THE INVERTER AND ITS CONTROL STRATEGY
3.4 IMPROVED OSAP CONTROLLER
3.4.1 Errors Introduced By Component Tolerance
Even though OSAP controllers are easy to implement and have good dynamics, there are several disadvantages. Based on the analyses of the OSAP controller in Section 3.2 and Section 3.3, a future value is substituted with a present order value, yielding a one-step error between the reference signal and output signal. In addition, OSAP controllers rely on the model having accurateL,C and R values to get precise control gains. The tolerance of practical components as well as the uncertain electrical characteristics of power grid will bring uncertainties to the controller. These factors definitely cause tracking errors [37]. Fig. 3.12 illustrates the simulation results of an OSAP voltage-control inverter under practical conditions. This inverter model has exactly the same topology and components as the stand-alone inverter in Section 3.2, but this time this stand-alone inverter is treated as a practical inverter, which means the value of the L and C has a 10% tolerance (in this simulation, both L and C are assigned values 10% less than their normal values). The peak value of the output voltage is about 22V, which is far from the reference voltage peak value 30V. In order to achieve zero-error tracking, a Proportional- Resonant (PR) controller based on the Internal Model Principle is introduced in the next section to overcome uncertainties and improve tracking results.
Figure 3.12: Simulation results of OSAP voltage-control inverter with uncertainties in component values
3.4.2 The Internal Model Principle
Proportional-Integral (PI) feedback control is widely used to eliminate steady-state errors and achieve zero-error tracking for linear time-invariant systems. However, the steady-state errors in periodic systems are dynamic and also periodic, the known drawbacks of PI controller are the
3.4 IMPROVED OSAP CONTROLLER 17
difficulty in removing the dynamic periodic errors and the limitations in bandwidth, which means eliminating low-order harmonics is impossible [45], [46], [47]. A controller called Proportional- Resonant (PR) control was introduced and has attracted attention due to its ability to overcome the known drawbacks of PI controllers.
Based on the Internal Model Principle, which states ”accurate control can be achieved only if the control system encapsulates, either implicitly or explicitly, some representation of the process
to be controlled”, a new control philosophy—Internal Model Control (IMC)—was championed
by Francis and Woham in 1976 [48]. This control theory states that if the controller scheme has been developed based on an exact model of the process, then perfect control is theoretically achieved. A simple example of IMC is shown in Fig. 3.13 [49]:
Gc(s) Gp(s)
Set-point Output
Figure 3.13: Internal Model control strategy
In this scheme, the controllerGc(s) is applied to control the plant Gp(s). Suppose the model
of this plant has been built asG^p(s), which means mathematically G^p(s) =Gp(s). By setting
Gc(s) to be the inverse of the plant model, we have:
Gc(s) =G^p(s) −1
(3.20) Obviously, the plant output would always be equal to the set point and zero-error tracking would be achieved, as long as the plant is perfectly known and there are no extended disturbances.
3.4.3 Resonant Controller
The model in Section 3.4.2 briefly illustrates the idea of internal model principle. [50] states that tracking a periodic signal in a linear plant is guaranteed if the controller contains the signal model to be controlled. Now we are back to the OSAP controller. Since the reference signals and the output signals dealt with in these inverters are all sinusoidal waveforms, the one-step error generated by the OSAP controller can also be represented by a sine-wave. In order to eliminate this error, if we embed the model of this sinusoidal error into the controller, zero-error tracking can be achieved. This controller is called a resonant controller.
The ideal PR controller is defined in Eqn. 3.21. A PR controller has exactly the same mathe- matical formulation in the s-domain as the standard sinusoidal waveform and this indicates the
18 CHAPTER 3 THE INVERTER AND ITS CONTROL STRATEGY
appropriate IMC has been applied in the PR controller. GP R(s) =Kp+Kr
s
s2+ω2 (3.21)
whereKp is the proportional gain, Kr is the resonant gain and ω is the resonant frequency.
Fig. 3.14 shows the Bode-plots of the PR controller, where the resonant frequency ω is chosen as 2π·50Hz, Kp = 1 and Kr is set as 1. It can be seen that the PR controller achieves very
high gain in a narrow frequency band centred around the resonant frequency (which is chosen as the grid frequency 50Hz) and provides high attenuation for other frequencies. This unique characteristic guarantees that with the right choice ofKp and Kr, only those signals centred at
the resonant frequency will be preserved. Thus, a resonant controller is the ideal choice for this grid-connected inverter dealing with errors at fundamental frequency.
Figure 3.14: Bode plots of PR controller
3.4.4 Improved OSAP Controller and Simulation Research
As stated before, a PR controller has the capability of removing periodic errors, and this feature matches our needs of eliminating the errors in OSAP controllers. By adding the PR controller in the OSAP control systems, now we have the improved OSAP controllers. Their topologies are shown in Fig. 3.15, one is the improved OSAP voltage controller and the other is the OSAP curretn controller.
Simulation researches are also carried out to demonstrate the theories about this improved OSAP controller. Here the improved OSAP voltage control strategy is conducted as an example. The simulation model is exactly the same model in Section 3.2.4. In Section 3.2.4, the inverter is controlled by a single OSAP voltage controller. A one sampling error is found, and this sampling error is introduced byVo(k+ 1) replaced by Vref(k). Another error is the tolerance of
3.4 IMPROVED OSAP CONTROLLER 19 - Vo(z) Gp(z) Vo_ref(z)+ u(z) Plant OSAP Controller PR Controller + + (a) - iac(z) Gp(z) iac_ref(z)+ u(z) Plant OSAP Controller PR Controller + + (b)
Figure 3.15: (a) Improved OSAP voltage controller (b) Improved OSAP current controller
the electronic components in reality. This error is shown in Fig. 3.12. The functionality of the PR controller in this improved OSAP controller is expected to eliminate both errors.
The model of this improved OSAP voltage-control inverter is shown in Fig. 3.16 and the value of each component is in Table. 3.3. Here the 10% tolerance ofL and C is still included, same as Section 3.4.1.
Table 3.3: Parameters of improved OSAP voltage-control inverter Output VoltageVo 21.2V Input Voltage Vdc 60V
Output Voltage Frequency 50Hz Switching Frequency 19200Hz
Inductor L 7mH Capacitor C 24µF
Resistor R 30Ω Kp 0.5