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“Importance of Reactive Power Management, Voltage Stability and FACTS Applications in today’s Operating Environment”

Sharma Kolluri

Manager of Transmission Planning

Entergy Services Inc

Engineering Seminar

Organized by IEEE Mississippi Section

Jackson State University

August 20, 2010

Voltage Profile during Aug 14th Blackout

- Voltages decay to almost 60% of normal voltage. This is probably the point that load started dropping off.
- However, the recovery is too slow and generators are not able to maintain frequency during this condition.
- Many generators trip, load shedding goes into effect, and then things just shut down due to a lack of generation.

A “Near” Fast Voltage Collapse in Phoenix in 1995

North American Electric Reliability Council, System Disturbances, Review of Selected 1995 Electric System Disturbances in North America, March 1996.

Recommendation#23

- Strengthen Reactive Power and Control Practices in all NERC Regions

“Reactive power problem was a significant factor in the August 14 outage, and they were also important elements in the several of the earlier outages”

-Quote form the outage report

Laws of Reactive Physics

- System load is comprised of resistive current (such as lights, space heaters) and reactive current (induction motor reactance, etc.).
- Total current IT has two components.
- IR resistive current
- IQ reactive current
- IT is the vector sum of IR & IQ
- IT = IR + jIQ

IT

IQ

IR

North American Electric Reliability Corporation

Laws of Reactive Physics

- Complex Power called Volt Amperes (“VA”) is comprised of resistive current IR and reactive current IQ times the voltage.
- “VA” = VIT* = V (IR – jIQ) = P + jQ
- Power Factor (“PF”) = Cosine of angle between P and “VA”
- P = “VA” times “PF”
- System Losses
- Ploss = IT2 R (Watts)
- Qloss = IT2 X (VARs)

VA

Q

P

North American Electric Reliability Corporation

Reactive Physics – VAR loss

- Every component with reactance, X: VAR loss = IT2 X
- Z is comprised of resistance R and reactance X
- On 138kV lines, X = 2 to 5 times larger than R.
- One 230kV lines, X = 5 to 10 times larger than R.
- On 500kV lines, X = 25 times larger than R.
- R decreases when conductor diameter increases. X increases as the required geometry of phase to phase spacing increases.
- VAR loss
- Increases in proportion to the square of the total current.
- Is approximately 2 to 25 times larger than Watt loss.

North American Electric Reliability Corporation

Reactive Power for Voltage Support

Reactive Loads

VARs flow from High voltage to Low voltage; import ofVARs indicate reactivepower deficit

Reactive Power Management/Compensation

What is Reactive Power Compensation?

- Effectively balancing of capacitive and inductive components of a power system to provide sufficient voltage support.
- Static and dynamic reactive power
- Essential for reliable operation of power system
- prevention of voltage collapse/blackout

Benefits of Reactive Power Compensation:

- Improves efficiency of power delivery/reduction of losses.
- Improves utilization of transmission assets/transmission capacity.
- Reduces congestion and increases power transfer capability.
- Enhances grid reliability/security.

Transmission Line Real and Reactive Power Losses vs. Line Loading

Source: B. Kirby and E. Hirst 1997, Ancillary-Service Details: Voltage Control,

ORNL/CON-453, Oak Ridge National Laboratory, Oak Ridge, Tenn., December 1997.

Static and Dynamic VAR Support

- Static Reactive Power Devices
- Cannot quickly change the reactive power level as long as the voltage level remains constant.
- Reactive power production level drops when the voltage level drops.
- Examples include capacitors and inductors.
- Dynamic Reactive Power Devices
- Can quickly change the MVAR level independent of the voltage level.
- Reactive power production level increases when the voltage level drops.
- Examples include static VAR compensators (SVC), synchronous condensers, and generators.

Common Definitions

Voltage stability- ability of a power system to maintain steady voltages at all the buses in the system after disturbance.

Voltage collapse - A condition of a blackout or abnormally low voltages in significant part of the power system.

Short term voltage stability - involves the dynamics of fast acting load components such as induction motors, electronically controlled loads, and HVDC converters.

Long term voltage stability - involves slower acting equipments such as tap-changing transformer, thermostatically controlled loads, and generator limiters.

What is Voltage Instability/Collapse?

- A power system undergoes voltage collapse if post-disturbance voltages are below “acceptable limits”
- voltage collapse may be due to voltage or angular instability
- Main factor causing voltage instability is the inability of the power systems to “maintain a proper balance of reactive power and voltage control”

Voltage Instability/Collapse

- The driving force for voltage instability is usually the load
- The possible outcome of voltage instability:
- loss of loads
- loss of integrity of the power system
- Voltage stability timeframe:
- transient voltage instability: 0 to 10 secs
- long-term voltage stability: 1 – 10 mins

Voltage stability causes and analysis

Causes of voltage instability

Increase in loading

Generators, synchronous condensers, or SVCs reaching reactive power limits

Tap-changing transformer action

Load recovery dynamics

Tripping of heavily loaded lines, generators

Methods of voltage stability analysis

Static analysis methods

Algebraic equations, bulk system studies, power flow or continuation power flow methods

Dynamic analysis methods

Differential as well as algebraic equations, dynamic modeling of power system components required

Over-excitation Limit

Lagging (Over-excited)

0.8 pf line

Stator Winding Heating Limit

Normal Excitation (Q = 0, pF = 1)

- Per unit MVAR (Q) +

MW

Turbine Limit

Leading (Under-excited)

Under-excitation Limit

Stability Limit

Q-V Curve with Detailed Load Model

Peak Load with Fixed Taps

120

200

100

80

60

40

Base Case

20

Mvars

Contingency

0

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

-20

-40

-60

-80

Voltage (p.u.)

Q-V CurvePossible Solutions for Voltage Instability

- Install/Operate Shunt Capacitor Banks
- Add dynamic Shunt Compensation in the form of SVC/STATCOM to mitigate transient voltage dips
- Add Series Compensation on transmission lines in the problem area
- Implement UVLS Scheme
- Construct transmission facilities

Fault Induced Delayed Voltage Recovery (FIDVR)

- FIDVR Definition
- Load Models

Fault Induced Delayed Voltage Recovery (FIDVR)

- What is it?
- After a fault has cleared, the voltage stays at low levels (below 80%) for several seconds
- Results in dropping load / generation or fast voltage collapse
- 4 key factors drive FIDVR:
- Fault Duration
- Fault Location
- High load level with high Induction motor load penetration
- Unfavorable Generation Pattern

Load characteristics

The accuracy of analytical results depends on modeling of power system components, devices, and controls.

Power system components - Generators, excitation systems, over/under excitation limiters, static VAr systems, mechanically switched capacitors, under load tap changing transformers, and loads among others.

Loads are most difficult to model.

Complex in behavior varying with time and location

Consist of a large number of continuous and discrete controls and protection systems

Dynamics of loads, especially, induction motors at low voltage levels should be properly modeled.

Induction motor characteristics

Impact of fault on transmission grid

Depressed voltages at distribution feeders and motor terminals

Reduction of electrical torque by the square of the voltage resulting in slow down of motors

The slow down depends on the mechanical torque characteristics and motor inertias

With fault clearing

Torque -per unit

Square-law load torque

Electric torque

Constant load torque

1.0

Speed – per unit

Fig. 1 Induction motor characteristics

- Partial voltage recovery
- Slowed motors draw high reactive currents, depressing voltage magnitudes
- Motor will reaccelerate to normal speed if, electrical torque>mechanical torque
- else, the motors will rundown, stall, and trip
- The problem is severe in the summer time with large proportion of air conditioner
- motors

Air conditioner motor characteristics

Characteristics

Main portion (80-87%) consumed by compressor motor

Electromagnetic contactor drop out between (43-56%) of the nominal voltage and reclose above drop out voltage

Stalling at (50-73%) of the nominal voltage

Thermal overload protection act if motors stall for 5-20 seconds

The operation time of thermal over load (TOL) protection relay is inversely proportional to the applied voltage at the terminal

Air conditioner should be modeled to analyze the short term voltage stability problem

Quite important for utilities in the Western interconnection

Load modeling

Old models – Loads are represented as lumped load at distribution feeder

Does not consider the electrical distance between the transmission bus and the end load components

The diversity in composition and dynamic behavior of various electrical loads is not modeled

Modeling

WECC interim model

20% of the load as generic induction motor load

80% constant current P and constant impedance Q

Transmission Bus

OLTC

Distribution Bus

Distribution Capacitor

Lumped Load (ZIP load)

Fig. 2 Traditional load model

Composite load modeling

Representation of distribution equivalent

Feeder reactance

Substation transformer reactance

Parameters of various load components

Discharge lighting

Electronic Loads

Constant Impedance loads

Motor loads

Distribution Capacitor

Transmission Bus

Bus 1

OLTC

Distribution Bus

Bus 2

Substation Capacitor

Feeder Equivalent

Distribution Feeder

Bus 3

Dynamic Loads

(Small motor, Large motor, trip motor loads)

Static Loads

(Constant impedance, constant current, constant impedance loads)

Distribution Capacitor

Fig. 3 Composite load model structure

- Alternating Current Transmission Systems Incorporating Power Electronic Based and Other Static Controllers to Enhance Controllability and Increase Power Transfer Capability.
- power semi-conductor based inverters
- information and control technologies

Major FACTS Controllers

- Static VAR Compensator (SVC)
- Static Reactive Compensator (STATCOM)
- Static Series Synchr. Compensator (SSSC)
- Unified Power Flow Controller (UPFC)
- Back-To-Back DC Link (BTB)

RTO System

S/S

BTB

BTB

UPFC

Power Generation

Voltage Control

Power System Stability

Power Flow Control

System Reliability

Inter-area Control

Inter-tie Reliability

Enhanced

Import Capability

S/S

Load

Load

STATCOM

STATCOM

Improved

Power Quality

Increased

Transmission Capacity

Inter-connected

Power System

Load

S/S

SSSC

FACTS ApplicationsStatic VAr compensator (SVC)

Variable reactive power source

Can generate as well as absorb reactive power

Maximum and minimum limits on reactive power output depends on limiting values of capacitive and inductive susceptances.

Droop characteristic

V

I

TCR

Firing angle control

XC

XL

Fig. 4 Schematic diagram of an SVC

Static compensator (STATCOM)

Voltage source converter device

Alternating voltage source behind a coupling reactance

Can be operated at its full output current even at very low voltages

Depending upon manufacturer's design, STATCOMs may have increased transient rating both in inductive as well as capacitive mode of operation

System bus

V

Transformer

I

X

E

DC-AC switching converter

Cs

Vdc

Fig. 5 Schematic diagram of STATCOM

Technology Applications at Entergy to Address Reactive Power Issues

- Large Shunt Capacitor Banks
- UVLS
- Series Compensation
- SVC
- Coordinated Capacitor Bank Control
- DVAR
- AVR

Determining Reactive Power Requirements in the Southern Part of the Entergy System for Improving Voltage Security – A Case Study

Sharma Kolluri

Sujit Mandal

Entergy Services Inc

New Orleans, LA

Panel on Optimal Allocation of Static and Dynamic VARS for Secure Voltage Control

2006 Power Systems Conference and Exposition

Atlanta, Georgia

October 31, 2006

Areas of Voltage Stability Concern

North Arkansas

Mississippi

West of the Atchafalaya Basin

(WOTAB)

Southeast Louisiana

Western Region

Amite South/DSG

Study Objective

- Identify Voltage Stability Problems in the DSG area
- Determine the proper mix of reactive power support to address voltage stability problem
- Determine size and location of static and dynamic devices.

1 - 50 MW

2 - 60 MW

3 - 128 MW

4 - 740 MW

5 - 750 MW

Michoud Units

1 - 65 MW

2 - 240 MW

3 - 515 MW

115 kV

115 kV

- 230 kV

230 kV

Downstream of Gypsy Area - Critical FacilitiesLittle Gypsy-South Norco 230kV line

Waterford-Ninemile 230kV line

- Area load growth
- 1.6% projected for 2003 - 2013
- Weather normalized to 100º F
- Projected peak load – 3800 MW
- Area power factor - Low
- 94% at peak load
- Worst double contingency
- Loss of the Waterford to Ninemile 230 kV transmission line and one of the 230 kV generating units at Ninemile or Michoud

Michoud

Ninemile

New Orleans area voltage profile on June 2, 2003

(with 2 major generators offline)

- Area Problems
- Thermal overloads of underlying 115 kV and 230 kV transmissionsystem
- Depressed voltages throughout New Orleans metro area potentially leading to voltage collapse and load shedding

Various Steps Used for Determining Reactive Power Requirements

- Step 1 – Problem identification
- Step 2 – Determining total reactive power requirements
- Step 3 – Sizing and locating dynamic devices
- Step 4 – Sizing and locating static shunt devices
- Step 5 – Verification of reactive power requirements

Tools & Techniques Used

- Various tools and techniques used for analysis purposes
- PV analysis using PowerWorld
- Transient stability using PSS/E Dynamics
- Mid-term stability using PSS/E Dynamics
- PSS/E Optimal Power Flow
- Detailed Models used
- Motor models and appropriate ZIP model for dynamic analysis
- Tap-changing distribution transformers, overexcitation limiters, self-restoring loads modeled in mid-term stability study

PV CurveNinemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line

Stability Simulation Ninemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line

Process for Determining Reactive Power Requirements

- Approx 700 MVAr of reactive power shortage identified in the DSG
- How much static and how much dynamic?
- Criteria for determining static and dynamic requirements
- Voltage at critical buses should recover to 1 pu in several seconds
- Voltage at critical buses should recover to 0.9 pu within 1.5 - 2 seconds
- Voltage should not dip below 0.7 pu for more than 20 cycles
- Generator reactive power output should be below Qmax
- Factors considered in sizing static/dynamic devices
- Short circuit levels, size & location of the stations, number and existing size of cap banks, back-to-back switching, etc

Optimal size and location

SVC Size and Location- Sites considered
- Ninemile 230 kV
- Gretna 115 kV
- Paterson 115 kV
- Size
- 300 MVAR
- 500 MVAR

Steps to locate Static Shunt Devices

- Static shunt requirements – 400 MVAR approximately
- Options available to locate the static shunt devices on the transmission or distribution systems
- OPF Program used to come up with size and location of shunt devices

OPF Application

- PSS/E OPF Program used
- Objective Function – Minimize adjustable shunts
- OPF simulated for critical contingencies

Simulation Results with the Capacitors and SVCNinemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line

SVC PerformanceNinemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line

Summary

- Process for determining static and dynamic reactive power requirements discussed
- OPF program utilized for sizing/locating static shunt capacitor banks
- Results verified using mid-term stability simulations
- Study recommendation – 400 MVAR of static shunt devices and 300 MVAR of dynamic shunt compensation

Porter 0/+300Mvar SVC

SVC Topology: 2 x 75MVAr TSC & 1 x 150MVAr TSC

Porter Static Var Compensator (SVC)

Maintains system voltage by continuously varying VAR output to meet system demands Controls capacitor banks on the transmission system to match reactive output to the load requirements.

Series Capacitor – Dayton Bulk 230kV Station

The Capacitor offsets reactance in the line, making it appear to the system to be half of its actual length. Power flows are redirected over this larger line, unloading parallel lines and increasing transfer capability.

DSMES Unit

Stores Energy in a superconducting coil

Automatically releases energy to the system when needed to ride through voltage dips caused by faults. This unit improves power quality and reduces customer loss of production.

Industry Issues

- Coordination of reactive power between regions
- No clearly defined requirements for reactive power reserves
- Proper tools for optimizing reactive power requirements
- Incentive to reduce losses

Summary

- The increasing need to operate the transmission system at its maximum safe transfer limit has become a primary concern at most utilities
- Reactive power supply or VAR management is an important ingredient in maintaining healthy power system voltages and facilitating power transfers
- Inadequate reactive power supply was a major factor in most of the recent blackouts

Load Projection

- 2010 peak: 1770 MW
- 2012 peak: 1852 MW

Sample PV Curve ResultLewis Creek Unit 1 & China-Porter 230kV Out - 2010

2010 Summer PV Curve Analysis

Approved Construction Plan Projects included:

*Relocate Caney Creek 138kV

Results: 2010 case without load shed

Case 3 Voltages (pu): Goslin: 0.810; Conroe: 0.855; Cleveland: 0.909; Jacinto: 0.924; Dayton: 0.944; Huntsville: 0.944

Case 4 Voltages (pu): Goslin: 0.757; Conroe: 0.800; Dayton: 0.913; Huntsville: 0.928; Cleveland: 0.928; Rivtrin: 0.941

2010 Summer Conditions - Dynamics Analysis

- Lewis Creek Unit 1 outaged in the base case
- 50% induction motor load is modeled
- Result: Shed Load Block 1 (183 MW)

Observations for 2010 Summer Peak Conditions

- Existing load shed logic in Western Region OK for 2010 Summer conditions
- Voltage at some critical buses drop below 0.7 pu for more than 20 cycles – Potential of motor load tripping
- Conclusions for 2010 Summer
- Reducing load shed blocks to 180 + 70 MW in Western Region has no negative impact

Results: 2010 case with load shed (Block 1)

Case 3 Voltages (pu): Goslin: 0.872; Conroe: 0.902; Cleveland: 0.934; Jacinto: 0.948; Dayton: 0.966; Huntsville: 0.968

Case 4 Voltages (pu): Goslin: 0.827; Conroe: 0.855; Dayton: 0.939; Cleveland: 0.951; Huntsville: 0.954; Jacinto: 0.964

Conclusions and Recommendations

- Retain the exiting UVLS logic
- Change the load blocks
- Block one: 180 MW
- Block two: 70 MW (existing size 111 MW)

Voltage @ 4/8 buses <0.90 pu

Armed all time

Drop load

One or more Lewis Creek units in-service?

OEL at Lewis Creek units

Voltage @ 4/8 buses

< 0.92 pu

Time Delay 3 seconds

Load Blocks:

Block 1: 175 MW

Alden: 50 MW

Metro: 35 MW

Oakridge:30 MW

Goslin: 60 MW

Block 2: 75 MW

In the vicinity of Block 1

Monitored Buses:

Metro 138kV

Goslin 138kV

Alden 138kV

Oakridge 138kV

Huntsville 138kV

Rivtrin 138 kV

Poco 138 kV

Conroe 138 kV

Reset the Process for next LVSH block

Load Blocks:

Block 1: 175 MW

Block 2: 75 MW

The above conditions need to be met for 3 scans to trigger load shedding

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