Hybrid Voltage Sag/Swell Compensators - IEEE Xplore

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Hybrid Voltage Sag/Swell Compensators A Review of Hybrid AC/AC Converters

JACEK KANIEWSKI, PAWEL SZCZESNIAK, MARCIN JARNUT, and GRZEGORZ BENYSEK

T

he parameters of electrical energy, such as supply voltage amplitude, are very important, especially from the viewpoint of the final consumer with respect to sensitive loads connected to the grid. Dynamic states in the power grid—voltage sags and swells—might cause faults and defects to develop in sensitive loads. To mitigate unwanted effects, many topologies of ac/ac converters are implemented as voltage compensators. This article presents a review of hybrid ac/ac converters designed to compensate voltage sags and swells with the aim of protecting sensitive loads against sudden and severe changes in supply voltage amplitude. In this article, only solutions without galvanic separation between source and load are described. To assess the properties and to compare different topologies of voltage compensators, some common parameters, such as range of voltage sag and swell compensation, reliability, quantity of switches and transformers, Digital Object Identifier 10.1109/MIE.2015.2404350 Date of publication: 21 December 2015

1932-4529/15©2015IEEE

December 2015 ■ IEEE industrial electronics magazine 37

and required power ratings of power electronic units in relation to power of load, are introduced. In addition, we discuss possibilities for compensation of voltage interruption, time of compensation, the efficiency, and the effect on the supply network of the described circuits. The results of the analysis have been collected and compared in tabular form and represented in graphical form. Furthermore, we show potential areas of application for particular solutions of ac voltage compensators. The parameters of electric energy, i.e., its quality, are very important, especially from the viewpoint of the enduser. According to [1], one can define several voltage parameters, such as nominal voltage of the system U N , and voltage perturbations, such as voltage sag [defined as a decrease of the rootmean-square (RMS) value of nominal voltage at a range of 90–1% of U N ], voltage interruption (defined as a decrease of nominal voltage below 1% of U N ; with an interruption of up to 3 min in supply voltage U S being classified as a “short interruption” and a voltage interruption longer than 3 min being classified as a long interruption), and voltage variation defined as ±10% of U N . Dynamic states in a power grid caused by faults—such as rapid load changes, turn-on/turn-off of distribution generators and renewable energy sources connected to the grid, switching effects, and atmospheric discharges—generate undesirable effects for the end user, such as voltage variations, voltage imbalance, voltage sags/swells, flicker effect, or voltage interruption [2]. The analysis of power quality issues [3], [4] shows that, in about 92% of all power system events, there are voltage sags with 40–50% of the nominal value and with a duration from 2 to 30 periods. Only about 4% of voltage events occur with a duration of 2 s–10 min. In the case of ac voltage supply changes, both downward and upward, there is a high risk of damage to devices that are sensitive to voltage perturbations. In the case of big plants and factories, voltage sags and swells may cause financial losses [5], [6]. In Europe alone, the annual cost with

respect to voltage perturbations is estimated at €100 billion. Considering the potential financial burden of voltage perturbations, it is reasonable and important to construct devices to mitigate unwanted effects of supply voltage, especially sag/swell and interruptions, because these issues are crucial from an economic standpoint. In the literature, we can find various types of devices to alleviate variation, sag/swell, or interruption in supply voltage, referred to as voltage sag/swell compensators [13], [22], [36], [39], ac voltage regulators [23], [25], [26], ac voltage stabilizers [19], ac voltage sag supporters [34], [35], voltage conditioners [11], dynamic voltage restorers (DVR) [27]–[33], etc. Taking into account the results of a power quality issue analysis [3], [4], the “perfect” ac voltage compensator should have the following main features. ■■ A wide range in change of output voltage—With regard to power quality issues [3], [4], the ac voltage compensator should fulfil requirements for range of change in voltage transmittance to compensate up to 50% of voltage sag, from U N to 2 U N . To compensate up to 50% of voltage swell, the requirement for range of change in voltage transmittance is from 0.5 U N to U N . The range of change in output voltage of an ac voltage compensator is specified by two parameters: range of voltage sag compensation (Vsag _comp) and range of voltage swell compensation (Vswell _comp) . ■■ Good dynamics—According to [4] and [5], the duration of most voltage perturbations in the power grid is from 2 to 30 periods. For this reason, the response to a disturbance of ac voltage by a compensator should be shorter than two periods. Moreover, to increase the efficiency and reliability of an ac voltage compensator, other important features are as follows. ••A minimum number of switches: During commutation and conducting time, power electronic switches generate power losses. For this reason, the minimum number of switches or the implementation

38 IEEE industrial electronics magazine ■ December 2015

of a soft switching commutation method is a very important issue, especially in terms of efficiency. In this article, a comparison of particular solutions of ac voltage compensators is made by introducing the quantity of power electronic switches (QS). ••A minimum numbers of transformers: In many topologies of ac voltage compensators, series transformers (TSs) are used to inject compensating voltage into the grid. Each element connected in series with the line increases the impedance of the short-circuit loop. Again, for the sake of comparison, the quantity of transformers (QT ) is introduced. ••A low sensitivity to damage of the power electronic elements: With regard to reliability, the ac voltage compensator should permit continuous work, even after the occurrence of a fault in the power electronic converter. In this article, the level of reliability (R) is assessed as high if the ac voltage compensator is still able to operate with supply voltage parameters even when the damage has occurred in the power electronic unit. If the ac voltage compensator can still operate after a fault in the power electronic unit, but with output voltage lower than supply voltage, the reliability is assessed as middle. If the ac voltage compensator cannot operate after a fault in the power electronic unit, the parameter R is assessed as low. ••A required rated power of the power electronic unit in relation to the rated power of the load: In hybrid compensators, this relation is an important feature (defining the rated power of the power electronic converter). The use of a high-rated power converter is associated with an increase in power losses by semiconductor switches, and, moreover, higher power ratings of power electronic units generate higher costs. The required power rating of a power electronic unit in relation to the power load rating is described by parameter Pp /Pload . If this parameter is equal to one (Pp /Pload = 1),

it means that the power rating of the power electronic converter (Pp) is the same as a load power (Pload) . It is desirable that only a part of the energy needed for voltage sag/swell compensation is transferred by the power electronic converter, in which case the value of parameter Pp /Pload will be less than 1 (Pp /Pload < 1) . The general division of ac voltage compensators for the mitigation of unwanted effects on the supply is shown in Figure 1. The most popular group of voltage compensators (electromagnetic) uses a conventional transformer (with electromagnetic coupling) with a tap changer [7]. The major disadvantages of this solution are poor dynamics, step changes, and a narrow range of output voltage adjustment [8], [9], and, additionally, the tap changers are often working in no-load mode; thus, to achieve variable output voltage, it is necessary to disconnect the load. Another group of compensators is based on electric devices (with electrical coupling). These solutions are based on a conventional ac voltage thyristor controller, an ac/ac pulse-width modulation (PWM) converter [matrix chopper (MC) and matrix-reactance choppers (MRC)], and ac/dc/ac converters [10]–[18]. These devices normally are connected in series between the source and load; thus, this type of connection introduces additional impedances (reactance elements and power-electronic switches) to the grid and a voltage drop across their elements (voltage drop across the series elements of the ac/ ac converter generate power losses in the converter). Moreover, because all the energy from the source to the load is transferred through the ac voltage compensator, the converter must work continuously, even during nominal supply voltage conditions (without sag/swell); thus, the power ratings and costs for this type of converter are very high. There are two ways to reduce losses. The first is by using a bidirectional insulated gate bipolar transistor (IGBT) module with better parameters (i.e., a lower

voltage drop on the collector–emitter junction). This will reduce power losses during the conduction mode of power electronic switches. The second is to improve the commutation conditions by implementing a better commutation strategy or soft switching method [19]. This will increase switching frequency and reduce the size of reactance elements in the MRC. Additionally, losses can be reduced by using ac voltage compensators with hybrid coupling. The electromagnetic and electric ac voltage compensators, their properties, advantages, and disadvantages are well known and widely described in the literature [7]–[18]. For this reason, in this article, only the ac voltage compensators based on hybrid solutions are described in detail. In hybrid ac voltage compensators, the electromagnetic coupling is realized by the transformer, and the electrical coupling is realized by the ac/ac and

ac/dc/ac converters. The ac voltage hybrid compensators can be divided into two basic groups: with galvanic separation [37]–[39] or without galvanic separation [20]–[36] between the source and the load. The hybrid compensators without galvanic separation (described in this article) are usually connected in series with the grid by a TS. A simplified schematic of a hybrid ac voltage compensator without galvanic separation between the source and the load is shown in Figure 2. In the case of topologies shown in [Figure 2(a)–(c)], only part of the energy is handled by power electronic devices, while the major part of the energy is transferred directly from the source to the load [Figure 2(a)–(c)]. This means a low value of Pp /Pload factor. Furthermore, in the case of topologies shown in [Figure 2(a)–(c)], the ac/ac converter works only during dynamic states (during a voltage sag/swell or interruption), and these solutions can operate even after suffering damage to the power

ac Voltage Compensators

Electromagnetic

Electric

Hybrid

Conventional Electromagnetic Transformer with Tap Changers [7]–[9]

-ac/ac PWM Converter -ac/dc/ac Converter (Thyristor Controller, Matrix and Matrix-Reactance Choppers [10]–[18])

Conventional Transformer + ac/ac, ac/dc/ac Converter [19]–[39]

With Galvanic Separation

Without Galvanic Separation

With dc Energy Storage

Without dc Energy Storage

With High Frequency (hf) Link

uN

uN

u

u

Voltage Sag

Compensation of: u uN t

Voltage Interruption

uN

u

Voltage Swell t

Voltage Fluctuation

t

t

FIGURE 1 – An example of ac voltage compensators and compensation possibilities.


ucomp

ucomp

dc/ac

uS

uL

(b)

ucomp

TS

TS ZL

ucomp

Load

ac/ac

ac/ac

uL

uL

Control Unit

uS

Control Unit (c) Measuring Transformer T1

uL

Control Unit

TR uS

ZL

–

Control Unit (a)

ZL

Load

ZL

Load

ac/ac

uS

TS

UDC +

Load

TS

(d)

Filter

Amplifier

F1

Arithmetic Circuit

Phase Shifter

uL1

+

SN1

AC1

PS1

A1

+

x2

Φ

AC2 uL

Peak Detector 2 uL +

2 uL′

uLpeak

Arithmetic Circuit

Amplifier A2

AC3

SN2 Uu

–

x

x2

uL′

Controler RE1

Voltage Limiter

PWM Modulator Gate Signals

VL1

PI

+

s(t ) Uref uL

(e)

uL′

uL

uL′

2

uL + uL′ 2

s(t )

uL + uL′

2

2

s(t ) 10 ms/div

10 ms/div (f)

FIGURE 2 – A hybrid ac voltage compensator without galvanic separation between source and load. (a)-(d) Simplified schematics, where u S = source voltage, u L = load voltage, u comp = compensation voltage, TR = transformer, and TS = series transformer; (e) an exemplary control circuit, where SN1, SN2 = summation nodes and PI = proportional-integral controller; and (f) exemplary voltage time waveforms on the control circuit.


electronic unit. In the case of the configuration shown in [Figure 2(d)], all of the energy from the source to the load is transferred through the power electronic unit, which means a high value of Pp /Pload factor (Pp /Pload = 1) . The exemplary schematic block diagram of an ac voltage compensator control system is shown in [Figure 2(e)]. This solution [Figure 2(e)] is based on the peak detector method [40] to detect voltage perturbation and the proportional-integral (PI) regulator. This method provides good dynamic properties and is often implemented in ac voltage compensators. For detection of voltage perturbation other detectors can be used, e.g., the RMS voltage or the phase angle analysis method [41], even genetic algorithms [42]. All of these regulators are simple to implement, especially when using digital technology and require only a few sensors (only load voltage sensors). The main differences between these methods is their dynamic properties. The RMS voltage detector is the slowest, while the phase angle analysis method is the fastest. Considering this analysis, the aim of this article is to review and compare the properties of various hybrid ac voltage compensators without galvanic separation between the source and the load. To make the comparison possible, the following parameters are considered: range of compensation of voltage sag (Vsag _comp), range of compensation of voltage swell (Vswell _comp), R, QS, QT, and rated power of the power electronic unit in relation to rated power of the load (Pp /Pload) . As we might expect from this list, the perfect ac voltage compensator should have the following main features: high reliability and the ability to compensate voltage sags, interruption, and swells up to 150% of U N ; moreover, these requirements should be possible with a small number of transformers and power electronic switches and with a ratio Pp /Pload # 0.5.

Hybrid Voltage Sag Compensators An interesting configuration of an ac voltage sag compensator was introduced in [35] [see Figure 3(a) and (b)]. The main advantage of this solution

is its simplicity, though its weakness is its ability to only compensate up to 50% voltage sag. The ac voltage sag compensator [Figure 3(a)] operates at a switching frequency of 1.5 kHz and is designed to operate at a nominal voltage of 6.5 kV. The control method is based on the peak detection method. Similar properties to [35] are present in the arrangement shown in [Figure 3(c)] [36]. Both solutions (Figure 3) have linear voltage transmittance characteristics dependent on pulse duty factor D [Figure 3(d)]. The main properties of both solutions are shown in [Figure 3(e) and (f)]. The output voltage U L achieves a value between U S and 2 U S (U S < U L < 2U S ), and this allows compensation of voltage sags up to 50%. The output voltage is described by the following equation: U L = U S + U2 N = US + D 2 US N1 ACcomp - AC/AC_conv U L = HU US N = c 1 + D 2 m, (1) N1 where N 2 /N 1 is the transformer ratio. Both solutions have similar properties [Figure 3(d)–(f)]: the ability to compensate up to 50% voltage sag, lack of ability to compensate voltage swells, and a power ratio of Pp /Pload = 1. The only difference between the described solutions (Figure 3) is in the quantity of bidirectional power electronic switches. The control circuits of both solutions are based on a signal processor and guarantee good dynamic properties. Moreover, both solutions operate with sinusoidal input current and sinusoidal output voltage.

Hybrid Voltage Sag/Swell Compensators The hybrid ac voltage sag/swell compensators (without galvanic separation between the source and the load) are shown in [Figure 4(a)] [20] and [Figure 4(c)] [21]. These solutions operate in the configuration shown in [Figure 2(a)]. Optionally, in both cases [Figure 4(a) and (c)], the ac/ac converter can be supplied through the

transformer, as shown in [Figure 2(c)]. In the case of the circuit shown in [Figure 4(a)], the electric coupling is realized by the Cúk B2 MRC, and the idealized voltage transmittance is given by

l

- CukB2 = H ACcomp U

UL US

. 1-

(1 - 2D) . (2) n $ (1 - D)

The Cúk B2 is a kind of bipolar MRC; this means that the output voltage U comp [Figure 4(a)] could be shifted in phase in relation to supply voltage (U S ) by r. The main disadvantage of all types of MRC is their susceptibility to matching conditions. The MRCs operate better if load impedance is approximately equal to the characteristic impedance of the MRC. Any large mismatch worsens static (e.g., voltage gain) and dynamic properties of the MRC. In the case of the circuit described in [21] [Figure 4(c)], the function of the ac/ac converter is fulfilled by a PWM ac/ ac full-bridge MC controlled by a PWM with a dead time. The idealized voltage transmittance of the circuit from [Figure 4(c)] is given by (3). In both structures [Figure 4(a) and (c)], the electric coupling is realized by bipolar converters

- bipolarMC H ACcomp = U

UL US

. 1 + (2D - 1) $ n . (3) With regard to voltage transmittance in both solutions [Figure 4(b) and (d)], it is possible to compensate voltage sags and swells. The range of change of voltage transmittance depends on the voltage ratio of the TS. Moreover, in comparison to voltage sag compensators [Figure 3(a)–(c)], the energy transferred by an ac/ac converter to the load takes place only in the case of supply voltage perturbations. Furthermore, in both cases [Figure 4(a) and (c)], the reliability is assessed as high because the presented circuits can operate with supply voltage parameters even after damage to the power electronic units. The main properties of serial voltage


but the main disadvantage of this solution [Figure 5(a)] is the necessity to use thyristors T1 and T2 as switches. The power losses during the conduction mode are increased by these thyristors. The main properties of a single-phase Z-sourcebased voltage sag/swell compensator are shown in [Figure 5(c)]. The control strategy is based on the peak detection method realized by a signal processor. Taking into account the properties of the solutions shown in [Figure 4(a) and (c)] and [Figure 5(a)], each of them can be applied as a custom power device to protect sensitive loads.

and (5) (when T1 is in the off-state and T2 is in the on-state) [30], - Z_source H ACcomp = U

UL D . . US (2D - 1) (5)

The solution shown in [Figure 5(a)] has the ability to compensate 60% voltage sags and 20% voltage swells [Figure 5(b)]. The reliability can be assessed as high,

N2

N2

TR

L

TR

ZL

N1

CL

N1 Load

S

L

UL (2 - 3D) - Z_source H ACcomp = . , U US (1 - 2D) (4)

uS

uL

uS

S

(a) LL

UL TR

LF

US

T3

ZL

T4

100–200%

CF

uac/ac

ucomp

uL 100%

N1 u

⋅ 100%

200%

N2 Load

CL

uL

(b)

ac/ac T2

T1

ZL

CL

Load

compensators based on Cúk B2 and bipolar MC are shown in [Figure 4(e) and (f)], respectively. A similar topology to the one previously described [Figure 4(a) and (c)], but this time using a single-phase Zsource inverter, is described in [22] [Figure 5(a)]. Assuming, as previously, that the voltage ratio of transformer TS equals n = 1: 1, the voltage transmittance of a sag/swell compensator based on a Z-source converter [Figure 5(a)] can be described as (4) (when T1 is in the on-state and T2 is in the off-state) [30],

D

0

uS

0 uL

t

uS

0.2

0.4

0.6

0.8

1

(d)

D = 0.6 D = 0.2 t

(c)

D = 0.9

Vsag_comp

Vsag_comp

100% R

High

50%

Low

2

Pp /Pload

1

0.5 0.75

100%

0.25

4

6

8

QS/ Phase

R

High Low

2

1 150% 2

50%

3

QT/ Phase

Pp /Pload 1

4

0.25 1 0.5 150% 2 0.75

200%

6

8

3

QS/ Phase

QT/ Phase

200%

Vswell_comp

Vswell_comp

(e)

(f)

FIGURE 3 – A single-phase voltage sag compensator: (a) and (b) with a series and shunt PWM-switched autotransformer and (c) with a PWM ac/ac converter; (d) the idealized voltage transmittance; and (e) and (f) diagrams showing the main properties.


uS

uS

iS

iS

uL

uL

iL

D = 0.2

iL

D = 0.7

UL ⋅ 100% US

ucomp

LF iS

S1 CC

LC CF

200%

iL

TS n:1 CL

ZL

Load

´ Cuk B2 MRC

100%

uL

(1 – 1/n)100%

S2

uS

0

D 0

0.2

0.4

0.6 (b)

(a) u

uS ucomp

S2

CF

CL

S3

UL ⋅ 100% US

ZL

Load

uS

t

n :1

Bipolar MC

S1

1

uL

ucomp TS LF

0.8

uL

(1 + 1/n)100% 100%

S4

(1 – 1/n)100%

LF 0

0

0.2

0.4

0.6

Vsag_comp

100%

For n = 1:1 50% Low

Pp/Pload 1

0.75

0.25 0.5

2 1

1

Vsag_comp

100%

High

0.8

(d)

(c)

R

D

4

150% 2

6

8

QS/ Phase

R

For n = 1:1

High

50%

Low

2

3

QT/ Phase

Pp/Pload1

4

6

8

QS/ Phase

1

0.25 0.5 150% 2 0.75

3

QT/ Phase

200%

200% Vswell_comp

Vswell_comp (f)

(e)

FIGURE 4 – A serial ac voltage compensator: (a) using bipolar Cúk B2 MRC, (b) the idealized voltage transmittance, (c) using bipolar MC, (d) the idealized voltage transmittance, and (e) and (f) diagrams showing the main properties.

There are also possible solutions based on the DVR structure (without a dc energy storage unit), where the electric coupling is realized by a MC [23]–[26], [30] or matrix converter [27], [28], both of which possess the

ability to control load voltage over a wider range in comparison to solutions with MCs and the ability to control the input power factor [16]–[18]. Figure 6 shows exemplary DVR topologies based on MCs [24], [30].

As one can see, the DVR shown in [ Figure 6(a) and (b)] (without dc energy storage), by using a bidirectional converter, has the ability to compensate voltage sag up to 50% and any level of voltage swell. The voltage transmittance


T1

TS ucomp 1:1 Lf ac/ac

S2a

L1

C1

Z–Source

HU

Cf

4 T1–On-State T2–Off-State 2

S2b T2

L2

T1–Off-State T2–On-State

S1a

ZL

C2

uS

Load

0 uL

–2 –4

S1a

0 (a)

T1–On-State T2–Off-State

T1–Off-State T2–On-State 0.2

0.6

0.4

0.8

1

D (b)

Vsag_comp 100% R

For n = 1:1

High Low

50% 2

Pp/Pload

1

0.75

0.5

0.25

6

4

8

QS/ Phase

Including Thyristors

1 150%

2 3

QT/ Phase

200% Vswell_comp (c) FIGURE 5 – A single-phase Z-source-based voltage sag/swell compensator: (a) a schematic, (b) the idealized voltage transmittance, and (c) a diagram showing the main properties.

of a single-phase DVR [Figure 6(a)] is given by - 1phDVR_MC H ACcomp U

=

UL US

. 1 + n 1 (2D - 1) n 2 ,

(6)

where n 1 and n 2 are the voltage ratios of the shunt transformer TR and serial TS, respectively. The voltage transmittance of a threephase DVR [Figure 6(b)] is given by - 3phDVR_MC H ACcomp U

=

UL US

. 1 + (2D - 1) N ,(7) where N is the voltage ratio of the serial transformer TS.

The range of change of voltage transmittance as a function of pulse duty factor D is shown in [Figure 6(c)]. Moreover, the range of voltage sag/swell compensation can be changed by using different values of transformer voltage ratios. Similarly to a conventional DVR, the bidirectional converters [Figure 6(a) and (b)] work only during voltage perturbations. This is an important issue because it makes it possible to reduce power losses in the ac/ac converter used. The main properties of single phase [Figure 6(a)] and three-phase [Figure 6(b)] topologies are similar [Figure 6(c) and (d)], and the main difference is in the quantity of switches and transformers. [Figure 6(a)] shows the main proper ties for the voltage ratio of transformers n TSa = n TSb = n TSc = N = 1. Both solutions can be applied as custom power devices. In the case of a single phase topology [Figure 6(a)] in the construction of a


bipolar converter, only two bidirectional switches are necessary; however, there is a need to implement a parallel transformer with center tap. In both topologies [Figure 6(a) and (b)], the control strategy is based on a PWM with dead time.

Hybrid Voltage Interruption Compensators The interphase ac voltage hybrid interruption compensator (based on the DVR concept) is proposed in [34] [Figure 7(a)]. In this solution the single phase MC is implemented as an ac/ac converter. Because the compensating voltage in phase a (U Ca) is constructed from supply voltages U Sb and U Sc (8), this solution allows the compensation of a single phase voltage interruption

U La = U Sa + U Ca = U Sa + U Sb D 1 + U Sc D 2,

(8)

n1 = 0.5 n2 = 2

ucomp

TR 1:n1

n1uS

uS

Lf

ZL

uMC C f

Sn

uSa

Sb Load

Sp Bipolar MC

uSb uL

TSa LSb

Load uLa

ZLb

ucomp_b

STSa

Load uLb

TSb LSc

Cb

Ca La

ZLc

ucomp_c

STSb

uSc

n1uS

ZLa

ucomp_a

LSa TS 1:n2

Lb

Load uLc

TSc STSb Lc Cc

uL uS

u

ucomp

Three-Phase Bipolar MC

(a) uMC t uMC

Vsag_comp

(b)

100% Fig. 6a

High Low

2

Pp/Pload

1

4

6

8

3

US

200%

Fig. 6b

0.25 1 0.5 150% 2 0.75

UL

QS/ Phase

50%

⋅ 100% 0–200%

R

100% QT/ Phase

00

0

0.2

0.4

0.6

0.8

1

D (d)

200% Vswell_comp (c)

FIGURE 6 – AC voltage compensators based on DVR: (a) a single-phase topology based on MC, (b) a three-phase topology based on bipolar MC, (c) a diagram showing the main properties, and (d) the idealized voltage transmittance.

where D 1 and D 2 are pulse duty factors. The voltage transmittance of the described circuit [Figure 7(a)] in complex form is given by

- Interphase_AC/AC H ACcomp U

= 1 + n (e j

-2r 3

D1 + e j

2r 3

D 2) . (9)

In the case of voltage sag on one phase, the load can still be supplied by injecting compensating voltage without further loading of the damaged phase. The main disadvantage of the circuit shown in [Figure 7(a)] is the requirement of two single phase ac choppers for each phase. In consequence, this topology contains a large number of bidirectional power electronic switches (an IGBT transistor and four diode bridge rectifiers) and passive elements. A further disadvantage of the described solution

is the inability to compensate voltage swell. The scheme with the main properties of the interphase ac/ac voltage sag supporter is shown in [Figure 7(b)]. An often encountered circuit for voltage sag/swell and interruption compensation, which belongs to the series of hybrid ac voltage compensator family, is the conventional DVR [29], [33]. The advantages of a DVR include the ability to compensate 40% of supply voltage sag and 30% of supply voltage swell [27]– [29], [31], [32]. A diagram of the main properties of a DVR with and without dc energy storage, for comparison with other solutions, is shown in [Figure 7(c)]. A DVR equipped with energy storage secures a much wider range of voltage sag/swell compensation compared

to a solution without dc energy storage, while the power rating of the DVR (dc/ac converter) with dc energy storage [29], [33], which compensates voltage interruption, must be matched to the load power.

Summary This article has presented a review of selected topologies of hybrid ac voltage compensators for mitigation of voltage sags/swells and interruptions in the ac power grid. The focus has been on solutions without galvanic separation between the source and the load. To compare properties of particular topologies of ac voltage compensators, the following parameters have been introduced: range of compensation of voltage sag (Vsag _comp), range of compensation of voltage swell (Vswell _comp),


uSb LSc uSc

S2

uSb Cf 1 Lf 3 uSc

Cf 3

S3

Cf 2 Lf 4

S4

Cf 4

TS2

AC/AC I

LSb

D2⋅n⋅uSc

uSa

D1⋅n⋅uSb

ucomp_a ZLa Load ucomp_a = n (D1uSb + D2uSc) uLa ZLb ac/ac II Load u ZLc Lb ac/ac III Load uLc Lf 1 Lf 2 S1 TS1

LSa

Vsag_comp e as -Phption 100% e l Sig rru R Inte 50% High Low

Pp/Pload 1

6

4

2

0.25 1 0.5 150% 2 0.75

QS/ Phase

8

QT/ Phase

3

200% Vswell_comp

(a)

(b) Vsag_comp 100%

uSa

For n = 1:1

R High Low

uComp_a

50% 2

uLa

4

0.25 1 0.5 150% 2 0.75 Pp/Pload E 1 w ne ith rgy dc Sto 200% rag e Vswell_comp

(c)

6

8

3

QS/ Phase

QT/ Phase

(d) FIGURE 7 – An interphase ac/ac voltage sag supporter based on a MC: (a) a schematic, (b) a diagram showing the main properties, (c) exemplary voltage time waveforms, and (d) a diagram showing the main properties of the DVR with and without dc energy storage.

R QS, QT, and required rated power of the power electronic unit in relation to the rated power of load (Pp /Pload) . In Table 1, the major parameters are collected and discussed. Considering the data collected in Table 1, one can see that there is no perfect topology that enables the compensation of all kinds of voltage perturbations—sags/swells and interruptions. In comparison with conventional (electromagnetic and electric) solutions, the described hybrid compensators provide a continuous character and a wider range of change of output voltage. Additionally, they ensure a better Pp /Pload ratio and the same lower-rated power of the power electronic converter in comparison to electric devices. The lack of dc energy storage increases the reliability of

these arrangements. Selection of type of compensator must be preceded by an analysis of the power quality (depth of voltage sags/swells, its duration, incidence, dynamics, etc.) at the place of installation. Other important aspects that must be taken into consideration are ratio of voltage sags to voltage swells, depth of voltage sags and value of voltage swells, duration of voltage sags/swells and interruption, types of ac grid (soft power grid and rigid power grid), causes of voltage perturbations, and side of perturbation (primary or secondary side of ac/ac voltage compensator). Another important issue is the required dynamic properties, which mainly depend on the type of control strategy used. The presented ac voltage compensators are controlled via a conventional PWM or a PWM with dead


time method. All of described solutions can be applied as custom power devices to protect sensitive loads against voltage perturbations. Considering the fact that renewable energy sources are playing a more important role in the power system, there is a danger that such sources could lead to uncontrolled power flows. Future research will analyze ac/ac hybrid converters (without energy storage, and with and without galvanic separation) with the ability to simultaneously and independently control the amplitude and phase of voltage, giving it the ability to control power flow. Additionally, research will be focused on implementation of new control strategies and analysis of properties in industrial applications, such as voltage stabilizers, power flow controllers, and interline power flow controllers.

TABLE 1 – Main properties of hybrid ac/ac converters operate as voltage sag/swell compensators without galvanic separation.

Solution

Configuration

Diagram with main properties

Possibility to obtain topology ThreeOnephase phase

Voltage transmittance H U = U L /U S [V/V] or o utput voltage

Energy storage

Remarks

Hybrid voltage sag compensators

Single phase voltage sag supporter with a PWM-switched autotransformer [35] [Figure 3(a)]

Figure 2(a)

Figure 3(e)

Yes

Yes

D ^ N 2 /N 1h

No

Single phase voltage sag supporter with PWM ac/ac converter [36] [Figure 3(c)]

Figure 2(d)

Figure 3(f)

Yes

Yes

1 + D ^ N 2 /N 1h

No

Hybrid voltage sag and swell compensators

Serial voltage controller with Cúk B2 MRC [20] [Figure 4(a)]

Figure 2(d)

Figure 4(e)

Yes

Yes

1 - ^^1 - 2Dh /n $ ^1 - Dhh

No

Serial ac voltage controller using bipolar MC [21] [Figure 4(c)]

Figure 2(a)

Figure 4(f)

Yes

Yes

1 + (2D - 1) $ n

No

Single-phase Z-sourcebased voltage sag/ swell compensator [22] [Figure 5(a)]

Figure 2(a)

Figure 5(c)

Yes

Yes

^^2 - 3D h / ^1 - 2D hh *

No

Single-phase DVR based on MC [24] [Figure 6(a)]

Figure 2(c)

Figure 6(d)

Yes

Yes

1 + n 1 (2D - 1) n 2

No

three-phase DVR based on bipolar MC [30] [Figure 6(b)]

Figure 2(a)

Figure 6(d)

Yes

Yes

1 + (2D - 1) N

No

^ D/ ^2D - 1 hh * *

Hybrid voltage interruption compensators

DVR with and without dc energy storage [27]–[29], [31]–[35]

Figure 2(b)

Figure 7(d)

Yes

Yes

U La = U Sa = U comp_a for phase L1

Yes/No

AC voltage hybrid sag compensator based on DVR concept [34] [Figure 7(a)]

Figure 2(a)

Figure 7(b)

Yes

No

1 + n (e j^-2r/3h D 1 + e j (2r/3) D 2) for phase L1

No

Ability to compensate single phase voltage interruptions, only three-phase topology

* T1 – on-state and T2 – off-state, ** T1 – off-state and T2 – on-state

Biographies Jacek Kaniewski (j.kaniewski@iee. uz.zgora.pl) received the M.Sc. and Ph.D. degrees in electrical engineering from the University of Zielona Góra, Poland. He is currently a researcher with the Institute of Electrical Engineering, University of Zielona Góra. His research focuses mainly on power quality and power electronics systems in power networks, especially the analysis and study of properties of ac/ac transforming circuits. He is

a consultant with many companies in the field of power quality. Pawel Szczesniak (p.szczesniak@ iee.uz.zgora.pl) received the M.Sc. and Ph.D. degrees in electrical engineering from the University of Zielona Góra, Poland. He is currently a researcher with the Institute of Electrical Engineering, University of Zielona Góra. His research focuses mainly on power electronics systems in power networks, particularly the analysis, modeling, and properties study of ac/ac

converters without dc energy storage. He is a member of the IEEE Industrial Electronics Society and IEEE Power Electronics Society. He has more than ten years of experience in the development of practical solutions to ac/ac converter applications and has been involved in the realization of many national research projects. Marcin Jarnut (m.jarnut@iee. uz.zgora.pl) is an M.Sc. graduate and holds a Ph.D. degree in electrical engineering from the University of Zielona


Góra, Poland. He is a member of the IEEE Power Electronics Society. He is currently a researcher in the Institute of Electrical Engineering at the University of Zielona Góra, Poland. For more than ten years, he has been involved in the development of power electronics solutions for power quality improvement. Currently, he is also involved in the development of smart power solutions. Grzegorz Benysek (g.benysek@ iee.uz.zgora.pl) graduated from the Faculty of Electrical Engineering at the Technical High School of Engineering in Zielona Góra in 1994. Since 2008 he has been associate professor and head of the Institute of Electrical Engineering at the University of Zielona Góra, Poland. His research focuses mainly on issues related to the use of power electronics systems in the power network and the elimination of the negative impact of distributed energy sources on the electric grid. He is a coordinator of the Lubuskie Eco-Car project and cofounder of EkoEnergetyka Zachod.

References

[1] Standard EN 50160, Voltage Characteristics of Public Distribution Systems EN 50160, 2002. [2] J. Milanowic´ and I. Hiskansen, “Effect of load dynamics on power system damping,” IEEE Trans. Power Syst., vol. 10, no. 2, pp. 1022–1028, May 1995. [3] Electrotek Concepts, Inc., “An assessment of distribution system power quality, vol. 2: Statistical summary report,” Elec. Power Research Inst., Rep. EPRI TR-106294-V2, May 1996. [4] W. E. Brumsickle, R. S. Schneider, G. A. Luckjiff, D. M. Divan, and M. F. McGranaghan, “Dynamic sag correctors: cost-effective industrial power line conditioning,” IEEE Trans. Ind. Appl., vol. 37, pp. 212–217, Jan./Feb. 2001. [5] M. Bollen, and L. Zang, “Analysis of voltage tolerance of ac adjustable– speed drives for three phase balanced and unbalanced sags,” IEEE Trans. Ind. Appl., vol. 36, no. 3, pp. 904–910, May/June 2000. [6] A. Honrubia-Escribano, E. Gómez-Lázaro, A. Molina-García, and J. A. Fuentes, “Influence of voltage dips on industrial equipment: Analysis and assessment,” Electr. Power Energy Syst., vol. 41, no. 41, pp. 87–95, 2012. [7] J. V. López, S. M. García, J. C. C. Rodríguez, R. V. García, S. M. Fernández, and C. C. Olay, “Electronic tap-changing stabilizers for medium-voltage lines optimum balanced circuit,” IEEE Trans. Power Delivery, vol. 27, no. 4, pp. 1909–1918, Oct. 2012. [8] N. Yorino, M. Danyoshi, and M. Kitagawa, “Interaction among multiple controls in tap change under load transformers,” IEEE Trans. Power Syst., vol. 12, pp. 430–436, Feb. 1997. [9] Q. Wu, D. H. Popovic´ and D. J. Hill, “Avoiding sustained oscillations in power systems with tap changing transformers,” Int. J Electrical Power and Energy Systems, vol. 22, pp.597–605, Nov. 2000.

[10] Z. Fedyczak, R. Strzelecki, K. Sozan´ski, “Review of three-phase PWM AC/AC semiconductor transformer topologies and applications,” in Proc. Symposium on Power Electronics Electrical Drives Automation & Motion (SPEEDAM ), 2002, s.B5, pp. 19–24, Ravello, Italy, 2002. [11] E. Lefeuvre, T. Meyenard, and P. Viarouge, “Fast line voltage conditioners using a PWM AC chopper topology,” in Proc. European Conference on Power Electronics and Applications (EPE), 2001, pp. P.1–P.9, Graz, 2001. [12] O. Ipinnimo, S. Chowdhury, S. P. Chowdhury, and J. Mitra, “A review of voltage dip mitigation techniques with distributed generation in electricity networks,” Electric Power Syst. Res., vol. 103, pp. 28–36, 2013. [13] O. C. Montero-Hernandez and P. N. Enjeti, “Application of a boost AC-AC converter to compensate for voltage sags in electric power distribution systems,” in Proc. Power Electronics Specialists Conference, PESC, 2000, vol. 1, pp. 470–475. [14] R. Smolenski, M. Jarnut, G. Benysek, and A. Kempski, “Power electronics interfaces for low voltage distribution generation—EMC issues,” in Proc. 2011 7th Int. Conf. Compatibility and Power Electronics (CPE), pp. 107–112. [15] L. Li and Q. Zhong, “Novel zeta-mode threelevel AC direct converter,” IEEE Trans. Ind. Electron., vol. 59, no. 2, pp. 897–903, Feb. 2012. [16] T. Friedli, J. W. Kolar, J. Rodriguez, and P. W. Wheeler, “Comparative evaluation of three-phase AC–AC matrix converter and voltage DC-link back-to-back converter systems,” IEEE Trans. Ind. Electron., vol. 59, no. 12, pp. 4487–451, 2012. [17] J. W. Kolar, T. Friedli, J. Rodriguez, and P. W. Wheeler, “Review of three-phase PWM AC–AC converter topologies,” IEEE Trans. Ind. Electron., vol. 58, no. 11, pp. 4988–5006, Nov. 2011. [18] P. Szczesńiak, Three-Phase Ac-Ac Power Converters Based on Matrix Converter Topology: Matrix-Reactance Frequency Converters Concept, London: Springer, 2013, p. 174. [19] J. C. C. Rodríguez, J. V. López, C. C. Olay, S. M. Fernández, R. V. García, and S. M. García, “Dual-tap chopping stabilizer with subcyclic AC soft switching,” IEEE Trans. Ind. Electron., vol. 57, no. 9, pp. 3060–3074, Sept. 2010. [20] Z. Fedyczak, L. Fra˛ckowiak, and M. Jankowski, “A serial AC voltage controller using bipolar matrix-reactance chopper,” COMPEL, vol. 25, no. 1, pp. 244–258, 2006. [21] J. C. Oliveira, L. C. Freitas, E. A. A. Coelho, V. J. Farias, and J. B. Vieira, Jr., “A PWM AC/AC full-bridge used like a shunt and a serial regulator,” in Proc. 7th European Conf. on Power Electronics and Applications 1997, Trondheim, 1997, pp. 2.186–2.191. [22] M.-K. Nguyen, Y.-C. Lim, and J.-H. Choi, “Single-phase Z-source-based voltage sag/swell compensator,” in Proc. Applied Power Electronics Conf. Exposition, 2013, pp. 3138–3142. [23] S. M. Hietpas and M. Naden, “Automatic voltage regulator using an AC voltage–voltage converter,” IEEE Trans. Ind. Appl., vol. 36, no. 1, pp. 33–38, Jan./Feb. 2000. [24] E. Babaei and M. F. Kangarlu, “Sensitive load voltage compensation against voltage sags/ swells and harmonics in the grid voltage and limit downstream fault currents using DVR,” Electric Power Syst. Res., vol. 83, pp. 80–90, 2012. [25] P. M. Garcia-Vite, F. Mancilla-David, and J. M. Ramirez, “Per-sequence vector-switching matrix converter modules for voltage regulation,” IEEE Trans. Ind. Electron., vol. 60, no. 12, pp. 5411–5421, Dec. 2013. [26] H. J. Ryoo, J. S. Kim, and G. H. Rim, “Series compensated step-down AC voltage regulator using AC chopper with transformer,” KIEE Int. Trans. Electr. Machine. Energy Convers. Syst., vol. 5-B, no. 3, pp. 277–282, 2005.


[27] J. M. Lozano, J. M. Ramirez, and R. E. Correa, “A novel dynamic voltage restorer based on matrix converters,” in Proc. Modern Electric Power Systems (MEPS 2010), Poland, Sept. 2010, pp. 20–22. [28] A. Shabanpour and A. Reza Seifi, “A dynamic voltage restorer based on matrix converter with fuzzy controller,” Adv. Electric Electron. Eng., vol. 10, no. 3, pp. 143–151, Sept. 2012. [29] M. Vilathgamuwa, A. A. D. Ranjith Perera, and S. S. Choi, “Performance improvement of the dynamic voltage restorer with closed-loop load voltage and current-mode control,” IEEE Trans. Power Electron., vol. 17, pp. 824–834, Sept. 2002. [30] E. Babaei, M. F. Kangarlu, and M. Sabahi, “Compensation of voltage disturbances in distribution systems using single-phase dynamic voltage restorer,” Electric Power Syst. Res., vol. 80, pp. 1413–1420, 2010. [31] J. G. Nielsen, M. Newman, H. Nielsen, and F. Blaabjerg, “Control and testing of a Dynamic Voltage Restorer (DVR) at medium voltage level,” IEEE Trans. Power Electron., vol.19, pp. 806–813, May 2004. [32] N. H. Woodley, L. Morgan, and A. Sundaram, “Experience with an inverter-based dynamic voltage restorer,” IEEE Trans. Power Deliv., vol. 14, pp. 1181–1186, July 1999. [33] P. Kanjiyga, B. Singh, A. Chandra, and K. AllHaddad, “SRF theory revisited to control selfsupported dynamic voltage restorer (DVR) for unbalanced and nonlinear loads,” IEEE Trans. Ind. Appl., vol. 49, no. 5, Sept./Oct. 2013. [34] S. Subramanian and M. K. Mishra, “Interphase AC–AC topology for voltage sag supporter,” IEEE Trans. Power Electron., vol. 25, no. 2, pp. 514–518, Feb. 2010. [35] D.-M. Lee, T. G. Habetler, R. G. Harley, J. Roston, and T. Keister, “A voltage sag supporter utilizing a PWM-switched autotransformer,” IEEE Trans. Power Electron., vol. 22, no. 2, pp. 626–635, Mar. 2007. [36] E. C. Aeoliza, N. P. Enjeti, L. A. Moran, O. C. Montero-Hernandez, and K. Sangsun, “Analysis and design of a novel voltage sag compensator for critical loads in electrical power distribution systems,” IEEE Trans. Ind. Appl., vol. 39, no. 4, pp 1143–1150, July/Aug. 2003. [37] E. C. Aeloiza, P. N. Enjeti, L. A. Moran, and I. Pitel, “Next generation distribution transformer: to address power quality for critical load,” in Proc. Power Electronics Specialist Conference (PESC), 2003, vol. 3, pp. 1266–2171. [38] T. B. Soeiro, C. A. Petry, J. C. dos S. Fagundes, and I. Barbi, “Direct AC–AC converters using commercial power modules applied to voltage restorers,” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 278–288, Jan. 2011. [39] J. Kaniewski, Z. Fedyczak, and G. Benysek, “AC voltage sag/swell compensator based on threephase hybrid transformer with buck-boost matrix-reactance chopper,” IEEE Trans. Ind. Electron., vol. 61, pp. 3835–3846, Aug. 2014. [40] H.-Y. Chu, H.-L. Jou, and C.-L. Huang, “Transient response of a peak voltage detector for sinusoidal signals,” IEEE Trans. Ind. Electron., vol. 39, no. 1, pp. 74–79, Feb. 1992. [41] M. Mansor and N. A. Rahim, “Three phase control for ‘PWM-switched autotransformer voltage-sag compensator based on phase angle analysis,’” Journal of Theoretical and Applied Information Technology, vol. 30, no.2, pp. 102–107, Aug. 2011. [42] P. Jitta, S. Kaitwanidvilai, and A. Ngaopitakkul, “Switching angle design for pulse width modulation AC voltage controller using genetic algorithm and distributed artificial neural network,” in Proc. Int. MultiConf. Eng. Comp. Scient. 2011, Hong Kong, Mar. 2011, pp. 16–18.