Ssr

Power System Stability

Summer 2000

6 Subsynchronous Resonance Torsional interaction Mainly thermal plants with long and flexible shafts Shafts may break Can be excited by switching in the network Induction generator effect at subsynchronous frequencies

Known since 1970

Olof Samuelsson

1

Power System Stability

Summer 2000

Basic phenomena Resonance dynamics involved in torsional oscillations: fnet

Network with series compensation: RLàRLC

fshaft

Drive shaft of power plant – mass-spring

Torque components 50 Hz–fnet – subsynchronous – negative damping 50 Hz+fnet – supersynchronous – positive damping

Induction generator effect Synchronous generators are asynchronous to currents with frequencies other than 50 Hz Negative slip and resistance can lead to resonance Less common

Time scale: 10-45 Hz

Olof Samuelsson

2

Power System Stability

Summer 2000

Famous disturbance First event in Mohave, 1970 Second event at the same plant, 1971 The weak shaft to the exciter broken by torsional oscillations The term subsynchronous resonance defined after this

Olof Samuelsson

3

Power System Stability

Summer 2000

Modeling considerations Network modes Shaft modes Rotating masses – multimass representation

Walve Fig. 6.2

Mass-spring model for SSR: I 0  ∆ ˙   0 I  ∆   0 0 J  ∆ ˙  = K D∆  +  I   

Mass-spring model for angle stability: I 0   ∆ ˙   0 = 0 M ∆ ˙  K

I  ∆  + Bu D ∆ 

Only difference is if K is tri-diagonal or not

Olof Samuelsson

4

Power System Stability

Summer 2000

Analysis methods Time simulation Requires simulation tool for electromagnetic phenomena, even if it is a symmetrical, electromechanical phenomenon PSCAD/EMTDC EMTP Matlab Power System Blockset Frequency scanning Bode diagram Modal analysis of linear(ized) model Mode shape – Machowski Fig. 6.39

Olof Samuelsson

5

Power System Stability

Summer 2000

Means for mitigation New lines instead of reactive series compensation Appropriate degree of reactive series compensation Meshed system SSR relay bypasses series compensation or trips generator NGH scheme – resistor across series compensation Damping controllers Thyristor Controlled Series Capacitor HVDC

Olof Samuelsson

6

Power System Stability

Summer 2000

SSR in Sweden Nuclear plants 40 m and 3000 rpm 70 m and 1500 rpm HVDC converter Long heavily series compensated lines

Meshed transmission system SSR damper Baltic Cable HVDC link TCSC at Stöde

Olof Samuelsson

7

Power System Stability

Summer 2000

SSR and Stöde TCSC Forsmark 3 normally mesh connected At a fault Forsmark 3 may be radially connected SSR conditions between Stöde Series Cap and Forsmark 3

Compensation reduction not possible Instead part of fixed series cap rebuilt to TCSC Series capacitor inductive at subsynchronous frequencies

Olof Samuelsson

8