This document discusses various control strategies for grid converters. It describes control structures that can be adopted using the αβ, abc, or dq reference frames. It then discusses AC and DC voltage control using multi-loop control strategies. It also discusses cascaded control of the DC voltage through the AC current using an outer DC voltage loop and inner current loop. Various voltage oriented control strategies are presented including using the dq reference frame with open and closed loop active and reactive power control as well as using the αβ reference frame. Direct power control is also summarized. Finally, droop control for microgrids is briefly mentioned.

1. Grid Converter Control

2. Fig. The main control issues of grid converters are transformed through the
instantaneous power theory, with reference to AC current and voltage controllers
Structures may be adopted for implementation of the control (αβ, abc, dq)

3. Space Vector Representation
Clarke Transformation

4. Park Transformation

5. Fig. Overall scheme of the LCL filter grid converter control showing all the functions
Fig. L-filter inverter connected to the grid

6. Mathematical Model of the L-Filter Inverter

10. AC Voltage and DC Voltage Control
Fig. Multiloop control strategy (CC stands for current control, VC stands for
voltage control)
The control of the AC voltage across the capacitor could be needed because the
system should operate in a stand-alone mode or in a micro-grid.
A multi-loop control should be adopted: the AC capacitor voltage is controlled through the
AC converter current. In fact, the current-controlled converter is operated as a current
source used to charge/discharge the capacitor.

11. Management of the DC Link Voltage
The DC voltage can be subjected to transient conditions due to the change of the
power produced by the generator. The increase of the produced power results in
voltage overshoot while its decrease results in voltage undershoot. From the point of
view of the DC voltage control, power changes result in voltage variations that
should be compensated by charge or discharge processes.
The DC voltage control is achieved through the control of the power exchanged by
the converter with the grid or through the control of a DC/DC converter.
In the first case
the decrease or increase of the DC voltage level is obtained by injecting more or
less power to the grid with respect to that produced by the WTS, thus changing the
value of the reference for the AC current control loop or the phase displacement of
the AC voltage across the capacitor of the LCL-filter

12. In the second case the grid converter does not play a role in the management of
the DC link.
From a control perspective the DC voltage control can be achieved only indirectly through
the grid current/voltage control. This indirect control is motivated by the fact that the zero
dynamics of the DC voltage, if the average switching functions of the converter are taken
as control input, are not stable. If the zero dynamics diverge this means that it is not
possible to stabilize the system using that control input.
In the first case, the fact that the DC voltage is different from its reference value
implies
that the amount of energy that the capacitor must receive to come back at the set-
point is

13. Cascaded Control of the DC Voltage through the AC Current
The control of the DC voltage through the AC current can result in the identification
of two loops, an outer DC voltage loop and an internal current loop.
The internal and the external loops can be considered decoupled, and thereby
the actual grid current components can be considered equal to their references when
designing the outer DC controller.
assuming that the previous stage is injecting a current io(t)

18. This can be interpreted by considering that the grid perturbation influence on the DC
voltage is filtered by a time constant that depends on the DC capacitor and on the
equivalent resistance of the DC bus. Hence, the lower the DC storage and the higher
the DC load the more DC voltage will be affected by the grid disturbances.
However, what it is interesting to note is that as C tends to infinity (largeDClink capacitance)
the pole tends to zero and makes the system always stable again. It is important to stress that
the DC link storage is designed not only in view of DC link filtering but also to offer a power
buffer in view of the maximum known variation of the power on the DC bus and of the desired
load ride-through protection during utility voltage sag events. These issues suggest that the
DC link storage should not be limited too much.

19. Voltage Oriented Control and Direct Power
Control

20. • Fundamental harmonic has constant components in the dq frame while the
other harmonics space vectors have pulsating components.
• The main purpose of the grid inverter is to generate or to absorb sinusoidal
currents; thus the reference current’s components in the dq frame are DC
quantities.
Use dq frame

21. Use of αβ frame
Similar results can be achieved in a stationary αβ frame, but the relation between
active/reactive power and the vector current components are more complex. In fact,
the active and reactive power produced by the grid converter are

22. Synchronous Frame VOC: PQ Open-Loop Control
The most straightforward implementation of the voltage oriented control can be done
using a current controller implemented in a dq frame (Figure 9.17) and active and
reactive power feed-forward control. The control of the DC voltage modifies the active
power reference. Then the active and reactive power command signals are translated
into d and q components of the reference current, using the following matrix:
where vg is the measured grid voltage. Figure 9.18 shows the resulting control
scheme.

24. Synchronous Frame VOC: PQ Closed-Loop Control
The active and reactive powers are calculated using measurements at the
PCC and their values are compared with their set-points.
Then PI-based controllers decide the reference d and q components of the
reference current while the control of the DC voltage acts directly on the
reference current i∗d . The closed-loop control allows the dynamics of
active/reactive power control to be decided as a consequence of a variation of
the grid voltage change.

26. Stationary Frame VOC: PQ Open-Loop Control
The active/reactive power control can also be implemented in a
stationary αβ frame, leading to an indirect voltage oriented control
(Figure 9.20). In the case reported in Figure 9.21 there is active and
reactive power feed-forward control and the DC voltage control acts
on the power reference. The PLL is still used for adapting the
frequency of the resonant controllers and extracting the first
harmonic of the grid voltages used for calculating the reference
current.

27. Stationary Frame VOC: PQ Open-Loop Control

28. Stationary Frame VOC: PQ Closed-Loop Control
In the case of the implementation of the power control in the αβ frame it is also possible
to have a closed-loop version. In the scheme shown in Figure 9.22 the active and reactive
powers are calculated using measurements at the PCC and their values are compared with
their set-points. Then PI-based controllers decide the amplitude and phase of the grid
current reference. The control of the DC voltage acts directly on the amplitude value. The
PLL is indispensable for providing the grid voltage reference phase with the capacity to
calculate the phase displacement of the current in view of the desired reactive power
injection. Also in thiscase, as in the case of the scheme of Figure 9.19, the closed-loop
control allows the dynamics of active/reactive power control to be decided.

29. Stationary Frame VOC: PQ Closed-Loop Control

30. Direct Power Control
• The direct power control has been developed in analogy to the
well-known direct torque control used for drives. In DPC there
are no internal current loops and no PWM modulator block
because the converter switching states are appropriately
selected by a switching table based on the instantaneous errors
between the commanded and estimated values of active and
reactive power [12] (see Figure 9.24(a)). The main advantage of
the DPC is in its simplealgorithm while the main disadvantage is
the need for a high sampling frequency to obtain satisfactory
performance. A modified version proposed in reference [13]
consists in the use of a modulator to synthesize the desired
voltage (Figure 9.24(b)). However, if the grid is stiff the active and
reactive power loops behave like classical d and q current loops.
In the case where the system has the capability of influencing the
grid voltage substantial differences may arise.

33. Micro-grid, Droop Control