Download 2. Proposed Control Strategy

Control Method of Islanded Microgrid to Improve Reactive Power Sharing
and Power Quality*
Huang Jing1, Liu Xiaohua1, Li Hongping2
1Foshan
Power Supply Bureau,Guangdong Power Grid Co., Ltd. Foshan, 528000;
2Sichuan University, Chengdu;
*Fund Project: Scientific and Technological Project of China Souther Grid Corp（GD2014-2-0230）
Abstract:
Due to the influence of feeder impedance, reactive power supplied by distributed generation (DG) based on
traditional droop control could not be shared according to their droop gains. This paper proposes a control strategy
which is based on virtual impedance, frequency/voltage and reactive power regulation for islanded microgrid. Virtual
impedance is added to existing control loops to improve the characteristics of output impedance, weaken the power
coupled caused by resistive component. Secondary regulation on the voltage and frequency deviations from droop
control, improves the quality of voltage and frequency. Secondary reactive power regulation directly controls the
distribution of reactive power. Reactive power sharing can be obtained independently from inverter output voltage,
and it can limit the reactive power sharing errors. A microgrid simulation model is built in Matlab/Simulink
simulation platform, and simulation analyses verify the validity and feasibility of the proposed strategy.
1
Microgrid and Traditional Droop Control
In recent years, in order to solve the problem of energy crisis and environmental pollution, a variety of
renewable energy and distributed generation has been widely concerned. Microgrid can be defined as a cluster of
elements such as energy storage devices, energy conversion devices, protection devices, loads and distributed
generations. Microgrid allows the DG units work in stand-alone mode and grid-connected mode. Due to the
difference of feeder impedance, reactive power supplied by distributed generation based on traditional droop
control could not be shared according to their droop gains and cause circulation current. When loads change
large, traditional droop control could make the grid voltage and frequency deviate from its ratings, affecting the
power quality of microgrid. To design a control strategy, which make the reactive power distribution is not
affected by line impedance mismatch, is very necessary.
2.
2.1
Proposed Control Strategy
Virtual Impedance
Traditional droop control is proposed when inverter output impedance and line impedance present
inductance. DG units always work in low voltage microgrid, and the line impedance present resistance, so
traditional droop control method will no longer apply. By adding virtual impedance can make the inverter output
impedance present inductance, reduce inverter output active and reactive power coupling, make droop control
method continue to be used in low voltage microgrid.
2.2
Secondary Control Strategy
In order to guarantee the quality of voltage and high precision of reactive power distribution,
voltage/frequency and reactive power secondary control is proposed. It can make the frequency and voltage
maintain rated value, and reactive power is shared according to their load capacity. When the load reactive power
increase (or decrease) lead to voltage amplitude decrease (or increase), through the secondary voltage regulation
control, hanging up (or down) droop control curve to restore the voltage to the rating voltage; When load active
power increase (or decrease) lead to frequency decrease (or increase), through the secondary frequency
regulation, hanging up (or down) frequency control curve to restore frequency to the rating. Secondary reactive
power control adjust the allocation of reactive power directly, reactive power distribution is not affected by the
voltage of DG unit. It can eliminate reactive power share error caused by mismatch of line impedance, to achieve
high precision distribution of reactive power.
the secondary voltage control first compare inverter output voltage with its reference and then adjust the
difference value using PI controller; next the result add with the output voltage U of the traditional droop control;
eventually it can receive compensated voltage U which is the input to the voltage and current double-loop
control, and preserves the rated output voltage. By the same token, second frequency and reactive power control
can control frequency and reactive power, to preserve the rated output frequency and reactive power sharing of
high precision.
3. Simulation results
To test and verify the validity and feasibility of the proposed control method, simulation model of two
parallel DG units is built in Matlab/Simulink platform.
3.1
Calculation Case 1
With the traditional droop control, DG can share same active power even if load changes. But due to
the mismatch of line impedance, DG cannot share same reactive power. DG1 share more reactive power
because of smaller line impedance than DG2. DG output frequency and voltage have large deviation from its
reference when load changes. When DG active and reactive power increases, the frequency and voltage are
smaller than their ratings.
DG with proposed control method can share same power of load. What is more, when load changes,
compared with the conventional droop control method, it has smooth transition process. Even if line impendence
is different between each DG, DG still can realize accurate allocation of reactive power that is DG1 and DG2
evenly distributed load reactive power. When load changes large, it can keep DG output frequency maintain the
reference value 50Hz and output voltage maintain the reference value 310 V.
3.2
Calculation Case 2
Because DG units join voltage/frequency and reactive power secondary control based on the traditional
droop control, it can turn into a stable operation state in a short time, continuing to supply power for load, when
a DG exit of microgrid. When a DG malfunction or exit the runtime, the network can continue to run normally.
4. Simulation results
Considering the reactive power allocation accuracy of inverters parallel system using traditional droop
control and the voltage and frequency deviations caused by droop control in microgrid, a balance control strategy
of reactive power and voltage/frequency for inverters parallel operation is proposed based on virtual impedance.
Through the secondary control regulation, it can maintain the voltage and frequency in the rating, and reactive
power distribution also can achieve higher accuracy. The proposed control strategy works without central
controller, avoiding the whole system cannot run normally when a DG unit fails. It turned out that the simulation
results are fairly well matched, thereby confirming the validity and feasibility of the proposed approach.
Keywords:
Islanded microgrid, voltage/frequency regulation, power sharing, droop gains.
Author’s brief introduction and contact information:
1) Author’s Name: Huang Jing
2) Author’s Affiliation: Foshan Power Supply Bureau, Guangdong Power Grid Co., Ltd.
3) Author’s Tel: 13438039532
4) Author’s E-mail: [email protected]
5) Author’s Address: Foshan, Guangdong Province, China.
6) Fund Project: Scientific and Technological Project of China Souther Grid Corp（GD2014-2-0230）