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Penetration of distributed energy resources (DERs) such as wind, solar and battery units are rapidly increasing in distribution power system (DPS). In view of power grid reliability and efficiency, cyber and physical secure of these units are highly important since it can directly impact power system operations. This paper investigates the dynamic behavior of power distribution during cyber, physical and natural strikes on reactive power control in DER units. Various fault scenarios including modifications in voltage-reactive power (Volt-Var) curve pattern such as slope of the curve, dead band; and changes in capacitive or/and inductive reactive power delivery to the power grid are presented. Further, fault scenarios are investigated with the presence of dynamic VAR (volt-ampere reactive) compensation (DVC) units in the system. A southern California 140-bus commercial distribution power system is selected as a test system and faults are analyzed in both Matlab/Simulink and ETAP simulation platforms. The results evidence that the modifications in reactive power control values at DER units significantly affects the voltage quality/security of the system and affect the number of switching actions of voltage regulators (VRs). In contrast, DPS connected with DVCs improves voltage quality and reduces the number of switching actions of VRs, also voltage quality/security is maintained during the faults. [ABSTRACT FROM AUTHOR]

Traditionally voltage control in distribution power system (DPS) is performed through voltage regulating devices (VRDs) including on load tap changers (OLTCs), step voltage regulators (SVRs), and switched capacitor banks (SCBs). The recent IEEE 1547-2018 from March 2018 requires inverter fed distributed energy resources (DERs) to contribute reactive power to support the grid voltage. To accommodate VAR from DERs, well-organized control algorithm is required to use in this mode to avoid grid oscillations and unintended switching operations of VRDs. This paper presents two voltage control strategies (i) static voltage control considering voltage-reactive power mode (IEEE 1547-2018), (ii) dynamic and extensive voltage control with maximum utilization of DER capacity and system stability. Further, effective time-graded control is implemented between VRDs and DER units to reduce the simultaneous and negative operation. The proposed voltage control strategies are tested in a realistic 140-bus southern California distribution power system through extensive time-domain simulation studies. The results show that voltage quality in a distribution system is effectively achieved through the proposed voltage control strategies with a significantly reduction in the number of switching operations of VRDs. In addition, proposed voltage control strategies increase reliability and security of the DPS during unexpected failures. [ABSTRACT FROM AUTHOR]

The authors intent using a boost multi-level converter for the doubly fed induction machine (DFIM) used in variablespeed pumped storage plants (VS-PSP). Voltage-source converters connected on the rotor side of the machine control the active and reactive powers of the unit. The proposed boost neutral point clamped (NPC) converter topology provides a voltage output two times larger than a conventional three-level NPC (3L-NPC) with similar DC-link voltage and equal number of switches. Hence, it increases the speed variation of the unit, which improves the efficiency during generation and pumping modes. Moreover, it reduces the starting period of the unit at the pumping mode, which is significant during mode changeover time. Furthermore, it reduces switching and conduction losses in the converter. It also reduces the total harmonic distortion in the output current, as it provides five output voltage levels. These improvements show that the boost NPC converter topology is better among VS-PSP project authorities. In addition, the reliability of the proposed topology is investigated, where converter redundancy is a challenging issue in asynchronous VS-PSP units. The proposed boost NPC was compared with the conventional 3L-NPC system by examining a 250 MW DFIM hydro-generating unit. [ABSTRACT FROM AUTHOR]

Doubly fed induction machine (DFIM) is increasingly being preferred in large rated variable speed pumped storage power plants (PSPP), because it provides better energy efficiency due to optimization of pump/turbine efficiency. Increased efficiency results from the flexibility of operating speed at different hydraulic heads. In doubly fed configuration, partial rated power converters feed the rotor circuit. Parallel-connected back-to-back voltage-source converter on rotor side is responsible for starting, braking (dynamic/regenerative), and speed (real power) control of the unit. This paper presents smooth starting and regenerative braking, and assesses performance under power converter and sensor faults of a 250-MW DFIM pump turbine unit to be commissioned in Tehri PSPP, India. Survivability of the unit during faults is analyzed based on current, speed, and dc-link voltage. Further, this paper investigates fault-tolerant operation of 250-MW DFIM unit at open-circuit fault in converters. A 2.2-kW DFIM is tested in the laboratory to corroborate the simulation. [ABSTRACT FROM AUTHOR]

Doubly fed induction machine (DFIM) is increasingly being preferred in sites where there is large variation in water head (e.g., Tehri hydropower complex in India having water head variation from 130 to 230 m), as it offers high dynamic stability and better energy efficiency at part loads. Smooth starting, unity power factor, and better rotor current profile are the reasons; back-to-back voltage source converters are used in rotor side of the machine. Speed and power factor of DFIM are controlled through power converters in accordance with surplus power available in grid and water head of dam. This paper presents dynamic behavior of a 250 MW DFIM hydro-generating unit, to be commissioned in 1000 MW Tehri pumped storage plant, operating in pumping mode. Further, the performance of pumping operation of DFIM is assessed under power converter and control circuit failures. Persevere ability of the machine is analyzed based on performance measures: Speed, current, reactive power consumption. From the analysis, energy storage loss of about 66.61 MU/year is estimated in a 1000 MW plant due to the inadequate power and control redundancy. A 2.2 kW DFIM is experimented at laboratory level to support the simulation. [ABSTRACT FROM AUTHOR]

Reliable operation of multichannel power electronic converters in a large pumped storage power plant (PSPP) is a main concern among policymakers and/or hydropower project developers. This paper aims to design a two-stage protection circuit operation strategy for a large-rated doubly fed induction machine (DFIM) employed for variable-speed PSPPs. This paper initially studies the dynamic behavior of DFIM under symmetrical short-circuit faults in both the 400-kV power system and at stator terminal of the generating unit. Based on the fault analysis, a two-stage protection circuit (dc-link braking chopper circuit and crowbar protection circuit) operation strategy is designed in consideration of practical limitations. The designed operation strategy ensures the continuous operation of large-rated DFIM under marginal grid disturbances. To investigate the effective operation of a two-stage protection circuit, a 250-MW DFIM with multichannel power converters is implemented in MATLAB/Simulink tool. An experimental test is carried in a 2.2-kW DFIM laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper proposes a new family of multilevel inverter topology that is able to generate seven voltage levels by utilizing one or two floating capacitors and 10 power switches. This novel boost switched-capacitor seven-level inverter possesses voltage boosting capability with an achievable maximum voltage level 1.5 times the input direct current (dc) voltage. The generation of higher output voltage does not incur high-voltage stress on any power switch in this topology, as the peak inverse voltages of all power switches do not exceed the input source voltage. In addition, capacitor voltage balancing is not essential since the floating capacitors are effectively balanced during the charging and discharging processes. Furthermore, the proposed topology eliminates the need for multiple isolated dc sources, and a single dc source is sufficient in both its single-phase and three-phase topologies. The operating principle and steady-state analysis of the proposed topology are elaborated. Experimental results from a single-phase prototype are then presented to verify the validity of the proposed topology. [ABSTRACT FROM AUTHOR]