Noninvasive Online Parametric Identification of Three-Phase AC Power Impedances to Assess the Stability of Grid-Tied Power Electronic Inverters in LV Networks

Abstract

Abstract-This paper presents a noninvasive online parametric identification of three-phase AC power impedances to assess small-signal stability of grid-tied inverter systems by using well-known impedance-ratiobased stability criteria. The identification technique is integrated into the control of an existing grid-tied inverter for the estimation of wide bandwidth AC grid impedances, on top of its original power conversion function. This is accomplished in practice by injecting a short-time smallsignal Pseudo Random Binary Sequence (PRBS), a digital approximation of white noise which is wide bandwidth in nature, on the inverter control loop so that all frequencies of interest at the impedance measurement point can be excited at once. Then, digital processing is performed in the integrated control platform where the parametric AC grid impedance is extracted from the measurement of voltage and current over the length of PRBS injection. Moreover, a procedure on how to identify the output impedance of the inverter is deployed so that the parametric source and load impedances can be used to verify the system stability by means of the generalized Nyquist stability criterion. The technique is validated via Hardware In the Loop (HIL) realtime simulation. The present work focuses on the identification of balanced three-phase AC impedances in dq reference frame and a dq diagonal-dominant stability analysis which is typical of LV Distribution Grids.Index Terms-Impedance Measurement, Inverters, Power System Stability, Stability Analysis, Stability Criteria I. INTRODUCTIONThe traditional AC Power System is transforming into a PowerElectronics-based AC Power System due to the deeper This work extends previously presented work at COMPEL 2016.integration of Renewable Energy Sources (RESs) and loads through grid-connected feedback-controlled power electronic. This transformation is schematically depicted in Fig. 1, representing an exemplary low-voltage (LV) radial feeder. In Fig. 1 (a), the traditional AC LV feeder is mainly a passive network because the majority of loads are either AC machines, resistive, or diode rectifiers, fed by the MV/LV transformer. In Fig. 1 (b), the counterpart Power Electronics AC LV feeder is an active network where inverters interface the distributed RESs and active rectifiers interface all the loads. In the situation of Fig. 1 (b), it is also envisioned that the MV/LV conversion will be taken by solid-state transformer technology [4]. As power electronics penetrates the AC grid, a new challenge is raising for both power electronic engineers and power systems engineers. This proliferation of grid-connected power electronics can have a destabilizing effect on the AC voltage due to interactions among grid-connected feedback-controlled power electronic converters and equivalent power grid impedances seen at the various Point of Common Couplings (PCCs) [5]-[11]. In the literature, this stability problem has been explained in two ways. The first explanation considers the destabilizing equivalent negative i...

Online Parametric Estimation of Grid Impedance Under Unbalanced Grid Conditions

“…However, the second model of the Z g considers the RLC model. It is represented by a series resistance and inductance (Z 1 = R g + sL g ) in parallel with a capacitance (Z 2 = 1/sC g ) [8]. It is worth mentioning that only the RL model is shown in Figure 1.…”

“…This cost effective approach requires no extra hardware, such as a network impedance analyzer. The grid impedance information can be used to enhance the operation of grid-connected inverters, e.g., online stability analysis [7,8], voltage control [9], adaptive current controller [10], low voltage ride through [11], anti-islanding detection of grid-connected inverters [12][13][14][15], and analysis of multi-parallel inverters system [16][17][18].…”

“…Moreover, it suits a practical implementation due to its small disturbance amplitude and immunity against background noise in the system [19,20]. Recently, the advantages of the PRBS have motivated researchers to adopt it for impedance estimation purpose while using power electronics converters [8,[21][22][23][24][25].…”

“…The implementation of the PRBS identification technique into 3-phase grid-connected inverters controlled in the synchronous reference frames (dq-axis) for grid impedance estimation purpose has been reported in several studies [8,[21][22][23]. The injection point of the PRBS in the control loop of the inverters differs between studies.…”

“…The injection point of the PRBS in the control loop of the inverters differs between studies. For instance, it is injected into the carrier frequency of PWM [21], inner current controller references at dq-axis [22,23], and several locations (hybrid injection) simultaneously [8]. The second approach has the advantage of more control of the PRBS, and it provides more accurate results for a frequency range within the controller bandwidth.…”