Effect of dynamic load models on WAC operation and demand-side control under real-time conditions

https://doi.org/10.1016/j.ijepes.2020.106589Get rights and content

Highlights

  • Investigation on the impact of non-linear loads on the WAC performance.

  • Development of an advanced wide area demand-side control scheme.

  • Simultaneous coordination of the demand-side and the generation-side.

  • Investigation on the effect of the size and flexibility of the controllable load.

  • Validation of the proposed scheme through real-time simulations.

Abstract

The vast majority of the proposed Wide Area Control (WAC) methodologies is tested and evaluated under the assumption that the system consists only of constant power loads. However, in practice power systems are dominated by non-linear loads, which can potentially affect the smooth operation of the wide area controller and deteriorate the system’s stability. This paper is firstly focused on examining the impact of various and commonly used non-linear load models (static and dynamic), such as the exponential, exponential dynamic, ZIP and ZIP-Induction Motor (ZIP-IM) load models on the WAC performance. Real-time simulations on the IEEE 39-bus dynamic test system indicate that the load recovery dynamics of the exponential dynamic load model have a severe impact on the WAC operation, leading the system to instability. Furthermore, it is illustrated that, loads with constant power/current/impedance characteristics have limited effect on the WAC damping capability, even with dynamics included. To address the impact of the exponential dynamic loads on the WAC performance, a novel wide area controller is proposed for coordinating simultaneously all the synchronous generators and controllable loads of the system. Finally, the effect of the size and flexibility of the controllable demand on the performance of the proposed scheme is also investigated, while the robustness of the proposed scheme is evaluated under measurement errors and erroneous generator parameters.

Introduction

During the last few decades, power systems are becoming heavily stressed, operating more and more often close to their stability limits. This situation makes power systems vulnerable to any disturbances that might occur. Amongst the new challenges which have emerged, a major concern is the appearance of inter-area oscillations [1]. These oscillations are associated with the transfer of large amounts of power between interconnected systems through long transmission lines [2]. Inter-area oscillations along with local oscillations, represent the two electromechanical modes of the dynamic power systems. Inter-area modes occur when coherent groups of generators are swinging against each other (in the case of a contingency) and they are characterized by low frequency (0.2–1 Hz) [3].

The traditional approach to damp any kind of power system oscillations is to utilize the generator’s local controllers (such as power system stabilizers (PSS)) or to install flexible ac transmission systems (FACTS) on the lines. However, these controllers are unable to ensure effective damping of the inter-area oscillations, since they employ local signals which lack the global information [4]. The advent of the synchronized measurement technology and its application to power systems enabled the observation of the inter-area oscillations and has provided the backbone for the development of Wide Area Monitoring and Control (WAMC) systems [1]. More specifically, Wide Area Control (WAC) aims to utilize the synchronized phasor measurements in order to develop and provide coordination signals to the local controllers, allowing them to damp effectively all the inter-area modes [5].

Although various methodologies have been proposed for the successful development of WAC, the vast majority of them are validated considering that only constant loads are dominant in the system (mainly constant power [1], [6] and sometimes constant impedance [7], [8]), which essentially are voltage or frequency independent. For example, in [9] the proposed WAC has been evaluated under different operating conditions considering among others various constant power load levels. In the same context, the authors in [10] have tested the effectiveness of the proposed latency compensator-based WAC when the load on specific buses changes by a constant amount. In [11], [12] both methods are evaluated by decreasing the constant power exchange on the systems’ tie-lines.

However, several loads in the power system are, in reality, dynamic and highly nonlinear. Various studies have illustrated the effect of considering dynamic loads (instead of constant loads [13], [14] or even static loads only [15]) on the power system stability. Therefore, it is important to test the robustness of the WAC methodologies under the existence of non-linear loads (static and dynamic), which might compromise the stability of the system. In [16], the development of a wide area controller intended for the simultaneous coordination of all the generators and renewables of the power system has been proposed. The evaluation of the proposed scheme considered (among others) the inclusion of dynamic voltage dependent loads in the system, when a symmetric fault occurs. In the same context, in [17], the performance of the proposed wide area controller is evaluated under a heavy penetration of dynamic voltage dependent loads. The utilization of a simplified induction motor model as dynamic loads is presented in [8], in order to illustrate the robustness of the proposed WAC under different load characteristics and load types. Furthermore, in [18] the development of a two-level hierarchical controller combined with a Smith predictor is presented. Its performance, in this case, has been validated by including a nonlinear load which consists of an induction motor and a voltage dependent static load (load dynamics are not included). The authors in [19] have presented the design and performance of a wide area controller in the event of a load change when different types of load models are considered into the design procedure, assuming that way that all the model types are always known and available for the WAC design. However, this assumption may not be valid, as most of the time the load models are unknown and in addition they tend to have a constantly changing nature. Therefore, it is important to examine under large disturbances the actual impact of different dynamic load models on a conventional WAC method, the design of which is not directly dependent on knowing the load types which exist in the system.

Dynamic load models are developed in order to represent accurately the behaviour of thousands of loads which are aggregated (as one load) to the high-voltage buses [20]. However, accurate load modelling can be a very challenging procedure since the loads’ behavior varies based on their nature (e.g. industrial, residential) and they are time-varying according to the period of the day/month/year [21], [22]. Due to this reason, some load models are accurate enough for some systems, while for others they are inadequate.

An interesting solution to mitigate the impact of dynamic loads on the system stability (and thus on the WAC operation as well), which is gaining attention recently, is their utilization by the wide area controller for enhancing the system’s damping performance. Up until now, loads have been treated as passive components, whose demand cannot be modified. However, the installation of smart appliances into the households and the introduction of the demand-side control concept have provided the means to obtain more responsive and controllable loads. The load controllability is even more enhanced due to the growing penetration of inverter-based loads into the system, which allows their easy integration into coordination schemes. In [23] a frequency control methodology is presented, which is based on a decision tree for managing Heating, Ventilation and Air Conditioning (HVAC) systems by adjusting their temperature set-points. The implementation of wide area controllers based solely on demand-side control has been illustrated in [24], [25]. More specifically, in [24] a hierarchical demand-side load modulation strategy has been proposed for improving the frequency response and the inter-area oscillation damping. In [25], the authors have developed a wide-area demand-side control method for modulating only the active power of the end-use loads in order to increase the small-signal stability of the power system.

Overall, it is unclear how much is the actual impact of the non-linear loads on the WAC performance, especially when large-scale test systems are considered. In addition, various static and dynamic load models are available, which can have different effect on the WAC operation. Therefore, the first objective of this paper is to examine the impact of the high penetration of non-linear loads on the WAC performance. This is accomplished by considering different load models at each case study (e.g., exponential, exponential dynamic, ZIP). The second objective of this work is the development of a novel WAC scheme which will be able to apply demand-side control on the controllable loads (CLs) of the system, while coordinating also the synchronous generators of the system. For evaluating the performance of this novel scheme, real-time Electro-Magnetic Transient (EMT) simulations are performed based on the IEEE 39-bus dynamic test system. The robustness of the proposed coordination scheme has been evaluated in the presence of realistic measurement errors and generator parameter uncertainties. Finally, it is worth noting here that data delays/dropouts were not considered in this study since (as indicated in [26]) their impact can be compensated successfully through the use of a linear predictor. In this sense, data delays are out of the scope of this paper.

In summary, the main contributions of this paper are: 1) a thorough investigation is carried out on the impact of non-linear loads on the WAC performance. The investigation considers an increased presence of various types of static and dynamic load models that have different load characteristics; 2) the development of an advanced wide area demand-side control scheme, that provides coordination signals for all the controllable loads of the system; 3) the implementation of a novel wide area controller for the simultaneous coordination of the demand-side and the generation-side for enhancing the system stability; 4) the investigation for identifying the effect that the size and flexibility of the controllable load have on the performance of the proposed scheme; 5) the validation and robustness evaluation of the proposed scheme through real-time simulations. Regarding the third contribution, to the authors’ best knowledge, this is the first work that proposes a common wide area control scheme for the simultaneous coordination of the demand with other system components (i.e. synchronous generators).

The rest of the paper is structured as follows. Section 2 presents the methodology used for the conventional WAC design. The investigated load models are presented in Section 3. The real-time simulation and the results considering the effect of the dynamic loads are discussed in Section 4. Section 5 shows the proposed methodology for formulating the novel wide area controller as well as its performance in the IEEE 39 bus system. A discussion on the future work and the conclusion are provided in Section 6 and Section 7 respectively.

Section snippets

Wide area control design

For completeness, this section overviews briefly the procedure for the development of a conventional wide area controller. The WAC formulation is based on the methodology presented in [27], which requires wide area measurements from all the generators of the system. More specifically, the generators’ terminal voltages (vt), the frequencies (f) and the power angles (δ) are needed. For this reason, Phasor Measurement Units (PMUs) are assumed to be installed on all generator buses. The wide area

Load models

Load modelling refers to the mathematical expression of the load’s behavior under steady-state and transient conditions. Load models are mainly used for recreating the system’s conditions for analysis or to predict its response during specific disturbances. However, there is an imperative need from both the research community and industry for accurate load modelling, since loads are constantly evolving and changing [29]. The main issue is that there is not a standard load model representative

Effect of dynamic loads on WAC performance

This section presents the real-time simulation results regarding the evaluation of the WAC performance under the penetration of the aforementioned four load models. For this purpose, an OPAL-RT real-time simulator has been considered to obtain in real-time and more accurately the effect of the load models on the WAC operation. More specifically, as shown in Fig. 2, three cores of the OP5700 were considered here, each one equipped with an Intel Xeon E5 8 Core CPU at 3.2 GHz and with 8 GB RAM.

Integration of wide area control with demand-side control

One of the main contributions of this work is the development of coordination signals for the demand side control that will be included into the WAC scheme. Regarding this contribution, to the authors’ best knowledge, this is the first work that considers the simultaneous coordination of the demand-side with the generation-side for maintaining the system stability. More specifically, the proposed wide area controller is developed by advancing the conventional methodology of Section 2.B in order

Discussion and future work

The application of the proposed WAC to large power systems is a great challenge. One potential drawback of the proposed WAC when it is applied to large systems (where multiple generators and loads need to be coordinated) is the increased number of measurements that need to be collected and processed, while a large number of WAC signals should be provided to the controllable assets. For the case of the synchronous generators, the authors have shown in [17] that this can be alleviated through the

Conclusion

In this paper, an investigation is carried out for identifying the effect of non-linear loads on the WAC performance, while for addressing their impact a novel methodology is proposed for the coordination of synchronous generators and controllable loads. Regarding the first objective of this work, the investigation considered the penetration of four types of load models (static and dynamic) into the IEEE 39-bus dynamic test system. More specifically, the models that have been examined are: 1)

CRediT authorship contribution statement

Lazaros Zacharia: Conceptualization, Methodology, Software, Formal analysis, Investigation, Visualization, Data curation. Markos Asprou: Supervision, Project administration. Elias Kyriakides: Supervision. Marios Polycarpou: Supervision, Project administration.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgement

This work has been supported in part by the European Union's Horizon 2020 research and innovation programme under grant agreement No 739551 (KIOS CoE) and from the Government of the Republic of Cyprus through the Directorate General for European Programmes, Coordination and Development.

This work was also co-funded by the European Regional Development Fund and the Republic of Cyprus through the Research and Innovation Foundation (Project: INTEGRATED/0916/0035).

References (40)

  • B.P. Padhy et al.

    “A wide-area damping controller considering network input and output delays and packet drop”

    IEEE Trans. Power Systems

    (2017)
  • M. Maherani et al.

    “Fixed order non-smooth robust H∞ wide area damping controller considering load uncertainties”

    Int J Electr Power Energy Syst

    (2020)
  • Y. Nie et al.

    “Unified Smith predictor compensation and optimal damping control for time-delay power system”

    Int J Electr Power Energy Syst

    (2020)
  • A. Noori et al.

    “Designing of wide-area damping controller for stability improvement in a large-scale power system in presence of wind farms and SMES compensator”

    Int J Electr Power Energy Syst

    (2020)
  • N. Y., Y. Zhang, Y. Zhao, and B. Z. L. Fang, “Wide-area optimal damping control for power systems based on the ITAE...
  • M. Bennett et al.

    “The impact of large-scale dynamic load modeling on frequency response in the U.S. Eastern Interconnection”

    Int J Electr Power Energy Syst

    (2020)
  • L. Zacharia et al.

    “Integration of renewables into the wide area control scheme for damping power oscillations”

    IEEE Trans. Power Systems

    (2018)
  • L. Zacharia et al.

    “Wide area control of governors and power system stabilizers with an adaptive tuning of coordination signals”

    IEEE Open Access Power Energy

    (2020)
  • F. Okou et al.

    “Smith predictor approach for the design of a robust wide-area measurements based hierarchical controller”

    IEEE PES General Meeting, San Francisco, CA, USA

    (2011)
  • N. R. Naguru, G. V. N. YatendraBabu, and V. Sarkar, “Design and performance analysis of wide area controller in the...
  • Cited by (0)

    View full text