Voltage sag and swell mitigation based on modulated carrier PWM

•Voltage sags and swells in 3-phase lines can be compensated using power from other live-phases.•Viewed from any one phase, the other phase voltages “appear to contain unwanted ripples”.•The carrier is synchronously modulated by such a fluctuating power supply.•The almost analog-feed forward controller is applicable to a variety of 3-phase DVR strategies.•No feed-back loops are present; only feed forward control is used which makes the DVR very stable.

The voltage sags or swells which are normally encountered in distribution systems can be compensated by three-phase Direct Voltage Regulators (DVR). In general, when the sag or swell in any phase is sought to be compensated by using the power available in the other phases, extensive signal processing is required, since the magnitudes and phase-angles differ very much. A new control procedure for generating the pwm signals is presented, which is mostly analog and not at all computation-intensive. Illustrations are provided in which the swell or sag can be brought down by using the power either from the other two phases or power from the same phase. The emphasis is not to establish the superiority or otherwise of these arrangements of diverting power from any one phase to the other phase for mitigating the sag or swell. The core of the work deals with the simplicity of the new pwm generation procedure employed for such diversion of power. The feed-forward nature of the control leads to stable operation. Its effectiveness has been demonstrated by simulations.

Graphical abstractDirect compensation of 3-phase voltage sags and swells is much more interesting than compensating single-phase systems. The reason being that, compensation can be done using the power supply available from other live phases. Viewed from any one phase, (say a-phase), the voltages derived from the other two phases viz., |vb| and |vc| “appear to contain unwanted ripples”, as seen in the Figure. Hence, it is quite tedious to derive a “sinusoidal” compensation voltage, from the ‘other live-phases’. DSP’s have been used so far, to compute the p.w.m. pulse-width, point by point for the entire cycle. However, synchronous-modulation of the carrier by the fluctuating power supply voltage makes the design of the controller very simple. The power supply along with the ripples is multiplied with the triangular carrier. This modulated carrier when used in conjunction with the sinusoidal control signal eA(t), enables the development of a very pure sinusoidal compensating voltage across the injection transformer. The pulses Sca and Sga, enable the application of the compensating voltage derived from the c-phase to the a-phase. Similarly, the p.w.m. pulses Sab and Sga connect the b-phase power supply voltage to the injection transformer of a-phase. The signals, eA(t), vB(t) and vC(t) and a few logic circuits were used for routing the gate-pulses. The feed forward-controller becomes almost analog and can be very easily realized using even discrete components. The p.w.m. procedure is very general and applicable to the various 3-phase DVR strategies or structures. While compensating an arbitrary swell (A 20%, B 40%, C 60%), the THD in the compensation voltages is less than 1.5% under full load. The THD in the compensated mains voltage is obviously still smaller.Figure optionsDownload full-size imageDownload as PowerPoint slide