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In this paper, aiming at the energy loss and harmonic problems in the conventional power accumulator battery pack testing system (PABPTS), an improved multi-functional energy recovery PABPTS (ERPABPTS) for electric vehicles (EVs) was proposed. The improved system has the functions of harmonic detection, suppression, reactive compensation and energy recovery. The ERPABPTS, which contains a bi-directional buck-boost direct current (DC)-DC converter and a bi-directional alternating current (AC)-DC converter with an inductor-capacitor-inductor (LCL) type filter interfacing to the AC-grid, is proposed. System configuration and operation principle of the combined system are discussed first, then, the reactive compensation and harmonic suppression controller under balanced grid-voltage condition are presented. Design of a fourth order band-pass Butterworth filter for current harmonic detection is put forward, and the reactive compensator design procedure considering the non-linear load is also illustrated. The proposed scheme is implemented in a 175-kW prototype in the laboratory. Simulation and experimental results show that the combined configuration can effectively realize energy recovery for high accuracy current test requirement, meanwhile, can effectively achieve reactive compensation and current harmonic suppression.

The power accumulator battery pack testing system (PABPTS), which is used for evaluating the performance of high power density power accumulator battery packs (such as lead-acid battery, ultra-capacitor, lithium-ion battery, Fe battery,

The basic requirements for PABPTS have been indicated in [

Based on aforementioned work [

The state of the art in harmonic detection and reactive compensation can be categorized into the frequency and time-domain ones. Abundant literatures on this topic can be found in [

Based on all the work mentioned above, we know that much of this work has been done in the fields of reactive compensation, harmonic detection and suppression, but few papers have been found illustrating a way to unify harmonic suppression, reactive compensation and energy recovery in one system. This paper will combine harmonic detection with instantaneous reactive compensation in an ERPABPTS. Battery testing system requires fast responses with minimum overshot, and combing direct power control (DPC) with harmonic suppression and reactive compensation would be more beneficial for high resolution battery testing system. Therefore, we first put forward and analyze the operation principle of the proposed multi-functional ERPABTS, then, considering the reactive power and harmonic distortion problems, the corresponding control strategy will be given and implemented.

This paper is organized as follows: the operation principles of ERPABPTS considering non-linear loads are reviewed in Section 2; a compound control strategy considering harmonic detection, suppression, reactive compensation and energy recovery is analyzed in Section 3; simulation verification of the unified control scheme is presented in Section 4; experimental results of the proposed scheme are given in Section 5; and some conclusions and future trends are presented in Section 6.

The detailed power circuit topology for the ERPBPTS is shown in

It is needed to note that in many circumstances a non-linear load (such as a diode-based rectifier) might exist at the point of common coupling (PCC), thus, the power grid will be influenced greatly, by factors such as line heat loss, noise,

The operation principle of the ERPABPTS can be described as when performing discharging current test experiments, the DC-DC converter functions as a boost chopper, T6 switches ON and OFF with pulse width modulation (PWM). High accuracy discharging current is obtained via the bypass diode of T5, which gives rise of the DC-link capacitor voltage. Without being properly controlled, the power transistors can be damaged by high-voltage break down. Integrating the energy of DC-link capacitors to the AC-grid is a way to solve this problem. In this paper, the temporary energy is released to the power grid through a VSC. Energy balanced control between DC-DC and DC-AC converter is the most important issue for implementation. In this way, the discharging energy can be recovered effectively, which, at the same, can realize high resolution discharging current tests. Current filter, over-current protection devices, and fast-acting fuses are also used for protection. Whenever a non-linear load exists on the grid-side, the reactive power component is automatically detected and compensated by the power converter, which can be regarded as a unified grid-connected inverter and an APF.

In the following sections, we will first elaborate the operation principle of a DC-DC converter for ERPABPTS, then, a compensation control strategy considering the non-linear load will be analyzed.

In the combined system, the DC-DC converter regulates the discharging current of the PBP, as a testing system, wide-range, high-resolution discharging and charging currents with fast response and minimum steady state error are the basic requirements. This function is used for mimicking the instantaneous startup and driving process in EVs. To improve the dynamic response, and decrease the steady-state error of the DC-DC power converter, an optimal non-linear controller and the topology of discharging part for battery pack were proposed in _{ref} is the reference discharging current, _{d} is the feedback discharging current, _{bat} and _{DC-link} are terminal voltage of power accumulator battery pack and DC-link capacitors, respectively.

In _{d} = |_{q} = 0,

The grid-side voltage transformation block is used to acquire the equivalent voltage under synchronous rotation coordinates. Similarly, the load current transformation block is adopted to obtain the equivalent current under synchronous rotation coordinates. However, _{f} and _{f} stand for the active and reactive power components, respectively; _{h} and _{h} denote the total harmonic power components. After we get the higher-order reactive component, an inverse Clark transformation block is used to acquire the active and reactive current components. Assuming that the phase-currents of the non-linear load are asymmetrical, and contains higher-order current harmonics which can be described as:

Based on

Considering that the grid-side voltage might be unbalanced, and can be written as:

Similarly, the expression of phase voltage under stationary reference frame using the Clark transformation would be:

Combing

Substituting

Thus, active and reactive power component at fundamental frequency would be:

Since the peak value of grid-voltage

According to _{ah}, _{bh} and _{ch}) needs to be compensated, and can be deduced by:
_{ah}, _{bh} and _{ch} are the reactive and harmonic current needed to be compensated. Assuming that _{pa}, _{pb} and _{pc} are the inverter-side phase current and are needed to be injected to the power grid, the reference current for the VSC at inverter side should be:
_{aref}, _{bref} and _{cref} are inverter-side reference current and are determined by the outer closed- loop of DC-link voltage. This procedure is shown in

In _{a}, _{b} and _{c} are the phase voltage of the AC-grid, respectively. _{α} and _{β} are the grid-voltage under stationary reference frame. _{La}, _{Lb} and _{Lc} are the phase current of the non-linear load. The three-phase currents are firstly transformed into two-phase stationary reference frame, and then are changed into two-phase synchronous rotating reference frame to get DC-current component (_{Ld} and _{Lq}). According to the instantaneous power theory, active and reactive power component (_{α} and _{β}) under stationary reference frame can be acquired. After being filtered by the designed band-pass filter, the DC-current active and reactive power component (_{DC} and _{DC}) under synchronous reference frame are derived. The reactive and harmonic power component can be obtained by subtracting _{α} and _{β} from _{DC} and _{DC}.

In order to acquire the higher order active and reactive power component, two second-order band-pass butt-worth filters are designed in this paper. Since the fundamental frequency of the grid is 50 Hz, hence, the center-frequency is chosen to be 50 Hz, the passing-band frequency is set to be 20 Hz, by using sptools block in Matlab, the continuous time transfer function of the filter can be designed as:

Sampling time is chosen as _{s} = 5 μs, then, the discrete-time transfer function of the filter in

Based on

From

To describe and verify the validity of the proposed scheme, simulation verifications are carried out. For this purpose this work is implemented by co-simulation of Matlab/Simulink and Powersim 9.0. Powersim has the advantages of fast computation and convergence capability, and is considered as a professional power electronics simulation software. A simcoupler block is set up for the dynamic data exchange between the two environments. The schematic diagram of the proposed control technique in a distribution grid are shown in

The following steps are performed to validate the performance of the unified ERPABPTS:

Without compensation, the non-linear load is only powered by the AC-grid. If without active current component injection, the compensation current keeps track of the harmonic current. The phase current of the non-linear load and the AC-grid are shown in

To compare the performance of the proposed scheme and conventional scheme, simulation results for the non-linear load with and without reactive compensation and harmonic detection are illustrated, firstly. Then, by using the proposed fourth-order low-pass Butterworth filter with sampling time _{s} = 20 μs, cut-off frequency _{c} = 55 Hz, the proposed reactive compensation and harmonic suppression scheme are provided. The parameters of the proportional-integral (PI) current controller in _{p} = 2, time constant:

In terms of energy recovery, the energy discharged from the battery pack should be recovered to the power grid. Thus, based on the simulation work done in Step 2, active current component should be injected for modulation, which is demonstrated in _{aref}, _{bref} and _{cref} are the additional currents needed to be recovered to the power grid.

_{q} is given by 100 A at 150 ms, the grid-side current amplitude reduced from 117 A to 88 A, which means that the additional current 29 A is used to compensate the reactive and harmonic current. _{cmp}), which is derived by the difference between the load current (_{La}) and fundamental frequency current (_{fa}), which is calculated by using the proposed fourth-order filter. From _{aref}_{bref} and _{cref} are added before PI controller in _{aref}, _{bref} and _{cref}) reference is determined by DC-link voltage to maintain constant. Hence, a unity power factor control scheme with reactive compensation is established. _{La}.

A multi-functional inverter, which is used for a grid-connected energy recovery Li-ion type power battery testing system experimental platform, is set up to verify the proposed design. A microcontroller (DSP-TMS320LF2808, Texas Instrument, Dallas, TX, USA) is used for fast computation and implementation. The hardware configuration setup of the power circuit is shown in

A unified harmonic suppression, reactive compensation and energy recovery power accumulator battery pack test system is proposed. The topology and control strategy proposed in this paper can eliminate harmonics generated by the nearby non-linear loads and realize reactive compensation. The proposed reactive power and harmonic detection scheme can effectively detect the fundamental frequency active and reactive power components. Compared with a conventional battery pack test system, the unified control scheme improves and extends the performance of the combined system. A TMS320LF2808-based DSP controller using the proposed method has been developed, and is implemented in a 175-kW prototype in the laboratory. The results show that the THD is reduced from 13.46% to 1.89% in grid-connected condition, which demonstrates the good performance for harmonic elimination and the reactive compensation scheme proposed in this paper.

Since many ERPABPTS sharing the same AC-grid are used in battery companies, it is suggested that the future work might focus on ways of solving coordination control and energy management problems existing in multi-ERPABPTS systems.

This work was simultaneously supported by the Fundamental Research Funds for the Central Universities of China (No. ZYGX2012J095), China Postdoctoral Science Foundation Funded Project (2013M542266), and also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2013-009458) and (No. 2013-068127). The authors would like to thank all the reviewers for their advices and suggestions on improving this paper.

The authors declare no conflict of interest.

Power transistor specifications adopted for the three-phase neutral point clamped (NPC)-VSC inverter. IGBT: insulated gate bipolar translator.

IGBT | SKM400GB128D/Semikron (Berlin, Germany) | 1,200 V–400 A |

Fast recovery diode | SKKD75F12/Semikron | 1,200 V–75 A |

Capacitor | YDK Technologies Company (Tokyo, Japan) | 450 V–5,000 μF |

Specifications adopted for the proposed ERPABPTS under discharging mode. PBP: power battery pack.

PBP | Battery terminal voltage _{bat} |
240 V |

Battery type | Li-ion | |

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AC Power grid | Grid voltage (line to line rms) _{g} |
380 V |

Line frequency _{n} |
50 Hz | |

Grid inductance _{grid} |
1 mH | |

| ||

LCL Filter | Inverter side inductor _{inv} |
1 mH |

Grid side inductor _{g} |
0.5 mH | |

Filter capacitance |
4.7 μF | |

| ||

DC-DC Converter | Nominal power | 175 kW |

Inductor for boost chopping |
4 mH | |

Switching frequency _{s} |
5,000 Hz | |

Dead time _{d} |
2 μs | |

| ||

DC-AC Converter | Nominal power _{e} |
175 kW |

Two series DC-link capacitor _{DC} |
16,000 μF | |

Initial DC-link capacitor voltage _{c0} |
500 V | |

DC-link voltage reference _{DC_ref} |
900 V | |

IGBT switching frequency _{inv} |
2,000 Hz | |

Dead time _{d} |
2 μs |

Power circuit configuration for energy recovery power accumulator battery pack testing system (ERPAPBTS) considering a non-linear load. DC: direct current; and VSC: voltage source converter.

Control strategy of the DC-DC boost chopper in discharging mode.

Block diagram of the proposed reactive compensation and harmonic suppression scheme. LPF: low-pass-filter.

Bode diagram of the proposed band-pass filter.

Block diagram of the proposed compensation scheme. PI: proportional-integral.

Phase current and spectrum of the non-linear load: (

Time response and spectrum of the grid-side phase current before and after integration of the non-linear load: (

Multi-functional ERPABPTS using the proposed scheme.

Time response of the (

Hardware setup and experimental results of the proposed scheme: (

Parameter specifications adopted for simulation. AC: alternating current; and LCL: inductor-capacitor-inductor.

Non-linear load | Inductance | 0.3 mH |

Capacitance | 470 μF | |

Resitance | 0.5 Ω | |

| ||

AC-grid | Lineto line voltage | 110 V |

Frequency | 60 Hz | |

Internal inductance | 0.1 mH | |

Internal resistance | 0.5 Ω | |

| ||

LCL filter | Grid side inductance | 0.5 mH |

Capacitance | 2 μF | |

Reactive damping resistance in series | 0.1 Ω | |

Inverter side inductance | 2 mH |