AC MICRO GRID ARCHITECTURE AND REAL WORLD TESTBED

AUTHOR: SNEHA CHERUVATTATH

Abstract:

Despite being slightly complex than DC, AC micro grids are still preferred for their benefits like easier arc interruption. However, it is necessary to ensure that while constructing a micro grid, all its major elements like storage, communication, interface are all aptly selected. Also there are certain principles like principle of hierarchy, maximum utilization of resources, power quality and partition are maintained. With the current research and progress in technology for AC micro grids, there is a good chance that such micro grids can be utilized to their maximum capabilities.

Introduction:

A micro grid is an energy system which is like a smaller version of the main grid. It consists of distributed energy sources and loads capable of operating with, or independent from, the main grid. The primary purpose is to ensure affordable energy for urban and rural communities, including loads like commercial, industrial, and federal government. They are a key factor in improving power quality, decreasing emissions, and reducing congestion, avoiding power losses, improved efficiency and overall system cost. They can be established even in remote areas for supplying electricity. The ability of micro grids to form intentional islands during disturbances improve reliability and resilience. Micro grid can be classified based on the types of loads (Campus, military, residential, commercial and industrial), size (small scale, medium scale or large scale), application (premium power, resilience and loss reduction), and connectivity (remote and grid connected) and based on voltages and current (AC, DC or hybrid).

Fig 1: Micro grid layout

DC vs AC:

DC micro grids are used in electric vehicles and telecommunications. Despite its attractiveness, DC micro grids have limitations like high expense of high voltage Dc-DC converters, use of large DC capacitors which are bulky and unreliable etc. In AC micro grids the DERS and loads will be connected via a common AC bus with the generating units and storage connected via DC to AC inverters and using AC to DC rectifiers in case of supplying a DC load. Open series faults are observed in devices like circuit breakers and fuses. Generally, arc interruption of arc is simpler than dc arc. This is because the maximum time the ac current sustains itself is only half a cycle and the waveform reaches a zero current crossing where the arc ceases. Conversely, while interrupting dc arc the circuit breakers should force the arc to become unstable. Also AC micro grids have better transient and dynamic response.

Fig 2(a): AC Micro Grid (b) DC Micro Grid

Current Progress:

The current research in AC micro grids includes converter based grid interface like the BtB (back to back converter). Since more number of micro grids are being integrated to the grid it is necessary to use an interface which reduces complexity and at the same time is flexible and reliable. Hence, BtB converter is a good option. They can provide independent active and reactive power control, isolation between two AC Sides, enhancing power quality, link between asynchronous micro grids etc. The basic operation is that if micro grid generates less power, then extra power is obtained via the micro grid side converter and battery is charged through DC –DC converter. If the micro grid is in islanding mode, battery will be the slack terminal and compensate for any power imbalance. The battery also supplies all the loads if under extreme conditions, all the generating units are lost.

Another promising area of focus is a centralized controller to stabilize islanded micro grids. Centralized controllers are used to optimize the operation of micro grids and to stabilize dynamics caused by loads. One research proposes the use of such controllers for grid connected and islanded mode. A predictive algorithm is used that breaks down any problem into steady state and transient which overall computation.

Another important progress in AC micro grids is high frequency AC micro grid. This is formed by a high frequency AC converter. This has several advantages like compactness, good reliability, and prolonged lifetime.

Along with these, there have been proposals to improve the energy storage system. A great way to improve on this is by using hybrid energy system components say batteries and ultra-capacitors. The main advantage is that such a system provides independent control and improves flexibility.

For improving over the current micro grid systems, the hybrid micro grid is being proposed. In this system, AC and DC buses are connected to the AC and DC portion of the network which forms a hybrid. It contains the merits of both the AC and DC micro grid. It can decrease the AC/DC or DC/AC conversions and the number of the power electronic devices. Some loads are powered by AC whereas some by DC. The hybrid network reduces frequency conversion and decreases the cost. Since in this way, many conversion devices and transformation aspects are reduced to a much simpler system and flexible control which overcomes losses and improves reliability of the power system.

Fig 3: Hybrid Micro Grid

Main components of AC Micro grids:

1. Location: The most underestimated component of any micro grid is the location. Before setting up any micro grid it is necessary to perform a feasibility study of the location including parameters like temperature, available resources etc.

2. Generation system: Currently there is a major push towards using more renewable energy and distributed energy resources (DER). DERs are gaining more popularity as they can be set up in various locations even with a complete lack of grid like rural areas. DERs generally include solar, wind, hydro, micro turbines, fuel cells etc.

3. Storage: Any micro grid is incomplete without a storage system. This is again subject to parameters like size and location of the micro grid. Batteries are usually the most frequently used due to its high power density and its reliability. But as mentioned before, now there are newer systems like superconducting magnetic energy storage, supercapacitors etc.

4. Control system: The control and supervision of the system is managed by a controller. When the grid is still connected the controller needs to provide power to the grid. Here, the main grid signal is used as a reference. Hence, system is in stiff synchronization in current control mode. Whereas when the grid is in intentional islanding mode, controller supplies constant voltage to the load. But since the grid is no longer connected to the micro grid, a new reference signal is needed. The new reference signal is formulated using a phase locked loop and a proportional integral (PI) controller. To convert from current control to voltage control, the system needs to be in intentional islanding which is achieved by designing an islanding detection algorithm. After islanding grid and micro grid are reconnected where a resynchronization algorithm is established to obtain synchronization. Here, the distributed

generation voltage is forced to track the grid voltage. When this is done, the reconnection takes place and the controller goes back to the current control mode.

Fig 4: Micro Grid Controller5. Protection of AC Micro grids: Considering protection of AC micro grids, in grid connected

mode, generally differential protection is adopted. This is mainly used for the bus protection with circuit breakers as a backup protection. This protection system needs coordination between various relays and automatic devices. It also provides circuit breaker failure protection. While charging if there is an over current or over voltage stage there is an instantaneous trip current protection. Whenever a fault occurs on the distributed generation line, the line is switched off and disconnected from the micro grid. Before re-closing it is necessary to regain synchronization between the micro grid and the distribute generation unit. If it becomes necessary to maintain the power quality for an important load, an intentional island is formed. The protection system here is similar to that of the grid connected operation, but the protection parameters values needs to be reset depending on the capacity of the micro grid which is smaller than a main grid. When a fault occurs in the power system, it becomes necessary to separate the grid and the micro grid. However it is necessary to keep a check as the uncertainty of a non-intentional island may lead to an eventual voltage and frequency collapse. By removing the unimportant loads, the non-intentional island can be made intentional and any damage to the system can be avoided.

6. Communication system: Any efficient power system requires more than just a controller system. It further requires an effective communication system. This includes satellite, wireless, optical fibre etc. Any good communication system helps improve flexibility, controllability and makes it easier to install new circuits. Nowadays communication system also use renewable energy. For example in the case of satellite communication, the satellites can be powered through PV modules on it. It has backup batteries for when the sun is blocked by Earth. Batteries can be charged by excess current when sunlight is available.

7. Integration: Interconnection of a micro grid to the utility can be through one of three ways. Firstly, through circuit breakers which is simple and inexpensive. But it is also slow and requires electrical characteristics to be same in both main grid and micro grid. Secondly, static switches can be used. They are costly and more complex. They may include a bypass switch can be used for maintenance reasons. Power flow can’t be controlled through this but

there will be conduction losses. Finally, the most flexible option is using a power electronic interface. Both real and reactive power flow can be maintained. Its response is determined by controller, topology and internal energy storage.

Micro grid for various applications:

City Building: For such cases, grid takes the forefront and renewable energy use is low, keeping in mind users convenience and the technical limitations. However it can be included by say, placing PV panels on the building roof. Critical loads like hospitals have a constant back up supply like diesel generator. So in general, a micro grid with a distributed energy resources is a smart choice.

Small town or village: Normally towns and cities are further away from a main grid and face frequent problems like power outages. The demand is also pretty low. There is a great potential here for including more renewable energy to provide power. Also more standalone systems can be implemented in these places. Ex: PV/hydro micro grid

Factory away from utility grid: Here, there is not enough power already available but unlike the previous case the demand is very high. Remote factories have enough place and incentives to invest in renewable energy. The micro grid can be set up to operate in grid connected mode with the ability to operate in standalone mode when there is a fault.

City Household: Since a city house has a fixed area, the requirement of renewable energy or distributed resources is less. Setting up a renewable source like PV with an energy storage helps reduce the money consumers pay. It also helps reduce peak demand hours.

Rural Farm: These generally tend to be isolated. So a stand-alone system can be implemented with a DER along with a strong, constant backup storage system.

Principles of structure Design:

Every micro grid application, operation and controls are different. It is important to ensure that the grid is secure and reliable. It is also pivotal to include as much as renewable energy as possible. The following are the structural principles to be kept in mind.

a) Principle of partition: It is necessary manage power flow in the transmission lines. It depends on the load. If the load is small a distributed resource can be set up to meet the demand. If he load is heavier like factories, a point of common coupling should be set up in the vicinity. And if a smaller micro grid is contained with a larger system, it should be formed by grouping DC supply and loads together and similarly with AC.

b) Principle of hierarchy: This is basically the selection of the voltage levels within the micro grid. It increases transmission capacity, reduces losses and voltage drop.

c) Full use of resources: It basically advocates using a single renewable resource to the maximum. For this, the availability of said resources needs to be investigated in the early stages of the planning itself.

d) Power quality assurance principle: This works based on two important factors. The first one is storage allocation. Energy storage is mainly for stabilizing the load. When power generation is greater than the requirement the excess energy can be stored and used for peak demand times which ensures that there is an uninterruptible supply. The second important factor is reactive power compensation. Due to inductive loads and power electronic devices there are a lot of harmonics created. So, static or dynamic power compensation devices are required.

Popular topology in micro grids:

Multi-ring structure: It consists of an outer ring which is connected to larger grid via transformer. The inner loop is multiple smaller grids. Many distributed power and loads are connected to the inner loop which can create a regional grid. The benefit is that if the main line fails the inner micro grids can operate in stand-alone mode. And it is easier to set up new systems by adding an inverter to the local network.

Single ring structure: It is useful for dispersed loads. Two users with same voltage specs can be connected with a bus. When one user undergoes power failure, the other will still have electricity supply.

Complementary ring structure: Here, medium voltage DC bus can be connected with AC network through a converter. And low voltage AC micro grid is connected to the medium voltage bus through step up/down transformer. Fault handling and reliability are highly improved.

Cascade structure: Used to improve point of load regulation, reduce cost and improve efficiency. They have at least two conversion stages with three or more voltage levels.

Simulation:

For the simulation, a test bed from a reference paper has been used. A 65 kW micro turbine and 21 units of 1.5 kW PV arrays are used. Capacitor banks have been on the low voltage side of the transformer rated at 11.4kV/380V. Single phase loads like resistors, inductors and motors. Three phase loads used are air conditioners. This has been simulated using Matlab/Simulink.

Fig 5: Simulation

Fig 5 : Load Voltage (as obtained in the reference paper)

Fig 6 : Load Current (as obtained in the reference paper)

Fig 7 : Load Voltage and Load Current (obtained after performing simulation)

Technical Challenges of AC micro grids:

AC micro grids undergo higher power losses than DC. This is primarily because, it involves more conversion systems.

It has a lower efficiency than DC systems. Integration for AC micro grids is more complex as it requires synchronization and re-

synchronization. It requires frequency and phase control. More expensive compared to DC

Conclusion:

Presently, most systems still use AC power. So, considering the benefits of AC micro grids as mentioned before it is useful. But these systems can be definitely improved upon by say making it a hybrid micro grid of both AC and DC, which can harness the benefits of both AC and DC micro grids. These coupled with advances in micro grid technology like controllers and converters will create an efficient micro grid.

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