Q5Power System Protection

(i) Synchro Phasors

Synchrophasors are phasor representations (magnitude and phase angle) of power system voltage or current waveforms, measured at widely-separated locations across the power system but time-stamped and synchronized to a common, precise time reference (typically derived from GPS satellite signals, providing microsecond-level synchronization accuracy), so that phasor measurements taken simultaneously at different substations can be meaningfully and directly compared against one another on a common time/angle reference frame — this is in contrast to conventional, locally-referenced phasor measurements (as used in ordinary numerical relays), which are each referenced only to their own local voltage/current waveform and cannot be directly compared in angle to a measurement taken independently at a distant location.

Phasor Measurement Units (PMUs): synchrophasor measurements are generated by dedicated devices called Phasor Measurement Units, which combine a precise GPS-synchronized time reference with high-speed sampling and phasor-estimation algorithms (similar in principle to the DFT-based algorithms discussed in the corresponding fault-detection-algorithm answer elsewhere in this paper) to continuously report time-stamped voltage and current phasor measurements, typically at a reporting rate of several tens of samples per second, substantially faster than the periodic (seconds-to-minutes interval) measurements traditionally provided by SCADA systems.

Applications: synchrophasor data, collected from PMUs distributed across a wide interconnected power system and aggregated by Phasor Data Concentrators, forms the foundation of Wide Area Measurement Systems (WAMS, discussed in an earlier answer), enabling real-time visualization of system-wide voltage angle differences and their rate of change (providing early warning of developing stability problems well before they would become apparent through conventional local measurements alone), improved state estimation accuracy and update rate, oscillation and power-swing detection across the interconnected grid, and detailed, precisely time-aligned post-disturbance event reconstruction and analysis — collectively representing one of the most significant recent advances in power system monitoring, control, and protection technology.

(ii) Bus Bar Arrangement Scheme

A bus bar arrangement scheme defines the specific topological configuration by which incoming generator/transformer feeders and outgoing line/load feeders are connected to a substation's bus bar(s), with the choice of scheme representing a deliberate trade-off among reliability (continuity of supply during a fault or maintenance outage), operational flexibility, and capital cost.

• Single bus arrangement: the simplest and least costly scheme, with all incoming and outgoing feeders connected to a single common bus bar via individual circuit breakers — offering minimal cost and complexity, but with the significant limitation that a fault on the bus bar itself (or maintenance of the bus bar) requires a complete outage of every feeder connected to it, making this scheme suitable only for less critical, lower-reliability-requirement installations.
• Single bus with sectionalizing: an improvement on the basic single bus scheme, dividing the bus into two (or more) electrically-separable sections via a normally-closed bus-section circuit breaker, so that a fault on one section affects only the feeders connected to that section, while the bus-section breaker can be opened to isolate a faulted section without a complete station-wide outage, at a modest additional cost over the basic single bus arrangement.
• Double bus arrangement: provides two separate, parallel bus bars, with each feeder connected to either bus via its own pair of isolators (allowing any feeder to be switched between the two buses), and typically a bus-coupler breaker linking the two buses together — this arrangement provides substantially improved flexibility (allowing one bus to be taken out of service for maintenance while all feeders are transferred to the other bus, without any supply interruption) and improved reliability (a fault on one bus need not affect feeders that can be quickly transferred to the healthy bus), at a correspondingly higher capital cost due to the doubled bus bar infrastructure and additional isolators required.
• Main-and-transfer bus arrangement: uses one main bus (through which feeders normally operate) plus a second, auxiliary 'transfer' bus intended specifically to allow any single feeder's own circuit breaker to be taken out of service for maintenance (by switching that feeder's connection temporarily to the transfer bus, fed via a single shared transfer breaker) without interrupting that feeder's supply, offering a cost-effective compromise providing breaker-maintenance flexibility without the full expense of a complete double-bus arrangement.
• Ring bus and breaker-and-a-half arrangements: more elaborate schemes used at higher-reliability, higher-voltage transmission substations, in which multiple circuit breakers are arranged such that any single breaker can be removed for maintenance, or can fail, without interrupting supply to any feeder and without requiring any bus-transfer switching operation at all — these schemes offer the highest reliability and operational flexibility among common bus arrangement schemes, at correspondingly the highest capital cost due to the larger number of circuit breakers and more complex protection/control arrangements required.

The selection of an appropriate bus bar arrangement scheme for a given substation depends on the substation's voltage level, its criticality within the overall network, the number of connected feeders, and the specific reliability and maintainability requirements set by the utility's own design standards and practices, with higher-voltage transmission substations and generating stations generally justifying the additional cost of more elaborate schemes (double bus, breaker-and-a-half, or ring bus) given the severe consequence of an extended outage at such critical network locations, while lower-voltage distribution substations more commonly employ simpler, more economical single-bus or main-and-transfer arrangements.