ABSTRACT:
This paper proposes a multifunctional cascade controller
structure for voltage-source converters. The proposed structure contains a
decoupling loop between the outer voltage control loop and the inner current
control loop, and operation in either voltage or current control mode is
possible. In voltage control mode, the current controller can be made completely
transparent. In the case of faults, the proposed structure enables inherent
overcurrent protection by a seamless transition from voltage to current control
mode, wherein the current controller is fully operational. Seamless transitions
between the control modes can also be triggered with an external signal to
adapt the converter to different operating conditions. The proposed structure
allows for integration of simple, accurate, and flexible overcurrent protection
to state-of-the-art single loop voltage controllers without affecting voltage
control properties under normal operation. The properties of the proposed
controller structure are validated experimentally on a 10-kVA converter system.
KEYWORDS:
1.
Ac-voltage
control
2.
Cascade
control
3.
Current
control
4.
Overcurrent
protection
5.
Voltage-source
converters
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig. 1. Block diagram of the experimental setup. CB stands for circuit breaker.
EXPECTED SIMULATION RESULTS:
Fig. 2. Experimental validation of the transparency of the current controller in the proposed cascade controller structure. The application example controller presented in Section IV is compared with its single-loop counterpart based on the controller proposed in [14]: (left) reference tracking under no load (middle) reference tracking under 1 p.u. resistive and 0.45 p.u. inductive load and (right) disturbance rejection in the form of load change from no load to 1 p.u. resistive and 0.45 p.u. inductive load.
Fig.
3. Experimental transition between control modes with (a) 1 p.u. resistive and
0.45 p.u. inductive load (b) 0.08 p.u. resistive and 0.45 p.u. inductive load.
Additionally, reference steps in both control modes are presented. VCM and CCM
stand for voltage and current control mode, respectively.
Fig.
4. Experimental emulation of a load fault by connecting a low-resistance load
in parallel with the steady-state load. Recovery from the fault, which is
triggered by a circuit breaker, is also shown. The fault emulation is shown for
the case where the converter is designed to trip in the event of overcurrent
(left), for the reference current limitation method proposed in [24] (middle),
and for the proposed structure (right).
CONCLUSION:
This paper presented a multifunctional cascade
controller structure for VSCs. The
proposed controller structure allows for operation in either voltage or current
control mode. In voltage control mode and under linear operation, the current controller
can be made completely transparent. Consequently, the properties of both
control modes are purely determined by their corresponding control loops, which
can be designed independently of each other. The transitions between control
modes are seamless and occur either due to converter overloading, i.e., the
controller inherently includes overcurrent protection, or by manually activating
the current control mode of the controller. The properties of the proposed
cascade controller structure are validated by means of experiments.
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