# System Operating Limits Methodology

c.  **Regional Differences**

> 2\. The following Regional Difference shall apply only in the Western
> Electricity Coordinating Council (WECC). The WECC methodology required
> in 603(a)(1) shall require that System Operating Limits be established
> for following system conditions, in addition to those listed in
> 603(a)(3)(i)
>
> i\. Single Contingencies *[\[Same as Category B\]]{.mark}*

A.  The following single contingencies must be evaluated:

a\. Single line to ground or 3-phase fault, with Normal Clearing, on any
faulted Generator, Transmission Circuit or Transformer.

b.  Loss of any Facility without a fault.

c.  Single pole block, with Normal Clearing, in a monopolar or bipolar
    HVdc system.

```{=html}
<!-- -->
```
B)  System Operating Limits shall be established such that for
    contingencies in 603(c)(2)(i)(A) operation within the System
    Operating Limit shall provide system performance consistent with
    that prescribed in 603(a)(3)(iii)--603(a)(3)(iv) above.

*[\[The above will be deleted if section 603(a)(i)]{.mark}* *[is changed
to exclude SLG bus sections and breakers (and the reference in]{.mark}*
*[(ii)(A)]{.mark}* *[below will be changed).\]]{.mark}*

ii\. Multiple Contingencies

A)  In addition to the single Facility contingencies listed in
    603(c)(2)(i)(A), the following multiple Facility contingencies must
    also be evaluated when establishing System Operating Limits:

```{=html}
<!-- -->
```
a.  Simultaneous permanent phase to ground faults on different phases of
    each of two adjacent transmission circuits on a multiple circuit
    tower, with Normal Clearing. If multiple circuit towers are used
    only for station entrance and exit purposes, and if they do not
    exceed five towers at each station, then this condition is an
    acceptable risk and therefore can be excluded. *[\[Category
    C5\]]{.mark}*

b.  Simultaneous permanent phase to ground faults on different phases of
    each of two adjacent transmission circuits on separate towers in a
    common right-of-way, with Normal Clearing. *[\[Same as
    WECC-S2\]]{.mark}*

c.  A permanent phase to ground fault on any transmission circuit,
    transformer, or bus section with Delayed Fault Clearing. *[\[Same
    as]{.mark}* *[Category C7, C8 and or C9, but]{.mark}* *[excluding
    protection system failure.\]]{.mark}*

[]{.mark}

d.  Simultaneous permanent loss of both poles of a direct current
    bipolar facility without an AC fault. *[\[Same as]{.mark}*
    *[Category C4\]]{.mark}*

e.  [The failure of a circuit breaker associated with a special
    protection system to operate when required following: the loss of
    any element without a fault; or a permanent phase to ground fault,
    with Normal Clearing, on any transmission circuit, transformer or
    bus section.]{.mark} *[\[Same as Category C7, C8 and C9 for
    protection failure only, and subset of]{.mark}* *[Planning Standard
    III.F.S1.\]]{.mark}*

f.  [A common mode outage of two generating units connected to the same
    switchyard, not otherwise addressed by Standard 603.]{.mark}
    *[\[Same as WECC-S3.\]]{.mark}*

[]{.mark}

g.  [A permanent phase to ground fault on a bus section or breaker with
    Normal Clearing.]{.mark} *[\[Same as Category C1 and C2;
    WECC-S4\]]{.mark}*

```{=html}
<!-- -->
```
B)  System Operating Limits shall be established such that for multiple
    Facility contingencies in 603(c)(2)(ii)(a)-
    603(c)(2)(ii)([e]{.mark}) operation within the System Operating
    Limit shall provide system performance consistent with the
    following:

> a\. All Facilities are operating within their applicable
> post-contingency thermal, frequency and voltage limits.
>
> b\. Cascading outages do not occur.
>
> c\. Uncontrolled separation of the system does not occur.
>
> d\. The system demonstrates transient, dynamic and voltage stability.
>
> e\. Depending on system design and expected system impacts, the
> controlled interruption of electric supply to customers (load
> shedding), the planned removal from service of certain generators,
> and/or the curtailment of contracted firm (non-recallable reserved)
> electric power transfers may be necessary to maintain the overall
> security of the interconnected transmission systems.
>
> f\. Interruption of firm transfer, load or system reconfiguration is
> permitted through manual or automatic control or protection actions.

g\. To prepare for the next contingency, system adjustments are
permitted, including changes to generation, load and the transmission
system topology when determining limits.

> C. System Operating Limits shall be established such that for multiple
> Facility contingencies in 603(c)(2)(ii)([f]{.mark}) -- [(g)]{.mark}
> operation within the System Operating Limit shall provide system
> performance consistent with the following:
>
> a\. Cascading outages do not occur.
>
> D. When planning systems and facilities, WECC may make changes
> (performance category adjustments) to the contingencies required to be
> studied and/or the required responses to contingencies based on actual
> system performance and robust design. Such changes will apply in
> determining System Operating Limits.