c8-13

24/08/2007

UPFC (Unified Power Flow Controller )

VIGO (Spain)

UPFC - Coupling of converters' DC terminals offers a fundamentally different range of control options

The UPFC combines together the features of two FACTS devices: the Static Synchronous Compensator (STATCOM) and the Static Synchronous Series Compensator (SSSC). 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 active power supplied to the line by the series converter, can now be supplied by the shunt converter, as shown in the Figure.

Donsión

24/08/2007


VIGO (Spain)

UPFC - Coupling of converters' DC terminals offers a fundamentally different range of control options

This topology offers three degrees of freedom, or more precisely - four degrees of freedom (two associated with each VSC) with one constraint (active powers of the VSCs must match). Therefore, a fundamentally different range of control options is available compared to STATCOM or SSSC. The UPFC can be used to control the flow of active and reactive power through the line and to control the amount of reactive power supplied to the line at the point of installation.

Donsión

24/08/2007


VIGO (Spain)

DC - ACCONVERTER

DC -ACCONVERTER

“STATCOM” “SSSC”

BUSBAR A BUSBAR B

I

VSERIES

VA VM= VA + VSERIES

While operating both inverters as a UPFC, the exchanged power at the terminals of each inverter can be imaginary as well as real.

Donsión

24/08/2007


VIGO (Spain)

Representative of the last generation of FACTS devices is the Unified Power Flow Controller (UPFC). The UPFC is a device which can control simultaneously all three parameters of line power flow (line impedance, voltage and phase angle).

The UPFC combines together the features of the Static Synchronous Compensator (STATCOM) and the Static Synchronous Series Compensator (SSSC). In practice, these two devices are two Voltage Source Inverters (VSI’s) connected respectively in shunt with the transmission line through a shunt transformer and in series with the transmission line through a series transformer, connected to each other by a common dc link including a storage capacitor.

The shunt inverter is used for voltage regulation at the point of connection injecting an opportune reactive power flow into the line and to balance the real power flow exchanged between the series inverter and the transmission line. The series inverter can be used to control the real and reactive line power flow inserting an opportune voltage with controllable magnitude and phase in series with the transmission line.

Thereby, the UPFC can fulfil functions of reactive shunt compensation, active and reactive series compensation and phase shifting. Besides, the UPFC allows a secondary but important function such as stability control to suppress power system oscillations improving the transient stability of power system.

Donsión

24/08/2007


VIGO (Spain)

The basic components of the UPFC are two voltage source inverters (VSI's) sharing a common dc storage capacitor, and connected to the system through coupling transformers. One VSI is connected in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer.

The series inverter is controlled to inject a symmetrical three phase voltage system of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is equal only to the losses of the two inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point.

Donsión

24/08/2007


VIGO (Spain)

The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power flow on the transmission line.

The UPFC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current into the transmission line. This current consist of two components with respect to the line voltage: the real or direct component , which is in phase or in opposite phase with the line voltage, and the reactive or quadrature component, which is in quadrature. The direct component is automatically determined by the requirement to balance the real power of the series inverter. The quadrature component, instead, can be independently set to any desired reference level (inductive or capacitive) within the capability of the inverter, to absorb or generate respectively reactive power from the line.

Donsión

24/08/2007


VIGO (Spain)

So, two control modes are possible:

VAR control mode: the reference input is an inductive or capacitive varrequest;

Automatic Voltage Control mode: the goal is to maintain the transmission line voltage at the connection point to a reference value.

Instead, the series inverter injecting the voltage controllable in amplitude and phase angle in series with the transmission line influences the power flow on the transmission line. This series voltage can be determined in different ways:

Direct Voltage Injection mode: the reference inputs are directly the magnitude and phase angle of the series voltage;

Phase Angle Shifter Emulation mode: the reference input is phase displacement between the sending end voltage and the receiving end voltage;

Line impedance emulation mode: the reference input is an impedance value to insert in series with the line impedance;

Automatic Power flow Control mode: the reference inputs are values of P and Q to maintain on the transmission line despite system changes.In general the shunt inverter will be operated in Automatic Voltage Control mode and the series inverter in Automatic Power Flow Control mode.

Donsión

24/08/2007


VIGO (Spain)

VVBUS MAXBUS MAXVVBUS MINBUS MIN

X)sin(V)VV(P uBSERIEA

sδ−δ+

=

UPFC Operation

VVSERIESSERIES

VVAAVVMM= V= VA A + V+ VSERIESSERIES

VSERIES MAX

VB

δδ u

X)sin(VVP BA

sδ

=

WITHOUT UPFC

X = LINE REACTANCE

WITH UPFC

Donsión

24/08/2007


VIGO (Spain)

Model of UPFC

The mathematical UPFC model has been derived with the aim of being able to study the relations between the electrical transmission system and UPFC in steady and transient conditions.

0

1

2

3 0

2

4

6

0

1

2

0

1

2

3 δs

δbPs

1 2 3 4 5 6

0.2

0.4

0.6

0.8

1Ps

Qs

δb

Vr∠0

I∠α

Vser∠δb

V ∠θ

UPFC

Vs ∠δsr1 x1 r2 x2

Vi ∠δi

xi

∞

Ps & Qs

Ish∠δsh

Donsión

24/08/2007


VIGO (Spain)

Vi∠θi Vj∠θjXs

Psi+Qsi Psj+Qsj

UPFC injection model

)sin(.X

V.VPP bij

s

jbsjsi δ+θ=−=

Injection ModelIt is shown that the active and reactive powers can be expressed as below:

bs

ibsi cos.

XV.VQ δ=

)cos(.X

V.VQ bij

s

jsj

b δ+θ−=

Vb

IiIij

Vj ∠θj- +

XsVm

UPFC connected between bus i and j

Vi ∠θi

Iijϕ

η δbVb

Vm

Vi

UPFC vector diagramDonsión

24/08/2007


Flicker Compensation in Arc Furnace Power systems Using the UPFC

UPFC

MPScc=4400 MVA Scc=1370 MVA

Sn=120 MVA

220 kV

55 km

83 MW

120 TM

VT:220 kV/100 V

Topas 1000

Unilyzer U-812

S. Carregado AC arc furnace

Electrical circuit of the arc furnace supply from Carregado Substation

T1T2

Donsión

24/08/2007



Scc=4400 MVA Scc=1370 MVASn=120 MVA

83 MW

120 TM

S. Carregado AC arc furnace

T1

HV/MV T2

MV/LVXp Xf Rf

Typical arc furnace system diagram

The complex nature of these phenomena does not favour a physical approach to the study of arc-length variation. Therefore, flicker investigations have been performed on the basis of the harmonic analysis. The arc furnace load looks like a voltage source of harmonics behind a series of impedance consisting of the secondary cables to the electrodes. A typical arc furnace model for simulation will include the furnace lead impedance and a constant voltage source behind it at each harmonic of concern. A typical arc furnace system is shown in the figure.

Donsión

24/08/2007



Under unbalanced conditions of electrode arcing, there could besignificant amounts of third harmonic and its multiples. Also, fifth and seventh harmonics that occur under balanced conditions could increase under unbalanced arcing conditions. Measurements of arc furnace voltage have indicated a varying harmonic output. The recorded fifth harmonic voltage has varied from 8%, 6%, and 2.5% of the fundamental voltage during beginning of meltdown, end of meltdown, and refining, respectively. The process is optimized to operate around the rated regime, where the active power is maximum . But one heating is in fact composed of at least three steps:

(1) The bore down, lasts one or two minutes. The electrodes have to dive deeply into the scrap to heat it, thus inducing a high instability of the arc (succession of arc extinction and short-circuit between electrodes and scrap).

(2) When the scrap is hot enough, the electrodes arc set higher to begin the melting phase (about 10 minutes). Due to collapses in the scrap, the arc is still quite unstable.

(3) As the scrap becomes liquid, the laden phase takes place for another 10 minutes. During this phase, the operating point is quite stable.

Donsión

24/08/2007



Flicker is usually linked to the variation of arc length which is proportional to the arc voltage value V. Hence. We represent the flicker phenomenon by imposing a 10 Hz sinusoidal variation on V which provides the worst case of arc furnace operation. It is useful to investigate the effects of flicker compensation by UPFC. In order to approach periodic flicker behaviour, simulations can be made attributing to arc length a sinusoidal law with frequency close to the most sensitive for flicker perceptivity. For example, the frequency of 10 Hz can be chosen, which lies in the centre of the sensitivity range, close to the minimum of the flicker perceptivity threshold curve for sinusoidal voltage fluctuations.Donsión

24/08/2007



The main cause of harmonic problems in arc furnace operations is the interaction of power factor correction capacitors with the inductive reactance of the system.

The power fluctuation which causes the voltage drop can be separated in two parts: Mean reactive power absorbed by the furnace, which could be compensated by fixed shunt-capacitor, and the instantaneous variation of the reactive power around its mean value with can only be compensated with a dynamic device. The Instantaneous Variation of the reactive power can be cancelled by means of several solutions. Donsión

24/08/2007



Two compensations can be distinguished:-Shunt compensation: the reactive power consumed by the arc furnace must be kept constant. This type of compensation does not protect the arc furnace, consequently, in case of shortcircuit the installation of the furnace undergoes high currents. A shunt compensation of active power does note seem very interesting because of the inductive nature of the feeding system.

- Series compensation: two types of compensation can be achieved by this structure: active and reactive compensation. To control the real and reactive power consumed by the arc furnace we need to inject series voltages of the appropriate magnitudes and angles. The instantaneous injected voltage (vpq) can be split into two components which are in phase (vp) and in quadrature (vq) with the source voltage. It is to be noted that the UPFC is located near the LV transformer. The voltage of the network at the connection point of the UPFC is used as reference to define the pq coordinate system for the instantaneous parameters. As a result the UPFC has four controllable parameters: the Vp and Vq components of the series injected voltage and the Ip and Iq components of the shunt current.

Donsión

24/08/2007



The main cause of harmonic problems in arc furnaceoperations is the interactionof power factor correction capacitors with the inductive reactance of the system.

Illustrative diagram for UPFC with arc furnace

Donsión

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