Helwan University

From the SelectedWorks of Omar H. Abdalla

2018

Security & Reliability Criteria for Power SystemPlanningOmar H. Abdalla

Available at: https://works.bepress.com/omar/55/

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1

Security & Reliability

Criteria for Power System

PlanningProf. Omar Hanafy Abdalla

Fellow of the Egyptian Society of Engineers

Helwan University

1

1. Generation Planning Criteria LOLP and LOLE

Reserve Margin

2. Transmission Planning Criteria Normal condition (N)

Contingency conditions (N-1)

Equipment thermal rating

Short-circuit levels

Dynamic criteria

Load shedding

Spinning reserve

3. Connection with Neighboring Systems2

Definitions

• Reliability —The degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired.

• Adequacy—The ability of the electric system to supply the aggregate electrical demand and energy requirements of the customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements.

• Security—The ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements.

3

Generation Reliability Criterion

• Reliability criterion approach is used to

determine the installed capacity and reserve

margin on the basis of probabilistic analysis

of the power system.

• The Loss Of Load Expectation (LOLE) criterion

is the main driver for long term planning of

the power system and therefore must be

carefully chosen.

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General Reliability Hypothesis

(Egyptian System Planning Case)

The values that are used for reliability studies are the following:

• Minimum reserve margin: 15%

• Loss Of Load Expectation (LOLE): 8 hour/year

• Cables and tie-lines of all voltage levels have an availability of 99.5%.

• Transformers have an availability of 99.5%.

• Reactive compensation means have an availability of 99.5%.

5

Reserve Margin or Installed Capacity

Margin

• The amount of unused available capability (MW) of an electric power system at peak load.

• Such capacity may be maintained for the purpose of providing operational flexibility and for preserving system reliability.

• The Installed Capacity Margin is calculated as the difference between the net peak load demand and the total net installed capacity and is expressed in MW.

• The Installed Capacity Margin can also be expressed as a percentage of the net peak load demand.

6

Loss Of Load Probability (LOLP)

or Loss Of Load Expectation (LOLE)

• A measure of the probability that a system demand will exceed capacity during a given period.

• The calculation of the LOLP may take also into account the state for which the system will be unable to feed the load demand due to congestion in the network.

• LOLE is usually expressed as the estimated number of hours over one year where LOLP is expressed as a probability figure between zero and one or as a percentage.

• 5 hours per year corresponds to 5/8760 = 0.057%.7

Generation Reliability in Various

Countries

8

ENTSO-E 2 methods (depending on TSO):

Probabilistic approach: LOLE = 3 hours/year

Deterministic criteria: Fixed reserve margin

Egypt LOLE = 8 h/y + 15% min reserve margin.

GCCIA LOLE = 5 h/y (1d/5y) on the interconnected

system

KSA LOLE = 4.8 h/y + 15% min reserve after loss

of largest 2 units in isolated areas.

Oman LOLE = 24 h/y

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Transmission Network Planning

Criteria

• To achieve the planning criteria objective it is essential that system simulations be performed in advance to identify the facilities necessary to assure a reliable transmission system in future years.

• Here, we describe the outage events which are used to determine if any thermal or voltage violations exist during peak and minimum demand conditions.

• The definitions of the normal and contingency conditions are discussed:

9

Normal Conditions

• The normal condition (base case) represents

the system with all elements in service.

• This is often referred to as N condition.

• The system must be able to supply all firm

demand and firm transfers to other areas.

• All equipment must:

– Operate within applicable ratings,

– Voltages must be within applicable limits, and

– The system must be stable.

10

Basic Assumptions Related to

N Criterion of Transmission Networks

• The rating limits of transmission lines should be intended as maximum permanent currents (or MVA).

• No overload of the transmission network is allowed.

• No generator will be above its continuous reactive capability.

• In normal operating condition, it is allowed to have long-term overload of transformers up to 10% of nominal current (or MVA).

11

Normal Operating Voltage Range

(±5%) - Egyptian System

Nominal Voltage

(kV)

Maximum

Limit (kV)

Minimum

Limit (kV)

500 525 475

400 420 380

220 231 209

132 138.6 125.4

66 69.3 62.7

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Generator Capability Chart

13

E. M. Viana, et al: “An Optimal Power Flow Function to Aid Restoration Studies of

Long Transmission Segments”, IEEE Trans. on Power Systems, Vol. 28, No. 1,

pp.121-129, Feb. 2013.

Generator Capability Chart

• Part A: Mechanical Source LimitPart A: Mechanical Source LimitPart A: Mechanical Source LimitPart A: Mechanical Source Limit

�� ≤ ��������

• Part B: Stator Current Limit

��� + ��� = !"#�����

• Part C: Over Excitation Limit

��� + �� +!"

�

$%

�

≤!"&����

$'

+ !"� (

$%

−(

$'

cos + − ,

�

14

Generator Capability Chart• Part D: Under Excitation Limit

��� + �� +!"

�

$%

�

≤!"&��-.

$'

+ !"� (

$%

−(

$'

cos + − ,

�

• Part E: Stability Limit

• ��� !"�

$'+ �� ≤ −

!"�

$%+ ��

/

• + = Generator internal angle

• , = Bus voltage angle

15

Real Generating Unit

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Normal Operating Frequency Range

• The nominal frequency of Egyptian Unified

Power System is 50 Hz and its permissible

variations range under Automatic Generation

Control (AGC) is 50 ± 0.05 Hz.

• Under normal operating condition the

permissible variations range 50 ± 0.2 Hz.

17

Single Contingency

Single contingency, often referred to as (N-1) event, involves the loss of one of the following elements:

• A generator with the associated loss of steam turbine in case of the loss of a gas turbine in combined cycle;

• A circuit of an overhead line;

• A cable circuit;

• A transformer;

18

Single Contingency

• The loss of a cable, a transformer or a single

circuit of OHL following a single phase fault

cleared in base time (120 ms);

• A compensation element (capacitor or

reactor banks, SVC)

• A bus section on transmission system (132kV,

220kV, 400kV or 500kV)

19

After the Single Contingency (N-1), the System

Must Respect the Following Conditions:

• The voltage at all nodes must remain within contingency voltage limits;

• The loading of transmission elements (cables, lines and transformers) must remain within the contingency limits;

• No loss of load allowed.

• The system should be transiently and dynamically stable;

• No activation of defence actions (UFLS, generator protection, interconnection protections, islanded of generators, etc.)

• The oscillations after the incident have to be correctly damped.

20

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Voltage Limits Under Contingency

Conditions (±10%)

Nominal

Voltage (kV)

Maximum

Limit (kV)

Minimum

Limit (kV)

500 550 450

400 440 360

220 242 198

132 145 119

66 72.6 59.4

21

Operating Frequency Range

During Contingency

During N-1 contingency conditions:

• The maximum and minimum permissible frequencies are respectively 50.4 Hz and 49.6 Hz.

In case a sever accident occurs:

• The frequency could transiently rise to 52 Hz or fall to 47.5 Hz.

Note:• If the system frequency rises above 51.0 Hz or falls below

48.5 Hz, manufacturers’ instructions concerning the generating units to remain in synchronism with the grid shall apply.

22

Equipment Thermal Loading

• Normal operating condition allows a long-term overload of transformers up to 10% of nominal current (or MVA).

• A temporary (short term) overload (less than 15 minutes) of transformers is allowed up to 20%.

• In normal operating conditions no overload of the transmission lines is allowed.

• A temporary overload of the transmission lines generally is allowed up to 20% for 30 minutes.

• For old lines no overloads are allowed.

23

Heat Balance Equation

• Overhead conductor ampacity can be calculated by the heat balance equation.

• In steady-state, the temperature of the conductor is determined by equating the total heat input (or heating effect) to the total heat output (or cooling effect).

• The following equation shows this main heat balance:

#�0 + �123�4 = �52.6 + �0�'

24

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Conductor Rating Formula

#� =7889 × ;� − ;( × 6 × ' × (9</ 9.>>7

+ � × ? × @ × ' × ;� + �A/ > − ;( + �A/ > − B × 1- × '

0 × ( + C ;� − �9

6 (m/s) Velocity of wind 0.6 m/s

1- (W/m2) Intensity of solar radiation 1100 W/m2

;( (oC) Ambient temperature 45 oC (EEHC)

;� (oC) Max conductor temperature in steady state 80 oC (EEHC)

' (mm) Diameter of conductor (Manufacturer data)

C (1/ oC) Temperature coefficient of resistance 0.00403 1/ oC (for Al)

0 (Ω/km) Conductor resistance at 20 oC (Manufacturer data)

B Coefficient of solar absorption 0.5 (IEC)

� Emissivity power ratio to black body 0.6 (IEC)

? (W m2 K-4) Stefan Boltzmann’s radiation constant 5.7 x 10-8 25

Measured Emissivity Factor (e)

26

No Conductor Condition Emissivity (e)

1 New Conductor 0.229

2 23 Years in Service 0.795

3 38 Years in Service 0.774

4 21 Years in Service 0.887

C. S. Taylor, and H. E. House: “Emissivity and its Effect on the Current Currying Capacity of Stranded Aluminum Conductors”, AIEE, pp. 970-976, USA, 1956.

Ampacity Parameters in Various

Countries

27

Design Parameter

Eg

yp

t

Om

an

US

A

Ch

ina

Jap

an

Fra

nce

Ind

on

esi

a

IEC

Ambient Temp. T1

(oC)45 50 - 35 - - 35 -

Max. Conductor

Temp. T2 (oC)80 80 90 70 90 85 75 -

Wind Speed (m/s) 0.6 0.5 0.61 0.5 0.5 1.0 0.5 1.0Solar Irradiance Si

(W/m2)

>

11001200 - 1000 1000 900 1250 900

Solar Absorptivity

D- 0.5 0.5 0.9 0.9 0.5 - 0.5

Conductor

Emissivity e- 0.6 0.5 0.9 0.9 0.6 - 0.6

Short Circuit Levels

The maximum short circuit levels of equipment should be calculated for three phase and single phase to earth faults based on the following assumptions:

• 1.1 p.u pre-fault voltage (according to IEC 60909 Standards)

• Calculated figures should not exceed 100% of switchgear rating

• All generating units and transmission elements are in service during peak conditions

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Short Circuit Levels (EEHC)

29

Nominal

System Voltage

(kV r.m.s.)

Short-Circuit Level

Short Time Withstand

Current (KA)

Peak Withstand

Current (KA)

11 25 (3sec.) 63

22 25 (3sec.) 63

66 31.5 (1sec.) 80

132 31.5 (1sec.) 80

220 50/40 (1sec.) 125/100

500 40/50 (1sec.) 125/100

Short-Circuit Current

30

Short Circuit Levels

• The power system fault current levels shall not

exceed the values listed in the Table.

• The short-circuit fault ratings for equipment

shall not be less than these values.

• The short circuit levels given above are for a

three phase symmetrical fault.

• Fault level calculations shall be based on the

highest voltages at each system voltage level,

as defined before.31

Dynamic Criteria

The system will be considered stable if the

following conditions are met:

A. Generator Synchronism

B. System Damping

C. Transient voltage and frequency performance

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Generator Synchronism

All generators in the system have to remain

in synchronism as demonstrated by the

relative rotor angles;

Neither sustained nor increasing oscillations

of the rotor angle are generated for any

generator of the system;

No loss of synchronism among machines

should be detected;

No sustained voltage oscillations should be

detected on any node.33

System Damping

• The stability curves describing a transiently-limited

time domain system trajectory, will be visually

inspected to insure that the oscillations are damped or

at least are as close to 5% damping ratio as is possible.

• This value, in fact, is internationally adopted to ensure

an acceptable operating condition of the system.

• A stability simulation is deemed to exhibit positive

damping if a line defined by the peak of the machine

relative rotor angle swing curve will intersect a second

line connecting the valley of the curve with an increase

in time.

34

System Damping

• Corresponding lines on bus voltage swing curves

will also intersect with an increase in time.

• A simulation, which satisfies these conditions,

will be defined as stable.

• A simulation, which appears to have zero%

damping ratio with acceptable voltage, will be

defined as marginally stable.

35

Eigenvalue AnalysisEigenvalue AnalysisEigenvalue AnalysisEigenvalue Analysis

I(,� = −? ± LM' = −NM. ± LM. ( − N�

((((Damping RatioDamping RatioDamping RatioDamping Ratio)))) N =?

?�TM'�

36

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Transient Voltage and Frequency

Performance

• Minimum transient frequency and duration, maximum transient voltage dips and duration, and post transient voltage deviations should be considered for each of the simulated scenarios.

• These parameters will be measured at load buses.

• The minimum frequency and the transient voltage dip should be investigated for each case in order to evaluate the needs for eventually emergency actions.

37

Dynamic & Steady-State Frequency

Deviations

38

Load Shedding Scheme

• A load shedding scheme is implemented in

the Unified Power System of Egypt based on

under-frequency relays and on local

measures and actions.

• This scheme consists of 8 stages, with no

time delay and different percentages of total

load tripped at each stage, as reported in the

following Table:

39

Load Shedding Scheme in the Egyptian

Power Grid – (Ref. Operation Code)

Frequency (Hz) Load Shedding (%)

49.2 6

49.1 3

49.0 4

48.9 7

48.8 20

48.7 20

48.65 15% of rest

48.6 15% of rest 40

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ENTSO-E Recommended

Under Frequency Load Shedding (UFLS)

41

49.4

49.2

49.0 48.8 48.6 48.4 48.2 48.0 47.8 47.6 47.4

5%

10%

15%

20%

25%

30%

35%

Individual

range

Load

shedding

amount

Frequency

(Hz)

48.547.5

Not

needed

Solidarity

range

Minimum

Load shedding

Mandatory

45%

50%

40%

Standard

Load sheddingVoluntary

49.4

49.2

49.0 48.8 48.6 48.4 48.2 48.0 47.8 47.6 47.4

5%

10%

15%

20%

25%

30%

35%

Individual

range

Load

shedding

amount

Frequency

(Hz)

48.547.5

Not

needed

Solidarity

range

Minimum

Load shedding

Mandatory

45%

50%

40%

Standard

Load sheddingVoluntary

Spinning Reserve (SR)

• The size of the secured incident corresponds to

the largest power loss resulting from a single

credible incident (including the loss of a

combined cycle: GT + corresponding share of

the ST).

• The spinning reserve should cover the secured

incident plus a margin of 10%.

• The reserve allocated to each unit shall not

exceed 5% of the size of the generating unit.

42

Power Reserve Requirement and

Criteria

According to Operation Rules, in the Egyptian

power system, frequency and active power

control is provided by the following means:

• Automatic response from generating units

operating in a free governor frequency

sensitive mode (Primary Reserve);

• Automatic Generation Control (AGC) of

generating units equipped with automatic load

frequency control (Secondary Reserve).

43

Power Reserve Requirement and

Criteria (Ref. EEHC 2008)

• To ensure network security in the Egyptian and other system composing synchronous grid (Jordan, Libya, etc.), after the most severe outage considered, the total needed Primary Control Reserve is pre-determined at a value of 250 MW composed of 150 MW thermal reserve and 100 MW hydro reserve.

• These were provided by 20 units of TPP and 10 units of HPP under the AGC;

• The goal of Secondary Control Reserve is to restore frequency and cross–border exchange to the set values.

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Table 39: Operational Criteria for Definition of

Operating Reserve Value (Ref. EEHC 2008)

Type of Operating

Reserve

Value

(MW)

Comments

Primary spinning reserve (within 30 sec):

250

100 MW Hydro generation

150MW Thermal generation

Secondary spinning reserve (within 14.5 minutes):

In Winter 350 Hydro-Thermal generation

In Summer 175 Hydro-Thermal generation

175 Interconnection lines

Cold reserve (within 15 minutes):

600 Gas Turbines45

Primary & Secondary Responses

• It shows that the primary response must be

available within the first 5 seconds after

frequency drop incident and should be fully

available for at least 25-30 seconds after the

event.

• Beyond that the secondary response should be

available and must be available till a maximum

of 30 minutes.

47

Generation Control

48

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AGC of Two-Area System

49Source: O. I. Elgerd, “Electric Energy Systems Theory – An Introduction”, McGraw Hill, Inc.

Interconnection with External Systems• The Egyptian Unified Power System is

presently operating in interconnected mode with the neighboring countries:

– Jordan, and

– Libya.

• In near future, the Egyptian power system will be interconnected to the KSA system through a DC link to facilitate power exchange of 3000 MW.

• Interconnection with Sudan is also expected in future through 2000 MW HVDC link.

50

Expected Peak 2030 Egyptian Power

System with External Interconnections

51

8

Share of SR in the Interconnected

System Operation

• Each Transmission System Operator (TSO) should contribute to the SR in accordance with its respective contribution coefficient (Ci):

5- =�U�.<-

�U�.<"2"�3

• This effectively means that each TSO must maintain a share of the total spinning reserve requirement of the interconnected system which is proportional to its share of the total online generation within the interconnected system at the time of coincident system peak.

52

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Egypt-Libya Interconnection

• Actually the interconnection with Libya is

realized through two 220 kV OHTL starting

from Salloum substation up to Toubroq

substation.

• The main parameters of the interconnection

lines are:

– Length 257 km

– Rated voltage 220 kV

– Rated power 460 MVA

53

Interconnection with Jordan

• The interconnection with Jordan transmission network is realized through 500/400 kV transformer substation at Taba using autronsformer units installed to create the 400 kV busbar.

• The interconnection line 400 kV starting from Taba substation and ending at Aqaba substation is composed of a OHTL and HVAC undersea cable to cross the Aqaba Gulf up to Aqaba substation.

54

The 500/400kV Interconnection with

Jordan Transmission Network

The 400 kV interconnection link has the

following main parameters:

• Length 43 km (approximately)

• Rated voltage 400 kV

• Rated power 1780 MVA

55

ARE-KSA HVDC Interconnection

The ARE-KSA transmission system comprises of the construction of a ±500kV, 1300 km HVDC transmission system between Badr SS near Cairo to Madinah East SS and with a tap to TabukSS in K.S.A.

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Right-of-Way Widths

• A minimum right of way width of 70

meters, i.e. 35 meters on either side from

the line center, shall be required for 500 kV

overhead lines.

• A minimum right-of-way width of 50

meters shall be required for 220 kV

overhead lines

• For 66 kV lines 26 meters.

57