Interfacing Converter Control Strategies

The previously discussed energy management strategies determine operating point (such as power references) for each DG, and at the same time, guarantee voltage and frequency regulations, load demand matching, etc. In the DG interfacing converter control system, the reference real and reactive powers are controlled through DG output current and voltage regulations. Therefore, the DGs output power control strategies are generally categorized as current control mode (CCM) and voltage control mode (VCM). These strategies are explained in the following sections.

CCM-Based Power Flow Control Strategy

In the CCM scheme, both active and reactive powers, are tracked in the closed-loop manner. The real power control loop and reactive power control loop generate reference currents. Note that the real power reference could be from energy management scheme or from a MPPT scheme. With these reference currents, the DG output current can then be controlled in the synchronous frame or in the stationary frame. In this control strategy, the grid-voltage angle information from phase-locked loop (PLL) is used to synchronize the inverter output current with the grid voltage.

The real power control loop reference current can also be generated by DC-link voltage control. This condition occurs mainly in two-stage converters (in either DC-DC + DC-AC or AC-DC + DC-AC), where real power is controlled by first stage (DC-DC or AC-DC converters). In other words, output real power of the inverter is controlled to regulate DC-link voltage where the power difference between the input stage and inverter output can be used to charge or discharge the DC-link capacitor [213, 214].

In general, the CCM-based power flow control strategy is popularly used in grid- connected operation mode, where the AC bus frequency and voltage are determined by the grid. However, in stand-alone operation of a microgrid, the CCM-based method cannot directly regulate the microgrid voltage and frequency, and therefore the VCM-based control strategy of at least one or more large DG units or energy storage units in a microgrid is necessary.

VCM-Based Power Flow Control Strategy

In this control strategy, output voltage of DG is controlled to regulate the DG output power, and the DG behaves like a synchronous generator. The droop control can be easily implemented on VCM-based DG units [215].

In this strategy, the output voltage phase angle is determined by active power controller, and the output voltage magnitude is controlled by the reactive power controller. The DG output three-phase voltages are regulated on their reference values with a closed-loop control system. In this strategy, the voltage closed-loop control system can have an inner current loop for transient and stability performance improvement [216]. In this control scheme, the active and reactive power controllers can be proportional controllers. More complex controller can also be used here to closely mimic the synchronous generator with excitation and torque dynamics [217].

Compared to CCM-based control, the main advantage of VCM-based control is that it can be used in both grid-connected and stand-alone operation modes, which makes the operation mode transition easy and smooth. Possible issues when utilizing this method are mainly related to the lack of direct control of DG output current, especially during fault- or grid-voltage disturbances. These problems can be avoided by implementing virtual impedance control at the DG output [164, 212].

Ancillary Services

Ancillary services for DG systems are becoming an important issue that may further improve the cost-effectiveness of DG systems. This is a promising idea, especially considering that many renewable energy-based DG systems (such as PV and wind) do not operate at the maximum rating all the time (PV systems are simply idle during the night). As a result, the available ratings from these DGs’ interfacing converters can be utilized to provide ancillary services such as flicker mitigation [218], unbalance voltage compensation [219, 220], harmonic control [221], power factor correction, etc. Here the harmonics compensation and unbalance voltage compensation are briefly discussed.

Harmonics Compensation

The power electronics interfaced DGs can be controlled like active power filters at the harmonic frequencies to mitigate system harmonics. As mentioned earlier, there are two types of control strategies in DG systems: CCM and VCM. The CCM- based control strategy is widely adopted in active power filters to mitigate harmonics [222]. As a result, CCM-based DGs can be easily controlled as shunt active power filters to absorb harmonic currents produced by nonlinear loads. To do this, DGs can be controlled to act as virtual resistances at the selected harmonic frequencies. In VCM-based DG systems, the current-controlled harmonic compensation schemes mentioned before are not applicable, as they cannot directly control the DG output current.

Unbalance Voltage Compensation

Using the DG interfacing converters to compensate the grid-voltage unbalance can be an important ancillary service for the utility, where the unbalanced loads could cause serious unbalanced voltage resulting in poor power quality and even protection responses. For unbalance compensation, DG mitigates/reduces voltage sag and unbalances by injecting additional negative sequence current. Therefore, the DG injected current contains both positive sequence and negative sequence components, where the positive sequence component can help to improve the power factor or voltage support as discussed earlier, while the negative sequence component could reduce negative sequence of voltage at PCC [219].

Finally, other than the abovementioned ancillary services, the DG systems or the microgrid as a whole can be used to improve the power system operation by providing the reserve functions [223]. For these reserve functions, the DG or microgrid can be controlled with frequency or voltage droop control and help the grid frequency and voltage regulations. This can be done by the DG systems alone or collectively with both the DG and load response control. With more controllability and flexibility in a microgrid system, valuable ancillary functions can be provided for better grid operation and better grid power quality.

Microgrid is becoming an important aspect of future smart grid, which features great control flexibility, improved reliability, and better power quality. The important aspects of the microgrid are the grid integration and energy management strategies, which enables sound operation of the microgrid in both grid-connected mode and stand-alone mode. The recent research trend on the DG interfacing converter is focused on better efficiency, reduced size, multi-port, and modular design. For the energy management strategy, a hybrid combination of communication-based and communication-less energy management technologies could be a good balance of system optimal operation, reliability, and resilience. The interfacing converter control schemes show that VCM-based methods are gaining more attention due to its ability to mimic the behavior of a synchronous generator. Finally, the ancillary service is becoming a promising topic to further assist the grid control, enhance the grid power quality, and, at the same time, to improve the cost-effectiveness of power electronic-based DGs and microgrids.

Generation Systems

Generation systems mainly operate in two different modes: maximum energy extraction/constant power operation and droop regulation. During normal operation, as most DG systems are based on renewable energy sources (RESs), the converters attached to generation systems are controlled to absorb as much energy as possible from the energy sources. In the case of other types of DG systems such as diesel generators, secondary level controllers determine their constant power reference.

When the voltage or frequency of the microgrid increases above the preestablished level, DG systems shift out of their MPP to reduce the power amount they supply to the system. In this mode, DG systems contribute in the v/f regulation of the microgrid through a droop slope. This means that depending on the available power that can be absorbed from the RES, the controllers will have to adapt their operation characteristics to meet the grid codes predefined by the system operator. Classically, most converters associated with DG systems have been configured to exclusively operate on the MPP. However, the transition toward a more decentralized electrical system requires the participation of these generators in the regulation of the grid [3, 12, 224].

Demand Response

Just like for distributed generation systems, for microgrids in demand response mode, loads absorb the power required by the attached device. When generation systems are producing all the power they can and no power can be absorbed from the main grid and energy storage systems are not able to provide more power, the voltage or frequency decreases below the predefined level and the power consumed by loads is consequently decreased. In the literature, this type of operation is a part of the so-called demand-side management, as the loads actively participate in the regulation of the microgrid by reducing their consumed power when required. A high research activity has been carried out in the past years highlighting the importance of the participation of loads in the management of different types of electric systems [3, 12, 225].

Connection to the Main Grid

Depending on the topology and type of microgrid, one can follow different approaches with respect to the connection to the main grid. On a classical approach, the connection to the main grid can be employed to contribute to the regulation of the microgrid for the entire voltage range [3, 12, 226]. Another solution would be to use the link to the main grid at specific cases where the voltage or frequency levels are above the maximum or below minimum levels, avoiding the malfunction or disconnection of other devices.