None-linear current control: hysteresis control

Hysteresis current control is classified as a none-linear current control method. It directly controls the switching duty of the power electronic modules to force the current output to track a desired value or waveform. Several advantages such as a higher stability margin and the better tracking ability over other current control methods have been reported in [58, 59]. Moreover, fixed-band hysteresis current control has a very simple controller algorithm and small computational burden yet it has another advantage. This control method does not introduce any voltage feed-forward to minimise the transient response. During normal operation, the grid voltage is only sampled for generating correct current reference to achieve a desired power. Therefore, it is potentially very useful in cases when the grid voltage is not available while the current should be limited in order to protect the power electronic devices e.g. start-up of the a.c. voltage-sensor-less control of the VSI.

The principle of hysteresis current control is simple, applying the assumption that the d.c. link voltage are sufficiently higher than the peak to peak value of the utility a.c. voltage (line-line) so that the switching action always achieves its desired effect. This is shown in Figure 2.1. The

sampled current values are compared with the current reference. When the sampled current is shown to be smaller than the current reference, a signal to turn on the upper switch (after turning off the lower switch) of the phase-leg is given to raise the current. Similarly a signal to turn on the lower switch of the phase-leg (after turning off the upper switch) is given for bringing down the current when it is above the current reference. It is obvious that the update of the switching state is synchronised to the sampling process. Ideally, with an infinite sampling and switching rate, this mechanism is able to control the current to track its reference value without error. However, in practical applications this is hardly achievable. Firstly, the state of conduction or blocking of power electronics devices will not change instantly due to the physical nature of the devices. Secondly a dead-band is introduced to protect the upper and lower arms from shorting-through the d.c. link. Therefore a delay of turning on the in-coming device is introduced. Thirdly, the switching frequency of the power modules cannot be infinitely high by considering the switching losses. For analogue controllers, the noise problem would degrade their performance and hence they are not considered. Due to these reasons, the current reference usually sits within a tolerance range for finite switching rate [60]. This is known as the hysteresis band, and the control strategy is illustrated in Figure 2.2.

Voltage source inverter

Coupling inductance Grid voltage Vdc/2

Vdc/2

Figure 2.1 Circuit diagram of a two-level single-phase inverter

Figure 2.2 Illustration of current hysteresis control

The draw-back of fixed-band hysteresis current control is well known as the production of a wide range of harmonic frequency components, which makes the filter design difficult. For the fixed band hysteresis control, due to change of the grid voltage within a fundamental cycle, the voltage drop across the coupling impedance is not constant and this makes the current rise/drop to have different rates at different time points. The sample will not be naturally locked to the event that the current value passes the hysteresis band. During the time that the current value is between the upper and lower hysteresis band, the states of the switching devices are held. The hysteresis band should be designed taking the coupling impedance, the sampling rate on the control board and the d.c. as well as a.c. side voltage into consideration to minimise the difference of the number of samples among the switch duty updates so that the switching frequency is approximately constant.

Given the 2-level converter topology, the output voltage at the input terminal of the filter should be either +Vdc/2 or –Vdc/2, with respect to the d.c. neutral. As the switching frequency of the

hysteresis control is not constant but dependent on the operating conditions, the terminal voltage cannot be ensured to be zero sequence free. This results in the interference between phases. The current flow through each phase is no longer decided by the load and the phase voltage in this phase but is also subjective to the effect of the line voltage. In [61], the effect of the phase interference is evaluated and improvement to the hysteresis switching aiming at constant switching frequency is made. However, for many control systems, the load or grid parameters could be essential but are usually hard to be obtained; the methods increase the complexity of the control system configuration.

The well-known drawback of the fixed-band hysteresis current control, i.e. the unpredictable harmonic content which makes the filter design difficult, is due to the unequal duration of each switching duty update and this is caused by voltage drop variation as illustrated above. Ideally, switching duty updates should be made on a regular basis. Although many methods were proposed to provide some the harmonic rejection features for current hysteresis control [58, 62, 63], these methods are mostly trying to adjust the hysteresis band by a.c. source voltage or prediction from the VSI output. By adjusting the hysteresis band, a more constant switching frequency can be achieved. The methods rely on the voltage measured at the PCC which is not desired in this research because the use of the voltage sensors will be eliminated. Some of the methods above aim at working out a predictive scheme to minimise the switching frequency variation at a cost of increasing the sampling frequency instantaneously to as high as 300 kHz [58], which is not realistic in many high power applications. Such controllers may increase the cost of building the power control system hardware and therefore they are not considered any further.