Dr. Chengxiong Mao,Professor School of Electrical and Electronic Engineering Huazhong University of Science and Technology (HUST) P. R.

Dr. Chengxiong Mao,Professor School of Electrical and Electronic Engineering Huazhong University of Science and Technology (HUST) P. R. China Received his B.S., M.S. and Ph.D. degrees in Department of Electrical Engineering, from HUST, in 1984, 1987 and 1991, respectively. Visiting scholar in University of Calgary, Canada, from Jan. 1989 to Jan. 1990, and in Queen s University of Belfast from Dec. 1994 to Dec. 1995 respectively. He was doing researches in Technical University of Berlin from Apr. 1996 to Apr. 1997 under the support of Humboldt Foundation. Research fields: power system operation and control, the excitation control of synchronous generator and applications of high power electronic technology to power systems. 1

Panel 4: DC Distribution Technologies PN4-05 Multi-level DC Distribution System and Key Technologies Chengxiong Mao cxmao@hust.edu.cn School of Electrical and Electronic Engineering Huazhong University of Science and Technology Oct. 2014

Outline Multi-level DC Distribution System and Key Technologies Background Multi-level DC Distribution System Grid Configuration Voltage Class Series Operation & Control Key Technologies DC Transformer DC Circuit Breaker DC Transducer EPT-based AC/DC Hybrid Distribution System EPT Concept Application Scenarios Demo System & Simulations Future Development 3

Background Multi-level DC Distribution System and Key Technologies Wind Farm PV Fuel Cell Energy Storage Electric Vehicle Microturbine DC Loads Cable Transmission Heavy Load Supply Conventional AC Distribution System Big Challenge Hybrid AC/DC Distribution System DC Distribution System DC Power System AC Power System DC Power System??? 5

Background Multi-level DC Distribution System and Key Technologies Advantages of DC Distribution System 1 Low fault rate in circuit Capability of monopole operation, which transmits part of the power without interruption Fast responding and quick recovery Inherent self-protection of power electronics preventing internal faults affecting external system 2 Lower transmission losses (no reactive power, lower resistance) More efficient for DC loads Stronger power supply relatively 6

Background Multi-level DC Distribution System and Key Technologies Advantages of DC Distribution System 3 In AC distribution, frequency and reactive power control are necessary in addition to voltage control. Buried cables are superior to overhead lines in view of reliability and urban landscape. AC transmission via buried cables is not practical because of severe charging current. DC distribution could avoid these problems. 7

Background Multi-level DC Distribution System and Key Technologies Advantages of DC Distribution System 4 By instant regulation of power electronic equipments 5 Output of distributed sources ranges from DC to aperiodic AC and their dynamic performances vary. So it's convenient to convert outputs to DC uniformly for interconnection and control 8

Background Multi-level DC Distribution System and Key Technologies DC Distribution System Also faced Many challenges 9

Multi-level DC Distribution System Grid Configuration The Critical Equipments: DC Transformer DC Circuit Breaker DC Cable DC Transducer High Performance High Reliability Low Cost 11

Multi-level DC Distribution System Grid Configuration Small DG Energy storage Rail Transit DC loads Low voltage Medium voltage AC loads DC-EPT DC-EPT DC/AC High voltage DC-EPT DC/AC AC loads DC-EPT Medium voltage Low voltage Electric vehicle Large renewable sources Topology of multi-level DC distribution system (a simple case) 12

Multi-level DC Distribution System Grid Configuration Features: Satisfy power supply requirements of different levels Allow integration of electrified railway, Electric Vehicle charging station, urban metro system, large renewable source and DG at user side Decouple different stages of DC network Decline power requirements of DC circuit breakers, since DC transformer(dc-ept) can restrict rising of fault current due to its fast-responding features 13

Multi-level DC Distribution System Voltage Class Series Constraints: High load density in the future Load Density of Major City in China in 2020 City Type Load Density(MW/km 2 ) Developed 10~40 Developing Undeveloped 5~10 3~5 14

Multi-level DC Distribution System Voltage Class Series The development level of related technology ( e.g. VSC-HVDC ) Profile of typical VSC-HVDC projects Project Nation Time Rating/MW Voltage/kV Length/km Gotland Sweden 1999 50 ±80 70 Cross Sound Cable USA 2002 330 ±150 40 Estlink Estonia- Finland 2006 350 ±150 105 Trans Bay Cable USA 2010 400 ±200 88 Shanghai China 2011 20 ±30 8 Nanao China 2013 200 ±160 32 Zhoushan China 2014 1000 ±200 134 Dalian China CIP 1000 ±320 60 DolWin1 German y CIP 800 ±320 165 INELFE France-S pain CIP 2 1000 ±320 64 15

Multi-level DC Distribution System Voltage Class Series Optimization of grid structure To optimize grid structure, DC distribution system should support DC loads without superfluous converters, so proper integration voltage class should be set for certain items. 16

Multi-level DC Distribution System Voltage Class Series Optimization of grid structure (cont d) Other equipment engaged in aperiodic, non-power frequency or non-threephase operations suggests to be supported by DC. 17

Multi-level DC Distribution System Voltage Class Series Transition from traditional AC distribution system In this case, the insulation requirement of DC voltage should be no higher than the original AC voltage. 18

Multi-level DC Distribution System Voltage Class Series Proposal of DC voltage class series: Proposal of DC Voltage Class Voltage Capacity Cross section Supply radius(km) (kv) (MW) (mm 2 ) Al Cu ±320 1024 1200 408 685 ±150 225 600 204 343 ±30 18 300 51 86 ±10 7 200 6.8 11.4 ±0.75 0.04 90 3.06 5.14 0.4 0.01 60 2.04 3.43 19

Multi-level DC Distribution System Explanations: Voltage Class Series ±320 kv,±150 kv are for the dense load demand in the future, they have stronger supply ability than 500 kv, 220 kv (in AC) respectively, and they are standard voltage-levels in domestic VSC-HVDC projects. ±30 kv meets the IEC standard that voltage ratio may be greater than 5 between 50~150 kv, and it allows integration of electrified railway and large renewable energy sources. ±10 kv has equal power supply ability with 20 kv(ac) while maintaining 10 kv(ac) power lines. ±750 V supports urban metro and small DGs while 400 V and 48 V support most household and enterprise appliance. Each DC voltage has corresponding voltage in AC system, thus making it easier for interconnection and transition. 20

Multi-level DC Distribution System Operation & Control Steady state operation & control Transient state operation & control Stability analysis Economical operation & control Protection Reliability evaluation Many AC distribution system operation & control methods can also be referenced by DC distribution system. 21

Key Technologies DC Transformer AC very easy AC DC?? DC Requirements for DC transformer: Safe Efficient Smart 23

Key Technologies DC Transformer Existing schemes for DC transformer: Conventional boost converters Cannot achieve high gain and high power Unidirectional, poor controllability Switched capacitor converters Need too many modules to achieve high gain, which implies significant loss and complexity Resonant converters Increased switching losses Poor power quality Difficulties with power direction reversal Most of these schemes are not suitable for DC distribution system because of limitation in power scale, efficiency and controllability. 24

Key Technologies DC Transformer EPT(Electronic Power Transformer):DC/DC Middle/high frequency transformer General structure of DC-EPT 25

Key Technologies DC Transformer Features : Internal MF/HF ac transformers to achieve potential isolation, bidirectional power flow and flexible phases and gain Using cascade H-bridge (LV) or Modules Multilevel Converters (MMC) to achieve high power and reduce volume and weight of transformers and capacitors Lowest switching losses are achieved with a step modulation to obtain high efficiency The primary and secondary converters are blocked when dc circuit fault occurs, thus the main circuit breaker can be rated lower 26

Key Technologies DC Transformer Middle/High Frequency Transformer At present, high voltage, high power, low loss & low cost middle/high frequency transformer is still very difficult. Magnetic core materials: Silicon Steel, Amorphous, Ferrite, Nanocrystalline, 10kV/42kVA/1kHz middle frequency transformers

Key Technologies Features of DC short circuit fault: DC Circuit Breaker No current zero crossing, so DC breakers need to tolerate ultra overvoltage and current caused by generating zero crossing fault penetration is much faster and deeper because of low impedance, hence it is necessary to clear the fault within a few milliseconds DC circuit breaker based on LC oscillation system 28

Key Technologies Existing types of DC breakers: DC Circuit Breaker 29

Key Technologies DC Circuit Breaker ABB Hybrid HVDC Breaker The hybrid HVDC breaker is designed to achieve a current breaking capability of 9.0 ka in an HVDC grid with rated voltage of 320 kv and rated HVDC transmission current of 2 ka. 30

Key Technologies DC Circuit Breaker Proposed DC Circuit Breaker for DC distribution System Produce current of which size and waveform can be changed flexibly to superimpose on the DC fault current Generate artificial zero crossing & reduce positive amplitude of the superimposed current Decrease effectively the electricarc and damage to the switching contact u d L 1 C 1 L 2 u1 u2 C 2 i I dc QB Topology of the DC circuit breaker Waveform of superimposition current i sw R 31

Key Technologies DC Transducer How to measure DC voltage & DC current quickly and precisely and in low cost is very important for multi-level DC distribution system. Hall effect current/voltage sensors

EPT-based Hybrid Distribution System EPT concept Electronic Power Transformer (EPT) A static device that transfers electrical energy from one circuit to another through medium- or highfrequency (MF or HF) electromagnetic inductionbased transformation and power electronic conversion technology. Also named Solid State Transformer (SST) or Power Electronic Transformer (PET). Conceptual diagram of EPT 34

EPT-based Hybrid Distribution System AC AC EPT concept DC DC In fact, EPT is now a general concept DC-EPT AC-EPT Hybrid-EPT Without dc-link DC grid application With dc-link AC grid application Interface for ac & dc EPT can satisfy the different requirements of different power sources and loads. 35

EPT-based Hybrid Distribution System Application Scenarios Interface for dc micro-source Interface for ac micro-source 36

EPT-based Hybrid Distribution System Application Scenarios Interface for AC/DC distribution system 37

EPT-based Hybrid Distribution System Application Scenarios EPT-based industry distribution system Single system Power generation by remaining heat, pressure & gas, etc. In the large-scale industry enterprises, the conventional captive power plants may be integrated to enhance the supply reliability. Multi system Several systems can be interconnected to form a cluster of systems, and the loads in each system could be supplied from others. 38

EPT-based Hybrid Distribution System Demo System & Simulations Main Parameters of the Demo System 39

EPT-based Hybrid Distribution System Output Output power of PV PV (W) (W) DC load power (W) DC load power (W) Case I: Standalone state 12000 9000 6000 3000 0 0 0.1 0.2 0.3 3500 3000 2500 2000 1500 1000 500 time (s) -200 0 0.1 0.2 0.3 time (s) time (s) Output voltage control operating mode 0 0 0.1 0.2 0.3 Voltage Voltage of AC of AC bus (V) AC load power (W) 200 100 0-100 EPT with PV & ESS time (s) 7500 6000 4500 3000 1500 time (s) time (s) 0 0 0.1 0.2 0.3 Demo system time (s) time (s) The PV and battery together guarantee the reliability of the power supply for critical loads AC load power (W) The power of PV varies from 10.1 kw to 6.8 kw at 0.2 s, while the DC bus is loaded 3 kw and the AC bus is loaded 7 kw. EPT can maintain the AC bus voltage. The entire system can satisfy the load demand regardless the variations from PV. 40

Output power of PV (W) EPT-based Hybrid Distribution System Case II: Grid-tied state 12000 9000 6000 3000 Output power of PV (W) time (s) 0 0 0.2 0.4 0.6 0.8 1.0 0 0 0.2 0.4 0.6 0.8 1.0 time (s) time (s) Real power tracking operation mode DC load power (W) 4000 3000 2000 1000 DC load power (W) time (s) Demo system The DC loads are set at 3 kw and the AC loads are set at 7 kw. The maximum output power of PV varies from 10.1 kw to 6.8 kw at 0.1 s, and back to 10.1 kw at 0.4 s. And half of AC loads are switched off at 0.6 s. Power flowing through EPT (W) 10000 7500 5000 2500 Power flowing through EPT (W) 0 0 0.2 0.4 0.6 0.8 1.0 time (s) time (s) Power provided by utility grid (W) 6000 3000 0-3000 Power provided by utility grid (W) 0 0.2 0.4 0.6 0.8 1.0 time (s) time (s) The system can always obtain the maximum output power from the PV, and the excess power is transferred to the utility grid. Both the PV and utility grid power the loads reliably and greenly 41

EPT-based Hybrid Distribution System Output power of EV (W) Output power of EV (W) DC load DC power load (W) Case III: EV Powering state 0-1000 -2000-3000 -4000-5000 -6000 0 0.25 0.50 0.75 1.00 1.25 1.50 1500 1000 500 time (s) 0 0 0.25 0.50 0.75 1.00 1.25 1.50 AC load AC power load (W) RMS RMS voltage of AC bus bus (V) (V) 4000 3000 2000 1000 0 0 0.25 0.50 0.75 1.00 1.25 1.50 Output voltage control operating mode EPT with EV time (s) 150 100 50 time (s) time (s) 0 0 0.25 0.50 0.75 1.00 1.25 1.50 time (s) time (s) EV can enhance the power supply reliability of the critical loads in an emergency Demo system EV can be integrated to serve as a temporary power supply. The critical loads are set at about 4.8 kw, including DC loads 1.3 kw and AC loads 3.5 kw. At 0.3 s, the AC loads are reduced to half. EV can power loads through the EPT in a critical condition, and the power supply reliability would be enhanced. 42

EPT-based Hybrid Distribution System By far, one 10 kv/400v/500kva industrial prototype of AC-EPT was been developed. This valuable experience may attribute to development of DC-EPT. Brief introduction: 10kV/400V/500kVA AC-EPT Industrial Prototype 43

EPT-based Hybrid Distribution System The prototype will be installed in Wuhan Iron & Steel (Group) Corp. 10kV distribution system to increase the power supply quality and reliability in 2014 Industrial Distribution System Input stage isolation stage A output stage MF 10kV/500kVA EPT Industrial Prototype B C 44

EPT-based Hybrid Distribution System 10 kv Laboratory Experimental Results Steady state input voltage & currents Steady state output voltages & currents 45

EPT-based Hybrid Distribution System 10 kv Laboratory Experimental Results Startup Loaded 46

Future Development DC Loads high efficiency Advanced DC Transducer high accuracy & low cost Future Development Multi-level DC Distribution System Power Electronics high voltage & high power low cost & low loss high reliability Advanced Control Strategy high performance Advanced Material less volume & low loss & low cost 48

Summary Multi-level DC Distribution System and Key Technologies Background Multi-level DC Distribution System Grid Configuration Voltage Class Series Operation & Control Key Technologies DC Transformer DC Circuit Breaker DC Transducer EPT-based AC/DC Hybrid Distribution System EPT Concept Application Scenarios Demo System & Simulations Future Development 49

Thanks! 50