Principle of Operation of Unified Power Flow Controller: Top 10 UPFC Functions and Applications

 A Unified Power Flow Controller, commonly known as a UPFC, is an advanced Flexible AC Transmission System controller used to regulate voltage and control power flow in high-voltage AC transmission networks.

The operating principle of a UPFC is based on two back-to-back Voltage Source Converters connected through a common DC-link capacitor. One converter is connected in shunt with the transmission system, while the second converter is connected in series with the transmission line.

The series converter injects a controllable AC voltage into the line, while the shunt converter regulates the common DC-link voltage and can provide independent reactive-power support to the connected bus.

This coordinated arrangement enables the UPFC to control:

• Transmission-line voltage
• Effective line impedance
• Voltage phase angle
• Active-power flow
• Reactive-power flow
• Bus voltage
• Power-system oscillations

The UPFC combines many of the operating capabilities of a Static Synchronous Compensator, or STATCOM, and a Static Synchronous Series Compensator, or SSSC. The two converters exchange active power through their common DC link, allowing comprehensive control of AC transmission power flow. 

Basic UPFC arrangement using coordinated shunt and series Voltage Source Converters connected through a common DC link.

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What Is a Unified Power Flow Controller?

A Unified Power Flow Controller is a converter-based FACTS device capable of simultaneously or selectively controlling several electrical parameters of a transmission line.

A conventional AC transmission line has limited controllability because active and reactive power flows are largely determined by:

• Sending-end voltage
• Receiving-end voltage
• Voltage phase-angle difference
• Transmission-line reactance
• Network topology
• Connected generation and load

The UPFC changes this condition by introducing two controllable quantities:

1. A shunt current at the connected AC bus
2. A series voltage in the transmission line

The magnitude and phase angle of the injected series voltage can be controlled. This allows the UPFC to influence line current, effective impedance, active power, reactive power, and voltage conditions.

The UPFC therefore provides more comprehensive control than a standalone shunt-compensation or series-compensation device.

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Basic Components of a UPFC

A typical Unified Power Flow Controller contains the following major components:

1. Shunt-connected Voltage Source Converter, VSC1
2. Series-connected Voltage Source Converter, VSC2
3. Common DC-link capacitor
4. Shunt coupling transformer
5. Series injection transformer
6. Converter valves
7. Harmonic filters, where required
8. Cooling system
9. Protection and control system
10. Bypass and isolation equipment

Each component performs a specific function within the complete UPFC arrangement.

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1. Shunt-Connected Converter, VSC1

The shunt converter is connected to the transmission-system bus through a shunt coupling transformer.

Its main responsibility is to maintain the voltage of the common DC-link capacitor. It accomplishes this by drawing active power from, or supplying active power to, the AC system.

The shunt converter can also provide independent reactive-power compensation at the connected bus.

Depending on the selected control mode, VSC1 can:

• Maintain the DC-link voltage
• Generate reactive power
• Absorb reactive power
• Regulate the connected-bus voltage
• Improve the local power factor
• Support voltage during network disturbances
• Operate similarly to a STATCOM

The shunt converter therefore performs two coordinated functions.

First, VSC1 manages the active-power balance between the AC network and the series converter.

Second, VSC1 provides shunt reactive-power support within its available converter rating.

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2. Series-Connected Converter, VSC2

The series converter is connected to the transmission line through a series injection transformer.

VSC2 generates an AC voltage of controllable magnitude and phase angle. This voltage is injected in series with the transmission line.

The injected voltage behaves as a controllable synchronous AC voltage source. Depending on its phase relationship with the transmission-line current and bus voltage, the injected voltage can influence:

• Active-power flow
• Reactive-power flow
• Effective line reactance
• Voltage phase angle
• Receiving-end voltage
• Transmission-line current

The series converter can exchange both active and reactive power with the AC transmission line.

The reactive-power component is generated or absorbed by the converter at its AC terminal. The active-power component must be balanced through the common DC link by the shunt converter.

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3. Common DC-Link Capacitor

The DC terminals of VSC1 and VSC2 are connected through a common DC-link capacitor.

The DC-link capacitor provides:

• A common DC voltage for both converters
• Short-term energy storage
• Active-power transfer between converters
• DC-voltage stabilization
• Energy balance during converter switching and transient operation

A critical UPFC operating principle is:

Active power can pass between the two converters through the common DC link, but reactive power does not flow through the DC link.

Reactive power is generated or absorbed independently at the AC terminal of each converter.

The capacitor is mainly an energy-balancing component. In a conventional UPFC, it is not intended to provide sustained active power to the transmission system without an additional energy source.

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Principle of Operation of Unified Power Flow Controller

The operation of a UPFC can be understood through the interaction of its series and shunt converters.

A simplified operating sequence is as follows:

1. The series converter generates a controllable AC voltage.
2. The series injection transformer inserts this voltage into the transmission line.
3. The injected voltage changes the transmission-line current.
4. The change in current modifies active and reactive power flow.
5. The series converter exchanges active and reactive power with the AC transmission line.
6. The active-power requirement of the series converter appears at the common DC link.
7. The shunt converter supplies or absorbs this active power from the connected AC bus.
8. The shunt converter regulates the DC-link voltage.
9. The shunt converter can simultaneously provide independent reactive-power compensation.

The active-power balance between both converters can be written approximately as:

$${𝑃}_{\text{shunt}}+{𝑃}_{\text{series}}+{𝑃}_{\text{loss}}=0$$Pshunt​+Pseries​+Ploss​=0

Where:

• ${𝑃}_{\text{shunt}}$Pshunt​ is the active power exchanged by the shunt converter
• ${𝑃}_{\text{series}}$Pseries​ is the active power exchanged by the series converter
• ${𝑃}_{\text{loss}}$Ploss​ represents converter, transformer, filter, and auxiliary-system losses

If internal losses are neglected:

$${𝑃}_{\text{shunt}}\approx -{𝑃}_{\text{series}}$$Pshunt​≈−Pseries​

This means that active power supplied by one converter is approximately absorbed by the other converter.

The series converter may either absorb active power from the transmission line or inject active power into it. The shunt converter provides the balancing active power through the common DC link and maintains the capacitor voltage at its reference value.

The shunt converter may also generate or absorb reactive power independently. Its reactive-power function is not directly determined by the reactive power exchanged by the series converter.

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Active-Power Flow Through a UPFC

Suppose the series converter is commanded to inject active power into the transmission line.

The energy flow occurs as follows:

1. VSC1 draws active power from the AC bus.
2. VSC1 converts the AC power into DC power.
3. The common DC link transfers power from VSC1 to VSC2.
4. VSC2 converts the DC power into AC power.
5. The series transformer injects the resulting voltage into the transmission line.

If VSC2 absorbs active power from the transmission line, this energy-transfer direction is reversed.

The common DC link therefore provides a path for bidirectional active-power transfer between the converters.

In normal steady-state operation, the net active power drawn by the complete UPFC from the AC system is primarily associated with its internal losses.

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Reactive-Power Exchange in a UPFC

The reactive-power operation of a UPFC is different from its active-power operation.

VSC1 and VSC2 can each generate or absorb reactive power at their respective AC terminals.

• VSC1 manages shunt reactive power at the connected bus.
• VSC2 manages reactive power associated with series-voltage injection.
• Reactive power does not pass between the converters through the DC link.

This is because the DC link operates with DC voltage and DC current. AC reactive power is associated with the phase relationship between AC voltage and AC current and is therefore handled locally by each converter.

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Power-Flow Equation of a Transmission Line

For a simplified lossless transmission line, active-power transfer can be represented by:

$$𝑃=\frac{{𝑉}_{𝑆}{𝑉}_{𝑅}}{𝑋}\sin 𝛿$$P=XVS​VR​​sinδ

Where:

• $𝑃$P is transmitted active power
• ${𝑉}_{𝑆}$VS​ is sending-end voltage
• ${𝑉}_{𝑅}$VR​ is receiving-end voltage
• $𝑋$X is transmission-line reactance
• $𝛿$δ is the phase-angle difference between sending-end and receiving-end voltages

This relationship shows that active-power flow can be influenced by changing:

1. Sending-end voltage
2. Receiving-end voltage
3. Effective line reactance
4. Voltage phase angle

A UPFC can influence all four variables through coordinated shunt compensation and series-voltage injection.

This capability makes the UPFC one of the most versatile FACTS controllers for AC transmission systems. The device can independently control transmitted active and reactive power while also supporting bus-voltage regulation. 

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Top 10 UPFC Functions and Applications

1. Active-Power Flow Control

One of the most important functions of a Unified Power Flow Controller is controlling the active power transmitted through a selected transmission line.

The series converter injects a voltage that changes the line’s effective voltage-angle relationship or effective impedance. This allows the UPFC to increase, decrease, or redirect active-power flow within the operating limits of the converters and transmission equipment.

Main applications

• Increasing power flow through an underutilized line
• Reducing power flow through an overloaded line
• Balancing loading between parallel transmission circuits
• Maintaining scheduled power transfer
• Redirecting power through a preferred corridor
• Improving utilization of existing transmission infrastructure

In a meshed transmission network, electrical power follows available paths according to network impedance and voltage conditions. The power does not automatically follow commercial contracts or operator preferences.

A UPFC allows the system operator to exercise greater control over this natural power-flow distribution.

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2. Reactive-Power Flow Control

A UPFC can control the reactive power transmitted through a line.

The series converter modifies reactive-power flow through controlled series-voltage injection. At the same time, the shunt converter can generate or absorb additional reactive power at the connected bus.

Main applications

• Reducing unnecessary reactive-power circulation
• Supporting transmission voltage
• Improving the voltage profile
• Reducing reactive loading on generators
• Improving the receiving-end power factor
• Controlling reactive-power exchange between network areas

Reactive-power control is particularly valuable on heavily loaded transmission corridors where voltage stability can become a limiting factor.

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3. Bus-Voltage Regulation

The shunt-connected converter can regulate the transmission-bus voltage by injecting or absorbing reactive current.

When bus voltage falls, VSC1 may provide capacitive reactive current. When bus voltage rises, the converter may absorb inductive reactive current.

The shunt converter can operate in:

• Automatic voltage-control mode
• Constant reactive-power mode
• Constant power-factor mode
• Coordinated network-control mode

The shunt converter performs a function similar to a STATCOM and can regulate the connected-bus voltage while maintaining the common DC-link voltage. 

Main applications

• Supporting weak transmission buses
• Improving post-fault voltage recovery
• Reducing voltage fluctuations
• Supporting heavily loaded transmission lines
• Reducing reliance on mechanically switched capacitor banks
• Improving the voltage profile near major load centers

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4. Effective Line-Impedance Control

The series converter can inject a voltage component that emulates capacitive or inductive series compensation.

Capacitive series compensation reduces effective line reactance and may increase power-transfer capability. Inductive series compensation increases effective reactance and may reduce or redirect line loading.

The modified active-power relationship can be expressed as:

$$𝑃=\frac{{𝑉}_{𝑆}{𝑉}_{𝑅}}{{𝑋}_{\text{effective}}}\sin 𝛿$$P=Xeffective​VS​VR​​sinδ

Where ${𝑋}_{\text{effective}}$Xeffective​ represents the effective line reactance after compensation.

Reducing effective reactance can increase active-power transfer, subject to:

• Thermal limits
• Voltage limits
• Stability constraints
• Protection requirements
• Converter rating
• Equipment short-circuit capability

Main applications

• Increasing transmission capability
• Reducing line overloading
• Balancing parallel circuit loading
• Improving system operating flexibility
• Supporting contingency operation

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5. Phase-Angle Control

A UPFC can emulate the function of a phase-shifting transformer by injecting a series voltage that changes the effective phase-angle difference across a transmission line.

Because active-power flow is related to $\sin 𝛿$sinδ, changing the effective phase angle changes the transmitted active power.

Main applications

• Controlling power exchange between interconnected areas
• Reducing unintended loop flows
• Directing power through selected transmission corridors
• Balancing parallel power-flow paths
• Managing power transfer through regional interconnections

Phase-angle control is particularly useful in meshed networks where power can flow through several parallel routes.

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6. Transmission-Congestion Management

Transmission congestion occurs when a line or corridor approaches a thermal, voltage, or stability limit while other transmission paths remain underutilized.

A UPFC can help redistribute power away from a heavily loaded circuit and toward transmission routes that have available capacity.

Potential benefits

• Reduced overloading
• Improved contingency performance
• Better use of existing network assets
• Improved transmission security
• Reduced renewable-energy curtailment
• Improved transfer between generating areas and load centers
• Possible deferral of selected transmission reinforcements

A UPFC does not physically increase conductor ampacity or equipment thermal ratings. Instead, it helps the system operator use the available network capacity more effectively.

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7. Transient-Stability Improvement

Transient stability is the ability of synchronous generators and interconnected power systems to remain in synchronism after a major disturbance.

Examples of major disturbances include:

• Transmission-line faults
• Generator outages
• Busbar faults
• Sudden load rejection
• Loss of a major interconnection
• High-voltage switching events

A UPFC can respond quickly by changing series-voltage injection and shunt-current output.

Possible stabilizing actions

• Modulating active-power transfer
• Supporting transmission voltage
• Improving power transfer after fault clearance
• Reducing generator acceleration
• Damping rotor-angle oscillations
• Supporting recovery to a stable operating condition

The UPFC’s dynamic control capability can provide benefits for both steady-state and transient power-system performance. 

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8. Power-Oscillation Damping

Power-system oscillations may develop after network disturbances or because of weak damping between interconnected generating areas.

Poorly damped oscillations can limit secure transmission capacity. In more severe cases, oscillations can threaten system stability.

A UPFC can include a supplementary damping controller that modulates its active-power, reactive-power, or series-voltage command.

Possible controller input signals

• Active-power deviation
• Frequency deviation
• Rotor-speed deviation
• Voltage-angle difference
• Transmission-line current
• Wide-area synchronized measurements

Main applications

• Damping local generating-unit oscillations
• Damping inter-area oscillations
• Improving dynamic security
• Increasing secure power-transfer capability
• Supporting weakly interconnected power networks

The damping controller must be carefully coordinated with generator power-system stabilizers, HVDC controls, other FACTS controllers, and network-protection systems.

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9. Renewable-Energy Integration

Large solar and wind projects can change transmission loading, voltage profiles, power-flow patterns, reactive-power requirements, and network dynamic behavior.

A UPFC can support renewable-energy integration by controlling the power transmitted between renewable-generation areas and major load centers.

Potential applications

• Managing variable power flow
• Reducing congestion near renewable-energy plants
• Supporting voltage at important transmission buses
• Improving use of existing transmission capacity
• Reducing renewable-energy curtailment
• Improving post-disturbance recovery
• Coordinating power flow across parallel transmission corridors

A UPFC does not eliminate the need for substations, transmission lines, protection upgrades, or network reinforcement. Its value should be demonstrated through detailed technical and economic studies.

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10. Improved Use of Existing Transmission Assets

Constructing new transmission lines can require considerable time, land, permitting, environmental assessment, capital investment, materials, and stakeholder coordination.

A UPFC may improve the operational use of existing transmission assets by:

• Balancing line loading
• Redirecting power flow
• Reducing congestion
• Supporting bus voltage
• Improving contingency performance
• Increasing operational flexibility
• Reducing losses under selected operating conditions
• Deferring network reinforcement where technically justified

The actual benefit depends on:

• UPFC location
• Converter rating
• Network topology
• Existing transmission capacity
• Control objectives
• Contingency requirements
• System operating conditions

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UPFC Control Modes

Shunt-Converter Control Modes

1. DC-Link Voltage-Control Mode

The shunt converter exchanges active power with the AC network to maintain the DC-link voltage at its reference value.

This active-power exchange supplies the power required by the series converter and compensates for internal system losses.

2. Constant Reactive-Power Mode

The shunt converter generates or absorbs a specified amount of reactive power.

3. Automatic Bus-Voltage-Control Mode

The reactive current of the shunt converter is adjusted automatically to maintain the AC bus voltage near a selected reference.

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Series-Converter Control Modes

1. Direct Voltage-Injection Mode

The magnitude and phase angle of the series-injected voltage are specified directly.

2. Active and Reactive Power-Control Mode

The converter automatically changes its injected-voltage phasor to maintain selected active-power and reactive-power references.

3. Phase-Shifter Emulation Mode

The converter introduces an effective phase-angle displacement to control active-power flow.

4. Series-Compensation Mode

The injected voltage emulates capacitive or inductive series compensation.

5. Automatic Power-Flow Control Mode

The converter continuously regulates line power flow according to operator commands, network conditions, and equipment limits.

The shunt and series converters can perform these functions simultaneously, subject to the voltage, current, thermal, and control limitations of the complete UPFC system. 

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UPFC Compared with Other FACTS Controllers

FACTS Controller	Connection Type	Primary Function	Main Distinction
SVC	Shunt	Reactive-power and bus-voltage control	Uses thyristor-controlled reactors and capacitors
STATCOM	Shunt	Dynamic reactive-power compensation	Uses a Voltage Source Converter
TCSC	Series	Variable series compensation	Controls effective line reactance
SSSC	Series	Controllable series-voltage injection	Provides converter-based series compensation
UPFC	Series and shunt	Voltage, impedance, phase angle, active- and reactive-power control	Uses coordinated converters with a common DC link

The UPFC combines STATCOM-like shunt compensation with SSSC-like series-voltage injection. The common DC link enables active-power exchange between the two converters. 

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Main Advantages of a UPFC

The principal advantages of Unified Power Flow Controller technology include:

• Simultaneous active- and reactive-power control
• Fast converter-based response
• Independent shunt reactive-power support
• Controllable series-voltage injection
• Bus-voltage regulation
• Effective line-impedance control
• Phase-angle control
• Transmission-congestion management
• Improved transient stability
• Power-oscillation damping
• Better utilization of transmission assets
• Support for renewable-energy integration
• Improved power-flow flexibility
• Potential reduction in transmission losses

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Limitations of a UPFC

Despite its flexibility, a UPFC has several technical and economic limitations.

1. High Initial Cost

A UPFC requires:

• Two high-power Voltage Source Converters
• Series and shunt transformers
• Cooling systems
• Harmonic filters
• Protection and control systems
• High-voltage switchgear
• Bypass equipment
• Civil and auxiliary systems

The equipment and engineering costs can be substantial.

2. Complex Control System

The UPFC control system must coordinate:

• DC-link voltage
• Series-injected voltage
• Shunt reactive power
• Bus voltage
• Active-power transfer
• Reactive-power transfer
• Converter current limits
• Protection functions
• Power-oscillation damping

3. Converter Losses

Semiconductor switching, coupling transformers, cooling systems, filters, and auxiliaries introduce electrical losses.

4. Harmonic Performance

Voltage Source Converters generate switching harmonics. The converter topology, modulation method, transformer arrangement, switching frequency, and filter design must be coordinated.

5. Protection Complexity

The protection system must distinguish between:

• Transmission-line faults
• Shunt-transformer faults
• Series-transformer faults
• Converter faults
• DC-link faults
• Cooling-system failures
• Internal control failures
• External network disturbances

6. Specialized Maintenance

UPFC operation and maintenance require personnel with experience in:

• Power electronics
• FACTS controls
• High-voltage transformers
• Protection systems
• Cooling systems
• Harmonic measurement
• SCADA and communication
• High-voltage safety

7. Reliability and Bypass Requirements

A dependable bypass and isolation arrangement is required so the transmission line can remain available when the UPFC converter system is under maintenance or unavailable.

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UPFC Protection and Monitoring

A practical UPFC installation may include protection against:

• Converter overcurrent
• Converter valve failure
• DC-link overvoltage
• DC-link undervoltage
• Semiconductor overtemperature
• Shunt-transformer internal faults
• Series-transformer internal faults
• Transformer earth faults
• Cooling-system failure
• AC overvoltage and undervoltage
• AC overfrequency and underfrequency
• Harmonic overload
• Control-system failure
• Communication failure
• Bypass-breaker failure
• Auxiliary-supply failure
• Fire and smoke

Important monitored quantities include:

• DC-link voltage
• Shunt-converter current
• Series-converter current
• Injected series voltage
• Bus voltage
• Transmission-line current
• Active-power flow
• Reactive-power flow
• Transformer temperature
• Converter-valve temperature
• Harmonic distortion
• Cooling-system condition
• Breaker and bypass-switch position

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Studies Required Before Installing a UPFC

The following studies may be required before determining the location, rating, and control strategy of a UPFC:

1. Load-flow study
2. Contingency analysis
3. Short-circuit study
4. Voltage-stability study
5. Transient-stability study
6. Small-signal stability study
7. Power-oscillation damping study
8. Harmonic assessment
9. Electromagnetic-transient study
10. Insulation-coordination study
11. Protection-coordination study
12. Reliability and availability assessment
13. Converter-rating study
14. Economic cost-benefit analysis

A UPFC may provide limited benefit if installed at an unsuitable location or assigned an inadequate converter rating. Its location and size should therefore be determined through detailed power-system studies.

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Frequently Asked Questions

What is the full form of UPFC?

UPFC stands for Unified Power Flow Controller.

What is the principle of operation of a UPFC?

A UPFC operates using two Voltage Source Converters connected through a common DC link. One converter is connected in shunt and maintains the DC-link voltage, while the other converter injects a controllable voltage in series with the transmission line.

How many converters are used in a conventional UPFC?

A conventional UPFC uses two Voltage Source Converters. VSC1 is connected in shunt and VSC2 is connected in series.

What is the function of VSC1?

VSC1 maintains the common DC-link voltage by exchanging active power with the AC network. VSC1 can also independently generate or absorb reactive power at the connected bus.

What is the function of VSC2?

VSC2 injects a controllable voltage into the transmission line through a series transformer. The magnitude and phase angle of this voltage are adjusted to control transmission-line power flow.

Does reactive power flow through the UPFC DC link?

No. Reactive power is generated or absorbed locally at the AC terminal of each converter. Only active power is exchanged between the two converters through the common DC link.

Is a UPFC a combination of STATCOM and SSSC?

Functionally, yes. The shunt converter provides STATCOM-like compensation, while the series converter performs SSSC-like series-voltage injection. The common DC link enables active-power exchange between them.

Can a UPFC control active and reactive power independently?

Yes. A UPFC can control active and reactive power simultaneously or selectively, subject to converter ratings, system conditions, and control limitations.

Can a UPFC increase the thermal rating of a transmission line?

No. A UPFC cannot physically change the thermal rating of a conductor. It can regulate and redirect power flow to improve the overall use of available transmission capacity.

What is the difference between UPFC and TCSC?

A TCSC primarily controls effective transmission-line reactance through thyristor-controlled series compensation. A UPFC uses coordinated series and shunt Voltage Source Converters to control voltage, line impedance, phase angle, active power, and reactive power.

Can a UPFC improve power-system stability?

Yes. A properly designed UPFC can support bus voltage, control post-fault power transfer, damp power oscillations, and improve transient and dynamic system performance.

What is the purpose of the UPFC DC-link capacitor?

The DC-link capacitor maintains a common DC voltage, supports short-term energy balancing, and enables active-power exchange between the shunt and series converters.

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Conclusion

A Unified Power Flow Controller is one of the most advanced and flexible devices available for controlling power flow in AC transmission systems.

The UPFC consists of two back-to-back Voltage Source Converters connected through a common DC-link capacitor. The shunt converter regulates the DC-link voltage and can provide independent reactive-power support. The series converter injects a voltage of controllable magnitude and phase angle into the transmission line.

The series voltage can be controlled to influence active-power flow, reactive-power flow, effective line impedance, and voltage phase angle. Active power required by the series converter is transferred through the DC link and balanced by the shunt converter. Reactive power is generated or absorbed independently at the AC terminal of each converter and does not flow through the DC link.

Through these coordinated functions, the UPFC can:

• Control active and reactive power
• Regulate transmission voltage
• Provide series compensation
• Control phase angle
• Manage network congestion
• Improve transient stability
• Damp power oscillations
• Support renewable-energy integration
• Improve the use of existing transmission infrastructure

UPFC application nevertheless requires detailed system studies, appropriate converter sizing, reliable cooling, coordinated controls, harmonic assessment, dependable protection, effective bypass arrangements, and a comprehensive maintenance strategy.

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Technical Disclaimer

This article is intended for educational and general technical-information purposes. It does not replace an approved power-system study, transmission-system operator requirements, equipment specifications, control-system designs, protection-coordination studies, manufacturer instructions, or applicable engineering standards.

UPFC design, modeling, protection, installation, operation, and commissioning should be performed by qualified power-system and FACTS specialists.

Updated September 2026: This article has been expanded with detailed explanations of UPFC construction, operating principles, active- and reactive-power exchange, control modes, applications, advantages, limitations, protection, and required power-system studies.