micro-grid to system synchronization based on pre...

Micro-grid to System Synchronization

Based on Pre-Insertion Impedance Method

(Version 1.0)

By

Peter Zhou

University of Alberta

Jan 30th , 2015

Outline 1. What is Synchronization?

2. Synchronization Concerns?

3. How Synchronization is Performed In Reality?

4. Synchronization Challenges

5. Project Proposal

6. Project Solutions

I. Theoretical Explanation of Transients

II. What is Acceptable Transient Level?

III. Practical V and f Range for Open Loop

IV. Procedure for Selecting |Z|

V. Impedance Bypass Considerations

7. Other Problems Caused?

I. Stability Consideration

II. Reactive Power Imbalance

III. Bypass transient due to Q Imbalance

8. Advantages of Open Loop

9. Future Work

1. What is Synchronization?

• Synchronization in simple terms is a process of connecting an

electrical island to another.

• The electrical island can be a generator, a micro-grid, or part of

a large power grid.

There are 3 synchronization scenarios to consider:

• Generator synchronizes to system

• Micro-grid synchronizes to system

• System synchronizes to another system

2. Synchronization Concerns?

To perform a successful synchronization, these parameters must

be matched closely across both sides of the breaker:

• Voltage Magnitude

• Phase Angle

• Frequency

Poor synchronization can:

• Result in High Synchronizing Transient Current

• Result in generator out of synchronism with system

𝐼𝑝𝑒𝑎𝑘 ∝ ∆𝑉

𝑍𝑒𝑞

∆𝑉

3. How is Synchronization Performed in Reality?

Generator to System: System ~

Feedback Control

Generator

• In practice, all synchronizations require a way of feedback control

• Through feedback control, ∆𝑉 across synchronizing breaker is minimized

Micro-grid to System: System Micro-grid

~

~

~

Multi-DG Feedback Controls

System to System:

• The synchronization process can become complicated.

• Require sophisticated coordination and tuning of many generators.


30 miles long, it is a present situation of a real

MW-scale Micro-grid located in a rural area of

Rio de Janeiro state of Brazil.

Consider in a rural area, DG is really far away from utility substation, feedback

control is not the best option.

Disadvantages:

Long tuning process

Costly to build

Low Reliability over long

distance due to attenuation/loss

• Restoration of islands is required after a blackout or major faults.

• Effective Coordination and time are crucial factors in play to minimize impacts on utility customers.

• Feedback control is difficult to implement during system restoration process.

Stabilize any surviving islands

Recover Generation

Energize Transmission

Restore loads

Synchronize islands to each other

An example of IESO’s restoration strategy


5. Project Proposal

• The idea is to use an impedance pre-insertion to reduce the transients effects

from synchronization.

𝐼𝑝𝑒𝑎𝑘 ∝

∆𝑉

𝑍𝑒𝑞 ∝

∆𝑉

𝑍𝑖𝑛𝑠𝑒𝑟𝑡+𝑋𝑑′′+𝑋𝑡+𝑋𝑒𝑞

Impedance

Pre-insertion

6. Project Solutions I. Theoretical Explanation of Transients

Assumptions:

Constant Z type Load

Grid as swing bus

Zsys is the system short circuit impedance

𝑋𝑑′′

SG Grid

ZeqZsys

I Transient

Zs Zse

SG Grid

SuperpositionAcross Breaker

Zstator Zsys

Local Load

Z Insertion

• Following analytical equations were derived based on the transient circuit of superposition.

• Closing transient is primarily a function of ∆𝑉, ∆𝛿, 𝑎𝑛𝑑 𝑍𝑒𝑞

• 𝑍𝑒𝑞 is equivalent impedance seen at breaker

/

/

/

( ) sin( ) sin( )

( ) sin( ) sin( )

( ) sin( ) sin( )

Rt LAsyncA A A

eq

Rt LBsyncB B B

eq

Rt LCsyncC C C

eq

VI t wt e

Z

VI t wt e

Z

VI t wt e

Z

𝐼𝑝𝑒𝑎𝑘 is the

max current

among

three phases

6. Project Solutions I. Theoretical Explanation of Transients


𝑍𝑒𝑞 ∝

∆𝑉

𝑍𝑖𝑛𝑠𝑒𝑟𝑡+𝑋𝑑′′+𝑋𝑡+𝑋𝑒𝑞

Impedance

Pre-insertion

6. Project Solutions II. What is the acceptable transient level?

According to IEEE standards C50.12 and C50.13, the synchronization criteria for

both cylindrical and salient-pole synchronous generators are:

• Angle ±10°

• Voltage 0 to 5 %

• Slip ±0.067Hz

If within IEEE standards, the maximum transients will be in an acceptable level:

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-1

0

1

I sync (

pu)

Synchronizing Transients

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

0

2

I S

tato

r (p

u)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

time (s)

Te (

pu)

0.932pu

1.91pu

1.91pu

• 𝐼𝑝𝑒𝑎𝑘=0.932 pu for

synchronizing transient

• ∆𝑇𝑒=0.91pu Reference

Torque deviation

6. Project Solutions III. Practical V and f Range for Open Loop

• Busses inside both electrical islands to be synchronized have their voltages and

frequencies inside a practical range that was decided by the utility protocol.

• The boundary conditions for voltage and frequency based on the practical range

determine the worst case transients possible for open loop.

• After knowing the worst transients, the impedance size can be determined either by

analytical approach or EMTP simulations.

• ∆𝜃 = 10° is the IEEE standard value. However, in most cases, faulty

synchronization could happen due to higher than expected slip and breaker

mechanism delay, a worst case of ∆𝜃 = 30° is assumed.

Vmin~Vmax (pu) Fmin~Fmax (Hz)

Micro-grid (bus 1) 0.9~1.1 59.7~60.2

System (bus 2) 0.9~1.1 59.95~60.05

Bulk Grid Generator,

Microgrid

1 2

Table 1: Practical V and f Ranges Based on AIES Standard

Based on AIES

standard, worst case

open loop

synchronization

scenario is

determined as:

• ∆𝑉 = 0.2𝑝𝑢

• ∆𝜃 = 30°

• ∆𝑓 = 0.35𝐻𝑧

6. Project Solutions IV. Procedure for Selecting |Z|

Start

Run Load Flow (breaker Open)

Adjust Xfmer tap, local loading to matchWorst case open loop Synchronization Criteria

(20%,30 ,0.35 )o Hz

Adjust Xfmer tap, local loading to matchIEEE Synchronization Criteria

(5%,10 ,0.067 )o Hz

Run EMTP Simulation with disturbance (Closing breaker)

Run EMTP Simulation with disturbance (Closing breaker)

> ?open loopI IEEEI

Add Impedance at synchronization link(Initial Guess of |Z| obtained from

Analytical solution)Yes

Re-run

No

StopObtain |Z|


• Bottom figure is a comparison between analytical and EMTP simulation results

of 𝐼𝑝𝑒𝑎𝑘𝑉𝑠 ∆𝜃. They come to close agreements.

• ∆𝜃 is the primary factor that affects 𝐼𝑝𝑒𝑎𝑘 when ∆𝜃 > 10°.

• ∆𝑓 = 0.35𝐻𝑧 has negligible impact on 𝐼𝑝𝑒𝑎𝑘.


• The impedance to be inserted is selected based on the intersection of synchronizing

transient curve with the acceptable reference line.

• As an example, Z=0.556 (pu) or 16.6 ohm is needed to limit the maximum transient level to

be within the acceptable level incurred by ∆𝑉 = 0.1𝑝𝑢, ∆𝜃 = 30°, 𝑎𝑛𝑑 ∆𝑓 = 0.35𝐻𝑧

6. Project Solutions V. Impedance Bypass • Impedance bypass is required to avoid excessive energy dissipation due to the impedance

insertion and restores the transmission line capacity back to its original state.

• After synchronization, the generator starts to have electromechanical oscillations.

• In order to bypass the impedance, it is desirable to wait for generator electromechanical

oscillations to reach a steady state first. This way, a minimum voltage is seen across

breaker 2, which guarantees a minimum bypass transient.

Micro-grid

GeneratorsBRK1

Z Insertion

BRK2

Bulk Grid

Steady PowerFlow

V

11 2 22 2

12 1 22 2

cos( ) sin( )

sin( ) cos( )

VP R V V XV

R X

VQ RV X V V

R X

Frequency

Power Output

of

oP

sf

1P

5% droop

P

f

6. Project Solutions V. Impedance Bypass • Steady state P flow is caused by a micro-grid frequency different from the system

frequency, which results in DG’s change in mechanical power.

• Steady state Q flow is caused by the unmatched voltage magnitudes across the

breaker.

0 5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

3

Inserted Impedance (ohms)

Sw

itchin

g T

ransie

nts

(pu)

Optimal Impedance = 16.6 ohm

10%, 0.35V f Hz

0, 0.35V f Hz

10%, 0.25V f Hz

0, 0.25V f Hz

10%, 30oV

Bypassing Under Different Conditions of ,V f

Synchronizing

Transient

Bypass Transient

• Impedance insertion leads to

further rotor angle advancements.

But by how much?

• ∆𝜃 𝑎𝑛𝑑 ∆𝑓 have most impact on

transient stability of generator

• Inclusion of governor model can

reduce the effects of ∆𝑓 as shown

due to changing Pm.

• Even in the worst case scenario,

the generator remains angular

stable. The max ∆𝛿 deviation is 5°

when maximum impedance 0.556

per unit is inserted.

7. Other Problems Caused? I. Stability Consideration

Micro-grid Frequency (Hz) System Frequency (Hz) ∆𝒇 (Hz)

60.2 (max) 59.95 (min) 0.25

59.7 (min) 60.05 (max) -0.35

The Inertia constant H of the generator will have an effect on the rotor angle deviation:

• Higher the H means higher the initial kinetic energy of rotor

• Initial kinetic energy can cause further rotor advancement in the transient swing due to ∆𝑓

• Doubling the H value with ∆𝑓 = 0.35𝐻𝑧 in the worst case has negligible impact on the

stability of generator.

7. Other Problems Caused? I. Stability Consideration

𝐻 = 2.52𝑠

• Reactive power imbalance is a concern after bypassing the impedance.

• Reactive power will flow from the side of higher voltage magnitude to the lower

voltage magnitude bus. A result from high |∆𝑉| difference across breaker.

Consequences:

1. Sudden over-excitation or under-excitation of the field current. Both situations

should be avoided to prevent heating or damage to the field windings and end

iron core.

2. Power quality concern. For example, voltage swells or sags that may affect

some sensitive loads in the area.

7. Other Problems Caused? II. Reactive Power Imbalance

Q fI

sI

End Region Heating

P

• Reactive power will flow from the side of higher voltage magnitude to the lower voltage magnitude bus.

• Reactive power imbalance not much a concern after synchronization because the inserted impedance is there to limit the amount of Q flow. However, after bypass, the Q flow will be a concern if voltage magnitudes not matched properly.

• One consequence of Q imbalance can result in a sudden over-excitation or under-excitation of the field voltage. Both situations should be avoided to prevent heating or damage to the field windings and end iron core.

• Second concern over reactive power imbalance is related to power quality. For example, voltage swells or sags that may affect some sensitive loads in the area. Although it is a secondary concern if no equipment damage occurs.

• Due to the reactive power imbalance, it is strongly suggested to run load flow after closing the synchronizing breaker to check any nearby generators have reached their reactive capability limit.

• Relief small island loading and/or restore more loading in the larger island could be a way to mitigate the problem by pre-determine the loading profile of both islands.

7. Other Problems Caused? II. Reactive Power Imbalance

Bypass transients is over the acceptable level at all impedance values when ∆𝑉 = 0.2𝑝𝑢 in the worst

case.

Recommendations:

• Adjust AVR settings to match the Q of local loads. (Voltage following mode for DGs)

• Run load flow first to pre-determine a load profile for both micro-grid and system so that

generators will not go beyond its Qmax & Qmin.

7. Other Problems Caused? III. Bypass Transients due to Q Imbalance

0 5 10 15 20 25 30 35 400.5

1

1.5

2

2.5

3

Inserted Impedance (ohm)

Peak C

urr

ent

(pu)

Analytical

Simulation

Reference LineSynchronizing Curve

Optimal |Z| = 18.87 ohm

Bypass Curve

20%, 30oV

20%, 0.25V f Hz

• Idea of impedance insertion can mitigate the synchronizing transients from

synchronization.

• If the busses of islands are inside the practical range of 𝑉 𝑎𝑛𝑑 𝑓, then open loop

synchronization without feedback control can be implemented with impedance pre-

insertion.

• This approach eliminates the need for a feedback control of generators. It is beneficial

in the following ways:

• No need to build a communication line when DG is far away from the switchyard.

• Improves reliability in the case of power outage.

• No need to spend the time to tune/control the DGs if bus V and f are in the practical range.

• Faster synchronization process results in faster system restoration.

8. Advantages of Impedance Based Synchronization

9. Future work:

• Impedance based synchronization method shall be investigated between system to

system.

• In the system to system synchronization scenario, can one determine the size of

impedance still by the analytical approach? If so, is there a way to simplify or

reducing the networks in a way to easily find the impedance?

Open Loop Micro-grid to System

Synchronization Based on Pre-Insertion

Impedance Method

(Final Version)

By

Peter Zhou

University of Alberta

Jan 30th , 2015

Outline

1. Synchronization Concerns

2. Current Practice for Performing Synchronization

3. Synchronization Issues

4. New Idea --- Open Loop Synchronization

5. Research Strategy

6. Solutions

7. Conclusions

8. Future Works


• Synchronization in simple terms is a process of connecting an electrical island to another.

• The electrical island can be a generator, a micro-grid, or part of a large power grid.

There are 3 synchronization scenarios to consider:

• Generator synchronizes to system

• Micro-grid synchronizes to system

• System synchronizes to another system


• damage of generator and prime mover due to High Synchronizing Transient Current

• generator out of synchronism with system

Synchronization without any control may result in

How to deal with them?

Challenge

𝐼𝑝𝑒𝑎𝑘 ∝ ∆𝑉𝑍𝑒𝑞

∆𝑉

• ΔV --- difference of voltage magnitude

• Δθ --- difference of voltage phase angle

• Δf --- difference of frequency

Adjust the voltage phasor difference across the breaker into an acceptable

range during synchronization process through feedback.


Limit Torque deviation

Ensure stability

Limit Transient Current

(IEEE criteria)


Generator to System: System ~

Feedback Control

Generator

Micro-grid to System: System Micro-grid

~

~

~

Multi-DG Feedback Controls

System to System:

• The synchronization process can become complicated.

• Require sophisticated coordination and tuning of generators on both sides.

3. Synchronization Issues

Issues:

A costly communication link must be build when a DG is far away from PCC.

Feedback control is not reliable in the extreme case of power outage.

Feedback control may need an inacceptable long time to tune the generators,

when system restoration at emergency is required.

Bulk Grid

~

Micro-grid

25

Synchronizing Control

Sync Panel

Communication Link



𝑍𝑒𝑞 ∝

∆𝑉

𝒁𝒊𝒏𝒔𝒆𝒓𝒕+𝑋𝑑′′+𝑋𝑡+𝑋𝑒𝑞

New Idea:

Using an impedance pre-insertion to reduce the transients effects from synchronization

instead of adjusting ∆𝑉 through feedback.

The impedance is designed to meet the synchronization requirement, based on a pre-defined

voltage and frequency range of the islands to be synchronized together, such that the open loop

synchronization can be achieved.

synchronization

requirements

∆Ipeak, ΔTe

must remain the

same level

IEEE or utility

criteria

Pre-established

Operating Region


Microgrid

( )gV pu

( )gf Hz

60.2

59.7

1.10.9 Pre-established Operating Region

( )sV pu

( )sf Hz

60.05

59.95

1.10.9

1 2

Z

Circuit Breaker


Watch

∆V,∆θ,∆f

System ~

Feedback Control

Generator

No

Feedback

Watch ∆θ

Meet Criteria for ∆V,∆f?

Yes

∆θ<Criteria?

No

Close Synchronizing

Breaker

Yes

Close

Loop

Pre-established

Operating Region


Microgrid

( )gV pu

( )gf Hz

60.2

59.7

1.10.9 Pre-established Operating Region

( )sV pu

( )sf Hz

60.05

59.95

1.10.9

1 2

Z

Circuit Breaker

Each party operates within pre-

established operating region

(i.e. power quality limits)

Verify if V and f are within the

region at the breaker location

Can’t perform

Synchronization No

Yes

Watch ∆θ

∆θ<Criteria?

No

Close Synchronizing

Breaker

Yes

Bypass

Impedance

Open

Loop

5. Research Strategies

Problems Strategies

What is the acceptable

transient level?

How to select the impedance to

control switching transient?

Can the generator reach stable

condition?

Is the impedance bypass

transient acceptable?

• Use IEEE C50.12 criteria for generator

synchronization to establish acceptable

transients level.

• Use common utility operating limits for

voltage and frequency to establish open

loop criteria.

• Evaluate the bypass transients to

establish the number of steps or

impedance values for bypassing operation

• Establish a method or criterion to

determine the impact on stability

According to IEEE standards C50.12 and C50.13, the synchronization criteria for

both cylindrical and salient-pole synchronous generators are:

• Angle ±10°; Voltage 0 to 5 %; Slip ±0.067Hz

Current and torque transients experienced by a generator under the above condition

is considered acceptable.

Therefore, the limits on transients can be established by determining the maximum

transient under the above conditions

For example, simulation study reveals (Based on C50.12 standards): • |∆𝑰𝒑𝒆𝒂𝒌|=0.537 pu for stator current transient

• ∆𝑻𝒆=𝑻𝒆𝒎𝒂𝒙 − 𝑻𝒆𝒔𝒔 =0.628-0.056=0.572pu

6. Solutions – Acceptable transient level

0 0.5 1 1.5

0

0.5

1

Time (s)

Ele

ctr

om

agnetic T

orq

ue T

e (

pu)

0 0.5 1 1.5-1

-0.5

0

0.5

1

Time (s)

Sta

tor

Cur

rent

(pu

)

• Each party is expected to operate within established limits that meet the power

quality requirements of respective systems, as illustrated below:

• Limit on Δθ: ∆𝜃 is monitored at the synchronization point. The limit on ∆𝜃 is

selected to be the same as that use for close-loop synchronization, which is 10

degrees difference.

Objective of Impedance Selection:

• Find minimal Z that leads to acceptable transients levels (inrush current and torque

limits) under above synchronizing conditions.

Vmin~Vmax (pu) fmin~fmax (Hz)

Micro-grid (bus 1) 0.9~1.1 59.7~60.2

System (bus 2) 0.9~1.1 59.95~60.05


Microgrid

1 2

6. Solutions – Impedance selection


Research Method for Impedance Selection:

Step 1. Determine all possible worst case synchronization scenarios based on

pre-established power quality limits of the two parties.

Step 2. Evaluate the worst case transients (Torque and Current) resulting

from synchronization scenarios above.

Step 3. Design an impedance value for insertion such that the highest

transient is within the acceptable level.


Research Method Step 1 (Worst case synchronization scenarios):

V1 (pu) V2 (pu) f1 (Hz) f2 (Hz) ∆θ=θ1-θ2 (deg)

case1 1.05 1 60.067 60 10

case2 1.1 0.9 60.2 59.95 10

case3 1.1 0.9 59.7 60.05 10

case4 0.9 1.1 60.2 59.95 10

case5 0.9 1.1 59.7 60.05 10

• Ideally, synchronization wants to take place when V1=V2, θ1=θ2, f1=f2, two

parties will synchronize with perfect parallelization with no switching

transients and no system disturbances at all.

• Case 1 represents the reference case for acceptable transients.

• Possible worst case synchronizations will take place in following cases 2-5

shown in Table 1, which the operating points of both parties deviate the most

from normal operation (1pu, 60Hz), and synchronize at a maximum allowable

voltage and frequency differences.

Table 1: Synchronization Scenarios

1V

1f

60.2Hz

59.7Hz

1.1pu0.9pu2V

2f

60.05Hz

59.95Hz

1.1pu0.9pu

4 2

35

3 5

2 4

11


Research Method Step 2 (Worst case transients evaluation):

System under study:

15 miles 15 miles

25/4.16kVYg/Yg

SGSGSUB

25kV

346MVA

CB 6.6MVA

2.0MW

0.65MVAr2.0MW

0.65MVAr

2.0MW

0.65MVAr

V1,f1V2,f2

Assumptions:

• micro-grid is operating at near full load as

a worst case.

Worst transient case:

• Highest current and torque

transient is located in case 2.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

case1 case2 case3 case4 case5

Torq

ue a

nd C

urr

ent

Devia

tion (

pu)

Te Ipeak

case2 1.1 0.9 60.2 59.95 10 Fig: Torque and current peak deviations when not

controlled by impedance insertion.


Research Method Step 3 (Impedance Design Procedure to Reduce Worst Case

Current Transient):

• Impedance Z is determined by analytical solution. The accuracy of the analytical

solution is verified by numerous transient simulations.

Case Study

File (reference case)

Case Study

File (worst case)

Establish V1,V2,f1,f2 and

∆θ across open breaker

(Initialize by loadflow)

Establish V1,V2,f1,f2 and

∆θ across open breaker

(Initialize by loadflow)

Calculation

Acceptable transient

level (I & Te)Calculation

1 2 1 2( , , , , , )acceptableZ f I V V f f

/

/

/

( ) sin( ) sin( )

( ) sin( ) sin( )

( ) sin( ) sin( )

Rt LAsyncA A A

eq

Rt LBsyncB B B

eq

Rt LCsyncC C C

eq

VI t wt e

Z

VI t wt e

Z

VI t wt e

Z


Analytical method for determination of Impedance |Z|

Following analytical equations were derived based on the transient circuit of superposition: For simplicity, assuming f1=f2 (same frequency)

Micro-grid

Generators Bulk Grid

Superposition

Across Breaker

V1, f1 V2, f2

1 2A B cV V V V V

1 2( )A V V 1tan ( / )wL R

• 𝑍𝑒𝑞 is the total equivalent series

impedance seen by the breaker.

• 𝑍𝑖𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛 is determined by subtracting

all other system equivalent impedances

such as transformer, line, and generator

sub-transient reactance.

Where:

1 2( , , , )insertion acceptableZ f I V V


Analytical & Simulation results for determination of Impedance |Z|

0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (s)

Sta

tor

Cur

rent

(pu

)

Phase A (simulation)

Phase A (Analytical)

0 0.1 0.2 0.3 0.4 0.5 0.60

0.2

0.4

0.6

0.8

1

Impedance Insertion Value (pu)

Sta

tor

Curr

ent

Devia

tion (

pu)

Simulation (case 2)

Analytical (case 2)

Reference caseZ=0.23pu

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

case1 case2 case3 case4 case5

Torq

ue a

nd C

urr

ent

Devia

tion

(pu)

Te Ipeak

Z Insertion=0.23pu

• Analytical solution results in a Z insertion value of 0.23 pu (21.8 ohm).

• Analytical current waveform is closely matched to the simulation within the first cycle after breaker

is closed.

• With this designed impedance, both current and torque transient disturbances are within the

reference level that comply with IEEE synchronization criteria.

Fig: Torque and current peak deviations when

controlled by a designed impedance insertion.

Note: cases 4 and 5 have relatively lower excitation compare

to 2 and 3, therefore the P-𝜹 transient curve is lower so the

power and torque transient swing is smaller.

7. Bypass Transients

Bypass Transient Evaluation Method:

• Impedance bypass is done after the system has reached to a new steady-state.

The process of bypass also produces current and torque transients.

• These transients must not exceed the allowable limits as well.

• Method adopted to evaluate the bypass transient is shown below.

1. Evaluate the steady state real and reactive power imbalance between the two

parties after synchronization, when both parties synchronizes within the

operating limits pre-established.

2. Evaluate the worst bypass scenario based on worst resulting voltage phasor

difference across the inserted impedance ∆𝑉 , since 𝐼𝑏𝑦𝑝𝑎𝑠𝑠 ∝ ∆𝑉 .

3. Determine and verify that whether if the worst bypass transient is below the

allowable limit.


Evaluation Step 1 (Evaluate Steady state P&Q Imbalance):

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

Active Power Imbalance (pu)

Reactive P

ow

er

Imbala

nce (

pu)

Z=0.23puZ=0.23pu1

2

3

4

• Real and reactive power imbalance is mainly due to a frequency difference and

voltage magnitude difference prior to synchronization. (∆𝑓 → ∆𝑃𝑚, ∆𝑉 → ∆𝑄)

• The real and reactive power imbalance will incur a current flowing through the

impedance, and therefore incur a voltage drop across the impedance.

• Therefore, it is true that higher the power imbalances, the higher the voltage across

the impedance and hence, the higher the bypass transient.

• Segment 1-3 and 2-4 is obtained by fixing V1 and V2

but vary f1 (59.7-60.2) and f2 (60.05-59.95)

• Segment 1-2 and 3-4 is obtained by fixing f1 and f2 but

vary V1 (1.1-0.9) and V2 (0.9-1.1)


Evaluation Step 2 (Evaluate worst bypass scenario):

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5


Reactive P

ow

er

Imbala

nce (

pu)

Z=0.23puZ=0.23pu1

2

3

4

• Since the ∆𝑉 across the impedance is dependent on |S|= 𝑃2 + 𝑄2, worst bypass scenarios

can be evaluated by the worst power imbalances shown by 4 cases in the figure.

• ∆𝑉 = 𝑍 × 𝐼 = 𝑍 × (𝑆/𝑉1)∗, with 𝑉1 ≈ 1𝑝𝑢, ∆𝑉 is proportional to both the impedance size

Z and the power imbalance |S|.

• Based on figure below, there are 4 possible cases where the bypass transient are the highest.


Evaluation Step 2 (Evaluate worst bypass scenario):

Micro-grid

Generators Z Insertion

BRK2

Bulk GridBRK1

1 1V 2 2V

V1 (pu) V2 (pu) f1 (Hz) f2 (Hz)

Case1 1.1 0.9 59.7 60.05

Case2 0.9 1.1 59.7 60.05

Case3 1.1 0.9 60.2 59.95

Case4 0.9 1.1 60.2 59.95

Impedance Switching (BRK1

Closes)

Real Power (pu) Reactive Power (pu) |S| (pu) ∆𝑽 𝒐𝒗𝒆𝒓 𝒁 (𝒑𝒖)

Case1 -0.068 0.446 0.451 0.1031

Case2 -0.214 -0.354 0.413 0.0946

Case3 0.128 0.377 0.398 0.0918

Case4 -0.024 -0.424 0.424 0.0982

Determine largest voltage

difference across impedance

(∆𝑉) (by power flow)

Worst bypass scenario is Case 1

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5


Reactive P

ow

er

Imbala

nce (

pu)

Z=0.23puZ=0.23pu1

2

3

4


Evaluation Step 3 (Verify worst case bypass transient is within allowable limits):

0 5 10 15-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (s)

Sta

tor

Cur

rent

(pu

)

0 5 10 150

0.5

1

1.5

2

Time (s)

Ele

ctric

Tor

que

Te (p

u)

Impedance

switching Impedance bypass

Impedance

switching

Impedance bypass

• Impedance is bypassed at a worst bypass

scenario according to case 1 conditions.

• Bypass transients (current and torque) is no

more severe than the allowable limits in the

worst case, in fact, it is quite small due to a

small voltage drop across the impedance.

• After the bypass, the stator current is over 1

per unit, which suggests a machine

overloading primarily due to the reactive

power imbalance. This is more of a power

quality concern and a secondary concern if

no sensitive equipment are damaged.

8. Transient Stability Assessment • Transient stability is the ability of the power system to maintain in synchronism when

subjected to a transient disturbance, in this case, micro-grid to system synchronization.

• In the proposed open loop synchronization approach, there are 2 expected switching

(disturbances) to the power system, one is the first impedance switching, and the second is the

impedance bypass switching. Both disturbances must remain in rotor angle stable, therefore,

will be assessed individually by examining their maximum rotor angle deviation ∆δ_max.

Transient Stability Assessment

Impedance Switching Bypass Switching

Worst case ∆δ_max deviation based

on power quality limits of open loop

synchronization scheme?

Using Equal Area criterion for

transient stability assessment

of bypass operation

Sensitivity Study to show the impact

of different loading levels and

machine inertia values on ∆δ_max

8. Transient Stability Assessment

Micro-grid

System

sys sysV

th

th

V

Z

0eP

Impedance

Switching

Equivalent Machine

Method to analyze Micro-grid to System Synchronization phenomenon:

• For the impedance switching, it is important to realize that there is no power transfer between the

micro-grid to system prior to breaker closes. After breaker is closed, although steady state active power

transfer is non-zero due to the governor droop setting of the machine, but in the transient time period,

Pe steady state can be assumed to be zero. In other words, Pm=𝑃𝑒𝑠𝑠=0.

• At the instant when breaker is closed, the transient Pmax is determined by Vth, Vsys, and Zeq seen

across breaker (assume loads are modelled as constant impedance).

• Sudden loading of the micro-grid generator is determined by the instant loading angle, which is the

instantaneous angle difference when breaker is closed.

• The rotor angle and rotor speed behaviors due to the impact of impedance switching can be best

explained by the Equation of Motion or the Swing Equation. Micro-grid can be best viewed as an

Equivalent Machine through the reduction of Thevenin equivalence.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Time (s)

Rot

or D

evia

tion

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Time (s)

Rot

or F

requ

ency

(Hz)

Wsyn

o max

min

ss

In practice ,due to dynamics of field and

damping winding, the electromechanical

oscillation is damped and stabilizedWr

max min

8. Transient Stability Assessment

-90 0 90 180

Power Angle (deg)

Pow

er

Tra

nsfe

r P

e

o

max o

min o

max

th sys

eq

V VP

X

Initial Angle difference at across breaker

Rotor angle starts increasing due to higher micro-grid frequency

than system frequency

Begin

Synchronization

(Breaker closed)

Impedance Switching Impact on Transient Stability (Power Angle Relationship):

2𝐻

𝜔𝑠𝑦𝑛

𝑑2𝛿(𝑡)

𝑑𝑡2= 𝑃𝑚𝑝.𝑢 − 𝑃𝑒𝑝.𝑢

𝐻

𝜔𝑠𝑦𝑛

𝑑𝛿

𝑑𝑡

2

𝛿𝑚𝑎𝑥𝛿0

= (𝑃𝑚𝑝.𝑢 − 𝑃𝑒𝑝.𝑢)𝑑𝛿𝛿𝑚𝑎𝑥

𝛿0

𝛿max = 𝑐𝑜𝑠−1 cos(𝛿𝑜) −

𝐻

𝑊𝑠𝑦𝑛2𝜋 ∙ 𝑓𝑔𝑒𝑛 − 𝑓𝑠𝑦𝑠

2∙𝑋𝑡ℎ + 𝑋𝑖𝑛𝑠𝑒𝑟𝑡 + 𝑋𝑠𝑦𝑠

𝑉𝑡ℎ ∙ 𝑉𝑠

8. Transient Stability Assessment Worst case ∆δ_max due to Impedance Switching (with constant Ef):

Cases F1 (Micro-grid Frequency)

Hz

F2 (System Frequency)

Hz

V1

(pu)

V2

(pu)

∆θ=𝜽𝟏 − 𝜽𝟐

(Deg)

1 60.2 59.95 0.9 0.9 +10

2 60.2 59.95 0.9 0.9 -10

3 59.7 60.05 0.9 0.9 +10

4 59.7 60.05 0.9 0.9 -10

Worst case

With Constant Ef

• 4 cases are worth to consider to investigate the worst case ∆δ based on power quality limits.

22.8 23.7

27.0 27.8

0.0

5.0

10.0

15.0

20.0

25.0

30.0

case1 case2 case3 case4

∆δ m

ax (

deg)

Fig: Impedance used equals 0.23pu, with Micro-grid

loading level= 85%.

• Maximum ∆δ occurs in cases 3 and 4 are

primarily due to an initial large speed deviation

between the Micro-grid to system.

• Cases 3 and 4 have a larger first swing because

∆f= - 0.35Hz, which the speed differential

represents the initial kinetic energy offset in the

rotor.

8. Transient Stability Assessment Worst case ∆δ_max due to Impedance Switching (with Excitation control AVR):

Cases F1 (Micro-grid Frequency)

Hz

F2 (System Frequency)

Hz

V1

(pu)

V2

(pu)

∆θ=𝜽𝟏 − 𝜽𝟐

(Deg)

1 59.7 60.05 0.9 0.9 -10

2 59.7 60.05 1.1 0.9 -10

3 59.7 60.05 0.9 1.1 -10

Worst case

With AVR

• 3 cases are worth to consider to investigate the worst case ∆δ based on power quality limits and AVR control

Fig: Impedance used equals 0.23pu, with Micro-grid

loading level= 85%.

27.8 26.1

17.7

27.3

19.8

36.4

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

case1 case2 case3

∆δ m

ax (

deg)

∆δ max (const Ef) ∆δ max (AVR)

• As can be seen from figure, with the addition of

AVR excitation control, during transient, it can

either help or aggravate the rotor angle

advancement depending on the relative

magnitude of system voltage.

• IEEE type 1 exciter and voltage regulator is

implemented. In case 3, rotor advancement

increases due to AVR is because although system

voltage is on the higher end (1.1pu), but the AVR

lowers the field voltage set point automatically

due to an injecting Q from the system.

Worst case

8. Transient Stability Assessment Sensitivity study of Micro-grid Loading level and Machine Inertia effects on ∆δ_max:

0 10% 20% 30% 40% 50% 60% 70% 80%

26

28

30

32

34

36

38

Percentage Loading of Micro-grid

Max t

ransie

nt

Roto

r D

evia

tion (

deg)

Z=0.23pu

Z=0.3pu

Z=0.4pu

• The mechanical Pm of the machine increases as the loading

of the Micro-grid increases.

• With AVR controlling the Q out of the machine by adjusting

the field voltage, in case 3, maximum power transfer

capability decreases after impedance switching due to a Q

flow from the system side.

• As a result, the steady state Load angle of the machine

increases, which also affects the transient rotor angle

deviation due to AVR.

Fig: Loading effects for case3 synchronizing

condition with AVR control

0 10% 20% 30% 40% 50% 60% 70% 80%20

25

30

35

40

45

50

Percentage of Micro-grid Loading Level

Max T

ransie

nt

Roto

r A

ngle

Devia

tion (

Deg)

H=1.26

H=2.52

H=5.04

• Effect of H can be explained through the fundamental

equation of motion.

• For a 3-phase to ground terminal fault, H can improve

stability by reducing the amount of rotor advancement when

rotor is accelerating. However, for synchronization, higher H

may aggravate the rotor advancement primarily due to the

initial kinetic energy stored within the rotor due to ∆f across

breaker.

• Thus, for small inertia machines within the micro-grid, the

machine will most likely to be stable for all loading levels if

synchronized within the power quality limits.

Fig: Sensitivity Study of the effect of Machine Inertia

(H) on transient stability, Z=0.23pu, case 3

8. Transient Stability Assessment Impedance Bypass Impacts on Transient Stability:

-90 0 90 180

Power Angle (deg)

Pow

er

Tra

nsfe

r P

e

Pmax increased

due to a decreased

in Zeq

ssPe

Micro-grid

System

sys sysV

th

th

V

Z

ssPe

Bypass

Switching

Equivalent Machine

0 1 2 3 4 5 6 7 8-20

-15

-10

-5

0

5

10

Time (s)

Rot

or A

ngle

(Deg

)

Impedance Switching

Bypass

• Bypass Impedance can be seen as a way to

improve the system stability by increasing

maximum power transfer.

• Bypass operation should be always rotor

angle stable since the steady state Pe is close

to zero before the bypass.

• In addition, the ∆δ max for bypass is

considered a much smaller disturbance than

the first switching due to 𝛿𝑜 ≪ 10° and ∆f=0.

9. Conclusion

• Open loop synchronization is based on pre-defined power quality limits can be

implemented with an impedance insertion at the PCC to limit the inrush current within an

acceptable level.

• In practice, voltage levels of either the Micro-grid or the system should meet the voltage

and frequency range requirements as defined by the power quality protocol at all times.

• Synchronization transient current and torque disturbances are quite similar to that of a

short circuit/fault problem, which can be analyzed with similar procedure.

• Transient stability analysis of Micro-grid to system synchronization problem may be

analyzed using the Equal Area Criterion and the Equation of Motion by an analogy to

single machine to infinite bus case.

• As a general finding, loading levels of the Micro-grid have negligible effects on the

transient current and torque due to synchronization.

• A properly designed impedance can limit the worst case transient inrush current without

incurring a significant impact on the transient stability (rotor deviation).

micro-grid to system synchronization based on pre...

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