Control of VSC:Part 2⚡️⚡️

Direct control:

Power transfer between converter and AC system is

From these equations the real and reactive power can be controlled by altering power angle and modulation index for magnitude of the converter voltage. Error in power angle is processed through power angle controller. Actual power is calculated from terminal quantities and compared with desired power angle to get an active power order. Similarly reference reactive power and actual is compared to have a reference modulating signal. PLL circuit is used to synchronize the converter output voltage with the AC grid. Control scheme for direct control mode is shown if Figure.(1) change in the power angle affects both P and Q hence an independent control of real and reactive power can’t be realized.

_{Fig: (1)Direct voltage control mode}

Vector Control:

VSC control mechanism that can be achieved by a three-layer mechanism. Outer layer control, inner current control loop and PWM signal generation. Control mechanism discussed here is for a 3-level neutral point clamped voltage sourced converter. For a 3-level neutral point clamped converter it is needed to have a 12-pulse generator. Pulses given to the IGBTs are resulting from the control mechanism of outer controller and inner current control loop. Reference voltage signals generated from the inner current controller is compared with triangular carrier wave for generation of triggering pulses . The overall control structure of VSC is shown in the Figure(2).

_{Fig(2) control structure for current control mode of VSC}

Inner current controller:

Inner current controller produces a required control signals to ensure the maximum limit of converter current. Hence line current through the converter switches can be tightly controlled. The whole control mechanism that is realized by transforming the 3-phase converter current and AC grid voltage into its corresponding dq-rotating frame.
Where phase locked loop plays an important role of synchronization with AC grid voltage. The control mechanism is experienced in inner current control loop to generate a voltage reference signal in dq-rotating frame. This can be further transformed back to 3phase abc-frame and given as a reference signal for PWM generation.
From the equivalent circuit of AC side of VSC from the part 1 figure 3 we have

The above equation can be realized using dq-rotating frame.

Where U_sd, U_sq, I_d and I_q are the AC grid voltage and current signals in dq-rotating frame. (R+ r_on) and L are total resistance and the inductance between the converter bus and AC grid.

_{Fig: (4)Inner current control loop}

Outer control layer:

Outer control layer consists of different controllers to generate current reference signals i.e. I_d^ref and I_q^ref . Those controllers are active power controller, DC voltage controller responsible for generation of I_d^ref and reactive power controller, AC voltage controller responsible for generation of I_q^ref . Every controller is equipped with a PI controller to have a reduced steady state error

Active and reactive power controller:

Control signals used for controlling the active power and reactive power are transformed from 3 phase abc-frame to d_q-rotating frame, a DC equivalent of AC signal. The control signals transformed using park’s transformation. Signals are transformed from time varying sinusoidal signal to a DC equivalent signal, to have a simple, reliable and efficient control strategy.

Active and reactive power that can be calculate from a balanced three phase system in d_q-frame is given below.

When d-axis is aligned with AC voltage phasor using a phase locked loop (PLL) model, the q-axis voltage in balanced condition is zero and d-axis voltage shows the magnitude of AC voltage.
Hence the last two equations can be rewritten as

This shows a decoupled controlled of real and reactive power.

_{Fig :(5) Active (a) and Reactive (b) power controller}

DC side voltage controller:

DC side voltage control depends on the set reference signal. DC voltage at DC bus remain at its reference value and regulates real power exchange between AC grid and VSC. DC voltage controller generates a current reference signal

i.e. I_d^ref. Power corresponds to capacitor is

Energy stored in capacitor

From the last two equations revels that power across capacitor is proportional to square of DC voltage. To avoid non linearity in DC voltage controller we use difference between square of DC voltage. Then it is passed through PI controller to have I_d^ref.

_{Fig :(6) DC voltage controller}

AC voltage controller:

Reactive power controller is used to have a control over PCC voltage level that can be maintained at desired level. Reactive power exchange between the VSC and AC grid is decided by the converter limits. It can regulate AC voltage by comparing it with reference value. Controller gives I_d^ref as its output signal.


_{Fig :(7) AC voltage controller}

Current limiter:

VSC have a limitation over the maximum current as per the switch ratting and it is also benefit to control the AC voltage at desired level. As synchronous generator it does not have overload capability. For safe operation of VSC the maximum limit of current so chosen that modulation index is equal to or less than one for SPWM

_{Fig :(8) current limiter}

Choice of the current limit depends on its applications. I_d^lim and I_q^lim achieved when the converter current exceeds the maximum limit. When a VSC connected to a strong grid, to generate more power priority given to the I_d^lim (active reference current) as shown in Figure 8(a). and I_q^lim is given maximum priority when VSC connected to the weak grid as shown in Figure 8(b). It supports AC side voltage by increasing the reactive power. And last strategy is the one when current exceeds limit equal priority is given to active and reactive current component as shown in Figure 8(c).

Summary:

In this chapter control design of VSC is discussed. A control strategy to control active and reactive power is discussed

Reference

“Improvement of Voltage Stability by Using VSC-HVDC” , H.F. Latorre,student member IEEE and M. Gandhari. Member, IEEE, IEEE T&D ASIA-S1EF 2009, SEOUL, KOREA, October 26-30,2009.

Text book “Voltage- Sourced Converters in Power Systems, Modeling, Control, and application” by Aminase Yazdani and Reza Iravani

“Application Research on VSC-HVDC in Urban Power Network” by jian Luo, jianguo Yao, Di Wu, Chuanxin Wen, Shenchun Yang, ji Liu Research Centre of State Grid Electronic Power Research Institute, Nanjing, China. IEEE Xplore

Yazdani. "Electronic Power Conversion, Voltage-Sourced Converters in Power Systems”, 01/25/2010

R. Teixeira Pinto, “Dynamics and Control of VSC-based HVDC Systems,” Master Thesis, Politecnico di Torino, .Torino, Italy, 2008

Last few moths I was so busy with study. I didn't get time for this second part. The first Part was post about three months ago.

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