Multifunctional Cascade Control of Voltage-Source Converters Equipped With an LC Filter


Cascade Control 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.


Cascade Control 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.


  1. Ac-voltage control
  2. Cascade control
  3. Current control
  4. Overcurrent protection
  5. Voltage-source converters



Fig. 1. Block diagram of the experimental setup. CB stands for circuit breaker.


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).


 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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