IEEE Std C37.245-2018 pdf download – IEEE Guide for the Application of Protective Relaying for Phase-Shifting Transformers.
Under normal conditions the power flow split between the EHV and HV paths may be appropriate. However, it may be possible that under contingency conditions, (say one EHV line out of service), the HV Line 3 might become overloaded. The PST angle may be adjusted to reduce the power flow through HV Line 3 to a level within its rated capacity and force the remaining power to flow through the single remaining EHV line. Thus the PST has allowed the full capacity of the HV and EHV transmission paths to be available. Since PSTs are often applied to facilitate maximizing transmission capability, the rating of the PST may be considered as part of a system. The ratings of the PST and its associated bushings and tap changer would normally be carefully specified to be compatible with the rating of the external equipment. Under extreme conditions, if the steady state rating of the PST becomes exceeded, system integrity protection schemes may need to be applied to avoid overstressing the PST in a similar manner as such schemes may be applied to avoid overstressing any part of a transmission network.
5.3 Installation configurations A variety of installation configurations may be provided for optimal operation of the PST. Some example arrangements are shown in the following simplified single line diagrams. It can be seen that there is usually a PST bypass facility provided to allow the PST to be taken out of service while retaining the availability of the remaining power transfer equipment (such as a transmission line) up to its capability with no phase shift. Whether or not circuit breakers or disconnect switches are used depends on the degree of operational flexibility required and cost impact of alternative switching arrangements. Normally for PSTs connected in parallel, only one bypass device is applied. For PSTs connected in series, each PST may be equipped with individual bypasses. The location of circuit breakers and associated CTs will affect the protection zones for the PST. Even if dedicated breakers are not provided, as shown in Figure 7(a), the PST will usually have its own dedicated protection zone defined by CTs around the PST. The presence of one or more dedicated PST breaker(s), as shown in Figure 7(b), Figure 7(C), and Figure 7(d), allows the PST to be isolated quickly by control action without the need to bring the PST to neutral position before bypassing.