A Novel Control Scheme for Enhancing the Transient Performance of an Islanded Hybrid AC-DC Microgrid

A Novel Control Scheme for Enhancing the Transient Performance of an Islanded Hybrid AC-DC Micro grid

Abstract: This paper proposes a novel ancillary feature in bidirectional interlinking converter (BIC) for enhancing the transient performance of an islanded hybrid AC-DC micro grid. The system under consideration includes an AC micro grid with a mix of inertial and non-inertial sources integrated with a DC micro grid through a BIC. The existing BlC control schemes do not judiciously utilize the inertial sources (like the synchronous generator (SG)) as short-term storage for enhancing the transient performance of the system. Since both the micro grids are interfaced through a BIC, this can be achieved by adaptively varying its output frequency during disturbance in the AC micro grid. In the proposed scheme, the BIC is operated as a virtual synchronous generator (VSG) and its output frequency is deliberately increased/decreased during the disturbance. The change in BIC output frequency results in circulating current between the BIC and the SG. Since the BIC voltage phasor leads/lags the SG voltage phasor, it results in the utilization of the inertia of SG as storage. It provides a cost effective solution by reducing the dependency of the scheme on storage thereby minimizing the storage requirement

while maintaining the transient stability of the system. The proposed control strategy is validated through simulations and experiments.

Existing system: The BIC works as a VSG in voltage control mode since its operation in current control mode cannot provide inertial support to maintain the ROCOF of the system during power transients in the AC sub-micro grid. To the best of authors’ knowledge, this kind of control scheme, which enhances the transient performance of the system by judiciously utilizing the existing inertial sources like the SG as short-term storage has not been reported in the literature so far. In this paper, an islanded hybrid micro grid with high penetration of non-inertial sources on the AC side has been considered. The prime mover of the SG can be a diesel engine /gas turbine. The details of this work are presented in the various subsequent sections of this paper.

Proposed system: Taking a cue from the ongoing discussion, this work proposes a new control scheme for the BICwhich enhances the transient performance of an islanded hybrid micro grid during power disturbance (due to generation-load mismatch) in the AC sub-micro grid. Making use of the fact that the SG can absorb or release power momentarily, the proposed scheme facilitates the use of physical inertia of the SG for absorbing or releasing the energy during power transients by increasing or decreasing the output frequency of the BIC. The BIC works as a VSG in voltage control mode since its operation in current control mode cannot provide inertial support to maintain the ROCOF of the system during power transients in the AC sub-micro grid.

Advantages: Micro grids are gaining popularity and importance because of the several advantages they are expected to offer which include increased reliability and power quality. The type of micro grid that should be preferred i.e.

AC or DC depends on several factors such as (i) types of loads (ii) types of sources interfaced and (iii) number of power conversion stages. As the norms and standards pertaining to micro grids emerge, it is expected that a logical step in the

future will be to have the AC and DC micro grids running side by side in the industries, people’s home or other installations/establishments.

Disadvantages: The AC and DC sides are expected to interact constantly, sharing power. The typical problems faced by a DC micro grid could be voltage instability due to constant power loads and pulsed loads, faults etc. On the other hand, an AC micro grid may face problems such as.

Rate of change of frequency (ROCOF) exceeding the threshold due to the lack of inertia for example during surplus power generation (reverse power flow); voltage sag/ swell due to switching on/off of large reactive power loads; voltage sag due to faults and instability due to constant power loads .

Modules: Hybrid AC – DC GRID: Micro grids are gaining popularity and importance because of the several advantages they are expected to offer which include increased reliability and power quality. The type of micro grid that should be preferred i.e. AC or DC depends on several factors such as (i) types of loads (ii) types of sources interfaced and (iii) number of power conversion stages. As the norms and standards pertaining to micro grids emerge, it is expected that a logical step in the future will be to have the AC and DC micro grids running side by side in the industries, people’s home or other installations/establishments. a futuristic hybrid AC-DC micro grid structure serving as the utility network. The hybrid AC-DC micro grid will hereafter be called hybrid micro grid while the AC and DC micro grids constituting the hybrid micro grid are referred to as AC sub-micro grid and DC sub-micro grid respectively.

Rate of change of frequency: The AC and DC sides are expected to interact constantly, sharing power. The typical problems faced by a DC micro grid could be voltage instability due to constant power loads and pulsed loads faults etc. On the other hand, an AC micro grid may face problems such as: rate of change of frequency (ROCOF) exceeding

the threshold due to the lack of inertia for example during surplus power generation (reverse power flow); voltage sag/ swell due to switching on/off of large reactive power loads; voltage sag due to faults and (d) instability due to constant power loads. The question that is asked is: “is it possible for the AC and DC sides of a hybrid micro grid to co-operate with each other in mitigating each other’s problems?� In this context, some researchers have described various autonomous or decentralized power management schemes to achieve smooth power transfer between AC and DC sub-micro grids thereby supporting each other during the steady state. A distributed coordination control strategy in multiple BICs for power flow between the two sub-micro grids has also been discussed.

Synchronous generator: In a hybrid micro grid, the capacity of the AC sub-micro grid is expected to be significantly higher than that of its DC counterpart as most of the high power loads are designed to operate on AC voltage. The AC sub-micro grid typically consists of a mix of inertial (synchronous generator (SG) driven by prime movers such as diesel engine, gas turbine etc.) and non-inertial sources (solar PV, wind energy generator etc. feeding power through power electronic interfaces). As the penetration level of non-inertial sources increases, the AC sub-micro grid gets vulnerable to instability due to lack of physical inertia (rotating mass) in the system. This issue gets more prominent during the islanded operation of the hybrid micro grid, especially during large disturbances which may lead to cascade tripping of the generators and shedding of the loads when ROCOF relay hits the threshold limit. Under these conditions, the stability of the hybrid micro grid can be enhanced by incorporating additional short-term energy storages such as ultra capacitor (UC), flywheel etc. in the system. This paper focusses on UC storage.

Basic Philosophy Of The Proposed Vsg Based Bic Control Scheme: The objective of the proposed control scheme is to enhance the transient performance of the hybrid micro grid through proper control of BIC parameters by utilizing the inertia of SG as storage during power disturbances in the AC- submicro grid. Therefore, it is necessary to make the SG absorb/deliver the surplus/deficit power. From the basic principle of parallel operation of two alternators, one of the alternators can be made to deliver/absorb more power during

power transients by changing the speed of the other alternator. As the BIC is made to mimic the behavior of the alternator, it is possible to utilize the physical inertia of the SG present in the AC sub-micro grid to deliver/absorb more power by appropriately changing the speed of the BIC considering the stability constraints.

Unlike SG, since the VSG is a power electronic converter, it gives the flexibility for the user to adaptively change the parameters (virtual inertia, set mechanical power etc.) at any instant of time resulting in the desired change in the speed (frequency). This is explained using the concept of the synchronizing power in the following paragraphs.