Adaptive and integrated protection of the microgrids with overcurrent relays

Many individuals or institutions produce their own electricity, after renewable energy sources become more efficient and more accessible. Moreover, transmitting electrical energy produced by central high-powered plants to rural areas that are far away from city centers is considerably costly operation. These basic reasons lead to microgrid concept. The objective of microgrids is producing electricity and transmitting it to surrounding loads, similar to the large networks. However, microgrids are smaller than these networks, in addition to; they contain many distributed energy resources. These distributed energy resources are generally low-power sources, such as solar panels, wind turbines, biogas plants, and various energy storage units. Microgrids can operate with only these kinds of low-power distributed energy resources called as microgenerators and various types of loads or they can operate while connected to a main network. Microgrids transfer the excess energy to the main network during grid-connected mode. If the energy produced by local sources is insufficient for local loads, microgrids take energy form the main network. The connection between microgrid and main network can be interrupted on purpose or under emergency. Thus, microgrid operates in island mode. If a microgrid is operated by proper control and protection systems, then operating in island mode cause no issues. Otherwise, the system will not operate in the correct way, major changes will occur in frequency and voltage values. Power flow in microgrids can alternate greatly because microgrids generally contain more than one source, operate with renewable energy sources produces quite variable power and have two different operation modes that are grid-connected and island mode. Therefore, control and protection of the microgrids are more difficult than the traditional networks that have radial structure. Traditional directional overcurrent protection is a widespread method for protection and it is generally sufficient for protecting large networks. Relays that are used in this protection method determine faults by analyzing the amplitude of current. Direction of the current aids the relay while it determines whether the fault occur in the protection zone or not. If inverse time overcurrent relays are used as the sample microgrid system, operation time of the relays can be set to change depending on the amplitude of the current. Coordination time intervals are used between the consecutive relays for the coordination of inverse time overcurrent relays. IS and TMS values are set properly to determine the minimum limit value for sensing the fault and to apply coordination time intervals. Set values of this type of relays that have no communication line between each other or a central control unit can only be changed manually, this operation can not occur automatically. Therefore, this traditional protection method can not be applied on the networks that have frequently alternating load flow. For frequently alternating load flow condition of the network, a protection system that can adapt itself to this condition should be applied. This protection system is called as adaptive protection system. Adaptive protection system tracks the current values and circuit breaker (CB) status of the network. If a major change is determined by the protection system, then the system calculates load flow again and changes the set values of the relays depending on the load flow. To apply adaptive protection system on a network, relays should communicate with a central unit. Otherwise, new set values calculated after changes can not be transferred to the relays. Moreover, communication should be fast because the fault can occur at any moment. After establishing a fast and wide communication network, applying integrated protection will be faster and easier than conventional protection methods. Integrated protection system is a relatively new type of protection system that all CBs connected to same bus are controlled by an integrated relay and all integrated relays in the protection system communicate with a central unit while operating. If integrated relays communicate each other, then the system will operate faster. In this way, integrated relays compares the measurements taken from their own buses and the measurements of the other integrated relays to detect the fault and location of the fault in a short time. Additional set value is not needed for the relay coordination for this system. When all these characteristics are taken into consideration, an adaptive-integrated protection system will be more successful than a traditional protection system consisted by directional overcurrent relays in terms of selectivity and reliability. On the other hand, it is obvious that implementation of a protection system connected to central unit with a communication feature is more difficult than implementation of a traditional protection system consisted by directional overcurrent relays without communication. Therefore, it should be revealed that to prefer adaptive-integrated protection system is more proper way to protect microgrids. To compare these two protection systems, a microgrid is modeled on the MATLAB environment using Simulink tools and both of the protection systems are applied on the microgrid. Sample microgrid network operates in grid-connected mode normally; it can also operate in island mode. To constitute a realistic example, the sample microgrid contains renewable energy resources such as solar power plant, wind turbine and biogas plant and loads that have various amount of power consumption. Normal operation condition of the sample microgrid is determined as operating at maximum load, then set values of the protection systems are calculated according to this condition. Whereas the set values remain stable for all conditions for the traditional protection system, current pick up values (IS) of the integrated relays by means of a predetermined algorithm for adaptive-integrated protection system. Four different cases are determined for the comparison of the protection systems. These cases are normal operation, switching of certain loads, switching of one of the local sources, island mode operation. For all these conditions, 3 phase, phase to phase, phase to phase ground and single phase to ground faults are performed on the Line 1. The coordination of the traditional protection with directional overcurrent relays are based on the primary protection and back-up protection concepts. Whenever a line fault is detected, CBs that located on the both end of the line are expected to trip. If primary protection can not clear the fault, then back-up protection will operate. There should be a certain time interval between primary and back-up protection that is determined depending on the properties of the network equipments to prevent CBs from faulty tripping. This time interval is determined 300 - 350 ms for the sample network. Line protection is based on the comparison of the current directions for the adaptive-integrated protection system. In this case, the integrated overcurrent relay (IOR) that detects the fault communicates with neighbor IORs for direction comparison. If two IORs located on the both end of a line detect current flowing through opposite directions, then the location of the fault will be confirmed. Both of the protection systems are operated correctly during first two cases, normal operation and switching of certain loads as expected. Main reason of the correct operation of traditional protection with directional overcurrent relays (DOR) despite the changes in the system is the limited effect of load switching on the fault current. However, traditional protection that consists of DORs fails in terms of selectivity and reliability during last two cases whereas adaptive-integrated protection system manage to operate successfully in both cases. As a result, this study presents that adaptive-integrated protection system is more successful than traditional protection system with directional overcurrent relays (DOR) for microgrids.