Key Technologies and Requirements for Connecting Micro-grid to Power System Hao Mukai Renewable Energy Department China Electric Power Research Institute 2014-9-25
Outline 1 Overview 2 Key Technologies 3 Pilot Projects 4 Requirements and Codes
Overview Motivation To reduce the impact on power grid of DG integration and promote the significant potential of smaller DG to meet customers and utilities needs, DGs can be best captured by organizing these resources into Microgrids. Definition DG MG A microgrid is an integrated energy system consisting of interconnected loads and distributed energy resources, as an integrated system which can operate in parallel with the grid or in an intentional island mode.
Characteristics of microgrid Small scale Microgrid is a cluster of distributed generation, storage systems, and loads. The capacity of microgrid is normally smaller than tens of MW. Environmentally friendly Self-government Two operation modes The micro-sources in microgrid are mainly based on clean and renewable energy, thus environment pollution and expensive fuel costs can be reduced. Relying on advanced power electronics, control strategies and communication technologies, microgrid is integrated as a single self-controlled entity by coordinated control between microsources, storage systems and loads. Flexible operation modes and smoothing transition between grid-connected mode and island mode enhance system reliability for critical local loads.
Benefits of microgrid Helping to manage the intermittency of renewables,thus the impact of DG integration on power grid is reduced. Stand-alone and grid-connected operations of microgrid help in generation augmentation, improving overall power quality and reliability. Providing energy supply for remote communities isolated from large power systems. Reducing transmission congestion and power transmission cost. Achieving optimization management of energy. 5
Outline 1 Overview 2 Key Technologies 3 Pilot Projects 4 Requirements and Codes
Key Technologies of Microgrid Microgrid operating in grid-connected mode To reduce the impact of DG integration,in grid-connected mode, the output power of micro-sources, energy storage systems and loads in microgrid should be coordinately controlled to make the power flow at microgrid interconnection point can follow the dispatching signals. Interconnection point power PV output power Storage system output power 7
Transition between grid-connected mode and islanded mode To provide uninterruptible power for important customers in microgrid, smoothing transition between gridconnected/islanded modes must be ensured. Transition from grid-connected mode to islanded mode Fast detection of main grid status Seamless transition control from grid-connected mode to island mode Synchronization control in reconnection of microgrid back to the main grid Transition from islanded mode to gridconnected mode 8
Microgrid operating in islanded mode In islanded mode, without the support of main grid, control strategy should be adopted to maintain system frequency and voltage in acceptable range. Control strategies of microgrid in islanded mode are as follows: Centralized control. The central control system provides commands to the entire system, which is a effective master-slave configuration between the central system and distributed devices.(the master unit is controlled in V/F mode and the slave ones are controlled in P/Q mode.) Distributed control. This strategy is accomplished with independent controllers communicating with one another. It uses intelligent devices that are strategically located to detect the conditions and initiate the required actions. Autonomous control. This strategy is accomplished with independent controllers without communication with other devices. (Droop control method is always adopted in this control strategy.) 9
Outline 1 Overview 2 Key Technologies 3 Pilot Projects 4 Requirements and Codes
Pilot Microgrid Projects Urban grid connected microgridzhangbei microgrid experimental platform (Established by CEPRI ) Voltage level: 380V Capacity: 140kW PV, 20kW wind turbine, 100kW/400kWh Lithium battery Load: 100kVA for office building of Zhangbei test site Research activities: Automatic seamless transition between grid-connected and islanded mode; EMS based on renewable power forecasting development; Monitoring system development. 11
Technology characteristics of CEPRI Micro-grid Realization of seamless transition between interconnected and islanded mode in China for the first time without interrupting power supply and transient transition within 20ms; Wind-PV-Storage configuration, without diesel generators or micro gas turbine, reflecting the energy saving and environmental benefits of microgrid; Development of micro-grid Energy Management System (EMS) based on power forecasting system, realizing variable operation modes in which multi-objective optimization control and power control at interconnection point ; Development of micro-grid smart monitoring system and realizing active/reactive power control of energy storage system, PV system and so on with the response time of less than 0.5s.
Off-grid microgrid Overview Hydro/PV/Wind/Battery Micro-grid in Cuoqin, Tibet (Designed by CEPRI ) Cuoqin county located in the southeast of Ngari Prefecture, Tibet, China, about 977 km away from Lhasa. The county's terrain is mainly hilly plateau with an average altitude of 4700m. Population:14 thousands. Far away from Lhasa power grid. Nepal Cuoqin Tibet Lhasa 13
Original power supply system of Cuoqin county Originally, in Cuoqin county, power is mainly supplied by PV system and diesel generators. PV:80kW Diesel generator :980kW (500kW+360kW+120kW) Hydro power :3 350kW (under construction) Load:520 ~670kW(2012 ~2015) Original power supply system 14
Original power supply system of Cuoqin county Existing Problems: The diesel generators run with low efficiency and reduced capacity caused by high atmospheric pressure The generation cost of diesel generator is too high (0.65$/kWh) The hydro power station under construction can not operate normally in winter due to freezing and the capacity of PV is too small to support the local load in winter. Parts of PV module and battery have been damaged and can't work normally. Rapid load growth can not be meet. 15
New micro-grid configuration of Cuoqin county The power stations being interconnected through ac bus is beneficial for system capacity augment. 16
Hydro power Capacity: 3 350kW Annual utilization hours: 3271 h PV Capacity of module:440kw Capacity of inverter:500kw Wind power Capacity: 2 30kW Diesel Capacity: 300kW Control:changed to PQ control Battery system Energy-type storage system Storage type: lead-acid battery; Capacity:2400kWh; Capacity of PCS:500kVA Power-type storage system Storage type:lithium iron phosphate battery; Capacity:300kWh; Capacity of PCS:500kVA 17
Operation of the new off-grid microgrid Through piecewise droop control method hydropower generators and energy storage system establish system voltage and frequency in microgrid. Energy storage system participates in regulate the system frequency and keeps system power balance together with hydropower generators. Energy storage system mainly damps fast and deep components of power fluctuation, which can cause the system frequency severe changes, and hydropower generators damp slowly and smoothly components. PV and diesel generators operate under PQ mode, smoothly adjust their output power according to the instructions of dispatching system. The output power of PV can be limited when the system energy is surplus, and the output power of wind power can only be controlled by starting and cutting wind turbines. 18
Key technology Droop control can make inverter-type DGs and traditional generators operate in parallel, sharing system power together. 19
Control goal of first frequency adjust: reach power sharing proportional to droop gains during load fluctuation. fi_ref f0 mp i i Ei_ref E0 nq i i f f f A_ref B_ref m A PA m B PB P P A B m m B A f 0 f m A m B B A P B P A schematic diagram Pmax P Experimental waves of first frequency adjust 20
Control goal of second frequency adjust: When the frequency deviation of microgrid goes beyond the permitted value, adjust the no-load frequency of droop characteristics to ensure power quality of the system. f f max f A f 1 ' f A f 2 f min ' A A P A P 1 P 2 B ' B P B schematic diagram f f max f 1 ' f B f 2 f f B min P Experimental waves of second frequency adjust 21
Control goal of third frequency adjust: Through droop gain analysis and f max f 1 f selection, the droop gains in f 2 primary control are determined. Change droop gains can change power sharing in primary control. f min P 1 ' P 2 P 2 P1_max P2_max Experimental waves of third frequency adjust 22
Outline 1 Overview 2 Key Technologies 3 Pilot Projects 4 Requirements and Codes
Relevant standards National standard: Technical requirements for connecting Microgrid to power system Design code for connecting Microgrid to power system Design code for Microgrid engineering Commissioning and acceptance code for connecting Microgrid to distribution network Operation control code for connecting Mirogrid to distribution network Test code for connecting Microgrid to distribution network Industrial standard: Test procedure for connecting Microgrid to power system 24
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