REPORT ON THE VOLTAGE STABILITY PROBLEM IN THE WOTAB AREA – RESULTS AND RECOMMENDATIONS
- Introduction
The WOTAB region is a high profile industrial area accounting for approximately 25% of the total load on the Entergy system. The WOTAB region peak load for the summer 2006 operating season is expected to be approximately 6550 MW. A significant portion of the power imports into the WOTAB region is based on the flow on the Richard-Wells-Webre 500 kV and the Mt. Olive-Hartburg 500 kV lines. These imports can lead to significantly high east-west flows on the Webre-Wells-Richard line and a north-south bias on the Mt.Olive – Hartburg line. Upon the loss of the Webre-Wells-Richard 500 kV line, most of the import into the area is transferred onto the 180 mile long Mt. Olive-Hartburg line. Power transfer over this long line leads to heavy reactive losses and subsequent degradation of voltages at Hartburg and Cypress 500 kV stations. This problem is exacerbated if a large generating unit in the WOTAB region is offline.
There is a total of approximately 3800 MW of native generation in the WOTAB region. The two largest units in the area are the Nelson U6 and Sabine U4 units. In addition, 550 MW of merchant generation in the WOTAB region with long term firm transmission service is dispatched in the case.
2. Problem Description
The critical contingency in the region is:
1)Nelson U6 out-of-service and the loss of the Webre-Wells 500 kV line accompanied by the operation of the switching scheme.
Voltage Stability studies have indicated that under expected peak load conditions and the outage of either Nelson U6 or Sabine U4, the loss of the Wells-Webre 500 kV line may lead to high transient voltage swings. Also, unacceptably low steady state voltages at the 500 kV as well as underlying system may follow these swings.
- Study Assumptions
a)2006SummerPeak model was used for the study. The total native load and generation in the WOTAB area was modeled to reflect expected 2006summer peak conditions.
b)The load power factor in the WOTAB area was assumed to be 0.945.
c)Dynamic voltage studies were carried out at 50% motor load. Out of this total motor load, a 50% split was made between large motors and small motors. The rest of the load was assumed to be a combination of 100% constant current and 0% constant impedance for real power, and 100% constant impedance for reactive power.
d)The study was performed with firm transmission contracts in the area.
e)The WSCC voltage stability criteria have been adopted by Entergy and areused for evaluating the magnitude and duration of voltage swings. According to WSCC criteria, the contingency analyzed is classified as a level C contingency. Therefore, the maximum allowable voltage dip was considered to be 0.30p.u., and the maximum allowable duration for which dip exceeds 0.20 p.u. was chosen to be 40 cycles. For the steady state, a threshold of 0.96 p.u. was used for the 500 kV voltage level and 0.92 for the underlying voltage levels.
- Study Results
Table 1 shows the list of cases analyzed. Each of the three casesrepresents different post-contingency flow level on Mt.Olive – Hartburg 500kV line. The plot for these cases is shown in Figures 1-3. In all of the cases, a WOTAB area load of 6550 MW was considered. The contingency analyzed in this study for all cases is the loss of the Wells-Webre 500 kV line with Nelson U6 out of service.Voltages were monitored at critical buses in the WOTAB area.
Case No. / Case Description / Post Contingency Flow on Mt.Olive – Hartburg 500kV line (MW) / Post Contingency Steady State Voltage at Hartburg 500kV Bus (p.u.) / Lowest Bus Voltage Dip & Swing in WOTAB area1 /
- With Firm Transaction
- Nelson U6 off-line
- Pre-Contingency Flow on Wells-Webre 500kV line is 1550MW
Duration of Dip <0.80p.u.: 50 Cycles
2 /
- With Firm Transaction
- Nelson U6 off-line
- Pre-Contingency Flow on Wells-Webre 500kV line is 1479MW
Duration of Dip <0.80p.u.: 38 Cycles
3 /
- With Firm Transaction
- Nelson U6 off-line
- Pre-Contingency Flow on Wells-Webre 500kV line is 1407MW
Duration of Dip <0.80p.u.: 27 Cycles
- Conclusions
Based on the voltage stability analysis results, it is determined that the WOTAB area is susceptible to transient voltage swings under certain extreme system conditions that can adversely impact the security of the system. These swings, which are in violation of the WSCC criteria, can potentially lead to a loss of system load. Moreover, sustained operation at low voltages can be a violation of good utility practices and can affect system reliability. Since dynamic swings occur within a few seconds of the triggering event, a load-shed scheme is not a solution for addressing the system problems related to the transient voltage swings. Therefore, in order to limit the voltage swings and prevent loss of system load, it is recommended to limit the flow on the Mt. Olive-Hartburg 500 kV line to 1050 MW when the Richard -Wells 500 kV line or the Wells-Webre 500 kV line is out of service.
Updated 6/26/2006 1