Regarding PMSG-based wind turbine generation system, this paper proposes a supercapacitor energy storage unit (SCESU) which is connected in parallel with the DC-link of the back-to-back converter to enhance its high voltage ride through performance. The analysis of the operation and control for the grid-side converter and SCESU are conducted. Based on real time digital simulators (RTDS), a model and a Hardware-in-the-Loop (HiL) platform of PMSG-based wind turbine with SCESU is developed, and the simulation results show that the SCESU absorbs the imbalanced energy and the grid-side converter absorbs inductive reactive power during the period of voltage swell and verify the correctness and feasibility of the high voltage ride through control strategy.
In the past, wind turbine generation system (WTGS) was allowed to disconnect from the grid during grid voltage sag or swell. However, with the high penetration of wind power in power systems in many countries, grid codes become stricter and require the fault ride-through capacity of WTGS, including high voltage ride through (HVRT) and low voltage ride-through (LVRT) profiles during which WTGS must remain connected and supply the expected reactive power to support the utility grid [
The requirement for HVRT capability of WTGS will be more stringent in many grid codes when the penetration level of wind power in power system is increasing higher and higher. For example, the Australian new national electricity rules require that WTGS must maintain continuous uninterrupted operation for temporary overvoltage for the magnitudes and durations specified as 1.1~1.15pu for 1200s, 1.15~1.2pu for 20s, 1.2~1.25pu for 2s, 1.25~1.3pu for 0.2s, and 1.3~1.4pu for 0.02s. Additional inductive reactive current of 6% of the normal current for each 1% increase at point of common coupling (PCC) voltage above 1.1pu is also required [
The operation time of WTGS (see Table
HVRT requirement of State Grid Corporation of China.
Grid voltage (pu) | Operation time |
---|---|
1.10~1.15 | 2s every time |
1.15~1.20 | 0.2s every time |
>1.20 | Take off |
Existing technologies for implementing HVRT of WTGS can be divided into two categories: using improved control scheme and the use of additional auxiliary device. In [
In [
In this paper, the supercapacitor energy storage unit (SCESU) is selected to improve the HVRT capability of the PMSG-based WTGS, and the coordinated control strategy is presented. It will be shown that SCESU can add the missing HVRT capability to the WTGS in order to become compliant to grid code, even if it is getting stricter. Furthermore, SCESU does not reduce generated power of WTGS and its control strategy is simple.
This paper is organized as follows. Section
The block structure of a grid-tied PMSG-based WTGS with SCESU which is connected in parallel with the DC-link capacitor of the back-to-back converter is shown in Figure
Structure diagram of PMSG-based WTGS with SCESU.
When VOC is employed to control the GRSC, the GRSC voltage, current, and output power can be represented as [
In order to analyse the influence of the voltage swell on the GRSC, the small-signal modelling of the GRSC active power is given as
Considering
By ignoring
When the unit power factor control based on VOC is used in the GRSC,
It can be found from (
The SCESU consists of a super capacitor (SC) and a bidirectional DC/DC converter, as shown in Figure
The SC capacity is calculated based on two typical voltage swells.
In this case, the reactive current reference is -0.78pu (calculated by (
Based on the same calculation process, the capacitance of SC is given in this case as
Because the bidirectional DC/DC converter is a conventional Buck-Boost converter, no further discussion on its design is given here.
In order to realize HVRT control, a set of control logics of PMSG-based WTGS with SCESU are developed based on the finite state machine design method, as shown in Figure
HVRT control scheme.
The detailed block diagram of the proposed control of GRSC is shown in Figure
Control scheme of GRSC.
The specific requirements for reactive power generation as function of AC voltage were not considered in [
The active current reference is set to protect GRSC current to be less than its maximum allowable value
Figure
Control block diagram of SCESU.
The preliminary simulation results obtained in RTDS/RSCAD are presented in [
A detailed simulation model is built in the RTDS/RSCAD software to assess the HVRT performance of a 1.5MW PMSG-based WTGS with SCESU when the grid voltage swells. In the simulated system, the DC-link voltage is 1.22kV; the capacity and voltages ratings of the step-up transformer are 1.6MVA and 0.69/35kV; and the pretrigger time in simulation is 20%. In the simulation tests, is it assumed that the PMSG is running at the rated operation state. The simulation tests are carried out for the two following scenarios.
In this test, the three-phase symmetrical voltage swells to 1.2pu at 0.8s and recovers to 1.0pu at 1.0s, lasting for 200 milliseconds. The corresponding test results are shown in Figure
Response of GRSC with SCESU under voltage swell to 1.2pu.
Voltage and current of PCC
DC-link voltage
Modulation of GRSC
Active and reactive power
Supercapacitor voltage
The grid voltage recovers at 1.0s and the system can operate in the steady state established previously. At 2.2s, the PMSG output is reduced to 0.75pu. In order to compensate for this reduction, the SC is regulated to discharge to release the stored energy and deliver it to the grid until the SC voltage is reduced to 0.3kV. It can be concluded from these results that the proposed scheme with SCESU can provide the system with the enhanced HVRT capability during voltage swell from 0.8s to 1.0s. The excess energy is utilized to compensate for the PMSG output reduction. The HVRT performance in this test meets the second requirement in Table
In this test, the grid voltage rises to 1.15pu at 1.0s and returns to 1.0pu at 3.0s, lasting for two seconds. The test results are depicted in Figure
Response of GRSC with SCESU under voltage swell to 1.15pu.
Voltage and current of PCC
DC-link voltage
Modulation of GRSC
Active and reactive power
Supercapacitor voltage
In order to further verify the feasibility of the proposed HVRT control, a control Hardware-in-the-Loop (HiL) test platform is investigated to reduce the high development cost and technical difficulties. In the HiL test platform, the PMSG-based WTGS and the power circuit of SCESU are modelled in RTDS, and the controller is implemented is a real-time digital signal processor (DSP). The interface between RTDS and DSP is shown in Figure
Connection diagram of RTDS and DSP.
Photo of the HiL platform.
The results of the HiL tests for the PMSG-based WTGS with SCESU are shown in Figures
HiL results of GRSC with SCESU under voltage swell to 1.2pu.
Voltage and current of PCC
DC-link voltage
Modulation of GRSC
Active and reactive power
Supercapacitor voltage
HiL results of GRSC with SCESU under voltage swell to 1.15pu.
Voltage and current of PCC
DC-link voltage
Modulation of GRSC
Active and reactive power
Supercapacitor voltage
This paper comes up with a potential HVRT solution for the PMSG-based WTGS based on the theoretical analysis of GRSC operation. In the proposed scheme, the SCESU is connected in parallel with the DC link of the back-to-back converter. SCESU stores the imbalanced energy between GESC and GRSC when the voltage swell occurs and releases the stored energy to the grid when the PMSG output decreases. The PMSG-based WTGS can remain operational and absorb inductive reactive power from the grid during voltage swell and the PMSG output reduction can be compensated using the energy stored in SC. The HVRT capability of the PMSG-based WTGS is enhanced by the proposed scheme and corresponding controllers. The HVRT performance meets the relevant requirement. The results obtained in the RTDS simulation and HiL tests verify the feasibility and effectiveness of the proposed HVRT solution.
The simulation data based on RSCAD/RTDS used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was supported in part by the National Natural Science Foundation of China (51767019, 51867020) and in part by the Natural Science Foundation of Inner Mongolia Autonomous Region (2016MS0504, 2015MS0544).