Distributed Generation: Benefits & Issues

Transcription

1 Distributed Generation: Benefits & Issues Johan Driesen K.U.Leuven ESAT/ELECTA Traditional low voltage grid Limited number of loads Energy supplied top-down from central power station Increased loading Increased power quality & reliability problems: due to non-linear (power electronic) and sensitive loads 2nd Int. Conf. on Energy Innovation / 6-7-'06 2/52

2 Why are grids changing? 3 technological drivers Power Electronics (PE) becomes ubiquitous in loads, generators and grids More power produced (and stored) near consumers: Distributed Energy Resources (DER) Increased importance of Power Quality (PQ): more disturbances and more sensitive devices 3 socio-economic tendencies Liberalization of energy markets: free choice of supplier More sustainable energy (renewable and high-quality ) Less guaranteed security of supply 2nd Int. Conf. on Energy Innovation / 6-7-'06 3/52 Definition of Distributed Generation Distributed Generation (DG) connected directly to the distribution system or installed at the customer's side of the meter; non-dispatched by the network operators; not centrally planned; small-scale generators ( 50 MW). Distributed Energy Resources (DER) also including storage, active loads 2nd Int. Conf. on Energy Innovation / 6-7-'06 4/52

3 DER technologies Distributed Generation: Reciprocating engines Gas turbines Micro-turbines Fuel cells Photovoltaic panels Wind turbines Energy Storage Batteries Flywheels Supercapacitors Rev. fuel cells Superconducting coils Rotor CHP configuration Armature Winding Holders Field Winding & Bobbin Powder Iron Toroids Housing Endcap 2nd Int. Conf. on Energy Innovation / 6-7-'06 5/52 Current DG penetration Forecast: 7.2% DG share in 2004 to 14% in 2012 (WADE) Wind target in EU: 40,5 GW at present to 75 GW in 2010 (EWEA) 2nd Int. Conf. on Energy Innovation / 6-7-'06 6/52

4 Grid of tomorrow? Local generation Local storage Controllable loads, DSM ( NegaWatts ) Power quality and reliability is a bigger issue System s future structure & size? Growth (?): o Consumption rises annually by several % o Investments in production: very uncertain due to liberalisation o Limited or no grid expansions are accepted by the public Balancing: o Short-term: make balance by introducing DG? o Long-term: more storage and/or activate loads? 2nd Int. Conf. on Energy Innovation / 6-7-'06 7/52 Power electronic dominated grids Source: KEMA 2nd Int. Conf. on Energy Innovation / 6-7-'06 8/52

5 SmartGrids Vision EU Technology Platform preparing FP7 Involving specialists from industry, academia, Vision paper published in nd Int. Conf. on Energy Innovation / 6-7-'06 9/52 SmartGrids Future Network Vision 2nd Int. Conf. on Energy Innovation / 6-7-'06 10/52

6 Enabling Technologies Active Distribution Networks Improved power flow: FACTS, WAMS, WAPS Power electronic technologies Smart Metering Communication for DSM, on-line services, energy management Stationary energy storage 2nd Int. Conf. on Energy Innovation / 6-7-'06 11/52 Concepts for the future Virtual Utilities: Configure and deliver -> Internet model 2nd Int. Conf. on Energy Innovation / 6-7-'06 12/52

7 Concepts for the future Microgrids: Low voltage networks with DG sources, local storage and controllable loads, automatic islanding 2nd Int. Conf. on Energy Innovation / 6-7-'06 13/52 Microgrid idea Technically, parts of the grid may even separate from central supply A Microgrid is a collection of small generators for a collection of users in close proximity No net power exchange, total autonomy (Energy Island)? 2nd Int. Conf. on Energy Innovation / 6-7-'06 14/52

8 Is a Microgrid new? History of electricity networks: It all started that way, before interconnection Today, are they a new concept? No? o The grid behind certain UPS systems are driven like a microgrid with one generator o Multiple UPS units in parallel is not impossible, but not often implemented due to complexity (control, earthing, ) Yes! o Intended for longer term operation, not just emergencies o Scope of the idea is wider: from a single dwelling to parts of the distribution network (longer connections, multitude of connections, ) 2nd Int. Conf. on Energy Innovation / 6-7-'06 15/52 What makes a Microgrid difficult to operate? Ancillary Services are all to be delivered internally Balancing the active and reactive power: electricity produced - system losses = electricity consumed storage Stabilizing the grid: frequency, voltage amplitude and phase angle Providing quality and reliability: unbalance, harmonics, Black-start capability Can a Microgrid become fully separate and deliver these services? How to organize (system operator fully distributed)? How to reward financially? 2nd Int. Conf. on Energy Innovation / 6-7-'06 16/52

9 What is an optimal deployment of DG technology? Distribution grid was never built for local power injection, only top-down power delivery Technical difficulties: Power Quality & reliability Efficiency Safety Control Economic aspects DG% Are there optima for the size and location of DER units (generation, storage, active load)? t 2nd Int. Conf. on Energy Innovation / 6-7-'06 17/52 Power quality & reliability Problem: Bidirectional power flows Distorted voltage profile Vanishing stabilizing inertia More harmonic distortion More unbalance Technological solution: Power electronics may be configured to enhance PQ DG units can be used as backup supply 2nd Int. Conf. on Energy Innovation / 6-7-'06 18/52

10 Voltage fluctuation with PV PV power is calculated from 5-s average irradiance data measured in Heverlee (B) P (kw) Q (kvar) Time (min) Power variation causes voltage fluctuations Voltage (p.u.) Time (min) 2nd Int. Conf. on Energy Innovation / 6-7-'06 19/52 Example: MV cable grid Substation connecting to HV-grid Location: Leuven- Haasrode, Brabanthal + SME-zone 2nd Int. Conf. on Energy Innovation / 6-7-'06 20/52

11 Impact of wind turbine SUB nd Int. Conf. on Energy Innovation / 6-7-'06 21/52 Voltage Rise Problem SUB Generally, power injected by DG improves voltage profile Over-voltage may occur at high level of power injection (especially, synchronous machines) Voltage profile along feeder 4 Little impact on the voltage rise with the generators operating as induction machine Synchronous DG raises system voltage higher due to additional reactive power injection Connect one DG unit at node 406 record system voltage profile at different power injections of DG unit Voltage Power factor relationship at node 406 2nd Int. Conf. on Energy Innovation / 6-7-'06 22/52

12 Voltage dips Voltage at node 2 branch 1-2 open Voltage dips at node 2 with synchronous DG case Voltage dips at node 2 with induction DG case SUB An induction generator starts up (node 108) A large induction DG starts 401 up causes severe voltage dips at connected node and nearby nodes affect sensitivity loads A soft-start circuit is needed for large induction DG 2nd Int. Conf. on Energy Innovation / 6-7-'06 23/52 Voltage stability limit Z line U r E s I P load + jq load Z load U r Normal Operating Point Critical Point Voltage stability margin P max P load 2nd Int. Conf. on Energy Innovation / 6-7-'06 24/52

13 Voltage stability node 111, impedance load Synchronous DG cases SUB SUB Induction 305 DG cases DG Location has significant impact 403 on 404 voltage stability limit DG unit: 3 MW Synchronous DG unit has better contribution to Calculate voltage voltage stability stability limits. limit at node 111, is the furthest point from substation (considered to be weakest bus in terms of voltage stability) 2nd Int. Conf. on Energy Innovation / 6-7-'06 25/52 Control, or the lack of Problem: Generators are NOT dispatched in principle o Weather-driven (many renewables) o Heat-demand driven (CHP) o Stabilising and balancing in cable-dominated distribution grids is not as easy as in HV grids active power frequency reactive power voltage 2nd Int. Conf. on Energy Innovation / 6-7-'06 26/52

14 Networked system operations Solutions: Higher level of control required to coordinate balancing, grid parameters? Advanced control technologies Future technologies, under investigation Distributed stability control Market-based control o o Contribution of power electronic front-ends (see example) Scheduling local load and production, by setting up a micro-exchange (see example) Management of power quality Alternative networks o o Customize quality and reliability level E.g. stick to 50/60 Hz frequency? Go DC (again)? Rely heavily on intensified communication: interdependency 2nd Int. Conf. on Energy Innovation / 6-7-'06 27/52 Example: fully decentralized control Standard method: droop control KUL method: Virtual Impedance method Emulate a voltage source with internal tunable impedance in the time domain Ref.: K.De Brabandere et PESC 04 Advantage: seamless transition from grid-connected to island and reconnect 2nd Int. Conf. on Energy Innovation / 6-7-'06 28/52 14

15 Experimental results: connection of two independent grids (islands) voltage before voltage after current before current after 2nd Int. Conf. on Energy Innovation / 6-7-'06 29/52 Example: tertiary control on local market DG units locally share loads dynamically based on marginal cost functions, cleared on market P 1 old MC P1 new MC equal marginal cost P 2 old P 2 new P P MC translated translated & mirrored P 2nd Int. Conf. on Energy Innovation / 6-7-'06 30/52

16 Safety Problem: Power system is designed for top-down power flow Local source contributes to the short-circuit current in case of fault o Fault effects more severe o Difficult to isolate fault location Bidirectional flows o Selectivity principle in danger: no backup higher in the grid for failing protection device Conservative approach on unintentional islanding Solution: New active protection system necessary G G 2nd Int. Conf. on Energy Innovation / 6-7-'06 31/52 SUB Short-Circuit Current - 3 syn. gen.: 3 MVA (high penetration at a feeder) - 3-phase short circuit ka ka With DG connection Base case With DG connection Base case s s 2nd Int. Conf. on Energy Innovation / 6-7-'06 32/52

17 Societal issues Problems: Environmental effects o Global: more emissions due to non-optimal operation of traditional power plants o Local effects as power is produced on-the-spot, e.g. visual pollution Making power locally often requires transport infrastructure for (more) primary energy o Problem is shifted from electrical distribution grid to, for instance, gas distribution grid! Solution: Multi-energy vector approach Open debate on security of supply 2nd Int. Conf. on Energy Innovation / 6-7-'06 33/52 Economic issues Problems: Pay-back uncertain in liberalized market o Chaotic green and efficient power production o Reliability or PQ enhancement difficult to quantify System costs o More complicated system operation o Local units offer ancillary services System losses generally increase Who pays for technological adaptations in the grid? Who will finance the backbone power system? o Too much socialization causes public resistance Solution: Interdisciplinary regulation, not only legal Need some real deregulation 2nd Int. Conf. on Energy Innovation / 6-7-'06 34/52

18 System losses example DG introduction does not mean lowered losses Optimum is 2/3 power at 2/3 distance Other injections generally cause higher system losses Load distributed along feeder cable HV subst. DG Power flow along cable Before DG Zero point After DG 2nd Int. Conf. on Energy Innovation / 6-7-'06 35/52 Evolution of losses Penetration level of DG First: PL DG increases Losses decrease Then PL DG increases Losses increase DG PL DG (%) = x 100 Synchronous DG units have the largest influence on the reduction of power losses. P P L P loss (%) 8 Induc PF Con PF Syn PF Penetration level (%) 80 Induc PF Con PF Syn PF Penetretion level (%) 2nd Int. Conf. on Energy Innovation / 6-7-'06 36/52 Reactive-voltage drop (%)

19 Balancing question, again Fundamental electrical power balance, at all times is the boundary condition: Electricity produced - system losses = electricity consumed storage All sorts of reserves will decrease in the future Role of storage? Storage also means cycle losses! Next step in enabling technologies Usable storage Activated intelligent loads (demand response technology), also playing on a market? Boundary condition: minimize losses 2nd Int. Conf. on Energy Innovation / 6-7-'06 37/52 How far can we go? Large optimization exercise, making following considerations: Optimal proliferation, taking into account local energetic opportunities, e.g. renewables options? Unit behavior towards grid: technology choice? Control responsibilities? Is the same level of reliability still desired? Level of introduction of new additional technologies (storage, activated loads)? 2nd Int. Conf. on Energy Innovation / 6-7-'06 38/52

20 Looking for the optimal solution Where to place DER? A? B? C? Optimization goals: o voltage quality penalty o minimum losses o minimum costs o Constraints: o net balance o load flow equations o acceptable voltage profile o Optimization non-convex multiple objectives mixed discrete-continuous long term vs. short term interactions 2nd Int. Conf. on Energy Innovation / 6-7-'06 39/52 DER Planning Single objective Min (Energy Loss) Multi-objective very straightforward does not represent the real problem Weight factors single objective Min (α Energy Loss + β Voltage penalty + γ Installation Costs) value of weight factors? useful if objectives are comparable, e.g. all costs ( ) True multi-objective trade-offs Min {Energy Loss; Voltage penalty; Installation Costs} selection based on Pareto ranking 2nd Int. Conf. on Energy Innovation / 6-7-'06 40/52

21 DER Planning Long term planning problem efficiently handled by using Genetic Algorithms Optimization based on evolutionary computing techniques Able to deal with mixed discrete (placement) continuous (sizing) problem Global optimum not guaranteed!! Rather a target-oriented tool Often difficult to obtain sufficient data sets use Markov models = Gen A Node i Gen B Network topology 2nd Int. Conf. on Energy Innovation / 6-7-'06 41/52 Test case 2 generator types used: PV & CHP each represented by a 5-string bit network chromosome = 20 x 2 x 5 = 200 bits (2 40 siting options) 2nd Int. Conf. on Energy Innovation / 6-7-'06 42/52

22 Test case Load data are based on actual measurements residential buildings period of 1 year on a 15-minute basis 4 load scenario types are available each scenario consists of 4 typical one-day load profiles 5 times repeated over the 20-node grid Summer, high load 2nd Int. Conf. on Energy Innovation / 6-7-'06 43/52 Test case high load (e.g. working day) low load (e.g. weekend) winter summer 2nd Int. Conf. on Energy Innovation / 6-7-'06 44/52

23 Test case Evolution of total power loss in the optimal solution of each generation in the Summer Low scenario Optimal and stable solution reached after 100 generations 2nd Int. Conf. on Energy Innovation / 6-7-'06 45/52 Test case Optimal solution in the Summer-Low scenario 2nd Int. Conf. on Energy Innovation / 6-7-'06 46/52

24 Test case Cross-comparison test : use solution of scenario i in fitness function of scenario j Total power loss [kwh] SH Load scenarios SL WH WL SH Using optimal DG of scenario SL WH WL No DG nd Int. Conf. on Energy Innovation / 6-7-'06 47/52 DER Planning Instead of single-objective optimization, the use of true multiobjective optimization is proposed Single objective only one optimal solution significance? Multi-objective pool of solutions trade-offs between objectives insight in the potential of the grid which objectives are conflicting? Decreasing objective value with each generation Pareto front between conflicting objectives 2nd Int. Conf. on Energy Innovation / 6-7-'06 48/52

25 DER Planning What is a trade-off? f 1 f 2 f 2 translation from variable space to objective space Pareto front unfeasible feasible trade-offs x 1, x 2 Rank topologies according to dominance: If topology A is at least equal in all objectives compared to B and better in at least one objective, then topology A dominates topology B f 1 Aim of the optimization: Identify as much non-dominated topologies as possible 2nd Int. Conf. on Energy Innovation / 6-7-'06 49/52 DER Planning Fitness of a DER topology is calculated by running a scenario with assumed loads and grid topology What if Storage is to be optimized? Load has an elastic behavior? Production is dispatchable? Control? Demand Response? Forecasting? What if Local Markets exist? Long-term planning interacts with short-term planning problem!! primary control secondary control economic optimum 2nd Int. Conf. on Energy Innovation / 6-7-'06 50/52

26 Conclusion Current grid: Interconnection Higher PQ level required DER looking around the corner History repeats: after 100 years the idea of locally supplied, independent grids is back Microgrids, being responsible for own ancillary services Maximum (optimal?) level of penetration of DER = difficult optimization exercise Whose optimum? Special (technological) measures are necessary ancillary services are a challenge o e.g. in system control, balancing role of loads? Not only technology push, but also customer pull required 2nd Int. Conf. on Energy Innovation / 6-7-'06 51/52 more information: check publications sections, e.g.: Pepermans G., Driesen J., Haeseldonckx D., Belmans R., D haeseleer W.: Distributed Generation: Definition, Benefits And Issues, Energy Policy, Elsevier, Vol.33, Issue 6, April 2005, pp or contact johan.driesen@esat.kuleuven.be Thank you! (now, let s s discuss)