1. A Symmetrical Hybrid Power Flow Controller
Jovan Bebic, P.Eng.
October 31/2003
© Copyright by J. Bebic 2003
Slide 1: A Symmetrical Hybrid Power Flow Controller
A Symmetrical Hybrid Power Flow Controller (HPFC) is the new FACTS controller I proposed and studied in my PhD thesis.
In short, I am proposing a topology that uses static converters together with passive components (shunt capacitors) to achieve the control performance equivalent to that of the Unified Power Flow Controller (UPFC).
In most cases, reactive power support already exists in the transmission system (either switched shunt capacitors, or SVCs). The proposed topology uses the shunt capacitors as the crude source of reactive power, and the converters for vernier control of power flow and for control of the reactive powers supplied to the line segments.
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2. Overview VSC Basics Converter Based FACTS Devices Proposed Topology
Steady-State Operation
Reachable Sets and P- Curves
Dynamic Control
Summary
Future Work
Slide 2: Overview
I will start by reviewing the basic concepts of voltage-sourced converters (VSCs) . In particular, I will identify the control degrees of freedom available when VSCs are applied in power systems.
Next, I will review the three exemplary FACTS compensators: SSSC, STATCOM and UPFC. I will briefly discuss how the control degrees of freedom associated with the VSCs are used to achieve the line control.
Then, I will present the new topology and compare it to the topology of the UPFC.
After that, I will display exemplary vector diagrams that provide insight into steady-state operation of the HPFC.
Next, I will present and discuss the P-delta curves of the HPFC.
Finally, I will summarize the key points through the comparison of the HPFC with the UPFC.
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3. VSC Basics Slide 3: VSC Basics
A basic building block of any VSC is the three–phase converter bridge shown here. The bridge has two DC terminals (indicated by + and -) and three AC terminals in the mid points of the converter legs. In simplest terms, by controlling the states of switches in the legs we can produce arbitrary voltage waveforms at the AC terminals.
Now, when a VSC is interfaced to a transmission system it has to (ONE) operate at the line frequency, and (TWO) produce a balanced set of sinusoidal voltages. Therefore, a VSC coupled to the transmission system has only two control degrees of freedom - it can vary the magnitude and the phase angle of its output voltages relative to the system voltages.
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These two "control degrees of freedom" can be mapped into freedom to exchange active and reactive power with the transmission system. The amount of reactive power exchanged is limited only by the current capacity of the converter switches, while the active power coupled to/from the line has to be supplied/delivered_to the DC terminals, as shown symbolically in the lower figure.
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4. Converter Based FACTS Compensators
UPFC
STATCOM
SSSC
Slide 4: Converter based FACTS compensators
Static Synchronous Series Compensator (SSSC) is a VSC connected in series to the transmission line. In most cases the DC voltage support for the VSC will be provided by the DC capacitor of relatively small energy storage capability, hence in steady-state active power exchanged with the line will have to be maintained at zero at all times. This constraint makes the SSSC functionally equivalent to a series connected capacitor. The SSSC does offer fast control and it is inherently neutral to sub-synchronous resonance.
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Next, STATic COMPensator (STATCOM) is a VSC connected in shunt to the transmission line. Again, active power constraint makes this topology functionally equivalent to a shunt connected capacitor or an SVC. The STATCOM does offer faster control and improved control range compared to the SVC, as its ability to supply reactive power does not depend on system voltage.
A UPFC combines the two compensators into one, and therefore it offers a fundamentally different range of control options. The DC terminals of the two underlying VSCs are now coupled, and this creates a path for active power exchange between the converters. Hence, the series converter can now inject a voltage vector in an arbitrary direction relative to the line current vector.
This topology offers four degrees of freedom and there is one constraint: active powers of the VSCs must match. The existence of this constraint makes the control of the UPFC a challenging problem.
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5. Proposed Topology
HPFC

6. Limits and Constraints
HPFC
HPFC

7. Equal Power Lines

8. Steady-State Operation
Slide 6: Steady–State Operation
This first vector diagram shows the operating point that corresponds to the reduced power flow. This is achieved by reducing relative angles between voltages VS and V1, and V2 and VR, respectively.
Resulting currents IS and IR and their difference (IM) are also shown in the diagram. With known shunt susceptance BM, voltage VM can be determined based on the value of IM.
Finally, voltages VX and VY are positioned to compose the required voltages V1 and V2.
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In this diagram we see the reconfigured internal voltages of the HPFC that result in the same line conditions (the same IS and IR). The difference relative to the first diagram is that the amount of shunt capacitance is reduced (the same IM now requires more voltage).
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This confirms the earlier claim that the HPFC can be used with switched capacitances to achieve the vernier control of the line.
This diagram illustrates the ability of the HPFC to achieve decoupled control of P, Q1 and Q2. In this diagram Q2 is changed (increased) relative to the first diagram. The original position of vectors are shown in dashed blue lines to help appreciate the difference between the operating points.
This vector diagram corresponds to the increased power flow. Notice that V1 now lies below VM, and V2 is above VM. Therefore, relative angles between VS and V1, and V2 and VR are increased and their sum is greter than the angle between VS and VR.
This diagram shows the ability of the HPFC to reverse the power flow on a transmission line. The angle between VS and VR is small, and this permits selecting V1 and V2 so as to achieve negative flow of power.
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9. Reachable Sets

10. P- Curves

11. System Properties A system of variable structure,
with infinitely many internal operating points, for every line operating point.
A nonlinear control problem
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12. Dynamic Control
Operator specifies

13. Controlling the Power Balance of Converters
HPFC

14. Summary Ö HPFC UPFC Power flow control
Improves first swing stability margin
Topological symmetry
Able to utilize existing shunt compensation
Slide 8: Summary
It has been demonstrated that the Hybrid Power Flow Controller (HPFC) offers control characteristics substantially equivalent to those offered by the UPFC. It can be used for power flow control in the transmission system, and it improves the first swing stability margin.
Topologically, it is a symmetrical device – it employs converters and coupling transformers of identical ratings. This is of value in applications where reliability requirements may require a spare transformer.
Most importantly, the HPFC topology can be used to upgrade the functionality of existing switched capacitors and SVCs. In such applications, substantial cost savings can be realized, as the converters are used as an addition to already existing equipment and the required converter MVA is therefore lower compared to the UPFC.
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15. Future Work Study machine based equivalents. Study dual topology.
Formulate “optimality” algorithms based on “reachable sets” algorithms.
Optimize the controller.
Apply the concept of “equal power lines” in load flow programs.