christian ohler, abb switzerland corporate research...

© ABB GroupJuly 20, 2014 | Slide 1

Physical Aspects of Power SystemsControl

Christian Ohler, ABB Switzerland Corporate Research

Purpose of this PresentationDescribe how power systems are controlled

§ (1) Fundamentals: Power system components, active andreactive power

§ (2) Frequency and voltage control

§ (3) Outlook: Impact of renewables?

§ Focus is on the interaction of the components – the systemaspect: why is the power system stable?

© ABB Switzerland Ltd, Corporate ResearchJuly 20, 2014 | Slide 2

Fundamentals: Power System Components,Active and Reactive Power

(1)


Power systems are made from four main components:Overhead lines, transformers, generators, breakers


High voltage overheadlines enable longdistance powertransmission with lowlosses

Transformers allow foradequate voltage

levels for generation,transmission,

distribution, andconsumption

Synchronousgenerators control realpower and reactivepower

Circuit breakers caninterrupt short circuits

and disconnect gridsegments with a fault

Three symmetric phases provide steady power fromalternating current and do not need return conductors


V2~

V1~

V3~

I1à

I2à

I1+I2+I3 = 0N

Current II1 I2 I3

120°I1

I2

I3

Phasor representation

Review of Phasor Calculation

Time-domain Phasor-domain

( ) t jy t Aew d+=( ) sin( )y t A tw d= +

Use coordinatesystem rotating withspeed w rad/sß

jy Ae Ad d= = Ð

The phasor of a sinusoidalfunction (current, voltage, …) is acomplex time-independentnumber with amplitude and«phase» angle

Why bother about active and reactive power?

§ The balance of active power controls the frequency of thegrid

§ The balance of mainly the reactive power controls thevoltage at every point of the grid


Definition of Active and Reactive Power


0

0

§ For each single phase there isthe instaneous power

§ p(t) = v(t) * i(t)

§ Active (real) powerPactive(t) = P*(1-cos(2wt)) where

P = Veff*Ieff*cos(f)

Veff = Vmax /√2 etc. , =

èOscillates around P, is nevernegative

§ Reactive powerPreactive(t) = Q* sin(2wt) where

Q = Veff*Ieff*sin(f)

èOscillates around zero

v(t)i(t)

p(t)

p(t)p_active(t)

p_reactive(t)

f

T

Reactive power is the power flowing into and out of themagnetic field around the conductors

§ Three phases

§ The sum of active power is timeindependent (doesn’t oscillate)

§ The sum of reactive power is zero, but theredistribution of the magnetic field is theconsequence of the oscillating reactivepower in each phase

è Reactive power flow can not be neglected!

§ Sign convention: inductors (lines,transformers, induction motors) consumereactive power, capacitors generate reactivepower


H

Reactive power is the power flowing into and out of themagnetic field around the conductors (Animation)


Source: P. Leuchtmann, ETH Zurich, 2014.

Magnetic field around the six conductors of two 380 kV overhead circuits

Poynting vectors of active and reactive power fluxshow their physical character


Poynting vectorenvelopes ofreactive power fluxin a balancedthree-phaseoverhead line

Poynting vectorenvelopes of activepower flux in abalanced three-phase overheadline

Source: Z. Cakareski, A. Emanuel, 1999.

Summary 1 – A substantial part of the power is usedfor the electromagnetic fields around the conductors

§ High voltage for low losses

§ Alternating current in order to be able to transform thevoltage and to interrupt

§ In each phase, reactive power oscillates back and forthbetween the source and the sink at the frequency of thepower system. Reactive power is the change of the energystored in the electromagnetic fields of the components inoperation.


Frequency and Voltage Control

(2)


The power that a single generator feeds into the grid isa function of rotor angle (and hence torque, heat, fuel)

§ Three phase stator connected to the power grid, stator field rotates withgrid frequency

§ Rotor of a synchronous generator and its DC field rotates with the samefrequency

§ The more torque the turbine provides to the generator shaft, the larger willbe the angle between rotor and stator field


dGovernor

valve

Source: Wikipedia

Operation avoids coming close to the maximum torquewhere the generator looses synchronization

§ P(d) = Pmax * sin(d)

§ Stable against small disturbances inmechanical torque or grid voltage ford<90°

§ Realistic operation point with sufficientsafety margin is 45°


maxP

d1 8 0 °9 0 ° 3 6 0 °2 7 0 °

The phase angle of the voltage determines the amountof active power transferred by a transmission line


ZL

VG VLIX=iwL

P(d) = VG*VL * sin(d)VG*VL

X

DVVL

I VG

d

In the interconnected power system thousands ofgenerators operate synchronously


The bicycle equivalent- but thinkrubberbands instead of rigidchains

Map of the Swiss transmission gridwith major power plants (1.5% ofEuropean ENTSO-E grid)

Some power plants contract part of their output forprimary, secondary, tertiary reserve (frequency control)


Pow

er

Timehours

Primary

Secondary

Tertiary

Primary frequency control§ The power plants contracted for primary frequencycontrol act as a proportional controler

§ The power plants contracted for secondaryreserve act as an integral controler

§ If the tertiary reserve is insufficient, the final optionis load sheddingL

Tripping of a nuclear power plant in Hamburg on June28, 2007 measured in Zurich


49.9

49.92

49.94

49.96

49.98

50

50.02

50.04

14.46 Uhr 14.51 Uhr 14.56 Uhr 15.01 Uhr 15.06 Uhr 15.11 Uhr 15.16 Uhr

Freq

uenc

y[H

z]

-1500

-1300

-1100

-900

-700

-500

-300

-100

100

Pow

er[W

]

Frequency [Hz]Power flow [W]

Tolerance band

15:02 KKW Krümmel istaken off the grid.Pnom = 1376MW i.e. almosthalf of the reserve power isneeded

ABB BESS supplies 1ppmof the overall primaryfrequency regulation

3 kW battery supplies 1 ppm

Overhead lines and cables have an impedance, hencethe receiving end voltage depends on the load.


VG

VLI

ZLoad

ZGrid

ZGrid / ZLoad

P / Pmax

I / Ishort

VL / VG

§ In this (unrealistic) example Z(Grid) and Z(Load) are both pure resistancesand the voltage at maximum power transfer is 50% of the sending voltage.

The voltage drop across a transmission line is mainlycaused by the reactive power transferred


ZL

VG VLIX=iwL

Q(DV) = * DV (for d=0)VL

X

DV

VL

I

VG

The maximum real power transfer capability is stronglydependent on the power factor of the load


P / Pmax

VL / VG0.90 lag 0.95 lag 1.0 0.95 lead 0.90 lead

Power factor

Locus of critical points

Source: After Kundur (1994)

The rotor excitation of synchronous generators controlsthe voltage at that grid location


Overexcitation

Underexcitation

Capacitive current

Inductive current

d

d

d

d

A variety of equipment types contributes to the localvoltage control by injecting or absorbing reactive power


Series compensation

On-load tap changers

(in Transformers)

Shunt reactorShunt capacitor

Synchronous condenser (machine without turbine)

Static VarControl

no load

The elongation due to resistive losses causes a sag ofoverhead lines

clearance

sag§ Resistive losses heats up conductor

§ Heating causes elongation

§ Elongation causes sag

§ Sag reduces clearance

§ Severe overtemperature causesannealing

Minimum

clearance

sag

maximum load

Short lines can be loaded to the thermal limit, longerlines are limited by voltage drop and angle stability

0 80 320 1000Line Length (kms)

Line

Load

abilit

yThermal limit

anglestability limit

UnusedThermal capacity

voltagedrop limit

Summary 2 – Stable frequency indicates globalequilibrium of active power, stable voltage needs localcontrol of reacive power

§ If frequency is too low – morepower needed

§ Power plants are throttled to beable to ramp power up or downimmediately if required

§ Tap changing transformers andswitched capacitors are thetraditional way to achievevoltage stability


Renewables Integration

(3)


The power electronic converter lets the solar cell looklike a synchronous generator

§ Power semiconductors switch on/off with high frequency

§ Output is smoothened with low pass filters

§ Power converters can

§ Maximize the output power of the solar cell

§ While generating or consuming reactive power


Integration of power from wind and sun into the powersystem is a challenge, but can be done

§ New location for feeding power into the grid

èGrid upgrades and voltage control withpower electronic converters

§ Larger reserve requirements (prediction error)

è Renewables can also take the role ofreserve (trade-off with energy yield)

§ Wind and sun are contingent, statisticalenergy sourcesà Need to give economicincentive to

§ grid upgrades

§ complementary power plant technologywith fast ramp rate, capacity credits

§ energy storage© ABB Switzerland Ltd, Corporate ResearchJuly 20, 2014 | Slide 30

Summary of this PresentationPhysical Aspects of Power Systems Control

(1) High voltage alternating current for low losses.àWehave to deal with reactive power.

(2) Frequency low.à More active power needed. Voltagedepends on local reactive power balance.

(3) PV and wind power can contribute to voltage control (andfrequency control), but their intermittent statistical natureincreases the frequency reserve requirements. And they arecontingent and statistical by nature.



Photo: Michael Ploss

christian ohler, abb switzerland corporate research...

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