Contents High Voltage Smart Grids – Flexible transmission ... · An overview of conventional...

16/03/2012

1

by

Professor Enrique AchaTampere University of Technology

Tampere, Finland

(Formerly with the University of Glasgow, UK)

High Voltage Smart Grids –Flexible transmission systems 1. An overview of conventional power transmission systems with

emphasis on reactive power compensation

2. Flexible AC Transmission Systems (FACTS) for voltage and power control

3. High Voltage Direct Current (HVDC) for long-distance transmission and asynchronous interconnections

Contents

Reading Material

Description of Power electronic valves and converters

Models of FACTS equipment for large-scale power systems applications: steady-state

The main objective of a bulk powertransmission network is to transport reliably large volumes of electrical energy from a number of bulk power producing sites to a number of bulk power consuming sites

More often than not the large electrical energy generating plants are located far away from the large consumer sites such as industrial parks and cities; indeed, sometimes the distances between generation and consumer sites are truly continental

Because of efficiency reasons bulk power transmission is carried out at high voltages. For a given power level, the higher the voltage the lower the current and, hence, the associated transmission power losses

Electrical Power Transmission Networks

Hydro power plant and dam

Steam generator and power plant

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AC TRANSMISSION LINES

400 kV transmission corridor:

- Several three phase transmission lines are running in parallel

- Each transmission line has three phases, namely, a,b,c . The conductors are suspended from steal structures through chains of insulating discs

- Each phase contains one or more conductors in parallel to enable a more advantageous design

- Each transmission line uses one or two shielding wires to protect the phase conductors against lightning strokes

DC TRANSMISSION LINES

220 kV DC line:

- Simpler designs- Smaller foot prints and rights-of-way

Due to reliability reasons, bulk power networks tend to have a high degree of interconnectivity

Each country has its own power grid which would comprise tens of thousands of high-voltage over-head transmission lines of varying complexity. In the USA alone, bulk power is moved over roughly 200,000 miles of high-voltage transmission lines.

Furthermore, the electrical power grids of individual countries are interconnected so as to form truly continental electrical power systems

Long-distance power transmission makes the daily operation of the grid difficult and ancillary voltage supporting equipment is required at points where no synchronous generators are available –nevertheless conventional power grids are very brittle and very susceptible to undergo wide-area voltage instabilities (i.e., black-outs)

Electrical Power Transmission

Electrical Power Transmission: one-line diagram

15 kV

400 kV

132 kV

Substation

Hydro

Transmission level

Sub-transmission level

Distribution level

Substation

Coal

Weak point

Weak point

33 kVWeak point

Substation

Nuclear

400 km

Large synchronousgenerator

Substation

High-voltageTransmission line

Sub-transmissionline

Bulk supply point

Load point

Power System symbols:

Capacitor bank

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High-Voltage Over-Head Transmission Lines:At 50 or 60 Hz operation

/20

Pmax

2Pmax

PSR

Phase angle

QSR

SRSR

RSSR

RSSR

RSSR

XVVQ

XVVP

cos1

sin

rad60p.u.05.1,95.0

RS

RS VV

Limits:

LongLong--distance transmission line (longerdistance transmission line (longer than 350 km)than 350 km)

j XRj B

j XR j XR j XRj2 B j2 B j2 B j2 B j B

ShortShort--distance transmission line (lessdistance transmission line (less than 50 km)than 50 km)

jXSRRSR

ISR

VS S VR R

jXL=j LjBC=j C

MediumMedium--distance transmission line (lessdistance transmission line (less than 350 km)than 350 km)

jXSRRSR

jBR/2jBS/2VS S VR R

Inherent Limitations of Transmission Systems

The ability of the transmission system to transmit power becomes impaired by one or more of the following steady-state and dynamic limitations:

- Angular stability- Voltage magnitude- Thermal limits- Transient stability- Dynamic stability

These limits define the maximum electrical power to be transmitted without causing damage to transmission lines and electric equipment.

Excessive loading in one or more points of the power network will set a voltage collapse in motion, in that part of the network which, if not contained appropriately, might trigger a wide-area voltage collapse

Inherent Limitations of Transmission Systems

By way of example, the contrived transmission systems shown below undergoes a gradual increase of system active load, to the point in which it becomes impaired by low voltage magnitudes, large voltage angles and excessive current flows

j0.2 p.u.

p.u.004.122V

|SL2| pf=0.95 lag

P12

Q12

The plot opposite shows the voltage-power characteristic at bus 2 where the voltage magnitude drops below 0.65 p.u. for a load of just under 2 p.u.

Today’s power grid, although largely based on the engineering principles laid out in the 1950’s, is highly reliable when properly maintained and operated


According to figures given by the USA Department of Energy, today’s electricity system is 99.97% reliable. However, the power outages and interruptions associated with the remaining 0.03% cost the American economy approximately $150 billion each year.

More importantly, an ill-contained power outage due to human error or equipment failure may develop into a wide-area voltage collapse or “blackout”

The Northeast blackout in the USA in August of 2003 left approximately 50 million people with no electricity, affecting business and industry and having a high social cost. On that occasion, the blackout was initiated by a high-voltage power line in Ohio coming into contact with an overgrown tree

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Excessive loading in one or more points of theelectrical power network

Damage to a power line or cable due to a short-circuit

An ill-judged planning operation scenario, e.g.insufficient spinning reserve, taking transmissionequipment out of operation in haste

Inadequate protection against atmosphericdischarges

Inadequate provisions against geothermic storms


A report by The Brattle Group in 2008 predicted that the electrical power industry in the USA would need to invest $298 billion in the transmission system between 2010 and 2030, to modernize the power grid

A subsequent report by The Brattle Group in 2010 put the price tag to meet mandates for integrating more renewable energy into the existing electric grid at $50 to $100 billion.

Blackouts – possible causes

Conventional Transmission System Reinforcement

Transmission System Reinforcements –Conventional Solutions

One way in which the transmission system may be reinforced very effectively is by building a second transmission line, in parallel with the original one, effectively halving the overall transmission circuit reactance

The plot opposite shows the voltage-power characteristic at bus 2 where the voltage magnitude drops to 0.65 p.u. but for a load of almost 4 p.u.

j0.2 p.u.p.u.004.122V

|SL2| pf=0.95 lag

j0.2 p.u.

An alternative reinforcement arrangement would be to install in-situgeneration. By way of example, a 100 MW with ±20 MVAR capacity is added at bus 2 of the original system. The voltage magnitude at bus 2 is originally set to 1.04 p.u.

The plot opposite shows the voltage-power characteristic at bus 2 where the voltage magnitude drops to 0.55 p.u. but for a load of almost 2.7 p.u.

j0.2 p.u.

p.u.004.1 p.u.04.1 2

|SL2| pf=0.95 lag

100 MW


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A bank of switched capacitors is installed at bus 2. These are mechanically connected and have the following values: XC1=0.25 p.u., XC2=0.50 p.u. and XC3=0.25 p.u.

The plot opposite shows the voltage-power characteristic at bus 2 where the capacitors are switched on when the voltage drops below 1 p.u., 0.95 p.u.and 0.95 p.u.

j0.2 p.u.

p.u.004.1 22V

|SL2| pf=0.95 lagXC1XC2XC3


A bank of series capacitors are installed at half-way the transmission line to reduce to half the electrical length of the line, i.e. 50% series compensation. These capacitors are permanently connected

The plot opposite shows the voltage-power characteristic at bus 2 where the voltage magnitude drops to 0.60 p.u.but for a load of almost 3.9 p.u.

j0.1 p.u.p.u.004.1 22V

|SL2| pf=0.95 lag

j0.1 p.u.

-j0.1 p.u.


Principles of Mid-Point(Reactive Power)

Compensation

Principle of Power Transmission

jX/2jX/2I

+

Vm

-

+

Vr

-

+

Vs

-

The voltage and current relations are:The voltage and current relations are:

/2/2

I

Vx=jXI

VmVr

Vs

j / 2

-j / 2

(cos 2 jsin 2)cos 2

2(cos 2 jsin 2)2 sin 2

j

s s rm

r

s r

V Ve V V VV VV Ve V

V V VIX X

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Principle of Power Transmission

2

max

2

max

at 2

2 at

VPXVQX

22

2cos 2 jsin 2 sin 2

2 cos 2 sin 2 jsin 2

s sVS V I VX

VX2

2

sin

1 cos

s

s

VPXVQX

Active and reactive power relations:Active and reactive power relations:

Maximum powers:Maximum powers:

21 cosVQ

X

2sinVP

XmaxP

max2P

0

2Phase angle

,P Q

Principle of Shunt Compensation

jX/2jX/2

+

Vm

-

+

Vr

-

+

Vs

-

Ism Imr


jX/2Imr

Vs

jX/2Ism

Vm

Vr

Imr

VmrVsm

/4/4

Ism

/4 /4

j / 2 2(cos 2 jsin 2) sin 2 j 1 cos 2j 2s ms

smm

V V VV Ve V IX XV V

Principle of Shunt Compensation

Active and reactive power relations:

2

2cos 2 jsin 2 sin 2 j 1 cos 2

2 sin 2 j 1-cos 2

sm sm smVS V I VX

VX

2

2

2 sin 2

2 1 cos 2

sm

sm

VPXVQX

Maximum powers:Maximum powers:2

max

2

max

2 at

4 at 2

VPXVQX

max2P

max8P

22

22

,P Q

22sin

2smV

PX

221 cos

2smV

QX

2

sinsV

PXmaxP

Phase angles

Principle of Series Compensation

+ Vc -

+

Vm1

-

jX/2jX/2

+

Vm2

-

+

Vr

-

+

Vs

-

I I


j / 2

-j / 2

j / 2

(cos 2 jsin 2)

(cos 2 jsin 2)j

(cos 2 jsin 2)

ss r c

r

c

V Ve VV V VIV Ve V

XV Ve V

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jX/2jX/2 I

+

Vm

-

+

Vr

-

+

Vs

-

-jXC/2 -jXC/2 Vx

-jXC/2 I-jXC/2 I

I

Vm

VrVs

If the series applied voltage Vc is in quadrature with respect to the line current, the series compensator can only supply or absorb reactive power, hence, the source may be replaced by an equivalent reactance:

eq 1

where is the degree of series compensation (0 1)

c

c

X X X X r

Xr rX


Current and active and reactive power Current and active and reactive power relations:relations:

2

22

2

2 sin 2(1- )

Re sin(1 )

2 1 cos(1 )

c c

c c

VIr X

VP V Ir X

V rQ I XX r

2

12 1 cosr

rVQ

X2

1sin

rVP

X

maxP

max2P

0 2

Phase angle

,P Q

max4P

max3P

max5P

max10P

0.2r0.4r

0.6r

0.8r0.9r

0r

Principle of Phase Angle Compensation

I+

II-

+-

Vr-

Vr+

VrVs

±jXI

+

V1

-

+

Vr

-

+

Vs

-

The voltage and current relations are:

1 cos jsinVr

V TV

1cos jsinr

II TI

Active and reactive power relations:

2

sinVPX

2

1 cosVQX

Principle of Combined Compensation

Vm±Vc2

Shunt compensation

Vm

ImVm-

Vm+

Vc1

Series compensation

-Vm

Vm-

Vm+

+Vc1

Phase angle compensation

Vc2Vm

Vc1

Vm-

Combined compensation

jXs

+

Vs

-

IsjXr

+

Vr

-

IrVm

jXc2

+Vc2-

jXc1 + Vc1 -Im

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Comparisons of Compensators

The ideal shunt controller acts like a voltage source, which draws or injects current into the network. It enables tight voltage control at the point of connection. It is also quite effective in damping voltage oscillations

The series controller impacts directly the driving voltage and, hence, the current and power flow. If the aim is to control current or power flow and damp oscillations, the series controller is several times more powerful than the shunt controller, for the same MVAThe phase shifter does increase the transmittable power of the uncompensated transmission line. The flat-topped curve indicates the range of the action of the phase compensation

Act

ive

pow

er (p

.u.)

With 50% of series capacitive compensation

1

2

With no compensation

With shunt compensation

With phase-shifter compensation

Phase angles (rad) 0 2 2

Smart Electrical Power Transmission

There is the current wisdom that with the ageing of the electrical power transmissioninfrastructure and its due replacement, there is an opportunity to use the newinvestment and the enabling technologies available today, to benefit the environmentusing clean electrical energy sources and a much enhanced security of supply withincreased energy efficiencies. All these challenging high aims are being collectedtogether under the umbrella title of SMART GRIDS

The aim would be: (i) to prevent wide-area voltage collapse by the co-ordinated supportof reactive power sources at different points in the network; (ii) fast active power controlin designated transmission paths; (iii) local and remote damping of power systemoscillations; (iv) minimum system curtailment under emergency conditions

The widespread deployment of power electronics equipment for voltage and active andreactive power regulation, including FACTS and VSC-HVDC converters

Incorporation of large-size renewable energy power plants, e.g., off-shore windgeneration

Wide area measurement & control using PMUs and ICTSs to co-ordinate the operationof power electronic controllers, fast-acting synchronous generators and variable speedwind generators

Smart Electrical Power Transmission

• Hydro-electric power plants

• Wind power plants

• Wave power plants

• Thermo-solar power pants

• Photo-voltaic power plants

• Power plants with fuel cells

• Biomass-powered micro-plants

• Energy scavenging trees

Renewable Sources of Electrical Power

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Ls

-

+

Graetz bridge

T6 T2T4

T5T3T1

Vbc

Vab

Vc

Vb

Va

Phase control

+VDC

-

Ls

VAC

FACTS and HVDC with Thyristor Valves

IDC

AC1 AC2

Back-to-back HVDC link

Point-to-point HVDC link

AC1 AC2

+ or - VDC

- or + VDC

IDC

IDC

RDC

RDC

FACTS and HVDC with Thyristor Valves

Thyristor-Controlled Reactor (TCR)

Three Phase Static VAR Compensator (SVC)

Thyristor-Switched Capacitor (TSC)

ITCRa

ITCR3

Branch 1 Branch 2n

VaVbVc

ITCRc ITCRb

ITCR1 ITCR2

IaIbIc

ICc ICb ICa

Branch 3

Bypass breaker

TCSC ModuleSeries capacitor

(1.99 mF)Varistor

Thyristorvalve

Reactor(0.470 mH)

Reactor(0.307 mH)

Bypass disconnect

One branch of a Three Phase ThyristorControlled Series Compensator (TCSC)

Q Q

The price to be paid for an improved performance of the power grid using power electronics-based equipment is that such equipment achieves its main operating point at the expense of distorting the voltage and current waveforms – they generate harmonics and quite possibly inter-harmonics as well

The most promising power converter in power transmission applications today uses fully-controllable semi-conductor valves, such as IGBT, and PWM valve control. A wide range of converter topologies are available – the most basic three-phase configuration is shown below

Electrical Power Transmission –Power Electronics-based Equipment

Tb-

C

cb

a

Dc-Db-Da-

Dc+Db+Da+

Tc+

Ta-

Tb+

Tc-

Ta+

Vbc

Vab

Vc Vb Va

o

+

VDC

-C

PWM control

TwoTwo--level voltage source converterlevel voltage source converter

vao

+VDC/2

-VDC/2

Voltage output (Voltage output (vvaoao))

bus k+

VDC- Iv

Ev

Vkma

STATCOM

bus mbus k

Shunt Series IcI

EvRIvR +VDC

-

VmVk

ma ma

EcI

VvR VcRvR cRUnified control system UPFC

Back-to-back

VSC-HVDC

bus mbus k

VvR

+VDC

- IvIIvR VmVkma ma

vR VvI vI

EvR EvI

FACTS and VSC-HVDC with IGBT Valves

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Variable Frequency Transformer

The figure opposite shows a cut-away drawing of a 100 MW VFT installed at Langlois, Canada where the main components are:- The rotary transformer- The drive motor- The collector

The collector system conducts current between the three-phase rotor winding and its stationary bus-bar

One power grid is connected to the VFT’s rotor and the other grid is connected to the VFT’s stator

Induction machine model

Electric drive model

bus kIk

LsRs

GcBm

bus mIm

Rr/SLr

VFT Equivalent Circuit

c

IDC

AC system

turbine side

+VDC/2

-

CBA

DC-DB-DA-

DC+DB+DA+

TB-

TC+

TA-

TB+

TC-

TA+

VBC

VABo

+VDC/2

-AC system

Grid side

Db+

Dc-

ba

Vbc

VabDc+

Da-Db-

Da+

gear box

IG

IGBT converters

SG

IGBT converters

IGBT converters

Renewable Sources of Electrical Power - Wind

6 MW rated machinerotor diameters of 126 m

tower heights of 135 m

15 kV

STATCOM

400 kV

132 kV

TCSC50% compensation

400 km

Mainland

Multi-terminal CSC-HVDC Link

1000 km

CSP plant

CSP plant

50 km

Off-shore wind farm

100 km

“dead” load

Off-shore wind farm

Multi-terminal VSC-HVDC grid

SVC

Island

PMU

PMU

PMU

System 5 f=60 Hz

System 1 f=50 Hz

System 2 f=60 Hz

System 3 f=50 Hz

System 4 f=60 Hz

Asynchronously Connected Sub-systems

PMU

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