A New Algorithm for Harmonic and Reactive Current Detection

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A New Algorithm for Harmonic and Reactive Current Detection [Copy link]

Abstract: The basic working principle of shunt active filter is analyzed, and a new algorithm for harmonic and reactive current detection is proposed, and a detailed theoretical analysis is made. This detection algorithm does not require a phase-locked loop and can accurately detect the harmonic and reactive components in the load current. This detection algorithm is simulated by MATLAB and implemented in an experimental device with TMS320F2407 DSP as the control core. Both simulation results and experimental results confirm the feasibility of this detection method.

Keywords: active filter; harmonics; reactive current

introduction

With the development of power electronics technology, power electronic devices are increasingly used, but the harmonics they generate also cause harm to the power grid, such as pollution and electromagnetic interference. On the other hand, modern electrical equipment is more sensitive to power quality and has higher requirements for power supply quality. Active filters can eliminate harmonics and improve the stability of power system operation, and their research and application are receiving more and more attention.

There are two basic principles for active filters to eliminate harmonics: one is to inject a current equal to the reactive power and harmonic current of the load and in the opposite direction into the grid to compensate for reactive power and suppress harmonics, which is called a parallel active filter; the other is to inject a fundamental compensation current into the secondary side of the series transformer, so that the series transformer presents a low impedance to the fundamental current of the grid and a high impedance to the harmonic current [1], thereby suppressing harmonics. This method is called a series active filter. In addition, there are series-parallel type, hybrid type, etc. However, no matter which one is used, the values of harmonics and reactive current must be detected first. The more mature current detection methods currently include the p?q detection method [3] and the ip?iq detection method [4] based on the instantaneous reactive power theory [2]. However, these two methods require two coordinate transformations, which are computationally intensive. The ip?iq detection method requires a phase-locked loop, which has the problems of complex implementation and low detection accuracy.

This paper studies a new algorithm for harmonic and reactive current detection.

1 Principles of harmonic and reactive current detection methods

Figure 1 is a system block diagram of a parallel active filter. Its basic principle is: calculate the harmonic and reactive current of the load through the detection link, then control the inverter circuit output, and inject a compensation current equal to the reactive and harmonic current of the load and opposite in direction into the grid, so that the grid current contains only the fundamental active component. In this way, the device can not only filter the harmonics, but also provide the reactive current required by the power system, which can greatly improve the utilization rate of electric energy and improve economic benefits. This paper proposes a new harmonic and reactive current detection algorithm. Figure 2 is a schematic diagram of the detection principle of the load harmonic and reactive current. The dotted box in the figure is the DC side voltage control part. As shown in Figure 2, first detect the actual load current and grid voltage, and calculate these six quantities to obtain the required three-phase load harmonics and reactive current.

For simplicity, it is assumed that the grid voltage is three-phase symmetrical and has no distortion.

The load current iA, iB, iC can be expressed as the sum of the fundamental wave and the harmonics, that is,

Considering the load asymmetry, the current is divided into positive sequence, negative sequence and zero sequence, and the fundamental current is

iA1=i1+sin(ωt－φ)+i1－sin(ωt+θ1－)+i10

iB1=i1+sin(ωt－φ－2π/3)+

i1-sin(ωt+θ1-+2π/3)+i10

iC1=i1+sin(ωt－φ+2π/3)+

i1-sin(ωt+θ1--2π/3)+i10 (3)

Where: i1+, i1-, i10 are the amplitudes of the positive, negative and zero sequence components of the fundamental wave;

The simulation results and experimental results are given.

φ is the power factor angle;

θ1 - is the initial phase of the fundamental negative sequence.

Harmonic currents are also divided into positive sequence, negative sequence, and zero sequence. The kth harmonic current can be expressed as

Where: ik＋, ik－, ik0 are the amplitudes of the positive sequence, negative sequence, and zero sequence components of the kth harmonic;

θk＋ and θk－ are the initial phases of the positive and negative sequences of the harmonics.

The instantaneous value p of the three-phase active power can be obtained by formula (5).

Formula (5) contains DC and a series of harmonic components. The harmonic frequency can be as low as 100Hz. After low-pass filtering, the harmonic components in the power can be filtered out, leaving only the steady-state value p (3UMi1 + cosφ/2), where i1 + cosφ is the amplitude of the fundamental positive sequence current active component. For phase A, the fundamental positive sequence current active component iA1 is = i1 + cosφsinωt. From formula (6), we can get

Similarly, the fundamental positive sequence currents of the other two phases can be obtained as

The work component iB1 is =i1＋cosφsin(ωt－2π/3), and iC1 is =i1＋cosφsin(ωt＋2π/3).

Subtract the active components iA1, iB1 and iC1 of the fundamental positive sequence current obtained above from the actual load current iA, iB and iC, and you can get the load harmonics and reactive current, which are used as the compensation current command of the three-phase inverter output.
In addition, the DC side voltage Ud of the inverter should be kept constant during the operation of the active filter. The DC side voltage control part is represented in the dotted box in Figure 2. As shown in Figure 2, the difference between the given value Ud* and the actual detection value Ud is input into the PI regulator, and the output is multiplied by the actual DC measurement voltage Ud, and the result is the active increment ΔP. ΔP is superimposed on the output of the low-pass filter in Figure 2, so that there is a certain fundamental active current in iC*, so that the DC side capacitor of the inverter obtains energy from the AC side, compensates for the operating power consumption of the active filter, and thus stabilizes Ud at the given value Ud*.

2 Simulation and Experimental Results

The SIMULINK module in MATLAB is used to simulate this detection algorithm, and the simulation results are shown in Figure 3. From the simulation waveform, it can be seen that the fundamental active current calculated by the detection algorithm is completely in phase with the grid voltage and is a standard sine wave, which means that the detected harmonics and reactive currents are completely accurate.

The capacity of the experimental prototype is designed to be 6kW, the voltage is three-phase 380V, and the load is a motor and an uncontrolled rectifier bridge. The control part is based on TI's DSP chip TMS320S2407, and the harmonic and reactive current detection and PWM pulse signal generation are all realized by corresponding software.

The main functional modules involved in the software are: event manager, A/D conversion module, and interrupt service program. The T1 timer is used to start the A/D conversion regularly, and the grid voltage, load current, grid current and DC side voltage are sampled in turn, and the sampling frequency is set to 10kHz. After the A/D conversion is completed, an ADC interrupt is generated, and the algorithm is implemented in the interrupt service subroutine to calculate the harmonics and reactive current, that is, the compensation current instruction. Among them, the low-pass filter adopts a second-order Butterworth filter with a cut-off frequency of 20Hz. The current control method adopts the triangular carrier modulation method, compares the compensation current instruction with the actual compensation current, and sends the difference to the digital PI regulator. The output of the PI regulator is modulated with a high-frequency triangular carrier, and the PWM module generates 6 PWM control signals, among which the triangular carrier is implemented by the timer with a frequency of 10Hz.

The 6-way PWM control signal is sent to the driving circuit, and finally the corresponding compensation current is generated through the IGBT and injected into the power grid. The simulation results and experimental results of the whole system are shown in Figure 4 and Figure 5.

The experiment and simulation have similar results. As shown in Figure 5, the actual load current contains a large amount of harmonics and reactive components
.

The new detection algorithm for harmonics and reactive current in power systems proposed in this paper can detect all harmful currents including fundamental reactive current, zero-sequence current, negative-sequence current and harmonic current. The simulation and experimental results verify the correctness and feasibility of this detection algorithm. This algorithm does not require a phase-locked loop or matrix transformation, and has the characteristics of accurate calculation and simple implementation. The LPC900 series microcontroller bridges the human-machine interface and provides a low-power compact solution for daily applications.

As consumers continue to integrate technology into their daily lives, Asian manufacturers have to adopt economical solutions in their systems to attract this market segment. To meet market demand, Royal Philips Electronics has recently launched a low-cost microcontroller LPC935, priced at less than $2, which features two built-in analog/digital converters.

The LPC935 is the flagship chip among the nine new microcontrollers in the LPC900 series. Through two analog-to-digital converters, it can convert and read data on two channels (a total of eight channels) at the same time. For example, it can read the measurement results of voltage and current at the same time, so that designers can perform real-time data analysis. These LPC935 converters can convert these signals in less than 4μs.

The LPC395 series costs only a fraction of competing products and is designed for a variety of household appliances such as coffee machines, washing machines , smart toys, etc. It bridges the human-machine interface and can complete analog/digital and digital/analog conversion between analog and digital computing fields.

Each new LPC900 microcontroller, including LPC904, LPC915/6/7, LPC924/5 and LPC933/4/5, has simplified external components and adopted a miniature integrated package, giving Asian designers and manufacturers the flexibility to choose to use analog/digital conversion or high-speed digital/analog output. Through the analog/digital and digital/analog conversion functions of the LPC series, these companies no longer need to use separate analog/digital and digital/analog converters on printed circuit boards. These new microprocessors can also provide the function of defining data boundaries, which can limit the range of values within which interrupts are generated, so that the CPU can have more time to handle other tasks.

The LPC900 series is based on a high-performance processing architecture that can execute instructions within 167ns at a frequency of 12MHz (600% higher than the traditional 80C51), and applies byte-erasable flash memory technology to enhance flexibility and improve performance. The LPC900 has a real-time clock (RTC) and three 16-bit counters to enhance timing functions. In addition, serial communication channels such as 400kHz byte-wide I2C bus, enhanced UART and SPI are also provided. Flexible power management functions can also extend the battery life of handheld application devices.

The voltage is partially distorted due to the load. After compensation, the grid current is basically sinusoidal and in phase with the voltage.

tsunamips

Where is the picture please?

tmstd

Oh! I can't see clearly what it is!