Modeling And Simulation In Unified Power Controller Engineering Essay

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1.1 Introduction

Flexible alternating -current transmission systems (FACTS) is a recent technological development in electrical power systems. It builds on the great many advances achieved in high-current semiconductor device technology, digital control and signals conditioning. From the power systems engineering perspective, the wealth of experience gained with the commissioning and operation of high voltage direct current (HVDC) links an static VAR compensator (SVC) systems, over many decades, in many parts of the globe, may have provided the driving force for searcher deeper into the use of emerging power electronic equipment and techniques, as a means of alleviating long standing operational problems in both high voltage transmission and low voltage distribution systems. [1]

Early developments of the FACTS technology were in power electronic versions of the phase-shifting and tap-changing transformers. These controllers together with the electronic series compensator can be considered to belong to the first generation of FACTS equipment. The unified power flow controller, the static compensator, and the interphase power controller are more recent developments. Their control capabilities and intended function are more sophisticated than those of the first wave of FACTS controllers. They may be considered to belong to a second generation of FACTS equipment. Shunt-connected thyristor-switched capacitors and thyristor-controlled reactors, as well as high-voltage direct-current (DC) power converters, have been in existence for many years, although their operational characteristics resemble those of

FACTS controllers. [2]

A number of FACTS controllers have been commissioned. Most of them perform a useful role during both steady-state and transient operation, but some are specifically designed to operate only under transient conditions.

FACTS controllers intended for steady state operations are as follows:

Thyristor-controlled phase shift (PS): this controller is an electronic phase-shifting transformer adjusted by thyristor switches to provide a rapidly varying phase angle.

Load tap changer (LTC): this may be considered to be a FACTS controller if the tap changes are controlled by thyristor switches.

Thyristor controlled reactor (TCR): this is a shunt-connected, thyristor-controlled reactor, the effective reactance of which is varied in a continuous manner by partial conduction control of the thyristor valve.

Thyristor-controlled series capacitor (TCSC): this controller consists of a series capacitor paralleled by a thyristor-controlled reactor in order to provide smooth variable series compensation.

Interphase power controller (IPC): this is a series-connected controller comprising two parallel branches, one inductive and one capacitive, subjected to separate phase shifted voltage magnitudes. Active power control is set by independent or coordinated adjustment of the two phase-shifting sources and the two variable reactances. Reactive power control is independent of active power.

Static compensator (STATCOM): this is a solid-state synchronous condenser connected in shunt with the AC system. The output current is adjusted to control either the nodal voltage magnitude or the reactive power injected at the bus.

Solid state series controller (SSSC): this controller is similar to the STATCOM but it is connected in series with the AC system. The output current is adjusted to control either the nodal voltage magnitude or the reactive power injected at one of the terminals of the series-connected transformer.

Unified power flow controller (UPFC): this consists of a static synchronous series compensator (SSSC) and a STATCOM, connected in such a way that they share a common DC capacitor. The UPFC, by means of an angularly unconstrained series voltage injection, voltage magnitude, and the active and reactive power through it. It may also provide independently controllable shunt reactive compensation. [3]

1.2 Benefits from Facts Technology

Control of power flow as ordered

Increases the loading capability of lines to their thermal capabilities

Reduces reactive power flows, thus allowing the lines to carry more active power

Reduces loop flows

Increases the TX capability of AC line

Control of power flow over the TX routes



2.1 Introduction

A Unified Power Flow Controller (or UPFC) is an electrical device for providing fast-acting reactive power compensation on high-voltage electricity transmission networks. The UPFC is a versatile controller which can be used to control active and reactive power flows in a transmission line. The concept of UPFC makes it possible to handle practically all power flow control and transmission line compensation problems, using solid state controllers, which provide functional flexibility, generally not attainable by conventional thyristor controlled systems. The UPFC is a combination of a static synchronous compensator (STATCOM) and a static synchronous series compensator (SSC) coupled via a common DC voltage link. It is capable of controlling simultaneously or selectively, all the parameters affecting the power flow in a transmission line. The parameters usually are voltage, impedance and phase angle. [4]

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). Such "new" FACTS device combines together the features of two "old" FACTS devices: 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 fulfill 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. As the need for flexible and fast power flow controllers, such as the UPFC, is expected to grow in the future due to the changes in the electricity markets, there is a corresponding need for reliable and realistic models of these controllers to investigate the impact of them on the performance of the power system. [5]

2.2 UPFC Characteristics

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. A basic UPFC functional scheme is shown in Fig.2.1. [6]

Fig. 2.1: UPFC Functional Scheme

The series inverter is controlled to inject a symmetrical three phase voltage system, vse, 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. 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 flows 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 ish into the transmission line. This current consists of two components with respect to the line voltage: the real or direct component ishd, which is in phase or in opposite phase with the line voltage, and the reactive or quadrature component, ishq, 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. So, two control modes are possible: [7]

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

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 vse 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. [8]

Fig. 2.2: Phasor Representation of the UPFC

2.3 UPFC Model

2.3.1 Instantaneous Power Flow Delivered by a VSI Into a Power System

An inverter connected to a power system, which is able of power exchange between the power system and the dc storage capacitor, can be represented by a three symmetrical sinusoidal voltage sources. A symmetrical three-phase system can be transformed into a synchronously-rotating orthogonal system. A new coordinate system, having the axes rotating at the synchronous angular speed of the fundamental network voltage w, is defined on the basis of the d-q transformation. In Fig 2.2 a VSI is supplied by a voltage system vxa, vxb, vxc, R and L are respectively the transformer equivalent resistance and inductances. [9]

Fig. 2.3: Equivalent Circuit of a VSI Connected to a Power

The instantaneous active and reactive power flowing into the power system delivered by the VSI, neglecting transformer losses and assuming fundamental frequency and balanced conditions are:

p(t) = 1.5 * vxd * ixd q(t) = 1.5 * vxd * ixq

2.3.2 Series and Shunt Inverter Control

In the report it has been chosen a UPFC model in terms of two ideal controllable voltage sources, connected respectively in series and in shunt to the transmission line as in Fig.2.3, to represent respectively the series and the shunt inverters. So, the two UPFC control systems must be developed in such a way to evaluate the amplitude and the phase angle of these two voltage sources on the basis of operating functions required UPFC. Assuming the series inverter is operating in Automatic Power Flow Control mode, the amplitude and phase angle of the equivalent series voltage source are determined in such a way to control the power flows on the transmission line and so to obtain the active and reactive power flows desired at the receiving end. Instead, for the shunt inverter operating in Automatic Voltage Control or in VAR Control mode, the amplitude and phase angle of the equivalent shunt voltage source are calculated in such a way to control the voltage on the connection point (v1) or to generate or absorb a specific reactive power at this point respectively and naturally to supply or absorb the active power demanded by the series inverter. On the basis of (3) the instantaneous power flow at the receiving end, assuming vrd equal to the receiving end voltage amplitude vr and vrq=0 results:

p(t) = 1.5 * vrd * idline q(t) = 1.5 * vrd * iqline

where idline and iqline are respectively the values of the d-q line current components. So, it's possible to calculate the reference values of the d-q line current components as follows:

i*dline = 2/3 * p* r/ vrd i*qline = 2/3 * q* r/ vrd

where p* r and q* r are the instantaneous active and reactive power flow required the receiving end. At the same way, the instantaneous active and reactive power flows provided by the shunt inverter at the connection point are:

p(t) = 1.5 * v1d * idsh q(t) = 1.5 * vld * iqsh

assuming v1d equal to the sending end voltage amplitude v1 and v1q=0 and with idsh and iqsh the d-q current components injected by shunt inverter into the transmission line. So, the reference values of these two current components are evaluated as follows:

i*dsh = 2/3p*sh / vld i*qsh = 2/3q*sh/ v1d

where p* sh and q*sh are the instantaneous active and reactive power flow required to the shunt inverter. In the paper these control systems are based on the classical "decoupled watt-var algorithm" using the d-q axis decomposition

Fig. 2.4: UPFC Equivalent Circuit

The input values for the series inverter control system for the independent control of active and reactive power at the receiving end (pr, qr) of the power system are: instantaneous values of the sending and receiving end voltages and the line current, and the reference values p* r and q* r.

In VAR control mode the input values for the shunt inverter control system to control the active and reactive power flow provided to the sending end (psh, qsh) are: instantaneous values of sending end voltage v1, the instantaneous value of the current injected by shunt inverter ish into the transmission line, the reference value q*sh and the value of the active power p*sh evaluated so to balance the active power exchange between the series inverter and the transmission line. [10]

2.3.3 Inverter Control Technique

There are two basic strategies that can be utilized to control the GTO switching of an inverter. One approach involves multi-connected out of phase inverters with a common dc source and coupled through appropriate magnetic circuits. Another approach is to use PWM switching techniques.

Fig. 2.5: UPFC - Series Inverter Control Scheme

Fig. 2.6: UPFC - Shunt Inverter Control Scheme

2.3.4 DC-Side Control

For normal operation of two VSI's in an UPFC, the dc voltage across the dc storage capacitor Cdc must be kept constant. This implies that the active power exchanged between the UPFC and the power system is zero at steady state operation:

pse + psh = 0

that is, the active power delivered by the shunt inverter psh is equal to the active power exchange between the series inverter and the transmission line, pse. Hence, a dc voltage control system must be realized to keep Vdc constant by taking the actual value of Vdc as the feedback signal against a dc reference signal V* dc, as in Fig.2.4. The resulting dc voltage error processed by a proportional controller is added to a pre-value of the active power exchanged between the series inverter and the transmission line, p* se evaluated on the basis of the reference value of d-q line current components. [8]

Fig. 2.7: dc Voltage Regulator Scheme

The output signal of this control system is the reference value for the active power which must provided by the shunt inverter, p* sh.



In the remaining two months, a more detailed analysis of FACTS controller will be carried out. The working principle of UPFC shall be studied in detail. The power flow model of the UPFC along with its simulation in MATLAB code will be done.

A numerical example of power flow control using one UPFC shall be done to illustrate the practical applications of these controllers.