Coordinated Control Strategies with and without Circulating Current in Unified Power Quality

Abstract

Under traditional unified power quality conditioner (UPQC) control, a UPQC series converter (SC) is mainly used to handle grid-side power quality problems while its parallel converter (PC) is mainly used to handle load-side power quality problems. The SC and PC are relatively independent. The SC is usually in standby mode and it only runs when the grid voltage abruptly changes. In this paper, novel UPQC coordinated control strategies are proposed which use the SC to share the reactive power compensation function of the PC especially without grid-side power quality problems. However, in some cases, there will be a circulating current between the SC and the PC, which will probably influence the compensation fashion, the compensation capacity, or the normal work of the UPQC. Through an active power circulation analysis, strategies with and without a circulating current are presented which fuses the reactive power allocation strategy of the SC and the PC, the composite control strategy of the SC and the compensation strategy of the DC storage unit. Both of the strategies effectively solve the SC long term idle problem, limit the influence of the circulating current, optimize all of the UPQC units and reduce the production cost. An analysis, along with simulation andexperimental results, is presented to verify the feasibility and effectiveness of the proposed control strategies.

I. INTRODUCTION

The integration of distribution generation (DG) sources and the grid is considered as the main method to save investment, reduce energy consumption, and improve both the reliability and flexibility of power systems. The power flows of distribution networks are from one direction to multi-directions due to the joining of lots of DGs. When the power of DGs fluctuates, it leads to some kinds of power quality problems. The development of custom power technology (CPT) provides methods to effectively solve power quality problems. As one type of CPT equipment, the UPQC combines series voltage compensation theory with parallel current compensation theory, which can deal with the asymmetry and harmony of the grid-side voltage and the asymmetry and harmony of the load-side current [1]-[7].

Under the traditional UPQC control, the SC solves grid-side power quality problems, while the PC solves load-side power quality problems. The SC and PC exchange power with the common DC unit, but their functions are relatively independent [8]-[15]. In fact, the SC is mainly in standby mode, and it runs only when there are sudden grid-side power quality problems like voltage sags or swells. Meanwhile, the PC simultaneously fulfills reactive power compensation and harmonic elimination and is usually in heavy-load status. Nowadays, control strategy research on UPQC capacity allocation is usually limited to the case of voltage sags [16]-[19]. There are few references on the capacity allocation between the SC and the PC when the grid voltage is normal. The minimum energy compensation method and the complete compensation method can realize active and reactive power output in series power quality compensation equipment [20]-[22]. In UPQC systems, the SC and PC can coordinate each other. The SC outputs reactive power to share the reactive power compensation function of the PC, which can realize the UPQC coordination control [23]-[25]. [23] and [24] introduce the same strategy which uses a series inverter to realize reactive power compensation. [25] presents a combinatory UPQC and SFCLs (superconducting fault current limiters) to reduce the UPQC cost, and it does not use the UPQC itself to solve the cost problem. None of them consider the circulating power problem. However, the reactive power output of the SC is accompanied by the active power output, which leads to an active power circulating current. The circulating current will unavoidably influence the UPQC capacity. It can also harm the UPQC series converter and parallel converter.

This paper analyzes the active circulating power and proposes a UPQC power flow coordinated control strategy based on the circulating current. This strategy changes the working condition of the SC and PC, reasonably allocates the power output of the SC and PC, utilizes the SC to alleviate the reactive power compensation burden of the PC especially in the case of normal grid voltage, realizes coordinated control of the SC and PC, and improves the power quality problems in distribution networks. The simulation and experimental results testify that the strategy is accurate, reliable, reasonable and adaptable.

 

II. PROPOSED UPQC STRUCTURE

The UPQC main circuit structure is mainly composed of the SC, the PC, and the common DC unit in Fig. 1. The SC and PC are common 3-phase inverters. The SC connects the grid-side with the series transformer and the thyristor module. The PC connects the load-side with the inductance. The common DC unit includes the storage and the DC/DC converter and the SCM (Super Capacitor Module) constitutes the storage unit.

Fig. 1.System structure diagram of UPQC.

The SC mainly solves the grid-side voltage quality problems including voltage sags, swells, interruptions, imbalances and so on. The PC mainly solves the load-side current quality problems including reactive power compensation, current harmonics, imbalances, and so on. Finally, the UPQC realizes the load-side voltage uL sine characteristic, the grid-side current is sine characteristic and the high power factor characteristic (The angle between us and is is small enough). The series transformer isolates the SC and the grid. The thyristor module provides a by-pass circuit. The storage and DC/DC converter fulfill the reactive power buffer, the energy transfer, the compensation energy provision; and make the SC and PC decouple and solve the voltage interruption problem which the traditional UPQC cannot fully handle. PWM Controller1, PWM Controller2 and PWM Controller3 are used to control the output of the three converters, which will be shown in detail in Fig. 5(b).

 

III. UPQC COORDINATED CONTROL STRATEGY WITH A CIRCULATING CURRENT

The UPQC coordinated control strategy explores the power output relationship between the SC and PC to alleviate the reactive power compensation burden of the PC. It solves many power quality problems and realizes the coordinated control of the SC and PC. The following analyzes the UPQC coordinated control strategy based on the circulating current theory and takes a single phase as an example.

A. Normal Grid Voltage Analysis

When the grid voltage is normal, the SC outputs reactive power. However, the SC outputs active power at the same time. Fig. 2(a) shows a working vectogram of the UPQC at present. φ is the power factor angle of the resistive and inductive load; are the grid voltage vector, the load voltage vector and the SC output voltage vector respectively; are the load current vector before and after compensation, and they can be decomposed to respectively; the vector direction of can adjust between , and the vector direction of follows; and parallel is the compensation current vector of the PC output.

Fig. 2.Working vectogram of UPQC.

In Fig. 2(a),

Define the following:

The SC output voltage vector series is obtained as:

The active power Pseries is derived as:

The reactive power Qseries is derived as:

The PC output current vector parallel is obtained as:

Where:

The active power Pparallel is derived as:

The reactive power Qparallel is derived as:

From Fig. 2(a) and eq. (5), (6), (8), (9), it can be deduced that the SC and PC provide reactive power simultaneously, the SC consumes active power Pseries from the grid, and the PC generates active power Pparallel and Pseries=Pparallel. In this way, there is a circulating current between the SC and PC which is up to Pseries or Pparallel. The circulating current does not influence the UPQC when it is relatively small (usually less than 10% of the system rated current). In real working processes, the SC and PC capacity can be properly adjusted according to the spot requirements. If the distribution relationship is not suitable, the circulating current is large enough to injure the series and parallel converters. In order to balance the SC and PC capacity, assume that Qseries=Qparallel, and that the basic condition of the UPQC coordinated control can be derived by:

Usually, , and the present condition is given as:

The circulation ratio is defined as Pseries/UI or Pparallel/UI . It can be concluded that the ratio decreases with a decrease of the angle φ from (5) and (11) and that the circulation current is allowed to exist under effective control.

The sum of Qseries and Qparallel can be obtained by (12) which is just about the reactive power requirement of the loads.

The active power output of the grid can be obtained by (13) which is just about the active power requirement of the loads.

Under the UPQC coordinated control, when φ= π / 4 , γ = π / 6. In addition, the SC output reactive power is 0.354UI, as does the PC; the circulation active power is 0.095UI; the capacity of the SC or the PC is 0.366UI which shows that the circulation power is relatively low. When the PC compensates the reactive power alone, the required capacity is 0.707UI. Therefore, the strategy decreases the PC capacity and reduces the equipment cost. In addition, the SC is always in the working status which is propitious to timely compensation when voltage quality problems occur.

B. Grid Voltage Sag Analysis

The storage and DC/DC converter provides active power when the voltage sags, and the active current can be invariant. In order to transit freely and avoid current impulses, the angle γref should keep invariant, which is realized by changing the output voltage vector of the SC. The vectogram is shown as Fig. 2(b). is the voltage after sags. Fig. 2(b) shows that the reactive power of both the SC and the PC is invariant; the reactive power distribution relationship of the SC and the PC is invariant; the current vector of the PC is invariant, which leads to an invariant active power of the PC; the load power requirement is provided by the grid sources, PC and SC; and the energy of the SC and PC is from the storage and DC/DC converter.

Define the following:

The letter k is the sag coefficient and the output voltage vector can be derived as:

The sum of the active power from the gird source, SC and PC is given by (16) which is just about the active power consumed by the loads.

C. UPQC Coordinated Control Optimization Condition

According to the UPQC coordinated control strategy, selecting a suitable angle γ can make the UPQC work in the best condition and realize the capacity optimization allocation to reduce the production cost. The following analyzes the UPQC coordinated control strategy based on the optimal allocation theory in the case of voltage sags.

Ignoring the power losses of the converters and lines, the SC capacity and the PC capacity can be obtained by (17) and (18) according to (6), (9) and (16).

Selecting a suitable angle γ can optimize the capacity allocation of the SC and PC to realize a high economic benefit, a low investment cost and a small equipment capacity. Therefore, the objective function of the optimization allocation is defined by the minimum f(γ) as:

μ1 is the unit capacity price of the SC which can be taken as ＄500/kVA; μ2 is the unit capacity price of the PC which can be taken as ＄400/kVA and it does not consider the harmonic capacity; μ3 is the unit capacity price of the DC/DC converter which can be taken as ＄300/kVA; C is the fixed cost which is ＄800; U=220V and I=50A. Put (17), (18), (19) into (20) and the function expression about γ will be obtained. The function f(γ) is at its minimum when k or φ changes and the best working condition of the UPQC coordinated control can be obtained by calculating the value γ in the case of a minimal f(γ).

D. DC Storage Unit

The main circuit topology of the DC storage unit includes a SCM, a bidirectional DC/DC converter and a discharge unit. The SCM provides energy when there is a voltage sag or instantaneous interruption. The bidirectional DC/DC converter is used to adjust the output voltage. The discharge unit is used to provide an energy release circuit when the DC voltage is higher than the threshold value. The storage capacity depends mainly on the SC compensation fashion, the voltage sag level and the duration. The storage unit selection should take into account the storage capacity and the DC voltage demand. The cost can be determined by calculating the number of super-capacitors required. The storage capacity also determines bidirectional DC/DC converter capacity.

 

IV. UPQC COORDINATED CONTROL STRATEGY WITHOUT A CIRCULATING CURRENT

UPQC coordinated control strategy without a circulating current. The definition of the variable is the same as Fig. 2(a). When there is no grid-side power quality problem, the SC outputs voltage which is vertical to the current and the PC outputs current which is vertical to the voltage . Thus, the SC and PC only output reactive power, which avoids an active circulating current. However, the present load voltage UL will exceed rated voltage U. The larger the angle γ, the bigger the voltage UL. Consider the voltage fluctuation, UL < 1.05U , and the angle γ = 17° when UL = 1.05U . Usually, γmax = 15° and UL=1.035U in order to leave a margin. When φ ≤ 15°, the system power factor is high and it does not need to compensate the reactive power. When φ > 15°, the control strategy can be adopted.

In Fig. 2(c):

The SC output voltage can be obtained by (23) in the UPQC control strategy without a circulating current.

According to sine theorem,

and δ' = 90° - φ.

By combining (21) and (22):

The PC output current can be derived as:

The active power Ps generated by the grid is given as:

The active power PL consumed by the load is given as:

According to (26) and (27), Ps=PL. That is to say, the grid provides active power to the load.

The reactive power Qc generated by the SC is derived as:

The reactive power Qp generated by the PC is derived as:

The reactive power QL consumed by the load is derived as:

(30) and (31) show that reactive power consumed by the load is shared by the SC and the PC. (27) and (31) show that cos γ is close to 1 with a small value of γ. This makes the load active power and reactive power close to UI cosφ and UI sinφ, respectively. In fact, an increasing UL leads to the an increase in the load power. Whereas, the bigger the angle γ, the bigger the reactive power of the SC which can share a greater burden of the PC. In the case of φ = 45° and γmax=15°, Qc=0.196UI and Qp=0.536UI. ΔQ = (0.732 - 0.707)UI = 0.025UI. It can be deduced that the active and reactive power increase by 0.025UI.

After determining γ(γ ≤ γmax = 15°) under real working conditions, the SC and PC output can be obtained by (23) and (25) in the UPQC coordinated control strategy without a circulating current. When voltage sags exist, this strategy can be the same as the UPQC strategy with a circulating current in case of voltage sags.

The strategy with a circulating current and the strategy without a circulating current are two different strategies which work in different cases and realize different functions. It is possible to choose either according to the requirements. If a control strategy that switches from one to the other is needed, this can be realized by selecting a corresponding γ and changing the output vectors of the series converter and parallel converter according to the two strategies.

 

V. SIMULATION AND EXPERIMENTAL VERIFICATION

In order to verify the effectiveness of these strategies, a simulation model and prototype system are constructed. In this section, the simulation results are based on PSIM software and the experimental results are based on a 100kVA three-phase prototype. Table I shows the related electrical parameters of the simulation and experiment. The system uses a step-up transformer (the ratio is 1:2) to debase the current requirement of the SC switches and the filter. The SC and PC capacities are both 50kVA and the capacity of all of the transformers is one-third of the SC capacity (about 17kVA) due to the three-phase system.

TABLE ISYSTEM ELECTRICAL PARAMETERS

The serial filter is an LC filter. When its corner frequency is much lower than the switching frequency, the filter function is obvious in the range of the nearby switching frequency. The switching frequency fs is 7.2kHz in this system and the corner frequency is designed to be one-tenth of the switching frequency. The ripple current of the filter inductance ΔIs is inversely proportional to the inductance and the current is about 15%~25% of the rated current. From the above analysis, (32) and (33) are obtained. UDC is 690V in this system. The filter inductance Ls is 1.27mH according to (32) and (33) and is given as 1mH in order to reduce the voltage drop. The filter capacitance Cs is 48.9uF and is given as 50uF. The design principle of the parallel filter is similar to that of the serial filter and the ripple current is usually 5%~10% of the rated current in order to reduce the harmonic fluctuation range. The filter inductance Lp is given as 3mH.

A. Simulation Results

According to Fig. 1, the relative simulation parameters are determined. The grid phase voltage is 220V. A voltage sag occurs between 0.1s and 0.2s and the sag value is 30% of the rated voltage. The sampling time is 1e-7s, the power angle φ is about π / 4 , and according to (11), γref = π / 6.

Fig. 3(a) and 3(b) show the simulation results under the UPQC coordinated control with a circulating current. Fig. 3(a) shows the voltage and current waveforms of phase A. During the simulation, the grid voltage Vsa and the current isa are sine characteristic and the angle between Vsa and isa is close to 0, which achieves a unit power factor of grid-side. The load voltage VLa maintains a sine characteristic and normal amplitude during voltage sags. Vca is the SC output voltage, iCa is the PC output current, and iLa is the load current. Fig. 3(b) shows the SC output active and reactive power (Ps and Qs) and the PC output active and reactive power (Pp and Qp). Ps and Pp are almost equal and opposite in direction which is the aforementioned circulating active current. The SC absorbs energy from the storage unit during voltage sags and generates reactive power for compensation. Qs and Qp are kept almost invariable during the simulation which is the same as the theoretical analysis.

Fig. 3.Simulation waveform under UPQC coordinated control with circulating current.

Fig. 4(a) and 4(b) show the simulation results under the UPQC coordinated control without a circulating current when there is no voltage sag. The variables are the same as in Fig. 3(a) and 3(b). Fig. 4(a) shows the voltage and current waveforms of phase A. They indicate that the angle between Vsa and isa is close to 0; the load voltage VLa is a little bigger than Vsa; the Vca vector is vertical and lead to the isa vector; and the iCa vector is vertical and lead to the VLa vector. Fig. 4(b) shows the SC output active and reactive power (Ps and Qs), and the PC output active and reactive power (Pp and Qp). Ps and Pp are small and are mostly made up of converter loss. Qs and Qp are kept almost invariable during the simulation and both converters compensate the reactive power.

Fig. 4.Simulation waveform under UPQC coordinated control without circulating current.

B. Experimental Results

In this section, a 100kVA three-phase UPQC prototype is designed to verify the UPQC coordinated control strategy. The system control diagram is shown in Fig. 5(a) which includes the control core with a digital signal processor TMS320F28335, a power supply, AC and DC voltage sampling, current sampling, analog digital conversion, a PWM output drive and protection control. The DSP executes the signal processing, mathematical calculations and logic judgment. It then outputs required signals according to the control strategy. These signals include the PWM signal (used by the SC, PC and DC/DC converter), the SCR signal (bidirectional thyristors switching), the KM signal (contactor state control) and the PDP signal (system protection). Fig. 5(b) shows the control structure diagram with a circulating current. Us(ωt), IL(ωt) and UDC are the grid voltage, load current and common DC voltage. are the corresponding reference values. is the sampling voltage of the series converter output. is the sampling current of the parallel converter output. KP1, KP2 and KP3 are the corresponding proportional coefficients of the PI regulators. τ1, τ2 and τ3 are the corresponding integral coefficients of the PI regulators. In this paper, KP1=12, KP2=10, KP3=0.05, τ1=0.01, τ2=0.02, and τ3=0.2. The PWM generator uses the triangular wave comparison method. The buffer and driver use 74F245 and M57962 to drive the IGBT. PLL means Phase Lock Loop and HPF means High Pass Filter. are the grid voltage and load current harmonic component. The other variables are the same as aforementioned. Fig. 5(c) and 5(d) are the UPQC control system and the main circuit system, respectively.

Fig. 5.UPQC system prototype.

Experimental waveforms of the UPQC coordinated control with a circulating current are shown in Fig. 6(a), 6(b), 6(c) and 6(d) which take phase A as an example. The grid voltage usa and grid current isa waveforms before compensation indicate that isa possess severe harmonics and that the system needs reactive power compensation. Fig. 6(b) includes waveforms after compensation without voltage sags, which indicate that the angle between usa and isa is nearly 0 and that the grid-side power factor is nearly 1 through the reactive compensation of the SC and PC. uLa and uca are the load voltage and the SC output voltage, respectively. Under voltage sags (about 20% of the rated voltage), the usa, isa, uLa, and uca waveforms in Fig. 6(c) show that the system maintains a high power factor and that uLa maintains a normal value during voltage sags. iLa and ica in Fig. 6(d) are the load current and the PC output current, respectively.

Fig. 6.Experimental waveforms of UPQC coordinated control with circulating current.

Experimental waveforms of the UPQC coordinated control without a circulating current are shown in Fig.7(a), 7(b) and 7(c) which take phase A as an example. Under the control, the grid-side power factor is kept high and the SC output voltage uca vector is vertical to the gird current isa vector. The grid-side neutral current isn is nearly 0. Regardless of whether voltage sags occur or not, uLa maintain a normal value.

Fig. 7.Experimental waveforms of UPQC coordinated control without circulating current.

Fig. 8(a) and 8(b) show grid current waveforms and THDs (total harmonic distortions) of the PC in the case of the UPQC working and not working. There exists a severe load imbalance. When the UPQC does not work, the THDs of the three-phase current isa, isb and isc are 13.8%, 16.8% and 17.5%, respectively, and the neutral current isn is large. After the UPQC compensation, the THDs of the three-phase current isa, isb and isc are 2.2%, 2.2% and 2.3%, respectively, and the neutral current isn is nearly zero. The three-phase currents are nearly simple and the imbalance problem is resolved. Fig.8(c) shows the current waveforms in the case of an abrupt load change (the resistance load in Table I was abruptly changed from 17Ω to 12Ω at t1). It can be found that the proposed strategy does not influence the other functions of the UPQC and responds quickly from the waveforms.

Fig. 8.Current experimental waveforms.

The simulation and experimental results verify that the UPQC coordinated control strategy with and without a circulating current can improve the power quality problems, and use the SC to compensate the reactive power. Through rational capacity allocation between the SC and the PC, the strategy realizes coordinated control among the SC, PC and DC storage unit.

 

VI. CONCLUSIONS

This paper presented a novel UPQC coordinated control strategy with and without a circulating current according to the UPQC running feature. The strategies unite the reactive power allocation strategy of the SC and PC, the composite control strategy of the SC and the compensation strategy of the DC storage unit which effectively solves the SC long term idle problem. The best working condition of the UPQC under the proposed strategies is deduced to optimize all of the UPQC units and reduce the production cost. Meanwhile, the strategies can be properly adjusted according to different customer needs, especially in the case of high power quality equipment to meet different norms. The strategies theory, simulation results and experimental results show that both of the strategies can feasibly improve the power quality problems and coordinately allocate the capacity output of the SC and PC.