Spyros I. Gkavanoudis

Fault ride-through capability of a microgrid with wtgs and supercapacitor storage during balanced and unbalanced utility voltage sags

The concept of microgrids is recently attracting considerable interest. However, in order to widely integrate microgrids within the distribution networks, a shift in the philosophy of interconnecting them with the utility grid seems necessary. A grid-connected microgrid is required to possess Faults Ride-Through (FRT) capabilities, as well as provide ancillary services during abnormal grid operation. In this paper, a control strategy for improving the ability of an inverter-based microgrid to ride though symmetrical and asymmetrical grid faults is proposed. The microgrid is formed of several Distributed Energy Resources (DERs), which utilize Wind Turbine Generators (WTGs) as primary renewable energy source, each combined with a Supercapacitor Energy Storage System (SCESS). During balanced and unbalanced grid voltage sags, aim of the proposed control strategy is to keep the microgrid connected to the grid, according to the FRT requirements, while maintaining an acceptable voltage profile within the common ac bus. Each DER is controlled to support the voltage within the microgrid by injecting reactive power, without any physical communication. During unbalanced utility voltage conditions, the DERs operate collectively in order to compensate the undesirable negative and zero sequence voltage components. Thus, a set of balanced three-phase voltages is provided within the common ac bus. Simulation results demonstrate that the microgrid can ride through heavily balanced and unbalanced utility voltage sags, while supplying its loads with a high quality voltage profile.

A new Fault Ride-Through control method for full-converter wind turbines employing Supercapacitor Energy Storage System

With the increasing penetration of wind power into electric power grids, Fault Ride-Through (FRT) capability has become an obligation for the wind generation units. Previous research has focused on the improvement of FRT capability as long as the generator is still connected to the grid. The existence of distributed generation units in the power system has consequences for the conventional protection devices, as they might become blind to overcurrents and short circuits. This paper proposes the disconnection of the wind power unit when a fault occurs in the grid and the voltage does not recover within a time limit. The energy produced during the fault is stored in a Supercapacitor Energy Storage System, allowing the generator to stay functional for a period of time, despite being disconnected from the grid. In this way, the normal fault-clearing process can take place. Just after reconnection, the generators are able to supply the grid with power, ensuring reliability of the power system.

An adaptive droop control method for balancing the SoC of distributed batteries in AC microgrids

This paper proposes a new multi-segment adaptive droop control method, applied in islanded AC microgrids with distributed energy storage systems (DESSs) and hybrid PV/Battery units, for balancing their state of charge (SoC). The power/frequency characteristic curves are adjusted locally in real-time, based on the SoC level of the respective battery. Aim of the control is to maintain a comparable number of charging/discharging cycles among the batteries, while preserving the power balance in the islanded microgrid. The control strategies are implemented in each battery without relying on communications or a central management algorithm. The microgrid frequency is used as communication parameter in order to adjust the charging/discharging rate of each storage system. Detailed simulation results verify the effectiveness of this method in an islanded microgrid consisting of several loads, powered by PVs, hybrid PV/Battery units and DESSs.

FRT capability of a DFIG in isolated grids with Dynamic Voltage Restorer and Energy Storage

In order to utilize Doubly Fed Induction Generators (DFIGs) as primary power source of an isolated system, they must be able to regulate the voltage and frequency of the system, as well as ride-through faults. This paper proposes a new control strategy for a DFIG operating in an isolated power system, accomplished by a Dynamic Voltage Restorer (DVR) and a Supercapacitor Energy Storage System (SCESS), in order to ride through symmetrical and asymmetrical faults. During faults, the DFIG continues to operate normally, while the surplus of power is stored in the SCESS. During asymmetrical faults, the DFIG and the DVR are properly controlled in order to feed the non-faulty phases uninterruptedly. When integrated in a power system with conventional synchronous generators, the proposed FRT control strategy improves the FRT capability of a DFIG, while providing frequency and voltage support to the system throughout the fault duration. Thus, the transient stability of the power system is significantly improved. The effectiveness of the proposed control method under different fault conditions is verified by detailed simulation results.

Fault clearing in a converter-dominated microgrid with traditional protection means

The increased penetration of converter-interfaced distributed generation in microgrids has emerged the serious problem of protection during faults, due to the lack of large current injection. This paper proposes a fault detection and clearing control method for three-phase symmetrical faults in a microgrid with looped topology. The fault detection method is based on measuring the microgrid impedance variation during the fault, through the injection of a slightly distorted current. When the fault is identified, the distributed energy sources (DERs) switch their control strategy from droop control to current source mode, in order to inject a current proportional to the measured microgrid impedance. In this way, the DER closer to the fault injects a relatively larger current. The fault clearing process is carried out with simple overcurrent relays, which have the same settings. The coordination of the protection means is implemented from the discrete current injection of the DERs. Furthermore, in order to ensure the current injection even in the case of lack of power from the primary renewable source, a supercapacitor energy storage system (SESS) is added on the DC-link. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. The effectiveness of the proposed control strategy is evaluated through simulation tests, conducted in PSIM software environment.

Impedance-based adaptive droop method in islanded microgrids with three-phase and single-phase converters for line loss reduction

During the last decade, the integration of Distributed Energy Resources (DERs) has considerably increased, establishing the microgrid concept. A common power sharing methodology applied in converter-dominated microgrids without physical communication among the DERs is the droop control method. According to this method, the active power-frequency and the reactive power-voltage droop curves are adopted, in order to fulfil the load demands with power of high quality and reliability. Conventionally, the droop coefficients are fixed and predetermined, according to the nominal apparent power of the converter. However, this power sharing may not be the optimum, regarding the line losses. This paper proposes a novel adaptive droop control method, where the droop coefficients are adjusted based on the indirect measurement of the microgrid impedance, aiming at operation with reduced line losses. The sensed microgrid impedance is different for each DER, since the line impedances between the DER and the loads are different. The proposed control strategy can be implemented in both three-phase and single-phase converters, while the location and the size of the loads can be arbitrary within the microgrid. The effectiveness of the proposed control strategy is validated by analytical simulation tests.

A Control Method for Balancing the SoC of Distributed Batteries in Islanded Converter-Interfaced Microgrids

In a low-voltage islanded microgrid powered by renewable energy sources, the energy storage systems (ESSs) are considered necessary, in order to maintain the power balance. Since a microgrid can be composed of several distributed ESSs (DESSs), a coordinated control of their state-of-charge (SoC) should be implemented, ensuring the prolonged lifespan. This paper proposes a new decentralized control method for balancing the SoC of DESSs in islanded microgrids, without physical communication. Each DESS injects a current distortion at 175 Hz, when its SoC changes by 10%. This distortion is recognized by every DESS, through a phase-locked loop (PLL). In order to distinguish the origin of the distortion, each DESS injects a distortion of different time duration. This intermediate frequency has been selected in order to avoid the concurrence with the usual harmonics. The DESSs take advantage of this information and inject a current proportional to the SoC. Implementing this strategy, a comparable number of charging/discharging cycles for each DESS are achieved. Furthermore, an active filter operation, implemented in the rotating frame for each individual harmonic, is integrated in the control of the distributed generation units, supplying nonlinear loads with high-quality voltage. The effectiveness of this method is verified by detailed simulation results.

Fault detection and clearing control strategy in an islanded microgrid with converter-interfaced sources

A significant factor for properly designing the protection scheme of a microgrid is the fault current contribution from the converter-interfaced distributed energy resources (CI-DERs). The fault current direction and magnitude is difficult to be defined, since the currents injected by the CI-DERs affect it considerably. The fault situation is even more complicated, considering the case of islanded looped microgrids with conventional protection devices. In such microgrids, all protection devices are settled to the same rated current. In order to overcome these issues, this paper proposes a new method for detecting and clearing faults, without utilizing any communication means. The fault identification process is carried out by measuring indirectly the microgrid impedance, while each CI-DER adapts the internal control method and injects a fault current proportional to the sensed impedance. Therefore, the source being closer to the fault injects relatively larger currents enabling the selective coordination of conventional protection devices. In order to enhance even more the selective cooridnation of the protection devices, a time delay is also incorporated in the fault control. The proposed strategy is validated in a looped microgrid, protected by simple overcurrent relays.