PL247330B1 - Control system for a three-phase four-wire AC/DC converter for use in a prosumer installation - Google Patents

Abstract

The subject of the application is a method of controlling a three-phase four-wire AC/DC converter (1) consisting of three voltage transducers (2), three current transducers (3), a microcontroller with a multi-channel analog-to-digital (A/D) converter of signals from the voltage and current transducers with an implemented algorithm, connected to a three-phase AC prosumer network and a DC energy storage device. In order to obtain the specified course of instantaneous values of the converter phase currents, the instantaneous values u_a, u_b, u_c of the phase voltages and the instantaneous values i_a, i_b, i_c of the total phase currents of the receivers and other prosumer sources are taken from the voltage and current transducers as analog signals and after their conversion into digital signals using the A/D converter of the microcontroller, the signals of the specified instantaneous values of the converter phase currents are determined using the microcontroller processor i_aref = I * sinωt - i_a, i_bref = I * sin(ωt + 120°) - i_b, i_cref = I * sin(ωt + 240°) - i_c, where I is the amplitude of three sinusoidal signals synchronized with the phase voltage waveforms, which can be calculated based on the active power balance at the AC and DC terminals of the converter from the equation of formula A, in which P_Z denotes the value of power transmitted via the converter from the AC network to the DC storage tank, where the currents i_a, i_b, i_c multiplied by (-1) and appearing as components of the reference currents i_aref, i_bref, i_cref cause the converter to fully compensate the currents of uncontrolled loads and sources.

Description

Description of the invention

The subject of the invention is a current method for controlling the operation of valves of a three-phase four-wire AC/DC converter operating in prosumer installations and serving to integrate storage tanks or DC sources using renewable energy with the AC power supply network in order to compensate for asymmetry of three-phase voltage values, distortions of instantaneous voltage waveforms in network nodes and reactive power.

The converters that are the subject of the invention are built from fully controllable elements such as GTO thyristors or IGBT transistors. They are current controlled using an algorithm.

The invention in question solves the problem of asymmetry of three-phase voltage values and distortion of instantaneous voltage waveforms in network nodes to which prosumer installations are connected, using for this purpose a current-controlled three-phase four-wire AC/DC converter of a photovoltaic (PV) source or storage (ESS). Currently, single-phase receivers and sources, often non-linear, located in prosumer installations, dominate in low-voltage power networks. The effect of their work is disturbances, i.e. asymmetry and distortion of currents, and consequently asymmetry and distortion of voltages in the supply network, the values of which exceed permissible levels. In such a case, prosumer devices are switched off by protections or are damaged. This results in the inability to use renewable energy and sometimes significant financial losses incurred by the prosumer. In addition, the above disturbances have a negative impact on the operation of the power grid and constitute a significant problem for the operator, who is responsible for maintaining the proper quality of electricity.

There are currently no known systems with a three-phase four-wire AC/DC converter, controlled by an algorithm, which cooperate with photovoltaic modules or an energy storage device and at the same time provide compensation for asymmetry and higher harmonics in the low-voltage network caused by the operation of receivers and sources in the prosumer installation. This applies to both single-phase and three-phase sources and receivers.

Three-phase four-wire AC/DC converter systems are known, but their control systems implement goals other than those presented in this application. The concept of using PV inverters enabling the introduction of active and reactive power in low-voltage network connections is known. The solution can be used to reduce load asymmetry caused by linear and non-linear receivers, including reducing the content of higher harmonics, but it requires significant oversizing of the converter and the use of an additional transformer.

There are known solutions using four-wire converters to compensate for reactive power, asymmetry and higher harmonics. The significant difference is that they are not used to integrate energy sources or storage devices in a prosumer installation. Known control algorithms for this type of converter are significantly different from the proposed solution, because they use the transformation of abc phase signals to the dq0 or alpha beta zero coordinate system. In known solutions for storage or source systems, the use of the converter is limited to controlling it with symmetrical and sinusoidal reference signals in phase with the supply voltage. The converter then operates in the AC network as a symmetrical and linear element used only in the process of transmitting active power.

The advantage of the invention is that it extends the functionality of the AC/DC converter to include: reactive power compensation, asymmetry compensation and higher harmonic currents. Its use to control AC/DC converters in prosumer installations means that each of these installations becomes an "ideal prosumer" that loads the power supply network with sinusoidal currents, symmetrical in phase or in antiphase with the network voltage waveforms. This allows for obtaining technical effects in the form of: sinusoidal, symmetrical and within the required limits of supply voltage values at the connection terminals of prosumer installations; minimal active power losses in the supply network wires caused by prosumer current flows; the possibility of connecting new receivers and sources in the existing network without overloading the network elements and exceeding the permissible power quality parameters, without the need to install additional power electronic devices, but only by modifying the control system of converters operating in prosumer installations.

The essence of the method of controlling a three-phase four-wire AC/DC converter (1) consisting of three voltage transducers (2), three current transducers (3), a microcontroller

PL 247330 BI troller with a multi-channel analog-to-digital converter (A/D) of signals from voltage and current transducers and with an implemented algorithm, connected to a three-phase AC prosumer network and a DC energy source or storage device, consists in the fact that in order to obtain a given course of instantaneous values of the converter's phase currents, the instantaneous values of u _a , Ub, u _c of the phase voltages and the instantaneous values of i _a , ib, ic of the total phase currents of the receivers and other prosumer sources are taken from the voltage and current transducers as analog signals and after their conversion into digital signals using the A/D converter of the microcontroller, the signals of the set values of the instantaneous phase currents of the converter are determined using the microcontroller processor iaref = / * sincd - i _a , ibref = I * sin (ad + 120) - ib, icref = I * sin (cd + 240°) - and _c , where I is the amplitude of three sinusoidal signals synchronized with the phase voltage waveforms, which can be calculated based on the active power balance at the AC and DC terminals of the converter from the equation: V3U _n cj _ec i * 4= — - fjfu _a * i _a + uk * ih + u _c * i _c + P,)dt = 0 _lr , „ _r ,, nsien _T Jo ^vaa do cc z/ _in which p _z denotes the value of power transmitted via the converter from the AC network to the DC storage tank, and the currents i _a , ib, ic multiplied by (-1) occurring as components of the reference currents iaref, ibref, icref cause the converter to fully compensate the currents of uncontrolled loads and sources.

The method according to the invention is illustrated in the drawings, where Fig. 1 shows the algorithm for calculating the instantaneous values of the converter phase currents iaref, ibref, icref, Fig. 2 shows a diagram of the network with prosumer devices participating in the control process, Fig. 3 shows three graphs with the course of the instantaneous values of the phase currents: 1. currents of the prosumer's nonlinear and asymmetric loads, 2. converter currents controlled by the algorithm, and 3. prosumer currents flowing from the supply network as a result of the algorithm.

Example of the implementation of the method of compensation of current asymmetry and distortions and compensation of reactive power in prosumer installations using a three-phase four-wire AC/DC converter connected to a three-phase AC prosumer network and a DC energy storage device constructed of two series-connected capacitors, controlled by a reference current calculated by the processor connected to the converter by control circuits of the Texas Instruments TMS320F2837xD Delfino Dual-Core MCUs microcontroller operating in accordance with the algorithm implemented in the microcontroller memory for compensation of current asymmetry and distortions and reactive power in prosumer installations, whereby the A/D converters of the microcontroller convert the analog input signals corresponding to the measured phase voltages supplying the prosumer installation and the total phase currents of its loads and uncontrollable sources (if any in the installation) into digital signals (Blocks A and B in Fig. 1), at a sampling frequency of greater than 20 kS/sec, ensuring obtaining digital information on the course of instantaneous values of measured voltages u _a , U b, u _c and currents i _a , i b, ic, which, together with the value of active power P _z resulting from the operation of the DC storage device (Block G in Fig. 1), are the input quantities of the algorithm (Blocks C and D in Fig. 1).

To calculate the output quantities of the algorithm that set the instantaneous values of the phase currents of the converter as reference currents iaref, ibref, icref (Block F in Fig. 1), the algorithm processes the values of the input quantities u _a , Ub, u _c and i _a , ib, ic in accordance with the dependencies i _are f = i * sinćut — i _a , i _bre f — I * sin(wt + 120°) — i _b , i _cre f — I * sin(wt + 240°) — i _c where I is the amplitude of the three sinusoidal signals synchronized with the phase voltage waveforms, which the microcontroller processor calculates based on the balance of active powers at the terminals of the AC and DC sides of the converter from the equation:

r- I 1 f ^T .

v ^^nnetworks * er γ. I + ^b * 0

V2 and Jo

Multiplied by (-1) currents i _a , ib, ic occurring as components of reference currents iaref, ibref, icref cause the converter to fully compensate currents of uncontrolled loads and sources, which eliminates negative phenomena caused by asymmetry and nonlinearity of prosumer loads and sources in the supply network. This requires a frequency band of transducers intermediating in measuring currents and voltages of at least DC to 20 kHz, sampling frequency of A/D converters and iteration of numerical calculations of 20 kS/sec. Generation by the converter of currents compensating currents i _a , ib, i _c of prosumer loads and sources causes consumption of instantaneous power p — u _a * i _a + u _b * i _b + u _c * i _c from the DC storage at the expense of energy stored in it. In order to maintain the energy balance in the storage, the converter simultaneously draws the same amount of energy from the AC network. As a result of compensation, this is done with symmetrical and sinusoidal currents flowing from the AC network defined by sinusoidal components

PL 247330 BI reference currents, and therefore does not cause negative phenomena. The amplitude of these reference current components also ensures the transfer of power P _from the DC storage charge and discharge algorithm set by the prosumer.

Claims (1)

Patent Claim 1. Method of controlling a three-phase four-wire AC/DC converter (1) consisting of three voltage transducers (2), three current transducers (3), a microcontroller with a multi-channel analog-to-digital (A/D) converter of signals from voltage and current transducers with an implemented algorithm connected to a three-phase AC prosumer network and a DC energy storage device, characterized in that in order to obtain a given course of instantaneous values of the converter's phase currents, the instantaneous values of u _a , U b , u _c of the phase voltages and the instantaneous values of i _a , i _b , ic of the total phase currents of the receivers and other prosumer sources are collected from the voltage and current transducers as analog signals and after their conversion into digital signals using the A/D converter of the microcontroller, the signals of the set values of the converter's instantaneous phase currents are determined using the microcontroller processor i aref = I * sinojt — i _a , i _bref = 1 * sin(iot + 120°) - i _b , i _cref = / * sin(wt 4- 240°) - i _c where I is the amplitude of three sinusoidal signals synchronized with the phase voltage waveforms, which can be calculated based on the active power balance at the AC and DC terminals of the converter from the equation: /- / if ^r ' ^Unnetworks * «τ ™ I 4 ^b * ^b 4 Uc * t _c + P _z ^)dt 0 V 2 i Jo where P _z denotes the value of power transmitted via the converter from the AC network to the DC storage tank, where the currents i _a , i b, ic multiplied by (-1) and appearing as components of the reference currents i _are f, ibref, w cause the converter to fully compensate the currents of uncontrolled loads and sources.