modeling and analysis of microgrid cluster simulation based on...

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Modeling and Analysis of Microgrid Cluster Simulation Based on RTDS

OUYANG Jianna Smart Grid dept.

Electric Power Research Institute of Guangxi Power Grid Co. , Ltd.

Nanning, Guangxi of China [email protected]

LIANG Shuo Smart Grid dept.



GUO Min Smart Grid dept.



CHEN Weidong Smart Grid dept.



Abstract—In areas with abundant distributed energy, the trend of microgrid cluster is becoming more and more obvious. In order to achieve the real-time simulation of operation status, and to verify the coordination control and energy management strategies of microgrid cluster, this paper builds up a microgrid cluster model based on the demonstration project of Sanli Yizhong, covering photovoltaic, fan, gas turbine, fuel cell, etc. The operation status of grid-tied and islanding are simulated and the fault information is collected, so as to verify the validity of the microgrid cluster model.

Keywords—Microgrid cluster, RTDS, Simulation, Modeling

I. INTRODUCTION

In order to reduce environmental pollution and achieve sustainable development, distributed generation (DG) technology based on renewable energy has become an important application of clean energy in power grids. Microgrid is an independent and controllable system, which consists of distributed power source, energy storage device, energy conversion device, monitoring protection device and local load. It has the advantages of independent autonomy, multi-energy complementarity, coordinated optimization, high efficiency and reliability. But its scale and operational capacity are affected by the permeability of DG, energy storage or diesel engine capacity, control systems and other factors [1], [2]. In order to solve this technical problem, constitute microgrid cluster based on multiple independent sub-microgrids in the area with large numbers of distributed power sources. Reinforcing the electrical connections between sub-microgrids based on reasonable connectivity and organizational structure [3].

The application of microgrid cluster solves the size limitation of DG, which dues to microgrid structure, control, protection, etc. On the other hand, it reduces the influence of DG operation effectively and improves the power supply reliability of the system [4], [5]. Because of this, the construction and operation mode of microgrid has gradually developed from a single small-scale demonstration to a large-scale multi-microgrid cluster in areas with rich energy resources.

Many developed countries in the world started earlier to develop the microgrid experimental platform. In the second half of 2006, Consortium for Electric Reliability Technology

Solutions (CERTS) worked with American power company to set up a microgrid physics simulation platform. It provided experimental support for microgrid energy management, power quality research, microgrid integrated control and protection device research [6]. Similarly, there have been certain research results in China. In reference [7] several microgrid models, such as photovoltaic cell model, simplified fuel cell model and diesel generator model are built with MATLAB/Simulink sortware. With these models and wind turbine model in MATLAB, a simple microgrid simulation platform is built. In reference [8] the digital-analog hybrid simulation experimental platform for the operation and control testing of microgrid is discussed. It consist of real-time digital simulator (RTDS) system, control system for DG and the integrated monitoring and control system of microgrid. It can verify the coordinated control strategies and energy management algorithms for the microgrid. However, the real-time simulation platform for the microgrid cluster has not yet been reported.

RTDS simulation system is a combination of advanced computer hardware and a large amount of computer software [9], [10]. This paper builds up microgrid cluster simulation platform, which includes typical models of distributed power sources such as photovoltaics, fans, gas turbines and fuel cell. Then, the operation status of grid-tied and islanding are simulated according to the typical structural characteristics of microgrid cluster. The results are carried out to verify the validity and applicability of the model establishment, confirm the rationality of model parameters, control methods and source-charge matching. It lays the experimental platform foundation for the study of transient safety stability control and protection technology of microgrid cluster.

II. OVERVIEW OF THE MICROGRID CLUSTER

DEMONSTRATION PROJECT

The microgrid cluster demonstration project consists of three sub-microgrids and one public distribution system. Sub-microgrid 1 is a wind and light storage system, sub-microgrid 2 and sub-microgrid 3 are optical storage systems, and the public distribution system is 10/0.4kV.Its structure is shown in Figure 1. The photovoltaic capacity of sub-microgrid 1 is 58kW, and the wind power capacity is 60 kW. The photovoltaic power supply in sub-microgrid 2 is divided into two parts, the capacity is 31.5kW and 28kW respectively. The photovoltaic power supply in sub-

This work was supported by Science and Technology Project of China Southern Power Grid under grant GXKJXM20160317.

microgrid 3 is also divided into two parts, the capacity is 31.5kW and 28kW respectively. The microgrid cluster, photovoltaic and wind power supplies are connected to the low-voltage distribution system. Diesel generator access on the cluster bus with the capacity of 30kW. If the microgrid cluster presents power supply characteristics, the surplus power is reversed to the public distribution system.

Distribution Network(Qin Tang)

35 kV Substation

10 kV Feeder

Breaker

Main Transformer(315kVA, 10/0.4kV)

Bus 2

Bus 1Multiple Microgrids

Gas Turbine

Cable

Cable

Sub-microgrid 1

Sub-microgrid 2

Sub-microgrid 3

Cable

Teaching Building

LaboratoryBuilding PV SVGFan

Fuel Cell

Fuel Cell

PV

PVFuel Cell

TeacherDormitory

TeacherDormitory

FemaleDormitory

MaleDormitory

Lighting

Light-ing

Can-teen

Comm-issary

Infir-mary

Bus

Bus

Bus

Fig.1 Organization chart of microgrid cluster demonstration project

The basic information of components in the microgrid cluster is shown respectively in Table1, Table2 and Table3.

Tab.1 Configuration of key components of sub-microgrid 1

Type Name Capacity

Load Teaching Building and Laboratory Building

Power supply Load: 33kW Interruptible Load: 161kW

Normal Load : 79kW Maximum load : 130kW

Power Photovoltaic 58kWh

Fuel Cell 81kwh+20kwh Fan 30kW+3*10kW

Others SVG 380V/45A/30kVar


Type Name Capacity

Load Teacher dormitory，Female

dormitory and Lighting

Power supply Load:3.3kW Interruptible Load: 182kW Normal Load :75.74 kW Maximum Load :108.2 kW

Power Photovoltaic 31.5kWp+28kWp。

Fuel Cell 97kwh


Type Name Capacity

Load

Teacher dormitory, Male dormitory, Lighting,

Commissary, Infirmary and Canteen

Power supply Load:3.7kW Interruptible Load:108kW Normal Load : 55.44 kW Maximum Load : 79 kW

Power Photovoltaic 31.5kWp+42kWp

Fuel Cell 121kwh

A. Microgrid Cluster Model based on RTDS

Building up a microgrid cluster model covering photovoltaic, fan, gas turbine, fuel cell, etc. As shown in Figure 2, the model is divided into two sub-systems. Sub-system 1 includes sub-microgrid 2 , sub-microgrid 3 and the public distribution system, covering main transformer, gas turbine, two sets of photovoltaic and fuel cell, local load and corresponding connected lines, etc. The photovoltaic and fuel cell are grouped and connected to the distribution system.

Each micro-source is equipped with a switch, which can be connected or disconnected with the bus. Each group of photovoltaic and fuel cell are also connected with a switch, which can be connected or disconnected with the whole system. Sub-system 2 is sub-microgrid 1, includes photovoltaic, fuel cell, fan, local load and corresponding connected lines. Each micro-source is also equipped with a switch, which can be connected or disconnected with the whole system.

(a) Sub-system 1

(b) Sub-system 2

Fig.2 Microgrid cluster model

B. Double-fed Fan

Sub-microgrid 1 contains a set of doubly-fed fan. The model structure is shown in Figure 3. The doubly-fed fan consists of asynchronous generator and voltage source converter [11]. In the model, the primary energy side uses a controllable current source, the output side is divided by two identical DC capacitors. Filtering is required after inverter. The filter circuit uses L-RLC filter to eliminate harmonics. After filtering, it connects with the grid through grid-connected transformer.

Asynchronous Generator

Grid-Connected Transformer

Bus

Rotor side Converter

Grid-side Converter

(a) Structure

(b) Simulation model

Fig.3 The structure and simulation model of Double-fed fan

C. Photovoltaic Power

Each sub-microgrid contains photovoltaic power, which has the same structure. The model structure is shown in Figure 4. The DC output from photovoltaic array is converted to AC by inverter. In order to filter out harmonics and improve power quality, filter device is needed. The filter device uses the L-RLC filter with good overall performance, which can eliminate unnecessary harmonics and avoid output voltage distortion by setting filter parameters rationally. In order to make the photovoltaic output connected with the grid successfully, it’s necessary to convert the voltage level through grid-connected transformer. In the case of maximum power point tracking control, photovoltaic power can make full use of solar energy. And the solar cells always output the maximum power.

(a) Structure


Fig.4 The structure and simulation model of Photovoltaic power

D. Gas Turbine

Sub-microgrid 1 contains a gas turbine. The model structure is shown in Figure 5. Gas turbine drives the impeller to rotate at high speed with continuous flowing gas. The motor can convert rotational kinetic energy into electrical energy, and then connect with the grid via a transformer [12].

(a) Structure


Fig. 5 The structure and simulation model of Gas turbine

E. Fuel Cell

Each sub-microgrid contains fuel cell, which has the same structure. The model structure is shown in Figure 6. The fuel cell in the model is replaced by a constant voltage source. The DC output is converted to AC by inverter. In order to eliminate unnecessary harmonics, avoid output voltage distortion and improve power quality, filter device is needed. Similar to the photovoltaic power, the filter device uses the L-RLC filter with good overall performance. After filtering, the voltage level can be converted by the transformer. Then fuel cell can connect with the grid successfully.

The microgrid cluster contain a large number of new energy generation devices, whose output is random and fluctuating. Fuel cell has the advantages of rapid response

and flexible regulation, which make it suitable for microgrid cluster. On one hand, it can be used to change or mitigate the randomness and volatility of DG, and maximize the use of new energy. On the other hand, it can improve the power quality, the safety and stability of the power grid.

(a) Structure


Fig.6 The structure and simulation model of Fuel cell

III. SIMULATION ANALYSIS OF MICROGRID CLUSTER

OPERATION

A. Grid-tied Operation Simulation

Simulate the operation status of microgrid cluster model built above. The simulation parameters are shown in Table 1, Table 2, and Table 3. Collecting the voltage and current waveforms of each distributed power source. In the case of grid-tied operation, the three-phase voltage and current waveforms of the Public Connection Point (PCC) are shown in Figure 7. The maximum voltage is 8.0 kV and the maximum current is 0.42 kA.

(a) Grid-tied voltage waveform of PCC

(b) Grid- tied current waveform of PCC

Fig.7 Grid-tied waveform of PCC

The voltage and current waveforms are well. Assume that the three-phase short-circuit fault occurs on the cable before sub-system 2, and the fault time is 0.3 seconds. The three-

phase voltage and current waveforms at the fault point are simulated. It’s shown in Figure 8.

(a) Grid- tied voltage waveform of the fault point

(b) Grid-tied current waveform of the fault point

Fig.8 Grid-tied waveform of the fault point

In the case of grid-tied operation, once a fault occurs, the voltage drops to zero basically. The current increases suddenly, fluctuates and oscillates strongly. After the fault is removed, the voltage value oscillates and returns to the normal operating state slowly. The current value also returns to the normal operating value. And the system does not destabilize.

B. Islanding Operation Simulation

In the case of islanding operation, the three-phase voltage and current waveforms of the PCC are shown in Figure 9. The maximum voltage is 3.6 kV and the maximum current is 0 kA.

(a) Islanding voltage waveform of PCC

(b) Islanding current waveform of PCC

Fig.9 Islanding waveform of PCC

Although lacking the support of large power grid, it can be seen that the voltage and current waveforms are well, the three-phase symmetry and power quality can still maintain a well level. Assume that the three-phase short-circuit fault occurs on the cable before sub-system 2, and the fault time is 0.3 seconds. The three-phase voltage and current waveforms at the fault point are simulated. It is shown in Figure10.

(a) Islanding voltage waveform of the fault point

(b) Islanding current waveform of the fault point

Fig.10 Islanding waveform of the fault point

Once a fault occurs, the voltage drops to zero. The current increases suddenly, fluctuates and oscillates strongly. Overshoot is obvious and severe distortion occurs. After the fault is removed, the voltage value rises slowly and returns to the normal operating state gradually. The current value also returns to the normal operating value, and the system does not destabilize. Compared with the grid-tied operation status, the islanding operation status costs longer recovery time.

IV. CONCLUSION

In order to achieve simulation modeling and operation analysis of microgrid cluster, this paper builds up a microgrid cluster model, which consist of two sub-systems based on the demonstration project of Sanli Yizhong. The model includes photovoltaic, fan, gas turbine, fuel cell, etc. Then the operation status of grid-tied and islanding are simulated. Setting fault condition and collecting information about the fault point to verify the effectiveness and applicability of the constructed model. It lays the experimental platform foundation for the study of transient safety stability control and protection technology of microgrid cluster.

ACKNOWLEDGMENT

This work was carried out by the teacher, Dr. Huang. We gratefully acknowledge his invaluable cooperation in preparing this application note. Thanks are due to my workmates, who offered me the confidence and dicuss with me about my paper.

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