manitoba hvdc research centre, a division of manitoba … · 2016-10-15 · title sub synchronous...

Title

Sub Synchronous Oscillations – An Introduction

October 2016

MANITOBA HVDC RESEARCH CENTRE, a Division of Manitoba Hydro International Ltd.

© Manitoba HVDC Research Centre | a division of Manitoba Hydro International Ltd.

Presented by: Dharshana Muthumuni

Title Presentation Outline

© Manitoba HVDC Research Centre | a division of Manitoba Hydro International

• General background

• AC Network characteristics

• Series compensation and impact on SSR

• Types/Classification of SSO

– SSR

• IG and SSTI

– Transient Torques

– HVDC – SSTI

– SSCI – Wind

• Simulation examples

Title

Introduction / Background


Title Introduction / Background


Transients in the Electric Network

• Transients following a system disturbance should damp out in a reasonable timeframe.



Transients in the Electric Network

Voltage and current transient frequencies

– High frequency transients

– ‘Traditional’ electromechanical oscillations (close to system frequency on system side, 0.1 – 5 Hz on mechanical side)

– Frequencies below system frequency (60 Hz)

• Sub Synchronous Oscillations

9.7 Hz



Network Characteristics

System Impedance Vs Frequency Plots

Following a system disturbance currents (and voltages) corresponding to network resonance frequencies can flow in the system (and hence into generators stator windings).

Base Case - Case1

Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...

...

...

0.0

0.2k

0.4k

0.6k

0.8k

1.0k

1.2k

1.4k

Z+(o

hm

s)

case0 case1

A Typical frequency characteristic



Network Characteristics

Following a system disturbance currents (and voltages) corresponding to network resonance frequencies can flow in the system (and hence into generators stator windings).

Main : Graphs

0.170 0.190 0.210 0.230 0.250 0.270 ...

...

...

-300

-200

-100

0

100

200

300

y

Eb

Base Case - Case1

Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...

...

...

0.0

0.2k

0.4k

0.6k

0.8k

1.0k

1.2k

1.4k

Z+(ohm

s)

case0 case1

Title Mechanical Oscillations


Constant speed mechanical rotation => 60 Hz on electrical side

1 Hz mechanical oscillations => 59 Hz and 61 Hz

8 Hz mechanical oscillations => 52 Hz and 68 Hz

• fm Hz mechanical oscillations => (60-fm) Hz and (60 + fm) Hz

Gen

Speed - W

Electric network

Title Mechanical shaft-mass system


• In steady state (constant torque transmission) each shaft rotates at a constant speed and with a constant ‘twist’.

• Following a disturbance (network or mechanical system side), the shafts will go through a ‘transient’ period.

• The oscillation frequencies will correspond to ‘natural frequencies’ of the mass-shaft system.

• Damping associated with the mechanical system and the damping provided by the response of the electrical system will determine the nature of the oscillations.

Speed - Wk

LP HP G IP

δ

Title Mechanical shaft-mass system


• Interaction between the mechanical and electrical system:

– Damping associated with the mechanical system and the damping provided by the response of the electrical system will determine the nature of the oscillations.

Title Mechanical Resonance Frequencies


Mechanical modes can be calculated from shaft data:

Inertias of individual masses (J)

Shaft Spring Constant (K)

Typical resonant frequencies of Thermal Generator Turbine units are in the range of 15-25 Hz.

Speed - Wk

LP HP G IP

δ

fi=1

2𝜋√𝐾𝑖.𝑊𝑏

2𝐻𝑖

60Hz - 25 Hz => 35 Hz

Title

Series compensation and impact on SSR


Title Series compensation and impact on SSR


Impact on network characteristics (System Impedance Vs Frequency)

Network resonance points can shift to the sub synchronous frequency range (<60 Hz)

Higher the level of series compensation, the resonant point moves toward the system frequency

• Increases potential risk of SSO related issues

Base Case - Case1

Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...

...

...

0.0

0.2k

0.4k

0.6k

0.8k

1.0k

1.2k

1.4k

Z+(o

hm

s)

case0 case1

Title Series compensation and impact on SSR


Higher the level of series compensation, the resonant point moves toward the system frequency

Frequency Scan - Generator Terminals

0

0.2

0.4

0.6

0.8

1

1.2

1.4

5 9

13

17

21

25

29

33

37

41

45

49

53

57

Frequency (Hz)

|Z+

| (o

hm

s) 45% compensation

55% compensation

60% compensation

75% compensation

𝑓𝑒 = 𝑓0√𝑋𝑐

𝑋𝑙

Title

Types / Classification of SSO


Title Types/Classification of SSO


1. Self Excitation (Induction generator effect)

Purely electrical phenomenon

Self-excitation of Sub Synchronous Oscillations

Generator ‘acts’ as a NEGATIVE resistance at the sub synchronous frequencies. If this

effective negative resistance is greater in magnitude compared to the positive resistance as

seen from the system, an unstable resonance condition can take place.

Gens

1.00 2.00 3.00 4.00 ...

...

...

0.700

0.750

0.800

0.850

0.900

0.950

1.000

1.050

1.100

Machine Output Volts p.u.

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50 Capacitor Volts, A

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0 Machine Current, A,out+

Generator Series cap.




Synchronous generators as well as induction machines can contribute to

this condition

Easy to explain (in a 20 min introduction) using an induction machine

example

1X

1V

1I 1R

mX

'

2X'

2I

s

R '

2agP

Electrical resonant frequency

System nominal frequency

Negative Slip (S)

S s

R '

2Higher effective NEGATIVE resistance)




Synchronous generators

– Field (and damper) winding characteristics contribute to IG

– At frequencies (w) below the system frequency (wo), the effective resistance is negative.

)]')[(1(

')()'(2

0

00

do

d

b

sTww

Tww

w

xxwR



2. Torsional Interaction

Electrical resonance frequency (fe) of system caused by series

capacitors close to natural torsional resonance frequency (fm) of

mechanical system (turbine-generator unit)

Condition for SSR-TI Risk =>: fe fo - fm

Due to interaction between the electrical (generator/line series capacitor…) and

mechanical systems (long shafts and masses of thermal units)

Result in shaft fatigue & undesirable stress, damage

Typical mechanical resonance frequencies (15 – 30 Hz)

The risk is with higher Series Compensation (SC) levels

This condition should be avoided through design considerations



2. Torsional Interaction

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 10 20 30 40 50 60 70 80

Electrical damping from the machine/electric-network

mechanical shaft oscillations – derived from EMT simulations

(damping analysis using EMT simulations).



3.Transient Torques

• Shaft oscillations following disturbances will result in shortening of the shaft life time due to material ‘fatigue’.

• The critical items that determines the level of severity are:

• Amplitude of the mechanical transient oscillations

• Decay of the oscillations (damping)



3.Transient Torques

Peak transient torque produced on each shaft segment can be used to determine the level

of risk.

Machines are designed to withstand for “terminal short circuit” and the peak torque

produced for this event can be used as a reference.

If the torque produced for an evet is larger than about 75% of the value for terminal short circuit, it is

considered as in the zone of vulnerability.

S-N curves (number of stress cycles as a function of stress amplitude before crack

initiation).

Source: EPRI report: “Torsional Interaction Between

Electrical Network Phenomena and Turbine-Generator

Shafts”

1

2

3

103 105 107

Shaft Torque (PU)

Cycles to failure)



4. Interaction with HVDC converter controls - SSTI

• An issue associated with conventional (LCC) HVDC schemes

• A parameter ‘Unit interaction Factor’ is used to determine the level of risk (a screening level indicator)

– How close the generator is to the HVDC converter

– Other parallel AC connections (improved damping)

• Radial connection of a generator to the rectifier AC bus is the likely worst case.

• Can be easily mitigated through proper design of HVDC control schemes and settings.

Rectifier

Rbus

Gens



4. Interaction with HVDC converter controls - SSTI

• SCTOT – Short circuit level at the HVDC terminal

• SCi – Short circuit level at the HVDC terminal with the generator under SSTI

investigation disconnected.

• UIF > 0.1 is recommended for further analysis

• UIF should be calculated for carefully selected (credible) N-x outage

conditions

Source: EPRI report: “Torsional Interaction Between Electrical Network Phenomena and Turbine-Generator Shafts”



5. SSCI involving T3 Wind Units

• Modern wind turbines use power electronic converters (connected to the generator) to improve performance

• Wind farms located far from the AC grid has necessitated ‘Series Compensated (SC)’ Transmission lines

• Problem: DFIG controls act to ‘amplify’ sub synchronous currents entering the generator

– Negative Damping

– Rotor side converter plays the dominant role

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5. SSCI involving T3 Wind Units

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

0 5 10 15 20 25 30 35

Dynamic frequency scan to estimate damping provided by the

DFIG to sub synchronous frequency transients

Title

Do I need to perform studies?


Title Performing studies


Identifying potential risks

WGR Location

Series Cap Locations

Title

Thank you


manitoba hvdc research centre, a division of manitoba … · 2016-10-15 · title sub synchronous...

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