1. Introduction
In recent years, with the increasing penetration of distributed energy and nonlinear loads, harmonics have attracted much attention of both grid operators and power users. Modeling of harmonic sources is essential in estimating the harmonic current contribution as a function of the background harmonic voltage. Authors in Reference [
1] summarized the progress in harmonic analysis in both time and frequency domains. Among the state-of-the-art harmonic modeling methods, Norton model is a commonly-used equivalent circuit of harmonic sources [
2,
3,
4,
5,
6,
7]. Authors in References [
8,
9] built Norton’s equivalent circuit for iron and steel plants and estimated their harmonic current contributions to the background grid. In Reference [
10], the authors considered the open-loop inverter of a double-stage PV system as a Norton equivalent. Paper [
11] estimated the shares and locations of harmonic sources in radial distribution systems using the Norton model as representation of the customer side. A discrete time-domain closed-loop Norton’s equivalent circuit is developed in Reference [
12] for a micro-grid. Paper [
13] analyzed Inverse Nyquist Stability Criterion for a grid-tied inverter system based on a Norton equivalent circuit of the inverter.
Parameters in Norton models can be derived in analytical forms based on circuit topology and control schemes associated with harmonic sources. In Reference [
14], the authors considered control-related parameters in building Norton model for a grid-tied inverter and justified the method through a case study on a 1.4 MW PV plant. Impedance-based Norton modelling is studied in Reference [
15] for a VSC-HVDC (Voltage Source Converter High Voltage Direct Current) system and the impact of the different parameters on the system dynamics and stability is also analyzed.
The analytical derivation of Norton parameters cannot match the operational conditions due to aging of the system modules as well as time-varying characteristic of the system parameters. For example, paper [
16] addressed that the accuracy of Norton parameter evaluation for a PV plant are affected by aging of the PV modules. To address the time-varying feature of harmonic characteristics, paper [
17] used iterative Norton modelling to make customer harmonic emission levels assessment. Independent component analysis is used to describe the stochastic feature of Norton parameters in Reference [
18].
In case of absence of control parameters involved in analytical derivation of Norton, small-signal impedance modeling is an alternative way of modelling parameter estimation. In Reference [
19], Authors excited the system with short sequences of current signals and identified its impedance by constructing the relationship between corresponding voltage and current characteristics. A study in Reference [
20] is conducted using this method to analyze the terminal characteristics of a MMC (Modular Multilevel Converter) system. In Reference [
21], a defined current pulse is injected into the medium-voltage network and the network impedance is then evaluated by analyzing the network voltage responses. Authors in Reference [
22] developed a harmonic matrix of the grid-connected converter using a Harmonic State Space (HSS) small signal model, with considerations of its time-varying feature. In Reference [
23], different control schemes are considered in the HSS-based small-signal impedance modeling for a MMC system.
However, since small-current excitation into the system is required in small-signal impedance modeling, it is difficult to be applied widely due to its practical complexity and costs. Furthermore, as the power quality monitoring system is implemented in many countries [
24], the field measurements of voltage and current are quite available. Therefore, identification of Norton parameters using measurements is a practical method. In References [
17,
25,
26,
27], Norton-based harmonic impedance is identified from measurement data using linear estimation method.
Norton model itself in essence is not able to describe the mutual coupling relationships among voltage and current in different harmonic orders. However, this coupling relationship does exist and presents significant impact on the grid [
28]. In Reference [
29], a cross-frequency admittance model is obtained but the model is found only suitable for specific operational conditions [
26].
To address the mutual coupling between harmonics in different orders, authors in References [
28,
30] proposed a frequency-domain coupled linear admittance matrix and presented its theoretical foundation and analytical derivations. The elements in the admittance matrix are independent of harmonic voltages in the ac side. Based on this model, authors in Reference [
31] established the frequency-domain harmonic analytical model for EV charger. Also, frequency-domain analytical model is constructed for home appliances [
32] and compact fluorescent lamps in Reference [
33].
As aforementioned, the control-related parameters involved in derivation equations are not only time-varying but also hardly obtained. Therefore, constructing the model using measurements is an alternative method. Authors in Reference [
26] conducted matrix manipulation for the model and found the manipulation is ill-conditioned. Aiming to solve the ill-conditioning problem, two simplified models are thus built, with one omits the conjugate part of the model and the other simplified into a Norton model. Authors in Reference [
27] simplified the model by assuming that there is no distortion in the background voltage. These simplifications decreased the accuracy of the model to some extent. To address this issue, this paper proposed an identification method of the coupled harmonic admittance based on least square estimation using measurement data.
This paper is organized as follows. The proposed harmonic admittance identification technique is described in
Section 2. In
Section 3, two case studies are presented to evaluate the effectiveness of the proposed method. Discussion of the advantage and disadvantage of the method is described in
Section 4. Finally, the conclusion is presented in
Section 5.
2. Coupled Harmonic Admittance Identification
The coupled harmonic admittance model aims to estimate the harmonic current contribution as a function of the background harmonic voltages. The model builds relationships among current and voltage in all harmonic orders [
28], which can be described in the following equation.
The compact form in Equation (1) can also be rewritten as follows.
where
is the vector of fundamental and harmonic voltages,
is the conjugate value of
and
represents the harmonic voltage component in the harmonic order of
h. Similarly,
is the vector of harmonic currents of different orders and
represents the
h-th harmonic current.
is the maximum harmonic order of interest. In the strict mathematical form, the highest order of current and voltage can be different. However, since the paper aims to estimate the admittance matrix based on measurement data, so the highest harmonic orders for voltage and current are considered the same.
The admittance matrix, that is, , together with the current source , are the linear parameters between current and voltage in all harmonic orders. The combination of and represent the coupled admittance between h-th harmonic voltage and n-th harmonic current.
It is clear that the model couples all harmonic voltages and current components, that is, each harmonic voltage has contributions to harmonic currents at all frequencies. Additionally, the coupled matrix indicates that the harmonic current is a function of both the voltage itself and its conjugate.
In order to estimate the unknown admittance
and current source
, [
26] derived a method based on matrix manipulation. However, it was found and also evidenced in our experiment that, this method is vulnerable to numerical difficulties since the manipulated matrix is ill-conditioned for most of the data. To conquer this problem, a method based on Least Square Estimation (LSE) is proposed in this paper.
Also, it should be noticed that Norton model is actually a simplification of the coupled admittance model. By removing the conjugate counterpart of the voltage, as well as the admittance between different frequencies, the Norton model can be described as below.
For comparison, three Norton-based identification methods are conducted in the experiments. Details of the methods are included in
Section 3.
Based on Equation (2), the
h-th harmonic current
and the
h-th harmonic voltage
are first divided into their real and imaginary parts, as indicated in (4). Note that
and
are the real and imaginary part of
, respectively. The same rule applies to
and
. Here
.
By separating the real and imaginary part of each element, Equation (2) can be represented in the following forms.
As indicated in the above equation, let . The same rule also applies to and .
After obtaining
M measurements of voltage and current harmonic components, Equation (7) can be obtained, according to (5).
Note that Equation (7) is in the form of
. To solve this linear equation, LSE can be used to obtain the estimations of unknown parameters in (5).
LSE is able to produce estimation values of
and current sources
and
. Using estimation values of
and
, Equation (9) can be calculated to obtain the real and imaginary parts of the two harmonic admittance matrix.
Similarly, LSE can also be used to derive the admittance and current source in (6).
Therefore, and can be derive. After identification of harmonic admittance matrix and current sources, estimation of harmonic current can then be obtained given the grid voltage with or without distortion.
As indicated in Reference [
28], the elements of coupled admittance matrix are sensitive to the changes of parameters such as firing angle of a converter. To describe the time-varying characteristic of related parameters, a recursive method is constructed to obtain the changing values of elements in the coupled matrix.
The time series of measurement are divided into M groups in chronological order. For each group of measurements, the two admittance matrix together with current source are constructed using the identification method mentioned above. Here, let
be the unified notation for two admittance matrix and one current source estimated using measurements of the
M-th group. Then, the final estimations for each group can be built in the following recursive form.
where
is a forgetting factor ranging from 0 to 1. Smaller value of this factor is more suitable for situations with parameters of less variations. Using (10), the effect of the past measurements is fading gradually and the most recent measurement contributes most to the estimation, thus achieving accuracy of the harmonic current estimation.