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DC distribution and power electronics applications
in Smart Grids
Jenni Rekola
LVDC20/PJ
DC/AC
AC
LVDC20/1/0.4 kV
DC/AC
0.4 kV 0.4 kV
20/0.4 kV 20/0.4 kV20 kV
0.4 kV 0.4 kV
L
DC
AC
AC
AC
AC
AC
D CDCDC DC
AC
ACA C
AC
P
Backup connection
Suburban district
Indoor DC
AC 20 kV
Urban district
Rural areas Urban areas
LAPPEENRANTAUNIVERSITY OF TECHNOLOGYFINLAND
LAPPEENRANTAUNIVERSITY OF TECHNOLOGYFINLAND
LAPPEENRANTAUNIVERSITY OF TECHNOLOGYFINLAND
LAPPEENRANTAUNIVERSITY OF TECHNOLOGYFINLAND
Detached houses
=
M=
~
=
=
=
M
– Computers– Electronic appliances– Lights
– AC motor drives– Induction heating– Etc.
DC motors
Large resistive loads
=
~
– DC Generators– Power storages
SC
Variable speedAC generators
G
Customer networks/load devices
~
=
DC
…200 Hz
G
GTO-breaker
~
=
Public MV network
==
± 750 V
20 kV
=
=
PV
To other customers
LVDC microgrid
1 INTRODUCTION
The structure of the electricity distribution network is changing. Good power quality is more important nowadays and customers are dependent on uninterrupted power supply. The price of energy will rise because of a shortfall in fossil energy sources and stringent emission regulations, which therefore make the higher efficiency of electricity distribution more important. There will be more distributed generation (DG) units like wind farms and solar power and more energy reserves like super capacitors and batteries connected to the distribution network. The age of a large part of the low voltage (LV) branches and the entire medium voltage (MV) network is close to a point where these have to be replaced by new ones and because of increased consumption of electric energy, the current distribution network will soon become inadequate to meet the demand.
Smart grids represent the most useful and efficient way of integrating renewable energy generation in the main grid. Power converters enable efficient and flexible interconnection of different players (producers, energy storages, flexible transmission and loads) to electric power system. PWM converter technology and a proper control allow manage controlled bidirectional power flow and full control of active and reactive power. The main applications of power converters in distribution networks, at the moment, are grid connection of small-scale generators and power quality enhancing devices like UPS and STATCOM. Security and communication systems can be integrated to the power converters and the converters can be controlled external by communication system. The operation of different grids at different power levels, and based on different technologies such as DC or AC, single-phase or multi-phase, is possible to combine by PWM converters. PWM converters keep the power system stable, reliable and secure. Powerful computational devices, distributed intelligence and integrated measurement devices of the PWM converters will be more important in the future smart grids.
DC distribution is used already in many special applications in width voltage range. DC is used in vehicles and shipboard systems, aircraft, traction systems and automotive industry. In the industry applications common AC/DC converter and DC link systems are used, which feed many motors due distributed DC/AC converters. Low 48 V DC is used in the telecommunication systems and in data centres. High voltage direct current (HVDC) power transmission systems are used in point-to-point configurations for the long-distance transmission of electrical power.
The low voltage direct current (LVDC) distribution is a new LV distribution concept which is one possibility to realize the future smart grid. It enables the meeting of new requirements of the electrical network. In the LVDC distribution, the medium voltage is first decreased to 1kV AC low voltage by a supply transformer, rectified to DC and converted
back to AC near the customer. The Low Voltage Directive 2006/95/EC enables the use of 1500 V DC in power transmission.
The goal of the LVDC network is to increase the efficiency of the LV network. The power transmission capacity of LV network should increase with the use of DC because resistive losses in DC cables are lower because of higher voltage level. All of the transmitted power is active power so more power can be transmitted with the same cables. Consequently, the length and complexity of the MV network can be reduced because of high power transmission capacity and lower construction and cable costs of the LVDC network.
The target of the LVDC network is to ensure higher power quality to the customers than in the present AC distribution. The DC system enables the compensation of voltage sags and short interruptions of the MV network and thereby the DC voltage can decrease by for example 25 % without any effects on the customer voltage. Nowadays more than 90 % of the outages experienced by the customers are due to faults in the MV network. If a blackout occurs in the MV network, the islanding operation of the LVDC distribution network would be possible if distributed generation units and energy reserves are connected to the LVDC network, therefore the LVDC distribution system has characteristics of an UPS, guaranteeing the continuity of the electricity distribution. Also reliability of the MV network increases, since LVDC network forms its own protection area. Interruption costs of network companies would decrease if the LVDC network was used.
DG units and energy reserves would be easier to connect to DC than AC in the low voltage network. Some of the distributed generation technologies, like solar panels and storage systems, like batteries and super-capacitors, generate DC power and are easy to connect to the LVDC network by DC/DC-converters. Efficiency of the system would increase because of a decreased number of AC/DC and DC/AC conversion stages. Wind turbines are easy to connect to the LVDC network, because there are no problems with the frequency synchronization.
Electrical protection systems of the LVDC network can be integrated into the power converters. Detailed measurements of the network state and the customer’s behaviour are more important in the future when the network is more complex because of distributed generation units and energy reserves. Measurements and energy metering can be integrated into the converters. Separate automatic meter reading (AMR) meters are not necessary. The communication link can be connected between the converters and the information system of the local electrical network company, so the converters can transmit real-time information about the distributions system status and quality of the electricity for the Network Control System. This measuring data can also be used for example to identify the route of the fault current and for the fault localization, since potential fault points can be computed using a fault current value and network data. This would simplify and accelerate the removing of a fault. The interruptions could also even be prevented by the active control of inverters when the measuring data is found to be abnormal, before the actual protection works. This is possible since the inverters can measure smaller changes in currents and voltages compared to the actual protection devices.
2 STRUCTURE OF THE LVDC NETWORK
The structure of the LVDC distribution network, analyzed in this study, is unipolar or bipolar. In the unipolar system, the line and load converters are connected to 1500 V DC as presented in Fig. 1 a. Two voltage levels, ±750V DC, and the neutral are used in the bipolar system (Fig. 1 b). The bipolar system can be fed by a two-winding transformer and one line converter, which forms a bipolar connection with the DC network as shown in Fig. 1 b or by a three-winding transformer and two line converters, which are connected in a unipolar way between the positive or negative pole and the neutral and both of them produce 750 V DC.
If two converters are used, they are supplied by a three-winding transformer whose transformation ratio is 1000/400/400V if the basic carrier-based sinusoidal pulse width modulation method (PWM) is used. The transformation ratio is 1000/460/460V if the modulation index is boosted 15 % by the injection of a third harmonic component to the sinusoidal modulation references or if space vector modulation is used. If only one line converter supplies 1500 V DC, the transformation ratio of the two-winding transformer is 1000/920V or 1000/800V, respectively. With the used transformation ratios, the MV voltage can increase by 10 % as it is possible according to the standard EN-50160, and the DC voltage level can still be kept constant. 5 % controlling tolerance is also taken into account in the transformation ratio.
= =AC DC
1500 V
0
AC
= =
=
=
A
B
DLVAC LVDC
AC
AC
AC
+750 V
-750 V
=C AC
MV
a) b)
Figure. 1. a) A unipolar and b) a bipolar LVDC distribution network
Customers can be connected between the positive (customer A Fig. 1b) or the negative pole (customer B) and the neutral as a unipolar connection, between the positive and negative poles (customer C) or between the positive and negative poles with the neutral connection (customer D) in a bipolar way. The bipolar system provides the use of converters rated 750 V DC and these inverters are cheaper compared to converters rated 1500 V DC. The bipolar system is more reliable, because it enables an opportunity to keep the other half of the network in operation even if there is a fault in the other half of the network. The standardization allows LV cables to be used in DC voltage. Present low voltage underground cables are possible to use up to 900 V DC (the defined maximum voltage between conductors is 1500 V DC and between earth and conductor 900 V DC), so these can be used in the bipolar LVDC-system according to the standards SFS-4879, 4880, 5800 and 5546. In LV aerial bundled cable standard is not said
anything about cable usage in DC voltage. The existing cables are mainly 4- and 5-wire cables which can be connected in parallel to take advantage of all cable conductors. Example connections for unipolar and bipolar DC systems are shown in Fig. 2. in case of 4-wire AXMK-cable.
a) b)
Figure 2. Possible cable connections in a) unipolar and b) bipolar LVDC transmission [Salonen 1] The structure of the LVDC network by Japanese researchers is presented in Fig. 3. Distributed generation, energy reserves and cogeneration gas engines are connected to the DC network. The three-wire composition contributes that the voltage to ground becomes low, and one of the single-phase output lines becomes a grounded neutral line. Load side DC/DC converters can be connected to 170 V DC or to 340 V DC.
Figure 3. Structure of the LVDC network [Kakigano] The number of DC power consuming devices has increased very fast during the past few years in homes and offices. These are for example computers, printers, battery chargers and other home electronics. Most of the word wide voltage appliances (lightning systems as
compact fluorescence lamps and LEDs, computers, TVs and motor drives) operate normally when these are supplied by DC (140 V…220 V) without any modifications. Researchers are analyzed, would it be more efficient to use DC distribution in house electrical installations. Number of power conversion step size will be reduced, but DC/DC converters are still needed, because devices need different DC voltage levels to operate.
Engelen et al. have done theoretical analysis about efficiency of the LVDC distribution and using DC in house installations. 600 V DC voltage level is used in the LVDC network and DC levels 20 V, 230 V and 325 V are used in house installations. According to theoretical analysis and simulations, the efficiency of utilization electrical energy is not improved if only the residential installations are replaced by DC because the losses in an AC system are already negligible. Instead the combination of DC power in houses and the LVDC distribution system would be more efficient than the present AC system if the efficiency of the power converters is almost as high as transformers (> 95 %) also at partial load conditions.
2.1 Protection of the LVDC network
The LVDC distribution network has to fulfil the limits of the standard SFS 6000 about electrical safety. The LVDC network has more fault situations compared to the traditional distribution system because of increased number of components connected to the network. The main challenges of the protection system concentrate to the customer-end inverter, customer network faults including inverter switch faults and double fault situations between separate networks. Fuses and automatic relays are difficult to use with power converters, because power converters are not able to produce short-circuit current as long time as fuses need to react.
The customer’s network is grounded TN system and the LVDC network is ungrounded IT-system. The grounded TN system is not possible to use without galvanic isolation between the LVDC network and the customer, otherwise the ground faults of the LVDC network and the grounded TN system would produce dangerous double fault through the ground. The 50Hz galvanic isolation transformer is very heavy, but the high-frequency galvanic isolation transformer would be more economical. In the grounded system the earth faults are short circuits and therefore protection against these can be done by short circuit protection devices. The galvanic isolation is not needed if the customer’s network would be also ungrounded IT-system but the maximum DC voltage level in this case is ±700 V DC and protection against earth faults is more complicated. If load converters are connected to the LVDC network as a bipolar way and single-phase half bridges or three-phase three-level converters are used, it is technically possible to use operational grounding (TN) in the customer network without galvanic isolation.
The DC system needs to be compatible with existing protection devices. The inverters short circuit current capability needs to fulfil used circuit breakers or fuse current requirements. The load converter cannot be dimensioned according to effective power, but short-circuit current determines its dimensioning. In a short-circuit situation the inverter is
able to supply only 120% of its nominal current, but a traditional fuse protection of a low voltage AC network requires fairly high fault current to operate correctly. A 50A fuse requires at least a 250A short-circuit current to operate in five seconds (SFS-6000). For this reason, the load converter has to be dimensioned larger than the power supply capacity would require. The fuses should be replaced by much faster protection relays if smaller converters would be used.
The switch faults in AC/DC converter can be introduced short circuit across bridge or alternative current to DC network. Against AC/DC converter switch faults can be protected with protective functions integrated in the converter and with short circuit relaying. The short-circuit current can be limited in the converter to protect converters and other components in the network. The uncontrolled diode bridge cannot control the current so the components should be oversized. The active rectifier behaves similar to diode bridge during short-circuit so the diodes should be oversized also in this case. The short-circuit current produced by active rectifier to the MV network has to be limited or otherwise the protection systems of the MV network can also react. If there are some faults in the components of converter, these can be detected by overcurrent relay or by measuring the DC voltage level.
The DC network faults can be covered with combined over current and short circuit protection and with earth fault protection. The DC network short circuit protection can be made with molded case circuit breakers (A in Fig. 4) which includes circuit breaker and over current relay. The circuit breaker can be located either AC or DC side of the converter. On the AC side the breaker protects against converter switch faults also. The bipolar system poles needs to have its own protection devices. The DC breakers (B in Fig. 4) are connected in front of all inverters. These are marked as fuses in the figure, but DC breakers are more cost efficient to use than fuses.
Figure 4. A proposed protection scheme for bipolar LVDC distribution system when grounded TN system is used [Salonen 2] In customer AC network short circuit protection can be used circuit breakers and fuses (D in Fig. 4) when inverter short circuit capability is as high as used devices require. Due to power electronic switch poor short circuit capability, the inverter switches should be over-dimensioned or the current limiter may be added to restrict switch fault failures and ensure protection device correct operation. The B type 30mA residual current devices (F in Fig. 4) can be used to increase human safety in double fault situations. Residual current device separates customer network form DC network in case of double fault situations. Against
DC/AC customer converter switch faults can be protected with functions included in converter and by using surge arresters (G in Fig. 4) and DC circuit breaker to prevent full DC voltage affecting in customer network. The customer-end network protection can be similar to the solutions used today if galvanic isolation is used. Special equipments are needed only for the DC network.
2.2 Power quality in the LVDC network
The LVDC network needs to be secure and reliable. As already mentioned Low Voltage Directive 2006/95/EC enable to use 1500 VDC in power transmission. Medium-voltage network 20 kV voltage has to lay down by transformer to low AC voltage (1 kV) before rectifying it to DC. Voltage is defined to be DC voltage if its ripple voltage is not more than 10 % of the nominal voltage, but in reality the ripple has to be much lower to limit the losses in DC cables.
The current harmonics cause losses in the network components and may incur damages or malfunctions in other devices connected to the same network. If the standards IEC 61000-3-2 (class A) and IEC 61727 are applied in the case of LVDC distribution, they limit the current harmonics which are allowed to produce to the MV network. The standard IEC 61000-3-2 enables the maximum THD of the line current to be 5 % up to the 40th harmonic component and the maximum values to every single harmonic component (order numbers 2..35) are also defined. The THD is defined according to equation (1) where X1,rms is the rms-value of the fundamental component of the signal x and Xrms,n is the rms-value of the nth frequency component of the signal x.
100%2
40
2 ,1
,
n rms
rmsn
XX
THD (1)
The standard EN-50160 enables the maximum THD of the customer voltage to be 8 % up to the 40th harmonic component and the maximum values to every single harmonic component (order numbers 2..25) are also defined. Other standards about power quality are presented in Table 1.
Table 1. Power quality Character Target value Standard/recommended value DC-voltage level ± 750 V DC,
galvanic isolated system, IT-TN ± 700 V DC, IT-IT
900 V DC between earth and conductor and 1500 V DC between conductors, SFS-6000 and SFS-4879
DC-voltage fluctuation Fluctuation < 10 %, DC-voltage range +10%...-25%
Max fluctuation 10 %, SFS-6000
Customer AC voltage and current Nominal voltage Fluctuation Frequency THDu
THDi
1~230 V AC or 3~400 V AC 98% of time ±0% and 100% of time ±10% 100% of time 50Hz ±0,1 Hz < 5 % ( 2 % in this study)
1~230 V AC, SFS-6000 95% of time ±10% and 100% of time +10%/-15%, SFS-EN 50160 99,5% of time 50Hz ±1%, 100% of time 50 Hz +4%/-6%, SFS-EN 50160 < 8 % (up to 40th harmonic) < 8%/20A if iref > 200A < 10%/20A if 25A< iref 200A If iref 25A, according to device standards IEEE 519-1992, Sener 1999
MV network voltage and current Fluctuation THDu
THDi
< ±10% < 8 %
2 % in this study
10kV or 20kV SFS-2664 < ±10% SFS 2664 < 8 % SFS-EN 50160 <5 % (up to 40th harmonic) IEC 61000-3-2 < 8 % Sener 1999
Power factor of the DC network from MV network point of view
0,8ind-1,08kap -
Converters over-voltage capability Transients Continuous
12 kV against ground, overvoltage protector 1500/3000V (against pole/ground)
12kV SFS-6000, IEC 60664-1 U0+1200V < 5 seconds U0+250V > 5 seconds
3 LINE AND LOAD CONVERTERS
The LVDC network can be supplied by one or two line converters. The 12-pulse half-controlled thyristor bridge, presented in Fig. 5a, is the simplest rectifier topology which can be used in the LVDC distribution. The delay angle of the thyristors is controlled only during start-up of the LVDC network to limit high starting current, otherwise the bridge is assumed to be used as a diode bridge. Only unidirectional power flow is possible with the half-controlled rectifiers. The bridge is supplied by a three-winding transformer, whose secondary windings are star and delta connected because the 30° phase shift between the secondary windings is needed.
The two-level line converter, presented in Fig. 5b, and three-level line converter, presented in Fig. 5c, enable better power quality to the MV network and enable bidirectional power flow between the MV and the LVDC networks. The fourth option is to use the three-level Vienna-rectifier, presented in Fig. 5d, which produces low current distortion and offers the possibility to control the power factor of the rectifier. However, only unidirectional power flow can be used. The neutral conductor can be connected from the DC link midpoint to the star point of the supply transformer if only one line converter is used and the neutral conductor is grounded, as shown in Fig. 5b.
S1
D1
C1+-
C2+-
iLconv
DD
Y
S1 D1
C1
C2
Lgrid Lconv
C
+-
+-
iLconv
S2 D2
a) b)
S1 D1
S2 D2
D5
Lgrid Lconv
C
C1
C2
+-
+-
iLconv
S3
S4
D3
D4
D6
D1
D2
D3
D4
D5
D6
S1Lgrid Lconv
C
C1
C2
+-
+-iLconv
c) d)
Fig. 2. The line converter topologies: a) 12-pulse half-controlled thyristor bridge, b) two-level line converter, c) three-level line converter and d) Vienna-rectifier
The PWM line converter does not produce low frequency harmonics to the MV network like the 12-pulse half controlled thyristor bridges. Therefore losses in the supply transformers and the MV network are lower. Bidirectional power flow can be realized by using PWM line converters and distributed generation units can be connected to the LVDC network. The LVDC network behaves as a resistive load for the MV network if the line converters are used with the unity power factor. The PWM converters can also inject reactive power to control the power quality of the MV network. The drawbacks of PWM line converter are higher costs, more complex structure and control. Furthermore LCL-filter is needed between the line converter and MV network to filter high frequency harmonics caused by modulation.
Customer AC loads are connected to the DC network by single-phase half or full bridges or three-phase load converters which converge the DC voltage to AC. The DC loads can be connected to the network through DC/DC–converters. The single phase load converters are presented in Fig. 6. In most applications in residential power supply there is necessarily no need for the three-phase supply and in these situations the single phase converters can be used. The drawback is that the single phase loads produce 2nd harmonic component to the DC link current.
a) b) c) d)
Figure 6. Single-phase two-level a) half bridge, b) full bridge and three-level c) half bridge, d) full bridge The half bridges are simple, low-cost and easy to control. Because of fundamental frequency current which circulates though the DC link capacitors and the connection of half bridge, the mutual voltages of capacitors fluctuate by 50 Hz base frequency. DC link capacitors need to be large to balance the voltage fluctuation. It is not possible to supply half-wave rectifying loads by half bridges because the voltage balance of the DC capacitors doesn’t persist without a balancing circuit. Half bridges are problematic from the electrical protection point of view, because an active limiting of short-circuit current, which flows through the DC link capacitors, is difficult.
The structure of the full bridge is more complex than the structure of the half bridge, but the passive components, which are needed both on the DC and AC side, are smaller. Small capacitors in the DC link are sufficient and components of the LC-filter are smaller. Full-bridges don’t produce harmonic currents at the switching frequency as the half bridges
do. It is possible to supply half-wave rectifying loads by full bridges without any problems in the voltage balance of the DC capacitors.
3.1 NPC converters
The LVDC distribution is studied before but three-level converters are not used. Three-level converters are widely used in medium- and high-voltage level because of smaller voltage stress of the power semiconductor switches. Three-level converters are not yet used commercially in low-voltage level because of more complex structure and higher costs. There are many different multilevel converter topologies but this study concentrates on three-level NPC (neutral-point clamped) -converters and Vienna-rectifies. Principle of operation and structure of NPC converter are presented at first time in 1981. Single-phase half-bridge converter consists of four power semiconductor switches, four anti-parallel connected diodes and two clamping-diodes as presented in Fig. 7.
Converter, which load phase terminal can be connected to two voltage potentials, udc/2 and -udc/2, is called two-level converter. In three-level converter, the load phase terminal can be connected to three voltage potentials, udc/2, 0 and -udc/2. The output terminal can be connected to the voltage potential udc/2 by the switches S1, S2 and the anti-parallel connected diodes D1 and D2, marked in Fig. 7. The output terminal can be connected to the voltage potential 0, i.e. to the midpoint of the DC intermediate circuit, by the switches S2, S3 and the clamping-diodes D5 and D6. Finally, the output terminal can be connected to the voltage potential -udc/2 by the switches S3, S4 and the anti-parallel connected diodes D3 and D4. The current paths of the one converter leg are presented in Fig. 7, where the positive current flow is presented in green and the negative current flow with red colour.
n
+-
+-
S1 D1
S2 D2D5
D6 S3 D3
S4 D4
u /2dc
n
+-
+-
S1 D1
S2 D2D5
D6 S3 D3
S4 D4
0
n
+-
+-
S1 D1
S2 D2D5
D6 S3 D3
S4 D4
-u /2dc
n
+-
+-
S1 D1
S2 D2D5
D6 S3 D3
S4 D4
u /2dc
n
+-
+-
S1 D1
S2 D2D5
D6 S3 D3
S4 D4
0
n
+-
+-
S1 D1
S2 D2D5
D6 S3 D3
S4 D4
-u /2dc
Figure 7. Current path in the one leg of the NPC converter Current through IGBTs and diodes is presented in Fig. 8. S2 is conducting during the whole positive half cycle and S3 during the whole negative half cycle. The switching frequency of the outer IGBTs S1 and S4 is the same as switching frequency (10 kHz), but the switching frequency of inner IGBTs S2 and S3 is the same as base frequency (50 Hz). Load is in this case purely resistive so the anti-parallel diodes of the IGBTs are not conducting.
a) b) c)
Figure 8. Current of a) S1, b) S2 and c) clamping diode D5
The produced output voltages of two- and three-level single-phase half-bridge and three-phase converters are presented in Fig. 9 when modulation index in all cases is 0,87, switching frequency 1 kHz, DC voltage 750 V DC and produced output voltage rms-value 230V. The fluctuation of the voltage is caused by too small DC capacitors. Two-level single-phase half-bridge produces two voltage levels to its output voltage and three-phase converter five output-voltage levels. Three-level single-phase half-bridge produces three voltage levels to its output voltage and three-phase converter nine output-voltage levels as shown in Fig. 9. The amplitude of the output voltage of single-phase three-level converter is half smaller compared to two-level converter and because of that, the voltage capability of used IGBTs can be half smaller. There are more voltage levels in the output voltage of three-level converter so the output voltage looks more sinusoidal, therefore smaller filter is sufficient to produce allowable distortion to output voltage compared to two-level converter.
a) b)
c) d)
Figure 9. Output voltages of single-phase a) two-level, b) three-level converter and three-phase c) two-level and d) three-level converter
Many different modulation algorithms have been developed for multilevel converters but multicarrier PWM, presented in Fig. 10, and space vector modulation (SVM) are the most used methods. Multicarrier PWM is based on traditional PWM technique but multiple carriers are used to control each power switch of the converter. The carriers can be arranged with shifts in amplitude relating each carrier with each possible output voltage level generated by the converter to reduce the produced ripple in the output voltage of the converter. These methods are known as level shifted PWM methods.
Figure 10. Multicarrier PWM
The principle of the SVM method is the same with the three-level converters than in the case of two-level converters, but there are more state vectors forming the switching sequence. In a three-phase three-level converter, there are 27 feasible switching modes consisting 24 active states and three null states compared to eight state vectors of two-level converter.
The DC link capacitor voltage balancing has to be taken into account in the control system when the three-level converters are used, because the output terminal of the converter is possible to connect to the midpoint of the DC intermediate circuit. The voltage balance can be destroyed because of fast change in the operating point of the converter or because of unbalanced loading of the capacitors.
Life-expectancy of three-level converter is at least as long as two-level converters. There are more semiconductors in three-level converter but failures happen in semiconductors rarely. Usually converter broke because of failures in passive components and fans. Drawbacks of three-level converters are more complex structure and the control system.
3.2 Vienna rectifier
The Vienna rectifier consists of three IGBTs and 18 diodes as shown in Fig. 2d. Kolar has introduced Vienna rectifier for the first time in 1994. The structure of the Vienna rectifier is simpler than the two- or three-level converters’. The produced current distortion and the power factor of the rectifier are possible to control by Vienna rectifier just as with two- or three-level line converters. The drawback is that only unidirectional power flow can be used. Current path in the one leg of Vienna rectifier are shown in Fig. 11. The output terminal of the Vienna rectifier can be connected to three voltage potentials, udc/2, 0 and – udc/2 just as the NPC converter. The amplitude of the output voltage is half smaller compared to two-level converter and because of that, the voltage capability of used IGBTs can be half smaller.
D1
D2
D3
D4
D5
D6
S1
D1
D2
D3
D4
D5
D6
S1
D1
D2
D3
D4
D5
D6
S1
D1
D2
D3
D4
D5
D6
S1
Figure 11. Current path in the one leg of Vienna rectifier The diode D2 conducts during the whole positive current half cycle and the diode D3 conducts during the negative current half cycle. The current path depending on the switching states is presented in Fig. 12. The voltage balance of the DC capacitors has to take into account just as with the NPC converters.
Figure 12. Current path in the Vienna rectifier depending on the switching states (sw(a,b,c)=1
on and sw(a,b,c)=0 off) when ia> 0, ib< 0 and ic< 0 [Kolar]
3.3 Required AC-filters
LCL-filters are connected to the front of the line converters and LC-filters after the load converters as presented in Fig. 5 and 6. The inductors for three-phase systems are formed of three single-phase inductors. The LCL-filters, presented in Fig. 13a, are designed as follows:
the line current THD is 2 %, the ripple current max amplitude of iL,conv is 20 % to limit heating of the inductors, the total inductance value Lgrid + Lconv is 10 % and the capacitance C is 5 % of the base values. The filter resonance frequency should be at least ten times higher than the nominal frequency and lower than half of the switching frequency, which is 10 kHz in this case, to avoid resonance phenomena. Therefore the filter resonance frequency is set to 500 Hz < fres < 5 kHz. The LC-filters, presented in Fig. 13b, for the load converters are designed as follows: the load voltage THD is 2 %, the ripple current max amplitude of iL,conv is 20 %, the inductance value is limited to 10 % and the capacitance value to 5 % of their base values. The resonance frequency of the LC-filter should also be located between 500 Hz and 5 kHz. There are three voltage levels in the output voltage of the single-phase three-level converter compared to the two voltage levels of the two-level converter, so the harmonic content of the output voltage and current is smaller with three-level converters. Therefore the filter inductances required with the three-level converters are almost half of those with the comparable two-level converters as seen in Fig. 13. This is very important because the passive filter components increase the weight, cost and losses of the converters.
a) b)
Figure 13. Required size of the filter inductors and capacitors in a) the LCL-filters with the line converters and in b) the LC-filters with the load converters The real inductance of the filter inductors decreases when the frequency increases. The inductances are supposed to be 0,5*L50Hz with the 10 kHz frequency and 0,4* L50Hz with the 20 kHz frequency. In Fig. 14, the inductance values are presented with 10 kHz or 20 kHz frequency and the nominal inductance values at 50 Hz are twice or 2,5 times larger than these. The weights of the real inductor cores are also shown in Fig. 13.
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L _conv [mH] 1.11 0.53 0.53 2.23 1.07 1.07L _grid [mH] 0.14 0.13 0.13 0.28 0.26 0.26C [µF] 10.46 10.46 10.46 5.23 5.23 5.23fres [Hz] 4414 4816 4816 4412 4812 4812m_core_Lconv [kg] 1.53 0.5 0.5 2.74 1.53 1.53m_core_Lgrid [kg] 0.5 0.38 0.38 0.5 0.5 0.5
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L [mH] 1.22 0.66 1.32 0.56 5 2.45 1.67 1.32C [µF] 2.98 2.58 4.38 4.38 6.55 5.96 2.8 4.38fres [Hz] 2640 3857 2093 3214 879 1317 2327 2093m_core [kg] 1.64 1.04 1.53 0.83 2.99 2.74 1.64 1.53
4 CONTROL METHODS OF THE LVDC NETWORK
There are multiple different ways to control the operation of the LVDC distribution network. Basically the DC voltage control schemes can be divided in two different groups: master-slave and droop control methods. In the master-slave control, one of the converters, referred to as a master, is responsible for controlling the DC voltage and it distributes power references to the other source converters. Fast communication between the converters is needed. The voltage droop control does not require any communication between the converters. Instead, the DC voltage is measured at each converter and all the source converters try to balance the total power consumed by the loads and the losses of the system.
In the research work done by Brenna et al., the distributed generation units, battery storages and diesel generation are connected to the LVDC network as presented in Fig. 14 a. The LVDC network is able to bidirectional power flow between the MV network and the LVDC network and the LVDC network can also work in an island mode. The purpose of the control system is to allow a simple way to optimize the energy management of the LVDC distribution network. The control method is based on the DC link voltage value because it is a common signal for all of the converters. The DC link voltage is directly influenced by the voltage of the MV network, by the demand and the power generated. The control is based on four different voltage thresholds, presented in Fig. 14 b. One power system component is predominant with respect to the other devices during each voltage thresholds (droop control scheme.).
UDG
Uconv
Udiesel
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a) b) Figure 14. a) Basic structure of the LVDC network and b) voltage thresholds [Brenna] During the normal operating conditions, the line converter controls the DC distribution voltage at the voltage threshold Uconv (800 V). The supervisor computer can change the number of the running cogeneration systems so that the power from the MV network is within the contract demand. The current control of the line converter is based on d-q decoupling control. The current reference is calculated from the DC voltage reference value and the feedback value. The control of the load inverters is based on a voltage feedback
control with a current minor loop. When the voltage sag occurred in the MV network, the DC voltage is controlled to be constant by the line converter and the current on the AC side of the line converter is increased to keep the power from the MV network.
In the case of AC faults or fault in the converter between the MV network and the LVDC network, the voltage of the MV network decreases to a certain level. The system disconnects from the MV network and the operation of the line converter is stopped. When the DC voltage is fallen below Ubat, the batteries start to supply energy to the loads connected to the LVDC network. The DC/DC converter which supplies the energy from the batteries to the LVDC network regulates the DC voltage to the value Ubat (760 V). During the islanding operation of the LVDC network when the energy from the batteries is not adequate, the diesel power system supplies the LVDC network and in this case the converter of the diesel generator regulates the DC voltage to the value Udiesel (770 V DC). The supervisor computer can stop power supplying to some specific loads if there is not enough power production in the LVDC network. After the MV network is recovered, the controller of the line converter detects the phase of the MV network by phase-locked loop (PLL).
In the case of power excess in the DC network, the voltage threshold value UDG is reached and the converters of the distributed generation units limit the generated power on that way that the DC voltage is kept at the value UDG if bidirectional power flow is not possible. Otherwise the supplementary generated power can be transmitted to the MV network.
The suitability of the control system is proved by simulations and measurements with the laboratory prototype. The LVDC system is stable and the DC voltage is kept constant even if there is some disturbance in the MV network or in customer’s network. A short circuit accident at one load side does not affect the power supplying to the other loads. According to simulations and measurements in the laboratory, it is possible disconnect and reconnect the LVDC network to the MV network without any affects to customer voltage.
5 LOSSES OF THE CONVERTERS
The goal of the LVDC network is to increase the efficiency of the LV network. The power transmission capacity of LV network should increase with the use of DC because resistive losses in cables are lower because of higher voltage level. All of the transmitted power is active power so more power can be transmitted with the same cables. Consequently, the length and complexity of the MV network can be reduced because of high power transmission capacity and lower construction and cable costs of the LVDC network. Unfortunately the needed AC/DC and DC/AC converters decrease the efficiency of the LVDC network and therefore the losses of the power converters should be minimized. Conduction and switching losses of the IGBT’s and diodes are presented in Fig. 15. The total transmitted power is assumed to be 20 kW and the power factor is one in all cases. The power is produced by one line converter connected to the 1500 V DC or by two line converters connected to 750 V DC. The power is supposed to be consumed by six single-phase load converters (3,3kW/converter) or two three-phase load converters (3,3kW/phase, 10kW/converter). The cooling and control losses are not taken into account because they are presumed to be almost constant in all cases, and they do not affect the comparison substantially. The dead times of the switching components and snubber circuits are not taken into account either.
a) b)
Figure 15. The summation losses of a) the line converters and b) the load converters with resistive loads. Switching losses of the IGBTs and diodes are marked with blue, IGBTs conduction losses with red and diodes’ conduction losses with green. The losses of the diode bridge are very small as seen in Fig. 15, but the diode bridge produces low order harmonics to the MV network, which causes additional losses in the transformers. The conduction losses of three-level converters are larger compared to the two level converters, as seen in Fig. 15, because there are two switching devices in the current path. At
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the same time, the switching losses of three-level converters are smaller compared to the two-level converters because of smaller maximum voltage stresses of IGBTs. The maximum voltage stress of IGBT in three-level converters is half of those with the comparable two-level converters, which is an important advantage especially on higher voltage levels, because of limited voltage capability of IGBT. The IGBTs with higher voltage capability also have higher switching losses. The total diode conduction losses of the three-level converters are larger because of the clamping-diodes (D5 and D6 in Fig. 5).
The total losses of six single-phase half bridges and two three-level converters connected to 1500 V DC are the same as shown in Fig. 15b. The losses of the single-phase full bridges connected to 750 V DC are higher compared to three-phase converters in Fig. 5b because of two converter legs instead of one to produce the same output power.
According to the results presented in Fig. 15, the total losses of the converters connected to 750 V DC are lower compared to the losses of the converters connected to 1500 V DC because the increased voltage capabilities of the IGBTs also increase the losses. The total losses of the three-level converters are lower than the losses of two-level converters in all cases. The total semiconductor losses are typically lower with three-level converters when the switching frequency is above 5 kHz.
The AC-filters also cause extra losses to the LVDC distribution network. Losses of the capacitors are insignificant compared to inductor losses so they are not taken into account in filter loss calculations. Inexpensive EI-shaped laminated iron core (M400-50) inductors are supposed to be used in the filter inductor loss analysis. The losses of the inductors depend on the operating point of the converters but also the type of the inductor, especially core material. The filter inductor losses are caused by hysteresis and eddy currents in the iron core and DC-resistance and eddy currents in the copper windings. Eddy currents cause skin and proximity effects in the copper windings of the inductor. Accurate losses at high frequencies (10 kHz and 20 kHz) are difficult to estimate because of the nonlinear behavior of the iron core filter inductors.
The inductor copper losses are very low and most of the losses occur in the iron core. The losses at 50 Hz are not remarkable, but the harmonics at 10 kHz and 20 kHz cause significant losses. The mass of the filter inductor required with the two-level converter is almost double the size of the inductors with the three-level converters and thereby also the losses are increased. The core loss distribution between 10 kHz and 20 kHz differs with different converters depending on the current harmonic spectrum. For example the harmonics at 10 kHz cause significant losses in the inductor of the two-level half bridge but in the case of the single-phase full bridges the first harmonics are located at double switching frequency.
The total efficiency of the converters and their filters at nominal and partial load are presented in Fig. 16. The high frequency losses are assumed to not depend on loading current and therefore the efficiency of the converters is low at low load. The efficiency of the three-level converters is higher also at partial loading conditions compared to two-level converters because of lower converter bridge losses, but especially because of smaller filter inductors. The efficiency of the load converters connected to the 1500 V DC decrease more severely at
partial loading, so it would be more efficient to use load converters connected to the 750 V DC especially if the loading conditions change at a large scale.
a) b)
Figure 16. Total efficiency of the a) line converters and LCL-filters, b) load converters and LC-filters at nominal and partial loading conditions
The customer’s electricity consumption varies extensively and most of the time the converters operate under low load conditions. The total loss energies of the converter bridges and filters during one year in the LVDC distribution are analyzed with the aid of typical load characteristics of Finnish customers. The customer load is assumed to be resistive operating with unity power factor. In the first case, presented in Fig. 17a, the customer has electric storage heating which causes a high electric load at winter nights but most of the time the load is under 10 % of the nominal 10 kW power. In the second case, presented in Fig. 17b, the customer does not have electric heating, so the maximum load is only 3.4 kW compared to 8.7 kW in the previous case. The amplitude of the current harmonics is supposed to be constant in all loading situations and only the amplitude of the fundamental frequency current component varies.
a) b)
Figure 17. Duration curves of a) customer with electric storage heating and b) customer with no electric heating The total power loss of the converter bridges and their filters with both of the load types during one year are presented in Fig. 18. The total transmitted power is assumed to be 20 kW as in Figs. 15 and 16.
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a) b)
Figure 18. The total losses of a) the line converters and b) customer converters in the case of a customer with electric heating (marked as E) and a customer without electric heating during one year. According to the results, the most efficient converter topologies in the LVDC distribution with the analyzed load characteristics are the three-level line converters and the three-level, three-phase load converters connected to 750 V DC. The total efficiency of the LVDC network without losses in the DC cable is 80 % at maximum and 33 % at minimum with the analyzed converters. The foremost part of the total losses is caused by high frequency losses of the filter inductor core, especially at low load. The losses would decrease if the iron core was replaced by other materials.
The efficiency of the converters and filter inductors in the LVDC distribution is analyzed. According to the results, the converters should be connected to 750 V DC to minimize the losses. The three-level converters are a more efficient option than the two-level converters because IGBTs with smaller voltage capabilities can be used and also the required filter inductors are almost half of those with comparable two-level converters. A significant part of the total losses are caused by filter inductors and therefore the mass of the required inductors should be minimized and the material which has lower losses at high frequencies should be used in spite of higher acquisition costs.
The goal of the LVDC network is increasing the efficiency of the LV network. Since the LVDC network is composed of several components, the total losses are still higher than the losses of the traditional LVAC network with one transformer. There are multiple ways to increase the efficiency of the LVDC network and these will be analyzed in more detail in the future. The efficiency would increase for example if the modular converter structure was used. The modular structure means that there would be multiple smaller converters and a part of them would be used depending on the loading situation. The load converter cannot be dimensioned according to effective power, but short-circuit current determines its dimensioning as discussed in chapter 2.1. and because of that, the efficiency of the converters decrease. If different kind of protection devices would be used, the smaller, more efficient converters would be used. The efficiency of the LVDC distribution would also increase if some loads such as electrical heating were fed directly by DC, because the efficiency of the simple DC/DC converter can be higher than that of the DC/AC converter which would result in the DC/AC converter with a smaller nominal power to be used.
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Transformers have high efficiency, about 97-98,5%, under full as well as partial loading conditions. The drawback is that transformers are expensive, large and heavy components to install. Two transformers are need in the LVDC distribution system, the first one to decrease the 20kV voltage to 1kV low voltage AC level and the second one for galvanic isolation between the LVDC network and the customer. The efficiency of the system would be higher if the customer’s network would be also ungrounded IT-system because in this case galvanic isolation transformer is not needed. If galvanic isolation is still used, the efficiency would grow if the 50Hz galvanic isolation transformer is replaced by isolated DC/DC converter and high-frequency transformer. The different technical solutions for galvanic isolation should be researched more carefully in the future. Prices of traditional passive components like transformers and passive filters are increasing in the future and, at the same time, the prices of power electronic components are decreasing and their technical progress is very rapid.
In sparsely populated rural areas the customers are many kilometres away from the nearest distribution transformer. The construction costs of the new medium voltage branch are very high, the voltage drop during the long distance is high and the power-quality of the customer doesn’t fulfil the requirements refined in the standard. The LVDC distribution would be a good option in these cases, the good power quality can be proved to the customer by inverter and the construction costs of the LV network are lower compared to MV network.
6 LVDC FIELD TESTS
Two LVDC test systems have been constructed in Finland. The first one is constructed by ABB and LNI Verkko to Orivesi and the other one by Lappeenranta University of Technology and Suur-Savon Sähkö in Suomenniemi. One of the most challenging research questions of the LVDC network is the protection system and it will be tested by these pilot systems. The other very interesting research question analyzed by field test is the reliability of power electronic and control electronic systems in cold and damp climate conditions. 120kVA ACS800 Island Converter by ABB is used in Orivesi field tests. The whole cabinet consist of two power stages: one line converter as rectifier and the other one as island converter generating island network. Simple block diagram is presented in Fig. 19. LCL-filters are connected at the front of the rectifier and after the island converter (black boxes in the figure). There are two transformers on both sides of the converters: the first one boosts the AC voltage so that the line converter can produce 500 V DC. The transformer also prevents the possible transmission of common mode interference to the LV network. The other transformer between the island converter and the customer provides a neutral point to the customer side of the grid and galvanic isolation between DC bus and the customer network. There is a possibility to bypass the converters for maintenance or in fault situation as presented in Fig. 19.
Figure 19. Simple block diagram of the converter cabinet [Niiranen] The cabinet is presented in Fig. 20. The converters are monitored and controlled remotely via wireless GPRS communication system by SCADA system. There is a back-up battery system for communication if black-out occurs in the network.
Figure 20. Converter cabinet [Niiranen]
The purpose of the converter is to impove the power quality in the end of weak distribution line. The power to the island converter is supplied though a 920 m LV distribution line, which consists of 620 m of AMKA50 and 300 m of AMC35 subsea cable as presented in Fig. 21. The customer loads consist of mainly single phase household equipment.
Figure 21. Diagram of the distribution network in Orivesi According to measurements, the power quality of the customers increased remarkably because of the island converter. There had been several short outages (autoreclosings 0,5s) in the supply network, but there was no outage in the island network. The capacitors in the DC-link were sufficient to keep the island converter running during these supply network incidents. Before the installation, the customer voltage variation exceeded the limits of the standard EN-50160 causing voltage fluctuation and flickering. By the island converter, the voltage variation was maintained less than 2 %, the voltages were balanced and short term flicker index was decreased so the requirements of the standard were fulfilled. The island converter produced benefits also the supply network: the reactive power is zero because of the control system of the active rectifier. Single-phase and distorting load were limited to the island network only so these didn’t load nor distort the supplying distribution line. The drawbacks of the island converter were high losses because of oversized converter and audible noise caused by the cabinet’s cooling system (fan and compressor). The converters had to be oversized to meet the safety requirements.
The other pilot system is constructed by Lappeenranta University of Technology and Suur-Savon Sähkö to Suomenniemi. Because of audible noise problems in the first pilot, there is not any heating or cooling system in the cabinets. The reliability of the converters in
changing environmental conditions compared to industrial environment will be determined after the field tests. 12-pulse half-controlled thyristor rectifier is used so only unidirectional power flow is possible to use and it is not possible to increase the power quality of the MV network by the rectifier. Two-level, three-phase customer inverters are used. The LC-filter and 50 Hz galvanic isolation transformer is connected between the LVDC network and the customer so the customers don’t need to change their electrical installations. The 50 Hz transformer is very heavy but it can be replaced by high-frequency transformer in the future.
The PWM modulation of the converters causes some audible noise when the switching frequency is 10 kHz. The modulation method can be replaced by hysteresis control in the future to limit the audible noise. The block diagram of the test system is presented in Fig. 22 and installation in Fig. 23. The long DC cable, about 1,7km, enables the possibility to analyze, if there are low or high frequency interferences in the DC cable.
Figure 22. Simple block diagram of the pilot The communication channel between the rectifier station and the inverters is realized by optical fibre. The pilot system is not connected to SCADA but it can be controlled though the webportal. All the installations are ready and preliminary tests are done, but more detailed measurements will be done during the spring and summer 2012.
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Figure 23. Pilot installation a) rectifier and b) customer inverter
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LITERATURE
[Salonen 1] P. Salonen, T. Kaipia, P. Nuutinen, P. Peltoniemi, J. Partanen, “An LVDC Distribution System Concept”, NORPIE, 2008
[Kakigano] H. Kakigano, Y. Miura, T. Ise: “Low-Voltage Bipolar-Type DC Microgrid for Super High Quality Distribution” in IEEE Transactions on Power Electronics, Vol. 25, pp. 3066-3075, Dec. 2010
[Salonen 2] P. Salonen, P. Nuutinen, P. Peltoniemi, J. Partanen, “Protection scheme for an LVDC distribution system”, CIRED, 2009
[Kolar] J. W. Kolar, F.C. Zach, “’A Novel Three-Phase Utility Interface Minimizing Line Current Harmonics of High-Power Telecommunications Rectifier Modules”, IEEE Transactions on Industrial Electronics, vol. 44, No.4 August 1997 pp. 456-467
[Brenna] M. Brenna, G.C. Lazaroiu, G. Superti-Furga, E. Tironi, “Premium power quality with DG integrated DC systems”, IEEE Bucharest Power Tech Conference, 2009
[Niiranen] Niiranen J., Komsi R., Routimo M., Lähdeaho T., Antila S, ”Experiences from a back-to-back converter fed village microgrid”, IEEE PES 2010
P. Peltoniemi, “Phase voltage control and filtering in a converter-fed single-phase customer-end system of the LVDC distribution network”, Doctoral thesis, Lappeenranta University of Technology 2010 K. Engelen, E-L Shun, P. Vermeyen, I. Pardon, R. D’hulst, J. Driesen, R. Belmans, “The feasibility of Small-Scale Residential DC Distribution System”, IECON, 2006 A. Pinomaa, J. Ahola, A. Kosonen, ”Power-Line Communication-Based Network Architecture for LVDC Distribution System”, ISPLC, 2011, pp. 358 R. Teichmann, S. Bernet, “A Comparison of Three-Level Converters Versus Two-Level Converters for Low-Voltage Drives, Traction, and Utility Applications” in IEEE Trans. on Industry Applications, vol. 41, no. 3, May-June 2005, pp. 855-865. P. Karlsson, J. Svensson, “DC Bus Voltage Control for a Distributed Power System”, IEEE Transactions on Power Electronics, vol. 18, no. 6, November 2003 J. Rodriguez, J-S. Lai, F. Z. Peng, “Multilevel Inverters: A Survey of Topologies, Controls and Applications”, Transactions on Industrial Electronics IEEE, Vol. 49, pp. 724-738, Aug 2002 Tomi Hakala, “The utilization potential of LVDC distribution in the Vattenfall distribution network”, BSc thesis, Tampere University of Technology, 2012
