Advanced Electricity Infrastructure Workshop Global Climate & Energy Project STANFORD UNIVERSITY, Nov. 1~2, 2007 Voltage Control with Distributed Generators to Enhance Voltage Stability Presenter: Fangxing (Fran) Li, Ph.D. Assistant Professor, University of Tennessee Other contributors: Tom Rizy, John Kueck, and Yan Xu (ORNL) 1
Outline Introduction to voltage collapse What is it and what causes the problem Prevention of voltage collapse using reactive power compensation Current research, development and demonstration (RD&D) effort at ORNL/UT Using DG to provide dynamic reactive power System and control models Testing & simulation results Future Plans Summary & Challenges 2
Voltage Collapse 3
Voltage Collapse: what and why? Most of recent blackouts are related to voltage collapse. A severe voltage depression without the system s ability to recover. The key cause is inadequate reactive power (VAr or Q) supplies. The following factors are usually involved: High impedance High load current Insufficient generation of Q Line outage Line congestion 4
Voltage Collapse: Technical (1) Sending End E S S P P +Q -Q Receiving End V P Q loss loss = = I I 2 R 2 X R<<X for transmission lines and transformers. Reactive power loss is high. S: Apparent power (VA) P: real power (Watt). It does actual work. Q: reactive power (Var). This component flows back and forth between source and load. It does not do actual work. 5
Real power and reactive power 2 1. 5 1 0. 5 0 P S - 0. 5 Q - 1 6
Transmitting Reactive Power Reactive power cannot be effectively transmitted across long distances or through power transformers due to high I 2 X losses. Source: Max Chau, Chairman, EEI Transmission Planning & Operations Working Group, 2004 It is an uphill battle! 7
Reactive power should be addressed locally. Source: Max Chau, Chairman, EEI Transmission Planning & Operations Working Group, 2004 8
Voltage Collapse: Technical (2) E Z LN θ I V R P R + jq R Z φ LD 9
Voltage Collapse: Technical (3) Var compensation Nose Points A single-line system E R+jX V P-V Curves Q c P+jQ 10
Voltage Collapse: Vulnerability (1) Power systems are operated under much more stress than in the past Generation pattern changed under competitive markets Continuous growth of consumption in heavy load areas Transmission expansion does not keep pace with Gen and Loads Future concerns 11
Voltage Collapse: Vulnerability (2) 1.1 Transmissio on Capacity (1989 = 1) 1 0.9 0.8 0.7 NPCC WECC FRCC MAPP MAAC SERC U.S. Total MAIN SPP ECAR ERCOT 0.6 1989 1994 1999 2004 2009 Transmission capacity relative to load has been declining in every NERC region since 1982 TRANSMISSION INVESTMENT (billion 1999-$/ 98054 5 4 3 2 1 0 Transmission investment has been declining for three decades -$117 million/year 1975 1980 1985 1990 1995 2000 12
Common Solutions Load shedding (only for urgent needs) Reactive power compensation switched capacitor banks (ability drops quickly when needed most) power electronics such as SVC Additional local generation like DG (dynamic Var injection; responses very fast!) 13
Voltage collapse prevention A single-line system Nose Points E R+jX V Q c P+jQ P-V Curves Q c =V 2 /X c for capacitors. Ability drops quickly when V is low. Blue: no compensation (Q c =0) Green: capacitor compensation Red: constant Q c injection 14
Dynamic Performance: voltage control With fast response Volta age With slow response Disturbance occurs Time 15
DG to support voltage DOE has goal of more penetration of DG 20% by 2020 Modern DGs are interconnected to the grid with power electronic interface With the right controls, PE can provide flexible, dynamic reactive power (Var) to support voltage We are exploring the possibility of using DG s dynamic Var capability to help improve voltage stability. PV, Wind, fuel cells, microturbines Rotating machine interface Synchronous condenser Reciprocating-engine synchronous generator 16
Current RD&D (Research, Development, and Demonstration) at ORNL/UT 17
Objective and Approach Objective evaluate the impact and benefits of DG on voltage collapse Approach develop the model of ORNL s X-10 power distribution system to perform voltage collapse studies develop dynamic models of different DG sources explore impact of DG placement location determine the degree of voltage collapse protection provided by DG identify general engineering guidelines for the design and operation to use DG to prevent voltage collapse Facility Distributed Energy Communications and Controls (DECC) Laboratory at ORNL serves as the test-bed since it is actually interfaced to the ORNL Campus distribution system. 18
DECC Lab is Interfaced with the ORNL Distribution System TVA Transmission System 161kV/13.8kV ORNL Substation 3000 Substation 13.8kV/2.4kV Other Substations Other Circuit Loads Panel PP2 2.4kV/480V 750kVA Transformer Cir rcuit #4 2.4kV/480V 300kVA Transformer DECC Lab Panel PP1 Ci ircuit #2 2.4kV/480V 750kVA Transformer Other Circuit Loads Panel PP3 Loads Loads 300kVar SC 30kW Capstone Micro-turbine 19 150A/88kVar Inverter Induction Motor
DECC Lab Layout Panel PP2 Transformer 4-4 Load Banks 1000A Service 600A Service Panel PP3 Transformer 2-3 Disconnect door Bldg. 3114 Inverter Test Area PP2 Synchronous Condenser Test Area Bay Area PP3 N 20 door
DG with Power Electronics Interface DG with an inverter (compensator) interface is connected in parallel at PCC (point of common coupling). v s L s R s i s v t i l Load PCC voltage is regulated by the inverter. v t i c i l v dc C f i c By generating or consuming reactive power, the inverter regulates the PCC voltage. DE switching signals v dc Controller L c v c Compensator 21
Synchronous Condenser (SC) SC only generates or consumes reactive power. System voltage is controlled by increasing or decreasing the SC current. SC current is controlled by the SC excitation voltage. 22
Control Diagrams for each DG Type Inverter control diagram SC control diagram 23 Inverter Inverter reference voltage v * c is always in phase with v t, so that the inverter only provides reactive power. The magnitude of v c* is controlled according to the error of PCC voltage. Synchronous Condenser (SC) 85 V** is the base excitation voltage for the SC used in the system. Excitation voltage is controlled according to the error of the system voltage. **The level at which the SC (an overexcited synchronous motor) neither absorbs nor generates reactive power.
Experimental Results (RL Load) 24
Local Voltage Regulation on May 1, 2007 Synchronous Motor Start 25
Regulation with balanced sags Aug. 28, 2007 26
Regulation with unbalanced sags 08/30/2007 Motor Start Current (Ph a) Inverter Current 27
Multiple DGs on the Same Circuit 28 SC and inverter are installed in a system at ORNL. Currently they are on different circuit for testing Simulated on the same circuit They control the same voltage. Inverter response time is faster than SC, therefore an integrated control is required.
Integrated Control Diagram 29 Inverter Faster response, but is current is limited. Responsible for the fast changing voltage ripples. SC Picks up what the inverter is unable to. Integrated System The overall power rating of the combined Inverter/SC system is increased. Faster response for the integrated system. No competing between the inverter & SC.
Simulation Results Regulated voltage Inverter current SC current Voltage is controlled at the desired level. Inverter current is limited to 100A peak (70Arms). SC and inverter share the reactive power required for the voltage regulation. The total power rating of the voltage regulation devices is improved (greater than the inverter and SC individually) 30
Micro-Turbine Available for FY08 100kW Micro-turbine Donated by SCE Plan to wire up in FY08 Capable of limited voltage & power factor correction ±2% voltage regulation Plan to connect to circuit #4 Panel PP2 has an available 350A CB Elliott TA-100 Micro-Turbine 31
New DG Technologies to be Added Photovoltaic (PV) Array assess opportunity for non-active power compensation 50kW PV Array to be added by end of the year Storage could be added later to provide output when it is cloudy/rainy/night 32
Summary The key cause of voltage collapse is inadequate reactive power supplies. DG, equipped with Var-capable power electronic device is an effective approach to provide dynamic reactive power. ORNL/UT is carrying out research work on dispatching DG for reactive power compensation to better regulate voltage and enhance system voltage stability. 33
Challenges Many utilities are still reluctant to DG interconnection because they are worried that DG may mess up their network and make them lose control of the grid. IEEE Standard 1547 is restrictive in DG interconnection Technical issues (e.g. protection) related to interconnection need to be addressed. Cost of dynamic Var from DG power electronic interface Has been improved and still needs further improvement 34
Questions and Answers 35