
• A. Thermal and magnetic setting of a circuit breaker
• B. Fuse selection
• C. Size of conductor for capacitor connections
• Size of capacitor for transformer no-load compensation:
• Fixed compensation
• Sizing of capacitor for motor compensation:
1. If no-load current is known
2. If the no load current is not known
• Placement of power capacitor bank for motor:
• Location 1 (the line side of the starter)
• Location 2 (between the overload relay and the starter)
• Location 3 (the motor side of the overload relay)
• Placement of capacitors in distribution system:
• A. Global compensation
• B. Compensation by sector
• C. Individual compensation
• Common capacitor reactive power ratings

Size of CB, Fuse and Conductor of Capacitor Bank

A. Thermal and Magnetic setting of a Circuit breaker

1. Size of Circuit Breaker

1.3 to 1.5 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

• 1.31×In for Heavy Duty/Energy Capacitors with 5.6% Detuned Reactor (Tuning Factor 4.3)
• 1.19×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8)
• 1.12×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor (Tuning Factor 2.7)

Note: Restrictions in Thermal settings of system with Detuned reactors are due to limitation of IMP (Maximum Permissible current) of the Detuned reactor.

2. Thermal Setting of Circuit Breaker

1.5x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors

3. Magnetic Setting of Circuit Breaker

5 to 10 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors
Example: 150kvar,400v, 50Hz Capacitor

• Us = 400V, Qs = 150kvar, Un = 400V, Qn = 150kvar
• In = 150000/400√3 = 216A
• Circuit Breaker Rating = 216 x 1.5 = 324A
• Select a 400A Circuit Breaker.
• Circuit Breaker thermal setting = 216 x 1.5 = 324 Amp

Conclusion: Select a Circuit Breaker of 400A with Thermal Setting at 324A and Magnetic Setting (Short Circuit) at 324A

B. Fuse Selection

The rating must be chosen to allow the thermal protection to be set to:
1.5 to 2.0 x Capacitor Current (In) for Standard Duty/Heavy Duty/Energy Capacitors.

• 1.35×In for Heavy Duty/Energy Capacitors with 5.7% Detuned Reactor (Tuning Factor 4.3)
• 1.2×In for Heavy Duty/Energy Capacitors with 7% Detuned Reactor (Tuning Factor 3.8)
• 1.15×In for Heavy Duty/Energy Capacitors with 14% Detuned Reactor(Tuning Factor 2.7)

For Star-solidly grounded systems:
Fuse > = 135% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics).
For Star -ungrounded systems:
Fuse > = 125% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics).

Care should be taken when using NEMA Type T and K tin links which are rated 150%. In this case, the divide the fuse rating by 1.50.

Example 1: 150kvar,400v, 50Hz Capacitor

• Us = 400V; Qs = 150kvar, Un = 400V; Qn = 150kvar.
• Capacitor Current =150×1000/400 =375 Amp

To determine line current, we must divide the 375 amps by √ 3

• In (Line Current) = 375/√3 = 216A
• HRC Fuse Rating = 216 x1.65 = 356A to
• HRC Fuse Rating = 216 x 2.0 = 432A so Select Fuse Size 400 Amp

Problems with Fusing of Small Ungrounded Banks

Example: 12.47 kV, 1500 Kvar Capacitor bank made of three 3 No’s of 500 Kvar single-phase units.

• Nominal Capacitor Current = 1500/1.732×12.47 = 69.44 amp
• Size of Fuse = 1.5×69.44 = 104 Amp = 100 Amp Fuse

If a capacitor fails, we say that It may approximately take 3x line current. (3 x 69.44 A = 208.32 A).
It will take a 100 A fuse approximately 500 seconds to clear this fault (3 x 69.44 A = 208.32 A). The capacitor case will rupture long before the fuse clears the fault.

The solution is using smaller units with individual fusing. Consider 5 No’s of 100 kVAR capacitors per phase, each with a 25 A fuse. The clear time for a 25 A fuse @ 208.32 A is below the published capacitor rupture curve.

C. Size of Conductor for Capacitor Connections

Size of capacitor circuit conductors should be at least 135% of the rated capacitor current in accordance with NEC Article 460.8 (2005 Edition).

Size of capacitor for Transformer No-Load compensation

Fixed compensation

The transformer works on the principle of Mutual Induction. The transformer will consume reactive power for magnetizing purpose. Following size of capacitor bank is required to reduce reactive component (No Load Losses) of Transformer.

Selection of capacitor for transformer no-load compensation
KVA Rating of the Transformer	Kvar Required for compensation
Up to and including 315 KVA	5% of KVA Transformer Rating
315 to 1000 KVA	6% of KVA Transformer Rating
Above 1000 KVA	8% of KVA Transformer Rating

Sizing of capacitor for motor compensation

The capacitor provides a local source of reactive current. With respect to inductive motor load, this reactive power is the magnetizing or “no load current“ which the motor requires to operate.

A capacitor is properly sized when its full load current rating is 90% of the no-load current of the motor. This 90% rating avoids over correction and the accompanying problems such as overvoltages.

1. If no-load current is known

The most accurate method of selecting a capacitor is to take the no load current of the motor, and multiply by 0.90 (90%).
Example:
Size a capacitor for a 100HP, 460V 3-phase motor which has a full load current of 124 amps and a no-load current of 37 amps.
Size of Capacitor = No load amps (37 Amp) X 90% = 33 Kvar

2. If the no load current is not known

If the no-load current is unknown, a reasonable estimate for 3-phase motors is to take the full load amps and multiply by 30%. Then multiply it by 90% rating figure being used to avoid overcorrection and overvoltages.
Example:
Size a capacitor for a 75HP, 460V 3-phase motor which has a full load current of 92 amps and an unknown no-load current.
No-load current of Motor = Full load Current (92 Amp) x 30% = 28 Amp estimated no-load Current.
Size of Capacitor = No load amps (28 Amp) X 90% = 25 Kvar.

Thumb Rule:

It is widely accepted to use a thumb rule that Motor compensation required in kvar is equal to 33% of the Motor Rating in HP.

Placement of Power Capacitor Bank for Motor

Capacitors installed for motor applications based on the number of motors to have power factor correction. If only a single motor or a small number of motors require power factor correction, the capacitor can be installed at each motor such that it is switched on and off with the motor.

Required Precaution for selecting Capacitor for Motor:

The care should be taken in deciding the Kvar rating of the capacitor in relation to the magnetizing kVA of the machine.
If the rating is too high, It may damage to both motor and capacitor.

As the motor, while still in rotation after disconnection from the supply, it may act as a generator by self excitation and produce a voltage higher than the supply voltage. If the motor is switched on again before the speed has fallen to about 80% of the normal running speed, the high voltage will be superimposed on the supply circuits and there may be a risk of damaging other types of equipment.

As a general rule the correct size of capacitor for individual correction of a motor should have a kvar rating not exceeding 85% of the normal No Load magnetizing KVA of the machine. If several motors connected to a single bus and require power factor correction, install the capacitor(s) at the bus.
 

Where do not install Capacitor on Motor:

Do not install capacitors directly onto a motor circuit under the following conditions:

1. If solid-state starters are used.
2. If open-transition starting is used.
3. If the motor is subject to repetitive switching, jogging, inching, or plugging.
4. If a multi-speed motor is used.
5. If a reversing motor is used.
6. If a high-inertia load is connected to the motor.

Fixed power capacitor banks can be installed in a non-harmonic producing electrical system at the feeder, load or service entrance. Since power capacitor banks are reactive power generators, the most logical place to install them is directly at the load where the reactive power is consumed.
Three options exist for installing a power capacitor bank at the motor.

Installing a power capacitor bank at the motor

Location 1 (The line side of the starter)

Install between the upstream circuit breaker and the contactor.
This location should be used for the motor loads with high inertia, where disconnecting the motor with the power capacitor bank can turn the motor into a self excited generator, motors that are jogged, plugged or reversed, motors that start frequently, multi-speed motors, starters that disconnect and reconnect capacitor units during cycling and starters with open transition.

Advantage

Larger, more cost effective capacitor banks can be installed as they supply kvar to several motors. This is recommended for jogging motors, multispeed motors and reversing applications.

Disadvantages

• Since capacitors are not switched with the motors, overcorrection can occur if all motors are not running.
• Since reactive current must be carried a greater distance, there are higher line losses and larger voltage drops.

Applications

• Large banks of fixed kVAR with fusing on each phase.
• Automatically switched banks

Location 2 (Between the overload relay and the starter)

Install between the contactor and the overload relay.

• This location can be used in existing installations when the overload ratings surpass the National Electrical Code requirements.
• With this option the overload relay can be set for nameplate full load current of motor. Otherwise the same as Option 1.
• No extra switch or fuses required.
• Contactor serves as capacitor disconnect.
• Change overload relays to compensate for reduced motor current.
• Too much Kvar can damage motors.

Calculate new (reduced) motor current. Set overload relays for this new motor FLA.FLA (New) = P.F (Old) / P.F (New) x FLA (Name Plate)

Application:

Usually the best location for individual capacitors.

Location 3 (The motor side of the overload relay)

Install directly at the single speed induction motor terminals (on the secondary of the overload relay).

• This location can be used in existing installations when no overload change is required and in new installations in which the overloads can be sized in accordance with reduced current draw.
• When correcting the power factor for an entire facility, fixed power capacitor banks are usually installed on feeder circuits or at the service entrance.
• Fixed power capacitor banks should only be used when the facility’s load is fairly constant. When a power capacitor bank is connected to a feeder or service entrance a circuit breaker or a fused disconnect switch must be provided.
• New motor installations in which overloads can be sized in accordance with reduced current draw
• Existing motors when no overload change is required.

Advantage

• Can be switched on or off with the motors, eliminating the need for separate switching devices or over current protection. Also, only energized when the motor is running.
• Since Kvar is located where it is required, line losses and voltage drops are minimized; while system capacity is maximized.

Disadvantages

• Installation costs are higher when a large number of individual motors need correction.
• Overload relay settings must be changed to account for lower motor current draw.

Application

Usually the best location for individual capacitors.

Placement of capacitors in Distribution system

The location of low voltage capacitors in Distribution System effect on the mode of compensation, which may be global (one location for the entire installation), by sectors (section-by-section), at load level, or some combination of the last two.
In principle, the ideal compensation is applied at a point of consumption and at the level required at any instant.

Placement of capacitors in distribution system

A. Global compensation

Principle

The capacitor bank is connected to the bus bars of the main LV distribution board to compensation of reactive energy of whole installation and it remains in service during the period of normal load.

Advantages

• Reduces the tariff penalties for excessive consumption of kvars.
• Reduces the apparent power kVA demand, on which standing charges are usually based
• Relieves Reactive energy of Transformer , which is then able to accept more load if necessary

Limitation

• Reactive current still flows in all conductors of cables leaving (i.e. downstream of) the main LV distribution board. For this reason, the sizing of these cables and power losses in them are not improved by the global mode of compensation.
• The losses in the cables (I²R) are not reduced.

Application

• Where a load is continuous and stable, global compensation can be applied
• No billing of reactive energy.
• This is the most economical solution, as all the power is concentrated at one point and the expansion coefficient makes it possible to optimize the capacitor banks
• Makes less demands on the transformer.

B. Compensation by sector

Principle

Capacitor banks are connected to bus bars of each local distribution Panel.
Most part of the installation System can benefits from this arrangement, mostly the feeder cables from the main distribution Panel to each of the local distribution panel.

Advantages

• Reduces the tariff penalties for excessive consumption of kvar.
• Reduces the apparent power Kva demand, on which standing charges are usually based.
• The size of the cables supplying the local distribution boards may be reduced, or will have additional capacity for possible load increases.
• Losses in the same cables will be reduced.
• No billing of reactive energy.
• Makes less demands on the supply Feeders and reduces the heat losses in these Feeders.
• Incorporates the expansion of each sector.
• Makes less demands on the transformer.
• Remains economical

Limitations

• Reactive current still flows in all cables downstream of the local distribution Boards.
• For the above reason, the sizing of these cables, and the power losses in them, are not improved by compensation by sector
• Where large changes in loads occur, there is always a risk of overcompensation and consequent overvoltage problems.

Application

Compensation by sector is recommended when the installation is extensive, and where the load/time patterns differ from one part of the installation to another.
This configuration is convenient for a very widespread factory Area, with workshops having different load factors

C. Individual compensation

Principle

• Capacitors are connected directly to the terminals of inductive circuit (Near to motors). Individual compensation should be considered when the power of the motor is significant with respect to the declared power requirement (kVA) of the installation.
• The kvar rating of the capacitor bank is in the order of 25% of the kW rating of the motor.
• Complementary compensation at the origin of the installation (transformer) may also be beneficial.
• Directly at the Load terminals Ex. Motors, a Steady load gives maximum benefit to Users.
• The capacitor bank is connected right at the inductive load terminals (especially large motors). This configuration is well adapted when the load power is significant compared to the subscribed power. This is the technical ideal configuration, as the reactive energy is produced exactly where it is needed, and adjusted to the demand.

Advantages

• Reduces the tariff penalties for excessive consumption of kvars
• Reduces the apparent power kVA demand
• Reduces the size of all cables as well as the cable losses.
• No billing of reactive energy
• From a technical point of view this is the ideal solution, as the reactive energy is produced at the point where it is consumed. Heat losses (RI²) are therefore reduced in all the lines.
• Makes less demands on the transformer.

Limitations

• Significant reactive currents no longer exist in the installation.
• Not recommended for Electronics Drives.
• Most costly solution due to the high number of installations.
• The fact that the expansion coefficient is not incorporated.

Application

Individual compensation should be considered when the power of motor is significant with respect to power of the installation.

Common Capacitor Reactive Power Ratings

Voltage	Kvar Rating	Number of Phases
216	5, 7.5, 131/3, 20, 25	1 or 3
240	2.5, 5, 7.5,10, 25, 20, 25, 50	1 or 3
480	5, 10, 15, 20 25, 35, 50, 60, 100	1 or 3
600	5, 10, 15, 20 25, 35, 50, 60, 100	1 or 3
2,400	50, 100, 150, 200	1
2,770	50, 100, 150, 200	1
7,200	50, 100, 150, 200,300,400	1
12,470	50, 100, 150, 200,300,400	1
13,800	50, 100, 150, 200,300,400

T & D - 15) Reducing Distribution Line Losses

Reducing Distribution Line Losses

One of the main benefits of applying capacitors is that they can reduce distribution line losses. Losses come from current through the resistance of conductors. Some of that current transmits real power, but some flows to supply reactive power. Reactive power provides magnetizing for motors and other inductive loads. Reactive power does not spin kWh meters and performs no useful work, but it must be supplied.

Using capacitors to supply reactive power reduces the amount of current in the line.

Since line losses are a function of the current squared,I²R, reducing reactive power flow on lines significantly reduces losses.

Engineers widely use the “2/3 rule” for sizing and placing capacitors to optimally reduce losses. Neagle and Samson (1956) developed a capacitor placement approach for uniformly distributed lines and showed that the optimal capacitor location is the point on the circuit where the reactive power flow equals half of the capacitor var rating. From this, they developed the 2/3 rule for selecting and placing capacitors. For a uniformly distributed load, the optimal size capacitor is 2/3 of the var requirements of the circuit.

The optimal placement of this capacitor is 2/3 of the distance from the substation to the end of the line. For this optimal placement for a uniformly distributed load, the substation source provides vars for the first 1/3 of the circuit, and the capacitor provides vars for the last 2/3 of the circuit (see Figure 1).

A generalization of the 2/3 rule for applying n capacitors to a circuit is to size each one to 2/(2n+1) of the circuit var requirements. Apply them equally spaced, starting at a distance of 2/(2n+1) of the total line length from the substation and adding the rest of the units at intervals of 2/(2n+1) of the total line length. The total vars supplied by the capacitors is 2n/(2n+1) of the circuit’s var requirements.

So to apply three capacitors, size each to 2/7 of the total vars needed, and locate them at per unit distances of 2/7, 4/7 and 6/7 of the line length from the substation.

Figure 1 - Optimal capacitor loss reduction using the two-thirds rule

Grainger and Lee (1981) provide an optimal yet simple method for placing fixed capacitors on a circuit with any load profile, not just a uniformly distributed load. With the Grainger/Lee method, we use the reactive load profile of a circuit to place capacitors.

The basic idea is again to locate banks at points on the circuit where the reactive power equals one half of the capacitor var rating.

With this 1/2-kvar rule, the capacitor supplies half of its vars downstream, and half are sent upstream. The basic steps of this approach are:1. Pick a size
Choose a standard size capacitor. Common sizes range from 300 to 1200 kvar, with some sized up to 2400 kvar. If the bank size is 2/3 of the feeder requirement, we only need one bank. If the size is 1/6 of the feeder requirement, we need five capacitor banks.
2. Locate the first bank
Start from the end of the circuit. Locate the first bank at the point on the circuit where var flows on the line are equal to half of the capacitor var rating.
3. Locate subsequent banks
After a bank is placed, reevaluate the var profile. Move upstream until the next point where the var flow equals half of the capacitor rating. Continue placing banks in this manner until no more locations meet the criteria.

There is no reason we have to stick with the same size of banks. We could place a 300-kvar bank where the var flow equals 150 kvar, then apply a 600-kvar bank where the var flow equals 300 kvar, and finally apply a 450-kvar bank where the var flow equals 225 kvar. Normally, it is more efficient to use standardized bank sizes, but different size banks at different portions of the feeder might help with voltage profiles.

The 1/2-kvar method works for any section of line. If a line has major branches, we can apply capacitors along the branches using the same method. Start at the end, move upstream, and apply capacitors at points where the line’s kvar flow equals half of the kvar rating of the capacitor. It also works for lines that already have capacitors (it does not optimize the placement of all of the banks, but it optimizes placement of new banks).

For large industrial loads, the best location is often going to be right at the load.

Figure 2 - Placement of 1200-kvar banks using the 1/2-kvar method

Figure 2 shows the optimal placement of 1200-kvar banks on an example circuit. Since the end of the circuit has reactive load above the 600-kvar threshold for sizing 1200-kvar banks, we apply the first capacitor at the end of the circuit. (The circuit at the end of the line could be one large customer or branches off the main line.) The second bank goes near the middle. The circuit has an express feeder near the start.

Another 1200-kvar bank could go in just after the express feeder, but that does not buy us anything. The two capacitors total 2400 kvar, and the feeder load is 3000 kvar. We really need another 600-kvar capacitor to zero out the var flow before it gets to the express feeder.

Figure 3 -Sensitivity to losses of sizing and placing one capacitor on a circuit with a uniform load. (The circles mark the optimum location for each of the sizes shown.)

Fortunately, capacitor placement and sizing does not have to be exact. Quite good loss reduction occurs even if sizing and placement are not exactly
optimum. Figure 3 shows the loss reduction for one fixed capacitor on a circuit with a uniform load. The 2/3 rule specifies that the optimum distance is 2/3 of the distance from the substation and 2/3 of the circuit’s var requirement.

As long as the size and location are somewhat close (within 10%), the not-quite-optimal capacitor placement provides almost as much loss reduction as the optimal placement.

Consider the voltage impacts of capacitors. Under light load, check that the capacitors have not raised the voltages above allowable standards. If voltage limits are exceeded, reduce the size of the capacitor banks or the number of capacitor banks until voltage limits are not exceeded. If additional loss reduction is desired, consider switched banks as discussed below.

Energy Losses

Use the average reactive loading profile to optimally size and place capacitors for energy losses. If we use the peak-load case, the 1/2-kvar method optimizes losses during the peak load. If we have a load-flow case with the average reactive load, the 1/2-kvar method or the 2/3 rule optimizes energy losses. This leads to more separation between banks and less kvars applied than if we optimize for peak losses.

If an average system case is not available, then we can estimate it by scaling the peak load case by the reactive load factor, RLF:

RLF = Average kvar Demand / Peak kvar Demand

The reactive load factor is similar to the traditional load factor except that it only considers the reactive portion of the load. If we have no information on the reactive load factor, use the total load factor. Normally, the reactive load factor is higher than the total load factor.

Figure 4 - Example of real and reactive power profiles on a residential feeder on a peak summer day with 95% air conditioning

Figure 4 shows an example of power profiles; the real power (kW) fluctuates significantly more than the reactive power (kvar).

T & D - 6) Flexible AC Transmission System – What and Why?

Flexible AC Transmission System (FACTS) have been evolving to a mature technology with high power rating. This technology has wide spread application, became a top rate, most reliable one, based on power electronics. The main purpose of these systems is to supply the network as quickly as possible with inductive or capacitive reactive power that is adapted to its particular requirements, while also improving transmission quality and the efficiency of the power transmission system.

With the progression and development in power electronics application not only improved the performance of AC systems but also make it feasible for long distance.

Facts can also help solve technical problems in the interconnected power systems.

Facts are available in:

1. Parallel connection
• Static Var Compensator (SVC)
• Static Synchronous Compensator (STATCOM)
  .
2. Series connection
• Fixed Series Compensation (FSC)
• Thyristor Controlled/Protected Series Compensation (TCSC/TPSC)

Parallel Compensation

Any type of reactive power compensation employing either switched or controlled units that are connected in parallel to the transmission network at a power system node.

Mechanically Switched Capacitors/Reactors (MSC/MSR)

Most economical reactive power compensation devices are mechanical switched devices:

Mechanical switched capacitors are a simple but low speed solution for voltage control and network stabilization under heavy load condition. Their utilization has almost no effect on the short circuit power but it increases the voltage at the point of connection. Mechanical switched reactors have exactly the opposite effect and are therefore preferable for achieving stabilization under low load conditions.

An advanced form of mechanically switched capacitor is the MSCDN. This device is an MSC with an additional damping circuit for avoidance of system resonances.

a) Mechanically switched capacitors (MSC) and mechanically switched reactors (MSR) connected to the transmission system; b,c) Static Var compensator (SVC) with three branches (TCR, TSC, filter) and coupling transformer

Static Var Compensator (SVC)

Static Var compensators are a fast and reliable means of controlling voltage lines and system nodes. The reactive power is changed by switching or controlling reactive power elements connected to the secondary side of the transformer. Each capacitor bank is switched ON and OFF by thyristor valve (TSC). Reactor can be either switched (TSR) or controlled (TCR) by thyristor valves.

SIEMENS - Turnkey Static Var Compensator (SVC) Project

When system voltage is low, the SVC supplies capacitive reactive power and raises the network voltage. When system voltage is high, the SVC generates inductive reactive power and reduces the system voltage.

Static Var Compensators perform the following tasks:

1. Improvement in voltage quality
2. Dynamic reactive power control
3. Increase in system stability
4. Damping of power oscillations
5. Increase in power transfer capability
6. Unbalance control (option)

The design and configuration of an SVC, including the size of the installation, operating conditions and losses, depend on the system condition (weak or strong), the system configuration (meshed or radial) and the tasks to be performed.

Static Var Compensator (SVC) Plus

The modular SVC PLUS is equipped with an IGBT multilevel converter and a storage capacitor on the DC side. From approximately +/- 25 MVA to +/- MVAr, all of the main equipment, including the IGBT converter, the control and protection system and the converter cooling system of the SVC PLUS, is installed in a container and factory pretested so that it is ready to be installed outdoor at the site.

For indoor installations, converter modules with approximately +/- 100 MVAr are available.

Siemens - Static Var Compensator (SVC) PLUS

Parallel operation of converter modules is also possible, resulting in higher ratings. The footprint of an SVC PLUS installation is smaller than a conventional SVC installation of the same rating.

Series Compensation

Series compensation is defined as insertion of reactive power element into transmission lines.

The most common application is the fixed series capacitor (FSC). Thyristor-valve controlled systems (TCSC) and thyristor-valve protected systems (TPSC) may also be installed.

Fixed Series Capacitor (FSC)

The simple and most cost effective type of series compensation is provided by FSCs. FSCs comprise the actual capacitor banks, and for protection purposes, parallel arresters (metal oxide varistors, MOVs), spark gaps and a bypass switch for isolation purpose.

Fixed series compensation provides the following benefits:

1. Increase in transmission capacity
2. Reduction in transmission angle

Thyristor-controlled Series Capacitor (TCSC)

Reactive power compensation by means of TCSCs can be adapted to a wide range of operating conditions. It is also possible to control the current and thus the load flow in parallel transmission lines, which simultaneously improves system stability. Further applications for TCSC including power oscillation damping and mitigation of sub synchronous resonance (SSR), which is a crucial issue in case of large thermal generators.

Additional benefits of thyristor-controlled series compensation:

1. Damping of power oscillations (POD)
2. Load-flow control
3. Mitigation of SSR (sub synchronous resonances)
4. Increase in system stability

a) Fixed series compensation (FSC) connected to the network; b,c) Thyristor-controlled series capacitor (TCSC) connected to the network

Thyristor-Protected Series Capacitor (TPSC)

When high power thyristors are used, there is no need to install conventional spark gaps or surge arresters. Due to the very short cooling down times of the special thyristor valves, TPSCs can be quickly returned to service after a line fault, allowing the transmission lines to be utilized to their maximum capacity.

View of a TCSC system

TPSCs are the first choice whenever transmission lines must be returned to maximum carrying capacity as quickly as possible after a failure.

Short-Circuit Current Limitations (SCCL)

Extension of HV AC networks, coupling of independent grids and adding of new generation increase the existing short-circuit power in many cases. If the designed short-circuit level of the existing equipment is exceeded, and extension of the network, without extremely costly replacement of the existing equipment, is not possible. This no-go criteria can be avoided by using the Siemens short-circuit current limiter.

Fast short-circuit current limitation (SCCL) with high-power thyristor

By combining the TPSC with an external reactor, this combination can now be used as short-circuit current limiter (SCCL).

In case of a system fault, the thyristor valve will be fired, by passing the series capacitor. The corresponding short-circuit current will be limited by the reactor to the design values.