22-09-2012, 03:31 PM
Active Compensation of Harmonics in Industrial ApplicationsA
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In industrial low and medium voltage mains, passive filters and PFC capacitors have traditionally been used to improve the supply quality. However, they cannot be rated only for the loads being compensated. They are affected by harmonic currents from other non-linear loads or by harmonics from the power system.
Compared with passive element compensators, an active harmonic compensator (AHC) can be used to improve the supply quality without worrying about all the problems associated with applying passive elements.
The proposed active harmonic compensation AHC for industrial networks can be successfully used with nonlinear loads and consumers with rapid fluctuations of reactive and active power consumption to improve the supply quality of other loads supplied from the same mains. Clear reduction of the voltage wave form distortion and the voltage changes (flicker effects) as well as the stabilisation of the mains voltage are the main advantages of the proposed AHC. These all make the application of the power electronics to improve the supply voltage quality in industrial networks more effective in comparison to passive filters and PFC capacitors.
AHC advantages
The use of active mains compensation holds a number of advantages compared to the passive. The AHC:
- Is easy to size to the application as the design is independent of line impedance
- Does not generate resonance
- Actively controls both harmonic and reactive currents.
AHC solutions become more relevant
Despite the advantages, AHC’s have a limited market share mainly due to high cost. However, a number of trends and factors indicate that this is about to change.
- New developments enable the use of mass produced hardware in the active filter, which significantly will reduce the cost.
- Due to the large amount of cobber and steel used in passive filter, the increase of material cost has a higher impact on passive solutions than on active solutions
Control system
Fig.2 shows a block diagram of the control unit of the AHC. The control scheme is based on a cascade control with a current control in the inner loop without mains voltage sensors. The current controller sets the output voltage of the voltage source inverters for each sampling period of the control system so that the line current has a reference value. The voltage controller allows the dc voltage to have an almost constant value. The output signal of the dc-link voltage controller determines the value of the active current of the mains load and losses of the power unit of the restoring system . The reactive current is calculated by the reactive power and flicker estimation module of the control unit (see Fig.2).
Current control
The control value of the current control loop is the supply current. This current is a result of the sum of the measured load current (see Fig.1) and the ac current of voltage inverter. These two three-phase system currents are added together and then are transformed to a signal of the two-phase quantities i Sa ,b . In Fig.2 this current is represented as i Sa and i Sb .
The reference value for the current controller i ref d ,q (d and q components) is transformed to the stationary reference frame a −b . The transformation of the vector i refd q, to the vector i1a b, is executed by ejw1t, derived from a phase locked
loop PLL (see Fig.2).
The selection of the switching sequence for every switching operation of the both voltage source inverters is achieved through the use of a sliding mode controller.
The selection of the switching sequence for every switching operation through the use of the sliding mode control is discussed in detail in [4-6]. This makes it possible to control the active filter without mains voltage sensors [7]. It significantly simplifies the hardware configuration of the active mains compensator, especially for medium and high voltage applications.
The output signals of two P-controllers with saturation represent two components of the mains voltage vector u Wa , u Wb which are used to detect the position of the voltage vector by PLL.
DC-link Control
With non-sinusoidal mains current of the voltage inverter, the dc-link voltage contains not only a ripple from transistor switching operations, but also a low frequency voltage ripple like the dc voltage at the dc link of the diode rectifier with capacitor. This low frequency voltage ripple must be filtered in the control loop by feeding back the dc voltage otherwise this voltage ripple would be increased by the proportional part of the voltage controller and it would be passed on to the line current control loop. Therefore the line currents would be distorted [8].
To decrease the influence of the dc-link voltage ripple on the current control loop, the cut-off frequency of the feedback low-pass filter must be f0=50÷75Hz. The low cut-off frequency of the feedback filter causes the large
delay time in the dc-voltage measurement and therefore the dc-link voltage control has a low dynamic performance.
To improve the time response of the dc-link voltage control, an adaptive control system is used, whose parameter values of the feedback filter and PI controller are changed in accordance with the value of the dc voltage error [8].
Simulation results
The simulation of AHC control shows very high dynamics. This is justified first by the high dynamic of the current controller, which is characteristic to the sliding mode control. Second, due to the implemented adaptive dc-voltage controller the AHC can overcome much faster the transient during the connection/disconnection of the AHC or the harmonic load change.
A simulation result is available in Fig. 3 (only a single phase is shown).
The harmonic current is generated by a typical three phase diode rectifier Adjustable Speed Drive. Since the displacement power factor is close to unity there is no requirement of reactive power compensation in this case, but only harmonic current mitigation. At time 0.16 the AHC is connected to the power system and starts mitigating the harmonic currents from the ASD. The transient takes almost one fundamental period, until the source current resembles sinusoid waveform.