Unified Power Flow Controller: Modes and Control System | Electricity

In this article we will discuss about:- 1. Functional Control Mode of UPFC 2. Operating Modes of UPFC 3. Control System.

Functional Control Mode of UPFC:

i. Shunt Controller:

a. Reactive (VAR) control side

b. Automatic voltage control mode

ii. Series Convector:

a. Direct voltage injection mode

b. Line impedance compensation mode

c. Bus voltage regulation and control mode

d. Phase angle regulation mode

e. Automatic power flow control mode.

UPFC is a multifunctional FACTS controller based on the back to back voltage source converter arrangement.

It offer 3 possibilities- in the first controller, one converter of the back to back arrangement is in series and the other is in shunt with the transmission line. This arrangement is known as Unified Power Flow Controller (UPFC). In the second controlled, both converter of the back to back arrangement are connected in series with, usually, a different line this arrangement is known as Interline Power Flow Controller (IPFC). In the third controller, both converter are connected in shunt, each with a different power system. The arrangement function as a back to back STATCOM.

Operating Modes of UPFC:

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 consists 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. The shunt inverter can be controlled in two different modes: VAR control modes & Automatic control mode.

Series Converter:

i. Reactive VAR Control Mode:

The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the Var reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, V_dc, is also required.

ii. Automatic Voltage Control Mode:

The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. For this mode of control, voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer. The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways.

iii. Direct Voltage Injection Mode:

The reference inputs are directly the magnitude and phase angle of the series voltage.

iv. Phase Angle Regulation mode:

The reference input is phase displacement between the sending end voltage and the receiving end voltage.

v. Line Impedance Compensation mode:

The reference input is an impedance value to insert in series with the line impedance.

vi. Automatic Power Flow Control Mode:

The reference inputs are values of P and Q to maintain on the transmission line despite system changes. The series converter can operate either in power flow control (automatic mode) or in manual voltage injection mode. The UPFC can operate in the automatic power flow control mode keeping the active and reactive line power flow at the specified values.

This can be achieved by the linearizing the line power flow. In power control mode, the measured active power and reactive power are compared with reference values to produce P and Q errors. The P error and the Q error are used by two PI regulators to compute respectively the V_q and V_d components of voltage to be synthesized by the VSC. (V_q in quadrature with VI controls active power and V_d in phase with VI controls reactive power). So, two automatic Power flow Control modes block diagram shown in Fig. 5.55(a) and Fig. 5.55(b).

3. Control System of UPFC:

The shunt converter operates as a STATCOM. In summary, the shunt converter controls the AC voltage at its terminals and the voltage of the DC bus. It uses a dual voltage regulation loop: an inner current control loop and an outer loop regulating AC and DC voltages. For a description of its control system, refer to the Static Synchronous Compensator (Phasor Type).

Control of the series branch is different from the SSSC. In a SSSC the two degrees of freedom of the series converter are used to control the DC voltage and the reactive power. In case of a UPFC the two degrees of freedom are used to control the active power and the reactive power. A simplified block diagram of the series converter is shown below Fig. 5.58.

The series converter can operate either in power flow control (automatic mode) or in manual voltage injection mode. In power control mode, the measured active power and reactive power are compared with reference values to produce P and Q errors.

The P error and the Q error are used by two PI regulators to compute respectively the V_q and V_d components of voltage to be synthesized by the VSC. (V_q in quadrature with V₁ controls active power and V_d in phase with V₁ controls reactive power). In manual voltage injection mode, regulators are not used. The reference values of injected voltage F_dref and F_qref are used to synthesize the converter voltage.

In manual voltage injection mode, regulators are not used. The reference values of injected voltage F_dref and V_qref are used to synthesize the converter voltage.

UPFC Control System (Single-line Diagram of a UPFC and Phasor Diagram of Voltages and Currents):

In order to understand the UPFC Control System the phasor diagram in the Fig. 5.56 and Fig. 5.57 given below is system.

This FACTS topology provides much more flexibility than the SSSC for controlling the line active and reactive power because active power can now be transferred from the shunt converter to the series converter, through the DC bus. Contrary to the SSSC where the injected voltage V_s is constrained to stay in quadrature with line current I, the injected voltage V_s can now have any angle with respect to line current.

If the magnitude of injected voltage V_s is kept constant and if its phase angle with respect to V₁ is varied from 0 to 360 degrees, the locus described by the end of vector V₂ (V₂ = V₁ + V_s) is a circle as shown on the phasor diagram. As is varying, the phase shift 8 between voltages V₂ and V₃ at the two line ends also varies. It follows that both the active power P and the reactive power Q transmitted at one line end can be controlled.