Technology Considerations for Cross Border Power

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Technology Considerations for

Cross Border Power Interconnections

SAARC Dissemination Workshop Lahore, Pakistan | 30 September – 01 October 2015

“The Past, Present and Future of High Voltage DC

(HVDC) Power Transmission”

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Objectives of Cross Border Power Transmission

• Import of cheaper electricity from

neighbouring country: Save investment on

local costlier generation

• Export of surplus power to neighbouring

country: earn revenues

• Reserve sharing: save investment on adding

generation capacity for peaking and

maintain reserves

• Enhance system strength to allow more

penetration of renewable energy and gain

environmental benefits

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Technical Considerations

• For Exporting Country (Source system)

• Enough Surplus Generation for export without causing

shortage in its own system

• Adequate Transmission System Capacity to bring the

exportable surplus power to the export point of Tie Lines

• Strong Regulatory Framework on Technical Reliability Criteria

to sustain all types of contingencies

• For Importing Country (Sink system)• Enough spinning reserve in generation or an automatic

loadshedding scheme in place to avert system collapse in

case of loss of Full or partial Tie-Lines

• Adequate Transmission System Capacity to absorb the

imported power deep into its network

• Strong Regulatory Framework on Technical Reliability Criteria

to sustain all types of contingencies

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Technical Issues with Fixed Power Export/Import

(Unidirectional or Bidirectional)

• Frequency Regulation and maintaining constant flow of

power on Tie-Line

• Source System:

• Maintain enough hot active spinning reserve (ASR) margins so that

loss of generation (Under-frequency) should transiently recover the

frequency of its own system and restore constant flow on Tie Line

• Control of over-frequency in case of load rejection in its own

system or due to loss of Tie Line (partial or total) through governor

action or trip-scheme of some specific generators

• Sink System:

• Maintain enough hot active spinning reserve (ASR) margins or

automatic loadshedding scheme so that loss of generation or loss

of Tie-Line (partial or total) should transiently recover the frequency

of its own system and restore constant flow on Tie Line without

overburdening the source system

• Control of over-frequency in case of load rejection in its own

system or due to loss of Tie Line (partial or total) through governor

action or trip-scheme of some specific generators

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Technical Issues with Fixed Power Export/Import

(Unidirectional or Bidirectional)

• Voltage Regulation and Reactive Power Management

• Source System:

• Must have enough reactive power reserve (in generators, SVCs or

other FACTS devices) to properly compensate the Tie Lines’

reactive consumption to maintain acceptable voltage profile at

sending end

• Control of over-under voltage through fast acting devices (TCR,

SVCs or FACTS devices)

• Sink System:

• Must have enough reactive power reserve (in generators, SVCs or

other FACTS devices) to properly compensate the loads’ reactive

power demand to maintain acceptable voltage profile at receiving

end

• Control of over-under voltage through fast acting devices (TCR,

SVCs or FACTS devices)

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Technical Issues with Reserve Sharing

• Dynamic Reserve Power Sharing (DRPS)

• Under normal condition, minimum or no power flows on the Tie-Lines

• Loss of generation or load rejection in any of the two or more countries

interconnected through Tie Lines would immediately trigger the DRPS to

rise to the occasion and redress the system need accordingly

• An optimum capacity of Tie Lines is determined through analysis by running

iterative simulations using Generation/Transmission Planning Software

specially meant for this analysis which bring out the tentative tie line

capacity for reserve sharing using LOLE criteria and Reserve Margins

criteria

• For frequency and voltage control, more fast response of controllers for

active power (AFC or AGC) and reactive power (SVC or FACTS) is required

than in fixed power transfer interconnections.

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Other Common Issues

• Inter Area Oscillations (Small Signal Stability Issues)• The inter-area oscillatory modes may be excited

• Even in a large system of one country

• but they are quite common when it comes to interconnected

systems of two or more countries

• These modes are inherent in the large systems and may result into

• Poor damping of oscillations after transient faults

• Oscillatory instability leading to system collapse

• Slow voltage recovery in a certain country gradually

leading to voltage collapse especially if the systems

are synchronised through AC Tie-Lines

• Cascading effect originating in one country may lead

the whole interconnected system to collapse if

automatic stability control strategy is not in place, an

acceptable brown out may avoid a blackout

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alSelection of Tie Lines

(Technology and Voltage Level)

• What is required of Tie Lines to be decided • The power to be transfered (MW)

• The distance (line length) of line route

• The terrain, high or medium or low altitude, mountainous or

plain or a sea

• Tie-Line Options• HVAC (400 kV, 500 kV or 765 kV)

• Shunt and or Series compensated HVAC• Switched Shunt compensation, SVCs, STATCOMs etc.

• Fixed Series compensation (FSC) or Thyristor Controlled Series

Compensation) (TCSC) or SSSC

• HVDC (ranging from ± 250 kV to ± 800 kV Bipoles or Single

Poles)

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Selection Preferences of Tie Lines

(Technology and Voltage Level)

• Short Distance – High Power Transfer• HVAC with shunt compensation

• HVDC Back to Back

• Long Distance – High Power Transfer• HVAC with Series Compensation (FSC or TCSC or SSSC)

• HVDC Bipole (in the range of ± 400 kV to ± 800 kV depending

on power to be transfered)

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Series Compensation

Sending

GMW

MVAR

MW

MVAR

Receiving

VsVR

XC

)Sin (12

q-q=

XL -- XC

VR . VS

PR)Sin (12

q-q=

X

VR . VS

PR

• Line reactance compensation device for long distance transmission

• XC can be connected in the mid-point or at both ends as 0.5 XC each

• Enhances power transfer capacity

• Decreases effective electrical length of transmission a line

• Reduces overall losses

• Influence Reactive Power Conditions of the system

• Automatic control of MVAR in proportion to line current squared (I2XC)

It increases with the increase of transmitted power and thus increases reactive

power balance of the system

• Prevents voltage collapse particularly on heavily loaded lines

• Thyrister controlled series capacitor banks for damping of oscillations and active

power flow control

• The degree of compensation lies between 20 to 70 %

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HVDC Line

DC to AC

ConverterAC to DC

Converter

POWER FLOW

DC LineTransformer

Receiving

Network

Sending

Network

Transformer

i

Direct CurrentAlternating Current

i

Alternating Current

i

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Comparison of HVAC and HVDC Lines

HVDC HVAC

HVDC is economical for long distance

large transfer of power

HVAC Series compensated circuits for

long distance transmission may cause

subsynchronous resonance (SSR)

Filter circuits, damping circuits and

Thyristor Controlled FACTS devices

(TCSC or SSSC) avoids SSR

Firewall against disturbances Disturbance travels quickly from one

system to another

Smaller right of way Bigger Right of Way

Upgradeable schemes Upgrading requires lot of space

Linking asynchronous networks Not Applicable

Control power flow: Electronic Control

of active/reactive power

Requires additional investment on

SVC or FACTS devices for faster

control of active/reactive power

Very fast response: high speed power

transfer possible

Not so fast because of mechanical

switching

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Comparison of HVAC and HVDC Lines

HVDC HVAC

Does not contribute in increasing fault

levels as much as an HVAC system

does, therefore does not increase

system strength

Larger contribution in fault levels

helps system strength especially

required for penetration of

Renewables

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Case Studies:

Gulf Cooperation Council Interconnection Authority

(GCCIA)

Based on Reserve Sharing between 6 countries of GCC i.e. Kuwait, Saudi

Arabia, Qatar, Bahrain, UAE and Oman

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Case Studies:

Gulf Cooperation Council Interconnection Authority

(GCCIA)Tie-Line Power, MW

Kuwait-Saudi

Arabia

1200 MW

Saudi Arabia B-B

HVDC

1200 MW

Saudi Arabia-

Bahrain

600 MW

Saudi Arabia-

Qatar

700 MW

Saudi Arabia-UAE 900 MW

UAE-Oman 400 MW

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•

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Case Studies:

Interconnection of 10 countries of

Economic Cooperation Organization (ECO)

• ECO comprises of 10 countries:

1. Pakistan

2. Iran

3. Afghanistan

4. Turkey

5. Turkmanistan

6. Tajikistan

7. Qazaqistan

8. Azerbaijan

9. Kyrghizia

10. Uzbekistan

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Case Studies:


• The interconnection study was carried out jointly by

NTDC, NESPAK and PPI in 2008-09

• Six Central Asian Countries shyed away to provide

their system data, and Afghanistan also could not

provide its inputs due to civil war situation

• Study was finally completed only for three countries,

Pakistan, Iran and Turkey

• The spot year of study was 2013

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Case Studies:


Supply-demand balance analysis had shown that Iran

would have huge surplus power for export mostly

during the winter season in future years.

Turkey and Pakistan will have most of the deficits in the

winter season.

Load in Iran was light in the winter season with

maximum surplus of generation capacity and deficits in

Turkey and Pakistan to be high in the same season.

Load flow, short circuit and transient analysis was

carried out by modeling winter peak load conditions

(2013) of the three countries.

Generation surplus in the Iranian system was around

16000 – 18000 MW during winter season in 2013

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Case Studies:


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Case Studies:


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Case Studies:


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Case Studies:


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Case Studies:


Two Alternatives of interconnection between Iran and

Pakistan through HVAC and HVDC have been studied.

Zahidan–Quetta 500 kV double circuit, with 50 % series

compensation provided at the mid-point switching station

alongwith ArgBam N-Zahidan double circuit of 400 kV,

Quetta-D.G. Khan 500 kV single circuit and Quetta R. Y.

Khan 500 kV single circuit will facilitate transfer of 1500 MW

from Iran to Pakistan (Alternative-I).

Alternative – II for Iran-Pakistan has been based on HVDC for

transfer of 2000 MW which envisage N-Kerman-Quetta ±500

kV HVDC Bipole with Quetta-D.G. Khan 500 kV single circuit

and Quetta R. Y. Khan 500 kV single circuit as the

reinforcements.

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Case Studies:


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Case Studies:


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Case Studies:


Load flow analysis of all the above Alternatives show that transfer of

2000 MW each to Turkey and Pakistan from Iran is at optimum level. All

the intact circuits are loaded within the rated limits and there is no

violation of voltage profile under normal as well as contingency

conditions under N-1 criteria.

In short circuit analysis there is some increase in the 3-phase fault

levels in the substations on the receiving-end i.e. those importing power

in Turkey and Pakistan.

Increase is not as much as it could cause any violation of exceeding

the fault levels above the short circuit ratings of the equipment of these

substations.

For Iran, as the interconnection alternatives are for its light load

conditions, the fault levels are lesser than what they would be during

the peak load conditions.

No concerns or limitations are foreseen related to the short circuit

levels in case of interconnection between the three power systems with

transfer of 2000 MW each to Turkey and Pakistan from Iran.

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Case Studies:


Transient stability analysis carried out for the interconnected

systems of Turkey, Iran and Pakistan with 2000 MW transfer

each to Turkey and Pakistan from Iran shows that the

proposed inter-ties between power systems of Turkey, Iran

and Pakistan, are strong enough to keep the three systems

intact under disturbed conditions occurring in either system.

Impact of disturbance in one system is mitigated to travel to

the other system because the inter-ties are HVDC bipoles

that allow each system to maintain its own equilibrium.

Swings of rotor angles are wider for the generating units

which are at or close to fault locations but all

swings/oscillations damp down to maintain the three systems

in synchronism.

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Case Studies:

1000 MW Import from Iran to Pakistan

• Three Alternatives were studied:

• Alternative –1 based on HVAC of two 500 kV single

circuit transmission lines between Zahedan and

Quetta 678 km, with 50 % series compensation

provided at the midpoint switching station

• Alternative –2 based on HVAC of two 765 kV single

circuit transmission line between Zahedan and Quetta

678 km, with midpoint switching station provided with

reactors to mitigate midpoint overvoltage.

• Alternative – 3 based on Zahedan-Quetta ±500 kV

HVDC with 678 km long bipolar line and converter

station of 1000 MW capacity at each end.

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Dispersal of Power to the National Grid

Quetta 500/220kV Substation

Double Circuit 220kV Transmission Line

Quetta Industrial

Mid Point Switching Station

(50% Series Compensation)

Two Single Circuit 500kV HVAC

Transmission Lines

N-Zahedan 400/500kV Substation


Transmission Lines

N-Zahedan 1000MW Power Plant

Case Studies:


Alternative-I: 500 kV HVAC

50 % Series Compensated

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D.G.Khan R.Y.Khan

Single Circuit 500kV HVAC Single Circuit 500kV HVAC

Transmission Line Transmission Line

Quetta 765/500/220kV Substation

Double Circuit 220kV Transmission Line

Quetta Industrial

Mid Point Switching Station


Transmission Lines

N-Zahedan 400/765kV Substation


Transmission Lines

Edimi 1000MW Power Plant

Case Studies:


Alternative-II: 765 kV HVAC

Mid-Point Switching Station

to install Reactors to

mitigate overvoltage at

midpoint

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Case Studies:


Alternative-III:

± 500 kV HVDC Bipole

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Case Studies:


• In all the three alternatives, the voltage and frequency excursions

and swings of rotor angles of the generating units are not wide and

they are damped down quickly to maintain synchronism and keep

the two system stable under all events of sever disturbances;

• The proposed three alternatives of interties between power systems

of Iran and Pakistan are technically strong enough to keep the two

systems intact under disturbed conditions occurring in either system;

• The 3rd Alternative has an edge over the other two in view of the fact

that impact of disturbance in one system is mitigated to travel to the

other system because the interties are HVDC bipolar line that allow

each system to maintain its own equilibrium;

• There appears to be no issues or concerns regarding transient

stability of the interconnected systems of Iran and Pakistan with

import of 1000 MW from Iran to Pakistan.

• Costs of both 500 kV HVAC and HVDC were comparable but due to

Iran’s preference for HVDC , Alternative-III was recommended

“SAARC Energy Ring” - (1) Power Grid

Planned 500 kV AC lines for CASA-1000

Existing 500 kV AC lines and substations

Existing 220 kV lines and substations

Proposed CASA-1000 DC line and converters

Proposed Delivery Point

Peshawar

Quetta

Gawadar

Mand Jackigur

Existing 70 MW

Under Construction 100 MW

Planned 1000 MW Amritsar

Planned 500 MW

Zahidan

Lahore

Khudzhent

Dushanbe

Sangtuda Shurkhan

Pol-e-Khomri

Kabul

Datka

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Case Studies:

An Overview of SAARC Interconnection Potential

Multiple Systems in SAARC Countries

• Internal Asynchronous Multiple Systems in some countries

Afghanistan

o Kabul and surrounding network (local generation)

o Kandahar and Hilmand system (local generation)

o Herat (Import from Iran)

o Balkh and Jowzjan (Import from Uzbekistan and Turkemanistan)

o Qunduz (Import from Tajikistan)

Bhutan

o Thimphu and surrounding (local generation)

o Chhukha and surrounding network (import from India)

o Trongsa, Mongar and surrounding network (import from India)

o Export of power to India

Nepal

o Internally synchronised from Mahendranagar to Anarmani with importing

interties from India at many places

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Case Studies:

An Overview of SAARC Interconnection Potential .)

Internally Synchronised Multiple Systems at SAARC Level

Bangladesh: with import of 500-1000 MW power from India through

back-to-back HVDC (Bheramara-Behrampur intertie with BB station

at Bheramara)

Pakistan: with little import from Iran for feeding Gawadar-Pasni Isolated

system

Sri-Lanka: Externally isolated

Maldive: Externally isolated

India: Exporting power to Bangladesh (HVDC-BB)

Exporting to Nepal and Bhutan radially (internally

asynchronous)

Importing from Bhutan

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Case Studies:

An Overview of SAARC Interconnection Potential

• CASA 1000 Multiple-Terminal HVDC will interconnect Afghanistan

and Pakistan

• Nepal, Bhutan and India should preferably interconnect through AC

because it will strengthen the networks of Nepal and Bhutan.

HVDC may be an option if High Hydel Power Potential in Bhutan is tapped

and the bulk of power requires HVDC intertie.

• Bangladesh-India BB-HVDC intertie capacity may be enhanced for

more power exchange as planned

• Sri Lanka-India submarine HVDC intertie seems quite a costly

proposition and not winning on Benefit/Cost Ratio

• Pakistan-India BB HVDC between Lahore and Amritsar has a good

potential and may be tapped. Would be in the interest of both

countries

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Thank You for Your Kind Attention

Documents

Technology Considerations for Cross Border Power