Contingency Ranking byTime Domain Simulations

ECE 422/522Russell Patterson

Micah TillTerry Jones

Project Goals

• Identify top five critical contingencies in Zones 2-A and 2-B

Project Goals

• Rank the top five contingencies by the stability criterion.

• Rank the top five contingencies by their Critical Clearing Time and compare the ranking with that based on values of

• Determine the effect of varying load models

Task 1: Rank N-1 Contingencies

• A study was created in TSAT using the Case Wizard• Contingencies were ranked by the stability criterion:

(1)• The results of this analysis are presented below

Contingency (η) Faulted Line Zon

eLM1-1 -68.43 172 to 83 circuit 1 2-A

LM1-2 -68.41 172 to 173 circuit 1 2-A

LM1-3 -65.31 169 to 168 circuit 1 2-A

LM1-4 -65.31 169 to 114 circuit 1 2-A

LM1-5 -63.99 171 to 170 circuit 1 2-A

Task 1: Rank N-1 Contingencies

• Bus 83 is the major bottleneck for power flow between Zones 1-A and 2-A

• As seen, all of the buses in Table 2 are electrically close to bus 83

• Many of these branches are negative reactance, which means they represent series capacitors

• In reality, some of the series capacitors would be bypassed during faults

almost 4GW

Briefly - Series Capacitors

• Used to increase the MW transfer capability of the path (XL – XC)

• 60% series compensation means XC = 60% of XL

• Too much compensation can lead to problems like overvoltage and subsynchronous resonance (SSR)

P=𝑉 𝑆𝑉 𝑅

𝑋 sin∅

Briefly - Series Capacitors

• subsynchronous resonance (SSR) can result if too much series compensation is applied

• More XC means higher fR

• SSR usually in the range of 10 to 50Hz

rad/s

Hz

IEEE definition: “Subsynchronous resonance (SSR) is an electric powercondition where the electric network exchanges energy with a turbine/generator at one or more of the natural frequencies of thecombined system below the synchronous frequency of the system.”

Reference: EPRI Power Systems Dynamics Tutorial

Briefly - Series Capacitors

• Series capacitors are bypassed when current through produces voltage across of 150-300%

• Initially bypassed by arrester (non-linear)

• Hard bypassed prior to exceeding MOV energy capability

V

I

Briefly - Series Capacitors

• Series capacitors directly in faulted line will be hard bypassed

• Series capacitors farther away will likely just be partially bypassed as their voltage peaks

• Those not hard bypassed will be quickly re-inserted after fault is cleared

• Hard bypassed will take as long as 5-cycles to reinsert (if at all)

Briefly - Series Capacitors

Series compensation at Dafang, China

Ref: “Series Capacitor By-pass Switch – ABB”

Briefly - Series Capacitors

How can series capacitor bypassing be handled in transient stability analysis?

• TSAT – create user defined model which can be accessed in dynamic editor as SERUDM (TSAT has a test case of a thyristor-controlled series compensator (TCSC) model)

• PSS/E has model called SCGAP2• PSLF has model called SCGAP

Task 1: Rank N-1 Contingencies

• To test the assumption that properly modeled series capacitor bypassing would change the results, a 3-phase fault was place close-in to bus 83 and the rotor angle of generator 112 was recorded.

• This was repeated with the capacitive branches shorted out (bypassed).

• Note that while the results will eliminate the effects of series capacitors, they still will not accurately reflect system response

Task 1: Rank N-1 Contingencies

• Initial run in black, run with series capacitors bypassed in red• Note that initial case loses stability shortly after fault inception

Task 2: Critical Clearing Time• TSAT was able to identify the CCT by gradually

increasing the clearing time until the stability criterion became negative.

• The closer the faulted line is to generator 112, the smaller the CCT

Contingency

CCT (cycles) Faulted Line

LM1-1 4.58 172 to 83 circuit 1

LM1-2 4.58 172 to 173 circuit 1

LM1-3 1.76 169 to 168 circuit 1

LM1-4 1.76 169 to 114 circuit 1

LM1-5 1.76 171 to 170 circuit 1

Task 2: Critical Clearing Time• Cannot transmit

power through zero voltage bus

P=𝑉 𝑆𝑉 𝑅

𝑋 sin∅

Task 3: Load Model Comparison

• The load models were varied as shown:

• Note that conversion to a constant impedance load was required below a certain threshold for TSAT to calculate solutions for all contingencies

• Results for LM2, LM3, and LM4 were compared to the CCT values from LM1

Load Model Description Conversio

n

LM1 P as 100% constant IQ as 100% constant Z V = 0.7 p.u.

LM2 P and Q as 100 % constant Z N/A

LM3 P and Q as 100 % constant I

V = 0.35 p.u.

LM4 P and Q as 100 % constant P

V = 0.85 p.u.

Task 3: Load Model Comparison

• Constant impedance (LM2) performs better than the mixed model (LM1)

• Constant current (LM3) and power (LM4) cannot solve unless certain buses are converted back to constant impedance (LM2)

• Even after this, LM3 and LM4 underperform LM1

Contingency

CCT (cycles)

LM1 LM2 LM3 LM4

LM1-1 1.77 2.89 1.20 0.50LM1-2 1.77 2.89 1.20 0.50LM1-3 4.58 6.64 2.05 0.50LM1-4 4.58 7.11 2.05 0.50LM1-5 1.77 2.89 1.20 0.50

Task 3: Load Model Comparison

Thank You!