Resistance Emulation Research Articles

Harmonic and reactive power compensation as ancillary services in inverter-based distributed generation

The introduction of distributed generation, DG, into low-voltage (LV) networks opens up the possibility of supplying ancillary services to aid network management and to maintain power quality. DG itself can cause voltage magnitude difficulties when injecting real power into networks with high R/X ratios, but control of reactive power injection can help overcome this. Continuous control and support only at the voltage limit conditions (to avoid unnecessary reactive power flows in normal conditions) are explored. Relatively high-impedance LV networks are prone to harmonic distortion from nonlinear loads. A variety of control methods that emphasise either harmonic-line flows or local-voltage distortion are examined, and a compromise method based on resistance emulation is shown to be effective. Experimental results from a single-phase laboratory network and 2 kVA inverter are used to illustrate how these additional control functions can be integrated into the existing control scheme for real-power management. Decomposition of observed voltages and currents into harmonic terms that are phasesynchronised to the grid voltage is a challenge in real-time systems. Kalman observers are used to achieve this with an additional advantage of avoiding explicit phase-locking while producing quadrature components useful in instantaneous calculation of reactive power and in providing feed-forward compensation terms

Current-injection-based 12-pulse rectifier using a single three-phase diode bridge

A three-phase rectifier that applies current injection to reduce the input current total harmonic distortion (THD) is proposed. The rectifier injects rectangular current, which results in the waveforms of the input currents the same as in the case of 12-pulse rectifiers, with the THD equal to 15.22%. The current injection system applies a voltage-loaded resistance emulator. To simplify the rectifier analysis, an equivalent circuit of the rectifier is derived, which significantly reduces the number of nonlinear elements. Analytical optimisation of the resistance emulator transformer turns ratio is performed for the case when the losses in the current injection system can be neglected. Influence of the losses in the current injection system is analysed numerically, and it is shown that they considerably affect the rectifier performance and design. Numerical optimisation of the resistance emulator turns ratio in the presence of losses is performed. Analytical results are experimentally verified on a rectifier with a rated power of 3 kW.

Low-Harmonic, Three-Phase Rectifier That Applies Current Injection and a Passive Resistance Emulator

A new three-phase diode bridge rectifier that provides low harmonic distortion of the input currents applying current injection technique is proposed in this paper. The rectifier applies a novel passive resistance emulator consisting of four diodes and a transformer with the volt-ampere rating of 3.57% of the rectifier-rated power. Optimization of the transformer turns ratio is performed in order to minimize the input current total harmonic distortion (THD). It is shown that with the optimal turns ratio the input current THD equals 3.72%. Dependence of the input current THD on the load current is analyzed, and it is shown that at low load currents the rectifier operates in the discontinuous conduction mode with the THD of 7.79%. The analytical results are experimentally verified on a 2 kW rectifier, indicating that the input current THD lower than 8% is provided within a wide range of the load current variations applying simple circuitry.

A Voltage-Sensorless Control Method to Balance the Input Currents of a Three-Wire Boost Rectifier Under Unbalanced Input Voltages Condition

This paper proposes a control method that can balance the input currents of the three-phase three-wire boost rectifier under unbalanced input voltage condition. The control objective is to operate the rectifier in the high-power-factor mode under balanced input voltage condition but to give overriding priority to the current balance function in case of unbalance in the input voltage. The control structure has been divided into two major functional blocks. The inner loop current-mode controller implements resistor emulation to achieve high-power-factor operation on each of the two orthogonal axes of the stationary reference frame. The outer control loop performs magnitude scaling and phase-shifting operations on current of one of the axes to make it balanced with the current on the other axis. The coefficients of scaling and shifting functions are determined by two closed-loop proportional-integral (PI) controllers that impose the conditions of input current balance as PI references. The control algorithm is simple and high performing. It does not require input voltage sensing and transformation of the control variables into a rotating reference frame. The simulation results on a MATLAB-SIMULINK platform validate the proposed control strategy. In implementation Texas Instrument's digital signal processor TMS320F240F is used as the digital controller. The control algorithm for high-power-factor operation is tested on a prototype boost rectifier under nominal and unbalanced input voltage conditions.

Digital Implementation of a Line Current Shaping Algorithm for Three Phase High Power Factor Boost Rectifier Without Input Voltage Sensing

In this paper the implementation of a simple yet high performance digital current mode controller that achieves high power factor operation for three phase boost rectifier is described. The indicated objective is achieved without input voltage sensing and without transformation of the control variables into rotating reference frame. The controller uses the concept of resistance emulation for shaping of input current like input voltage in digital implementation. Two decoupled fixed frequency current mode controllers calculate the switching instants for equivalent single phase boost rectifiers. A combined switching strategy is developed in the form of space vectors to simultaneously satisfy the timing requirements of both the current mode controllers in a switching period. Conventional phase locked loop (PLL) is not required as converter switching is self-synchronized with the input voltage. Analytical formula is derived to obtain the steady state stability condition of the converter. A linear, low frequency, small signal model of the three phase boost rectifier is developed and verified by measurement of the voltage control transfer function. In implementation Texas Instruments's DSP TMS320F240F is used as the digital controller. The algorithm is tested on a 10-kW, 700-V dc, three phase boost rectifier.

A new power converter for fuel cells with high system efficiency

This paper presents a new high-efficiency grid-connected single-phase converter for fuel cells. It consists of a two-stage power conversion topology. Since the fuel cell operates with a low voltage in a wide voltage range (25 V–45 V) this voltage must be transformed to around 350–400 V in order to be able to invert this dc power into ac power to the grid. The proposed converter consists of an isolated dc–dc converter cascaded with a single-phase H-bridge inverter. The dc–dc converter is a current-fed push-pull converter. The inverter is controlled as a standard single-phase power factor controller with resistor emulation at the output. Experimental results of converter efficiency, grid performance and fuel cell dynamic response are shown for a 1 kW prototype. The proposed converter exhibits a high efficiency in a wide power range (higher than 92%) and the inverter operates with a near-unity power factor and a low current THD.

Low-harmonic thyristor rectifiers applying current injection

Application of the third harmonic current injection in three-phase full-bridge thyristor rectifiers is analyzed. Spectra of the rectifier output terminal voltages are derived. Results of the optimal current injection for diode bridge rectifiers are generalized for the thyristor rectifiers, and the waveform of the optimal injected current is derived. It is shown that for the optimal third harmonic current injection, the current injection network takes 8.571% of the rectifier input power from the thyristor bridge output terminals, regardless the thyristor firing angle. A current injection network with a passive resistance emulator that provides recovery of the power taken from the thyristor bridge output terminals is proposed. Volt-ampere ratings of the magnetic components are derived. An algorithm to control the switches in the resistance emulator is presented. Analytically obtained results are experimentally verified.

A novel low-harmonic three-phase rectifier

A novel three-phase low-harmonic rectifier is proposed. The rectifier consists of a diode bridge, three bi-directional switches that are switched at the double of the line frequency, and a current shaping network. Two current shaping networks are proposed, the first one consisting of two capacitors and three resistors or switching resistance emulators, and the second one consisting of two capacitors, two resistors and a transformer. The power processed by the resistors or resistance emulators is shown to be equal 8.81% of the input power. The rectifier is suitable for application at medium and high power levels, since only the small amount of the input power is processed by high-frequency switching converters, and as a retrofit upgrade of existing rectifiers, requiring connection at only five points. Design guidelines are presented. Switching resistance emulation is discussed, and its feasibility is demonstrated. Analytical results are experimentally verified on a 2 kW rectifier.

Three-phase rectifiers that apply optimal current injection

A class of current injection based three-phase high power factor rectifiers is proposed. Low distortion of the input currents and high power factor are obtained applying near optimal current injection. The optimal current injection is discussed, and requirements it imposes to the current injection network are derived. A class of simple current injection networks that provide near optimal current injection, consisting of a transformer, two capacitors, and a number of resistors or resistance emulators is proposed. The power processed by the resistors or resistance emulators is shown to be 8.81% of the input power. Design of the current injection network is discussed, and dependence of the input current distortion on capacitance of the applied capacitors is analyzed. Volt-ampere rating of the transformer is shown to be only 0.16% of the input power. Effects caused by the output current ripple are studied, and a method for their compensation is proposed. Analytical results are experimentally verified. Switching resistance emulation is discussed and its feasibility is experimentally demonstrated.

Two three-phase high power factor rectifiers that apply the third harmonic current injection and passive resistance emulation

Two high power factor rectifiers that apply the third harmonic current injection principle and passive resistance emulation are presented in this paper. Two methods of current injection, simultaneous current injection in all three of the phases, and current injection to only one of the phases, are discussed. Conditions for the optimal third harmonic current injection are derived, structures of the rectifiers that apply passive resistance emulation, enabling recovery of the power taken by the current injection network, are presented. The proposed resistance emulator consists of a transformer, two diodes and a capacitor. Choice of the passive components is discussed, and their values are proposed, volt-ampere ratings of the magnetic components are derived. Influence of the higher order harmonics is discussed. Dependence of the input current total harmonic distortion on the load level is presented for both of the current injection methods. Analytically obtained results are verified on 1.5 kW experimental rectifiers.