Power transfer between converter and AC system is
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 Usd, Usq, Id and Iq are the AC grid voltage and current signals in dq-rotating frame. (R+ ron) 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. Idref and Iqref . Those controllers are active power controller, DC voltage controller responsible for generation of Idref and reactive power controller, AC voltage controller responsible for generation of Iqref . 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 dq-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 dq-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. Idref. 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 Idref.
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 Idref as its output signal.
Fig :(7) AC voltage controller
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. Idlim and Iqlim 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 Idlim (active reference current) as shown in Figure 8(a). and Iqlim 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
- “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.
- Yazdani. "Electronic Power Conversion, Voltage-Sourced Converters in Power
- 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.