Power Flow Efficiency of a DC Distribution Grid

Transcription

1 Power Flow Efficiency of a DC Distribution Grid A test case with a High Penetration of Renewable Energy Resources in Different Operational Modes TuanDat Mai, Jeroen Tant, Johan Driesen ESAT/Electa, KU Leuven, Heverlee, BELGIUM. tuandat.mai, jef.beerten, jeroen.tant, johan.driesen@esat.kuleuven.be Abstract A models of a DC local distribution grid with penetration of distributed energy resources (DER) is studied that are expected to increase significantly in the future. Because the existence AC distribution networks are not optimally designed for bidirectional energy flows caused by the distributed energy resources (RES, storage energy systems, PHEVs), a replacement of delivering DC power in conductors for existing AC network is assumed. Therefore, a DC distribution grid with participation of renewable energy resources will be presented in different operational modes to evaluate the efficiency of power control solution using DC signal in terms of power flow and total losses. A comparison with AC power flow is the same grid is also introduced to clarify the viable alternatives of a low-voltage DC grid for bidirectional power flow in future. Keywords- DC grid; efficiency; distribution; energy flow; I. INTRODUCTION Electricity was discovered from 19 th century and quickly developed to be one of major energies, which is widely used. Development of electric power has been started both in AC and DC power in 1880s. Due to advantages of AC power transmission compared to DC power, AC electric networks are deployed in all of the architecture of power network in 20 th century. Since the increasing demand of energy requires having more power generation while conventional generations cause to badly impact on environment, more green and distribution energy sources such as PV and wind turbines have been installed in utility grid. Nevertheless, AC local distribution networks are not designed to adopt DC power generation and draw bidirectional power flow. Additionally, development of power electronics is recently getting achievements that AC-DC energy conversion is more flexible and more efficient. The success of recent HVDC [1] projects proved the efficiency in terms of long-distance DC power transmission, while DC loads in datacenters are good example of DC power s efficiency in local areas [2], [3]. A DC local distribution grid is a suitable solution that PV arrays, PHEVs and energy storage systems without AC- DC round-trip can work together in a low voltage utility grid. The connection of DER to a DC distribution bus does require neither frequency synchronization nor reactive power compensation. It hence gives one of advantages of DC bus compared to AC systems. However, the architecture of HVDC transmission and MV/LV DC distribution has slightly differences. While a HVDC exists in mesh topology and requires a tight AC/DC interconnection, a DC distribution grid is mostly constituted by radical topology. On the other hands, strengthening grid reliability is the key requirement for HVDC while coupling on-site DC generation, DC loads, energy storage systems, PHEVs and efficiency in a local network is the highest considerations for DC utility. Offering more flexibility with a lower price for DERs and DC-power electrical devices is also a consideration in DC distribution grid. Previous researches concluded that a reduction of approximation 5-20% in electricity charges depending on bus voltage could be expected when data centers with an AC distribution bus migrates to a DC distribution system [2], [4]. In order to achieve a lower price of energy use, efficiency of power a one of benchmarks. Therefore, a DC distribution grid with participation of PV generation will be presented in different operating cases to evaluate the efficiency of power control solution using DC signal in terms of power flow and total losses. A comparison with AC power flow is the same grid is also introduced to clarify the viable alternatives of a low-voltage DC grid for bidirectional power flow in future. II. ASSUMPTION AND MODEL A. Model and role of converters: Power converters including AC/DC, DC/AC and DC/DC types are required in order to control power absorbed by loads in an optimal way in a DC power architecture. Due to different characteristics of them, several models of converters and loads are assumed. 1) Active Front-End Converter: provides a bidirection energy link between an AC bulky power source and a DC feeder. It converts AC power to DC power when energy production grenerated by local DERs is in lack. Contraditonally, it tranfers energy back to the AC utility if there is a redundacy of energy generated by DERs. 2) Three-phase rectifier: is used to connect a AC power gerator to a DC feeder such as wind generators. The DC

2 voltage at the DC feeder is regulated depending on operational modes of DC power circuitry. Due to small inertia of low-power wind generators, it is not necessary to account a bidirection-flow converter which converts DC power back to mechanical power for inertia reserves. 3) Uni-direction DC converter:converts DC energy captured by PV arrays to a suitable DC voltage level and supplies to DC feeders. Non-rechargable DC power sources such as PV could be connected to DC bus by using this type of converter to benefit from advanced efficiencies and power control. 4) Three-phase inverter:converts the DC power from a feeder to three phase AC power that supplies power to rotating mechanical loads such as AC motors. In fact it can be considered as a variable-frequency drive without a rectifier stage thanks to the exsitence of DC power in the feeders. 5) Single-phase inverter: functionates as one output phase of a three phase inverter which supplies for separate single-phase AC loads such as house-hold appliances. 6) Bidirectional DC converter: steps up or steps down DC voltage and links power to a DC power terminal. The power terminal can be considered as a loads for charging a battery energy storage/phevs ; as well as a power supply for discharge a battery energy storage/phevs. B. Proposed Modes of Operation in a DC grid The DC voltage at internal distribution buses is passively controlled depending on the two factors difference between the total renewable energy production and total demand; and the situation of bulky power interconnection. The operation of the proposed DC local distribution grid is hence categorized in four modes which are slightly modified compared to [5] and represented infig. 1. 1) Mode 1- islanding mode with battery discharging The DC grid operates in islanding mode and the DC bus voltage is regulated by battery discharging, which means the generated PV power is less than local load demand. Slack bus voltage is regulated at 95% of DC bus voltage in grid mode. 2) Mode 2- grid-connected mode The DC grid operates with a power interconnection to a higher power distribution system via an Active Front-End converter. The output voltage of the AFE is kept as a slack bus while generated DERs power is less than the demand. Converters for PV arrays and rotating generators work in MPPT and bi-directional DC/DC converter for energy storage does not work. PHEVS can be simultaneously charged in this mode. 3) Mode 3- grid-connected mode with battery charging The DC grid is connected to a higher power distribution system via an AFE converter similarly to Mode 2. The output of AFE is still kept as slack bus while generated PV power is greater than the local load demand; hence they are controlled in droop control. Battery storage systems are charged if they are not full. The charging current is also controlled to keep power flow nearly equal to zero at slack bus, which minimize loss of AC/DC conversion. 4) Mode 4- islanding mode with DC voltage regulation The DC grid operates in islanding mode. DC bus voltage is regulated by converters for DERs. As MPPT power of distribution generators is greater than the actual demand of loads, the redundancy power is for charging battery energy storage. If they are fully-charged, they are switched to burst mode and the node generated the highest power works as slack-bus at the highest acceptable voltage in this mode. Slack bus voltage is kept at 105% compared grid-mode at the bus where PV produces the highest power. The coefficient difference between these modes is defined at 5%, which doesn t cause the malfunction during switching from one mode to another mode. C. Assumption of losses in DC networks: Energy losses are main factors which is influence to the efficiency of a system. Losses in the proposed network can be classified into 3 groups including converter losses, losses of wiring and losses of non-mppt operation of DERs. 1) Converter losses: are refered to losses of power electronic switches. It can be categorized to static losses, switching losses and driving losses [6]. For the ease of calculation in next steps, it is assumed that all converters operate at 10kHz switching frequency at 25 o C. a) Static losses: On-state losses is actually conduction losses caused by power switches and reverse diodes which is dependent on the load current, the junction temperature and the duty cycles. Blocking losses accounts for a small share of the total power dissipation and it can be neglected in case of low voltage and low environmental temperature. b) Switching losses: Switching losses consist of turn-on losses and turn-off losses. They are dependent on load current, electric load type, DC link voltage, freewheeling diode, junction temperature and switching frequency. All of these Fig. 1. Operational modes of the proposed DC grid Fig. 2. Efficiency of different power converters

3 parameters are considered as constants except the DC link voltage and the load current. c) Driving losses: can be neglected since conveters operates around 400V at DC bus and switching frequency of 10kHz. By taking account the loss specification of converters, efficiency of different converters is introduced in Fig ) Branch losses: can be calculated as rline Iline where rline is resistance value of the branch and Iline is current carried by the branch. 3) Losses due to the non-mppt operation of RESs: is defined as the difference between power produced at MPPT condition and the actual generation power. These losses occur only in Mode 4 when all of battery energy storage systems are fully-charged and few converters switch to burst mode. Due to these factors, total losses at each node vary depending on the percentage of loads at certain conditions and mode of operations of the DC grid. D. Assumption of differences in AC network model and DC network model: In an AC distribution system, three phase network is commonly used. One of three feeders is sequentially contributed and supply AC power to households. However, demand at each house is not balance that leads to non-zero current in the neutral conductor [7]. Therefore, losses and voltage drops are greater than a three-phase balance load. As a consequence, the lowest loss case in AC system, which is three phase-balance load, is assumed and can be represented by its equivalent single line diagram. The three phase active power is defined as the following. V I cos P 3 3 l l where P3 : active power V, I : line to line voltage and line current l l line line cos : power factor of load In DC networks, the power at generators and loads is calculated by P V I pol where P D C DC DC line : power V D C, I : voltage from positive/negative lin e polarity to neutral and line current pol : number of polarity (1 for mono-polar and 2 for bipolar) In both AC and DC networks, cable EXVB 1kV is used for distributing power. While AC systems uses 3 conductors as three AC phase power and one conductor as neutral phase, DC system twins 2 conductors as the positive conductors and double 2 other conductors as the negative/neutral conductor. Since there is no reactive power produced or consumed in a DC power system, the loads modeled in DC is of the same real power magnitude[7]. Zwire r AC XLPE j xxlpe length Z r j x length where wiredc XLPE XLPE Z : total impedance wire rxlpe, x : resistance and reactance values of XLPE the cable specified in datasheet length : length of the feeder E. Assumption of Power Line Communication (PLC): It is assumed that the demand at loads and the production at each generator are known. The data can be transferred and accessible by converters in the DC infrastructure via PLC. III. POWER FLOW AND EFFICIENCY CALCULATION Start Demand power RES production Mode of operation Loss calculation at converters Power flow calculation NR method Total loss at nodes and lines Efficiency calculation Stop Fig. 3 Flow chart of power flow and efficiency calculation TABLE 1. PARAMETERS IN DISTRIBUTION GRID Power demand kw Length of the main feeder 725 m EXVB 1kV_ 4x150 (main feeder) Cable type EXVB 1kV_ 4x95 (main feeder) EXVB 1kV_ 4x16 (to loads/pvs) ABB EcoDry Basic Transformer 250kVA, 24/0.4KV IGBTs 1200V, forced air cooling AFE: 400V/250 KVA DC/DC converter: 400V/10KVA Power electronic switch DC/AC converter: 400V/10KVA DC/DC bi-directional: 400V/100kVA Energy storage Pb-acid battery, 34 packs 12V

4 Due to having a suitable DC bus available in the distribution architecture saves losses and chance of failure [2], the standard DC bus voltage is defined at 400 V to cope with household electric appliances, conventional converters and distribution cable infrastructure. Defining the power difference of demand and production is firstly processed in the main routine. If mismatch demand of the DC local grid is fulfilled by storage systems either by AC bulky power, DC grid operates at Mode 1 or 2 depending on the condition of interconnection. If the total produced power by PVs is greater than the local loads demand, DC grid operates at Mode 3 or 4, while the extra power is highly contributed for battery charging. Afterwards losses corresponding to each converter are calculated to define the certain amount of injected power of generators and real consumed power of local loads. Outage converters are disconnected from terminals. Data for the next step of calculation are power at distribution nodes, which is called Point of Loads (POL). If DC grids operates under Mode 1 or 4, the status of grid coupling converter- AFE converter s outage are considered as stations without AC grid connection. The power flow calculation is mainly involved by Matpower, MatACDC [8], [9], [10], [11] and some additional routines to obtain real power solutions of the DC networks. The file format of test case is compatible to MatACDC, a free MATLAB based open source program for AC/DC power flow analysis[12]. Before involving Newton- Raphson power flow calculation, input data is converted into pu. The solutions of power and voltage drop are used to calculate losses of the distribution grid and the efficiency as well. The main routine stops after converting pu result to SI units. Fig. 4. Schematic diagram of the DC semi-urban network [15] for the scenario of high-density penetration of PV installation [13]. Four cases corresponding to 4 proposed operation modes are evaluated in this paper. - Test case 1: The DC local grid is operating under islanding mode while production from PV is less than the total demand Ppv < Pdemand. Voltage drop in Test case 1 is shown in Fig Test case 2: The AC/DC converter at bus 1 is running to supply extra power from AC bulky system to the DC grid while production from PV is less than the total demand P pv < P demand. As a consequence, converter at bus 1 is chosen as slack bus, there by controlling the voltage on the network. Other converters control power. - Test case 3: DC local distribution grid is still in gridconnected operation. Due to production from PV is greater IV. TEST CASES AND RESULTS A typical Belgian household distribution network is introduced in Fig. 4 and Table 1. It consists of 62 loads/generators except node 1 connected to the AC bulky power system. Energy storage systems are placed at the intersection or at one end of the feeder. At each generator or loads, there is a power converter to control power flow and DC voltage. 48 PV sources are integrated with household s loads and provides from 1.81 to 4.40 kw of electric power. The PV peak power of 10-kW per three houses is assumed Fig. 5. Voltage drop in AC architecture and DC architecture

5 Fig. 6. Branch losses at different cases than the total demand P pv > P demand in this case, converter at bus 1 still controls DC voltage as a slack bus while others converters control power production/injection. It is similar to case 2. Redundancy power produced by PVs is partly transferred back to the bulky system through the DC/AC converter at bus 1. - Test case 4: There is not energy link between the DC local grid and the AC bulky system. The DC local distribution is under islanding operation while there a power redundancy of PVs production as compared to total demand. Therefore, the generator producing the highest power operates as a slack-bus, while other converters operate as controlling power. At bus 61, the converter operates in non- MPPT condition and reduces their efficiency since batteries are fully-charged. When the DC grid is connected to the AC bulky system in Case 2 and Case 3, voltage at bus 2 is kept at 400 V. Although the lowest voltage is V in Mode 2, it drops less than in AC case in which voltage is V. Thanks to production support from PVs, voltage at loads is around 400V in Case 3. There is also a very small voltage drop in Mode 1 and Mode 4 shown in Fig. 5 and Fig. 8. In terms of losses, converter losses mostly contribute in DC grids and they are the highest in Case 3 compared to other cases, which can be seen in Fig. 7. Since most of loads operate as nearly-zero power nodes, power transferred between these nodes is minimized. Branch losses are shown in Fig. 6 branch losses in case 2 are the higher than other 3 DC cases because most of power draws from slack bus to distribution load. However, it is small compared to AC case thanks to the wiring structure and DC power characteristic. Fig. 8. Differences of voltage drop compared to base voltage The total required power in Case 2 is approximately the least values compared to other 3 cases. However, Case 2 has less efficiency gain than Case 3 and 4. Efficiency in Case 2 and Case 4 is subsequently equal to 82.09%, 84.08% and 84.11%. It is consequence that around 19kW of PV production is either sent back to grid or charged battery in energy storage systems. For the comparative study shown in Fig. 9 the efficiency rating of the DC grids is not completely more superior to AC grids; even there are several inverters and constantpower electronic loads such as variable frequency drives and adjustable lighting systems in AC systems. AC and DC distribution systems can have the same merit when the loads are equal in ratio, the distribution grid is connected to the AC bulky system and there are few of inverter loads in AC case. The difference of efficiency in AC case with inverters, Mode 2, Mode 3 and Mode 4 is slightly around 2 % in total. The efficiency of these operating cases gains from 0.4 to 2.5% compared to AC case with inverter, while much lower than AC case, which is roughly 92% if there is not any DC on-site generation at distribution network. In fact, a new technology transformers applied in this paper would be competitive to increase the overall efficiency. Due to lower efficiency in battery discharging, the efficiency of 76.70% in Case 1 is the worst in this comparative result. V. CONCLUSION In this paper, a test case of a Belgian DC local distribution grid with a participation of DERs and storage energy systems have been introduced in different operating mode including islanding and grid-connected. Both voltage drop, total losses and efficiency of the DC architecture is Fig. 7. Contribution of demand and losses in total required power of different test cases

6 power flow calculation MatACDC. The DC powerflow analysis is partly inherited from his previous PhD works. REFERENCES [1] H. Knaak, Modular Multilevel Converters and HVDC / FACTS : a success story The modular multilevel technology, in Proceedings of the 14th European Conference on Power Electronics and Applications (EPE 2011), 2011, no. Lcc, pp [2] P. T. M. Vaessen, DC power distribution for server farms, Fig. 9. Efficiency of power in different test modes evaluated in simulation and compared each other. The paper also investigates AC power distribution architecture with a high efficiency transformer and compares it to the promising DC power distribution alternative. Efficiency is highly depends on working load compared to the rated capacity of converters, as well as modes of operations of DC networks. AC distribution grid is more advantage where the penetration of DERs and DC-powered loads is low. The result reveals that the efficiency gain of DC power distribution architecture is not much higher than in AC case. Compared to previous research, DC efficiency on data centers which consist of all electronic DC loads would achieve 5-20% efficiency gain while the efficiency gain of a DC local distribution grid in this paper is limited from 0.4 to 2.5%. In fact, the calculated result matches to the newest conclusion on high efficiency on data centers given by APC in [14]. It can be explained that the efficiency can be improved in DC grids by increasing number of DC electronic loads and by keeping a link between AC bulky system and DC local distribution grid. AC distribution grids also have its merit if modern- high efficient distribution transformer is applied in the conventional AC systems. Alternatively, gain of efficiency is highly varies depending on the efficiency-curves of each components in DC networks. However, the future DC local distribution grid unlikely depends on the efficiency gain in steady state power flow. DC power distribution holds the most advantage for the connection of emerging technologies for on-site power generation and energy storage as a significant amount of this equipment delivers power in the form of DC or alternatively as high frequency AC, which then requires an intermittent DC conversion. The efficient issues of primary reserves and power flow redistribution via power electronic conversions in transient are looking forward. VI. ACKNOWLEGMENTS The authors gratefully acknowledge Jef Berteen, a colleague who developed an open source code for AC/DC [3] Onboard DC Grid - Breakthrough for DC Technology, no. April p. 1, [4] S. Levy, Why DC Power is the Next Wave in Power Generation, Storage and Applications Prepared by. Tennessee Solar Energy Association, p. 49, [5] L. Zhang, T. Wu, Y. Xing, K. Sun, and J. M. Gurrero, Power control of DC microgrid using DC bus signaling, in 2011 Twenty- Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC), 2011, pp [6] Application Manual Power Semiconductors, [7] M. Starke, L. M. Tolbert, and B. Ozpineci, AC vs. DC Distribution : A Loss Comparison, in IEEE/PES Transmission and Distribution Conference and Exposition, 2008, pp [8] R. D. Zimmerman, C. E. Murillo-Sánchez, and R. J. Thomas, MATPOWER : Steady-State Operations, Systems Research and Education, IEEE transaction on Power Systems, vol. 26, no. 1, pp , [9] M. Padhee, P. K. Dash, L. S. Member, K. R. Krishnanand, and P. K. Rout, A Fast Gauss-Newton Algorithm for Islanding Detection in Distributed Generation, IEEE Transactions on Smart Grid, vol. 3, no. 3, pp , [10] E. Veilleux, S. Member, B. Ooi, and L. Fellow, Power Flow Analysis in Multi-Terminal HVDC Grid, in Power Systems Conference and Exposition (PSCE), 2011 IEEE/PES, 2011, pp [11] K. Purchala, L. Meeus, D. Van Dommelen, and R. Belmans, Usefulness of DC Power Flow for Active Power Flow Analysis, in IEEE Power Engineering Society General Meeting, 2005, p. 6. [12] [13] A. Woyte, V. Van Thong, R. Belmans, and J. Nijs, Voltage Fluctuations on Distribution Level Introduced by Photovoltaic Systems, IEEE Transactions on Energy Conversion, vol. 21, no. 1, pp , [14] W. Paper, N. Rasmussen, and J. Spitaels, A Quantitative Comparison of High Efficiency AC vs. DC Power Distribution for Data Centers, White Paper. APC - Schneider Electric, p. 21, [15] J. Tant, G. S. Member, F. Geth, D. Six, P. Tant, and J. Driesen, Multi-Objective Battery Storage to Improve PV Integration in Residential Distribution Grids, IEEE Transaction of Sustainable Energy, 2012.