Parallel PI Voltage-H Current Control of a Neutral Leg - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 216

PI V ave

+

+

u

C i C

Parallel PI voltage- H ∞ current control of the independently-controlled neutral leg Figure 13.6

13.3 Addition of a Voltage Control Loop
The neutral point usually drifts because of the mismatches of capacitors and/or the non-
linearity of switches, even if the current i C is maintained at 0. In order to avoid this from
happening, a voltage control loop can be added after measuring the capacitor voltages V + and
V − , as shown in Figure 13.1. Since the balance between the two capacitor voltages is desired,
which can be achieved by making V a v e very small, the average voltage V a v e is used to form
a voltage control loop. The current controller is designed to maintain i C to be nearly zero,
that is, to remove any components having a frequency higher than 0. The voltage controller
is just to bring the constant voltage deviation of the neutral-point voltage back to zero, that
is, to work with DC components. Hence, the functions of the two controllers are decoupled
in the frequency domain. As a result, the voltage controller and the current controller can be
put together to form a parallel voltage-current control structure, as shown in Figure 13.6. The
voltage controller can be chosen as a PI controller, which has a high DC gain. The output
of the voltage controller enters the current control loop, taking the role of the measurement
noise n in Figure 13.4. Hence, adding the voltage controller does not affect the stability of
the current loop. As long as the voltage control loop is stable, which can be easily achieved
by tuning the PI controller, the whole system is stable. It is worth noting that the derivation
of the analytic stability condition could be difficult but the stability of the system can easily
be achieved according to the above analysis. This control strategy not only leads to a stable
neutral point (by the current controller) but also leads to a balanced neutral point with respect
to the DC-link terminals (by the voltage controller).

13.4 Experimental Results
The system consists of an inverter board, a three-phase LC filter, a board consisting of voltage
and current sensors, a dSPACE DS1104 R&D controller board with ControlDesk software,
and MATLAB R

/Simulink R SimPowerSystems TM software. The inverter board consists of
two independent three-phase inverters and has the capability to generate PWM voltages from
a constant 42 V DC voltage source. The first inverter was used to generate a three-phase output
voltage and one leg of the second inverter was used to control the neutral point. By changing
the load of the first inverter, different neutral currents can be generated. The parameters of
the system are the same as those given in Table 13.1. The neutral current i N and the inductor
current i L were measured and the current i C was calculated; the DC-link voltage and the
voltage across the capacitor C N − were measured to calculate the average voltage V a v e .
The controllers were implemented to evaluate their steady-state and transient performance.
The steady-state performance was tested under three scenarios when balanced resistive,