The STATCOM Controller

CHAPTER FOUR

THE 24-PULSE STATCOM AND UPFC

4.2 The 24-Pulse Two-Level STATCOM

4.2.2 The STATCOM Controller

The two control requirements of a STATCOM can be described as follows:

1. Phase Control: In order to ensure that the shunt current flowing in and out of the STATCOM is always in quadrature with the line voltage via the reactive coupling, the inverter's output ac voltage is required to be in phase with the system voltage.

2. Magnitude Control: The magnitude of the quadrature current injected by the STATCOM is a function of the inverter's output ac voltage which is dependent on the dc capacitor voltage. The charging and discharging of the dc capacitor requires exchange of active power between the transmission line and STATCOM. This active power exchange is made possible by slightly shifting the angle of the inverter ac voltages so that they are no longer perfectly in phase with those of the transmission line. The sign of this phase shift between the inverter and transmission line voltages determines whether the dc capacitor voltage charges up or discharges.

Fig. 4.2 shows the high-level control block diagram of a stand-alone, two-level inverter-based STATCOM developed by Sen [27], where the magnitude of the ac output voltage from the inverter is varied by controlling the magnitude of the inverter's dc capacitor voltage. Given the two control requirements of the STATCOM described above, Fig. 4.2 is made up of two corresponding sub-sections, namely, phase control and magnitude control. The output of this high-level controller then provides the input for the gate pattern logic i.e. the low-level controls which determine the firing signals for each switching device within the inverter. Each of the two sub-sections of the high-level SSSC controller is described as follows.

I

Inverter

I

8

1 Gate

} - - - -... I Pattern Logic

a 8

Rotating Frame Transformer

Phase-Locked Loop

I I I I I

I I

I

Phase Control :

^I

L ~---I

I 1

1---'

I

Magnitude Control :

I I

I Error I

Amplifier I

I I I I I I I I I I I

- - - JI ^I

I I

I

8

I I

VI

I

Fig.4.2: Control block diagram of a stand-alone two-level inverter based STATCOM [27].

4.2.2.1 Phase Control

The phase control of the inverter's output ac voltage is achieved by means of a phase locked loop, whose principle of operation was explained in detail in Chapter Three (section 3.2.3.1) and will 'thus only be briefly described here. The instantaneous phase-to-neutral voltages ^VIa and ^VIe at the STATCOM terminals are measured and converted into their ds and qs components in a stationary two-axis coordinate frame using the transformation matrix [C]. These ds and qs components are then transformed into the d and q axis coordinates of a synchronously rotating coordinate frame using the time-varying transformation matrix [Cl]. The voltage Vq is used as the error signal to the phase locked loop. By maintaining Vq = 0, the phase locked loop ensures that its output angle

e

is synchronized to the phase of the transmission line voltage ^VIa' Hence, in the STATCOM, the steady-state angle of the phase a inverter voltage is equal to this phase-locked angle

e

so as to ensure that the STATCOM's ac voltages are in phase with the transmission line ac voltages. This form of synchronization also ensures that the transmission line ac voltage vector lies exactly along the d-axis of the synchronously rotating coordinate frame.

The Phase Control sub-section of the full STATCOM control scheme shown in Fig. 4.2 is now shown in more detail in Fig. 4.3, based on the explanations described above.

[C] [Cl]

v =PLLerror

\----.1 KPLLP~

+ Fig.4.3: An expanded diagram of the Phase Control sub-sectionfrom Fig. 4.2.

4.2.2.2 Magnitude Control

e

As described above, the magnitude of the STATCOM's ac output voltage, and hence the reactive power it generates or absorbs is controlled by adjusting the magnitude of the inverter's de voltage.

The transfer of active power into the STATCOM to change its de voltage is achieved by

,

temporarily adding an offset phase angle ex to the steady state angle

e

of the STATCOM's ac output voltages. The particular control loops used to adjust the magnitude of the STATCOM's dc (and hence ac) voltages via this angle ~ depend on the mode of operation of the STAfCOM.

For example, the STATCOM can be configured to regulate the magnitude of the transmission line voltage directly, or simply to provide a set-point value of reactive current injection into the transmission line. This latter mode of operation of the STATCOM is considered here, and the associated magnitude control loops shown in Fig. 4.4. The operation of this magnitude control is outlined below.

The coordinate transforms [C] and [Cl] discussed earlier are used to convert the measured shunt currents ia and ic at the ac output of the STATCOM into currents id and iq in the synchronously rotating dq coordinate frame. Because the d axis of this coordinate frame is phase-locked to, and aligned with, the transmission line voltage vector, the q axis STATCOM current in this coordinate frame is, by definition, the reactive current at the output of the STATCOM. Fig. 4.4 shows that this measured q-axis (reactive) current output i_lq from the STATCOM is compared to a reference input J_lq *, which represents the desired reactive current injection from the STATCOM, with a negative sign of J_lq

*

representing output of reactive power from the STATCOM (STATCOM in capacitive mode); the error between J_lq

*

and ilq is used to drive a PI controller whose output is the temporary phase offset a to be added to the instantaneous angle

e

to form the required angle

e

^{I of}

the STATCOM's ac output voltages.

Ic]

ia~

[Cl]

e

'(fromPLL)

a

e

Fig 4.4: An Expanded and Detailed Diagram of Magnitude Control sub-section from Fig. 4.2.

Consider the situation when the magnitude of the capacitive reactive current i_lq output at the STATCOM terminals is less than the magnitude of capacitive current commanded at the controller input Jlq *, (i.e. Jlq * is more negative than ilq ) such that the error C/ in the PI controller is negative.

In this case the PI controller creates a negative offset angle a, retarding the phase of the STATCOM ac voltages relative to the transmission line voltages, thus admitting positive real

power into the STATCOM to charge up its dc voltage. As the dc voltage charges up, the STATCOM's ac voltages increase in magnitude relative to the transmission line voltages, thus increasing the size of the capacitive reactive current output from the STATCOM towards the reference valueI_lq*. When the STATCOM ac voltages reach the correct amplitude (relative to the line voltages) to generate the commanded value of reactive current I_lq*, the error Eland phase offset a return to zero and the STATCOM ac voltages return to being in phase with the transmission line voltages.

Similarly, if the magnitude of the capacitive reactive current ilq is larger than the commanded valueIlq

*

(i.e.Ilq

*

is less negative than ilq ),the resulting positive error Elin the PI controller adds a positive phase offset a to the STATCOM ac voltages, advancing them relative to the line ac voltages, resulting in negative real power into the STATCOM (i.e. flow of real power out of the STATCOM to discharge its dc voltage). As the ac voltages of the STATCOM decrease in magnitude relative to the line voltages the reactive current output from the STATCOM decreases back towards the reference valueI_lq*.