Wide area monitoring, protection and control

Wide Area monitoring system

It is a collective technology to monitor power system dynamics in real time, identify system stability and help to design and implement counter measures. Stability includes a number of grid management aspects including: state determination and grid imbalance (voltage stability, phase imbalance); disturbance recording; network safety; and generation control. Improved planning will also result from the data gathered from WAM. Measurement is achieved through the optimal placement of PMUs across the transmission network and distribution network. Other monitoring (existing data collected through SCADA) is combined to give an overall picture of grid stability.

Phasor current, phasor voltage, frequency and rate of change frequency measurements are taken by Phasor Measurement Units (PMUs) at selected locations in the power system and stored in a data concentrator every 100 milliseconds. The measured quantities include both magnitudes and phase angles, and are time-synchronised via Global Positioning System (GPS) receivers with an accuracy of one microsecond. The phasors measured at the same instant provide snapshots of the status of the monitored nodes. By comparing the snapshots with each other, not only the steady state, but also the dynamic state of critical nodes in transmission and sub-transmission networks can be observed. Thereby, a dynamic monitoring of critical nodes in power systems is achieved.

 

A number of widely distributed PMUs in the power system may be utilized to implement a Wide Area Measurement System (WAMS). The architecture of a typical WAMS is shown in Figure 1. It consists of widely distributed PMUs in the power grid. These PMUs send the measurements to Phasor Data Concentrator (PDC), known as nodal PDC. In general one PDC caters to a fixed number of PMUs. The PDC collects and sort the data based on the time tags. It keeps the data required for local applications and transmits the data for advanced applications to a super PDC through a dedicated communication network. These three layers of a WAMS may be categorized as: data acquisition, data management and applications layers. The PMU based WAMS is one of the most important technologies expected to play a key role in the smart grid. The WAM technology may be utilized for the following:

1. Preventing Blackouts: PMU data provide information about the system at a common instant of time that can be used for real-time dynamic analysis. The real time information will be extremely useful in continuous monitoring and early detection of abnormalities. Timely initiation of corrective action will help in restricting the disturbance to a smaller area.

2. Improved State Estimation: The set of complex voltage phasors across its buses specifies state of the system. State estimator utilizes telemeter data from Remote Terminal Units (RTUs) to generate an optimal estimate of the system state. However these measurements do not contain the phase angles due to the difficulty associated with the synchronization of measurements. Consequently the phase angle has to be estimated with the slack bus as reference. However with the advent of Phasor Measurement Units (PMUs) this difficulty can be removed as the PMU measures voltage and current phasors synchronized through GPS. Therefore the existing SE can be improved by using data from a few PMUs installed at critical locations. Note that due to technical and economical constraints it may not be feasible to install PMUs at every bus of the system.

3. Transmission Line Congestion management: The traditional approach to real-time line congestion management is based on the Nominal Transfer Capability, computed off-line using conservative hypothesis concerning thermal, voltage or stability limitations. However the WAMS will allow computing the Real-time Transfer Capability for the actual operating conditions. Therefore it will result in better utilization of transmission line capacity. It is also expected that in future the PMU data will be integrated with smart sensors measuring line temperature,  sag etc then the operational limit may further be increased.

4. Accurate calibration of Instrument Transformers: The monitoring and control of a power grid depends heavily on the measurement of current and voltage signals derived from the secondary circuits of instrument transformers. The synchronized data available from WAMS may also be used to obtain accurate calibration of current and voltage transformers in a network.

WAMS Components

Wide Area Measurement components and functions are described below.

Phasor Measurement Units: Wide area measurement systems (WAM) rely on a set of phasor measurement units (PMUs) installed at diverse locations across an electrical transmission network. A PMU is a device that is connected to a feeder at a substation and takes current and voltage measurements in order to determine phasors (as defined by the IEEE C37.118-2005 standard). A phasor is a measurement of the magnitude and angle of an electrical waveform, either current or voltage. Voltage or current signal is sampled and converted into phasors, that are synchronized and compared across the electrically connected power system using an accurate GPS time reference. These time-synchronised phasors are called synchrophasors. When these sets of synchrophasors are send to a central location from each PMU, the state of the network from a wide area perspective can be seen. The angle in a phasor is a relative unit; therefore for analysis a reference zero angle is usually selected, generally this is a voltage phasor. Phasor angle differences provide useful information concerning system stress or modes of oscillatory disturbances in the power system. The PMU provided the critical synchronized time-tagged information that enabled a clear understanding of the events leading to the blackout.

Phasor Data Concentrator:  A PDC collects phasor data from multiple. PMUs or other PDCs, aligns the data by time tag to create a synchronized dataset, and then passes the data on to applications processors. PDCs typically buffer the input data streams  and include a certain “wait time” before outputting the aggregated data stream. A PDC also performs data quality checks, validates the integrity or completeness of the data, and flags all missing or problematic data.

Communications Media: The phasor measurement units use  existing fibre  network to communicate with the central server that hosts the PDC applications.

GPS: It provide a time reference to PMUs.

Functions of WAMS

Grid Model Validation:  The measurements provided by the WAM system can be used to give a more accurate view of the state of the network, in addition to measurements already provided by protection devices. It can be used to verify the network model and SCADA data used to perform power flow analysis.

Stability Analysis: Power flow can be predicted under different scenarios with more accurate measurements from the WAM system. With more accurate measurements of bus states, the network can be operated closer to the stability threshold resulting in a higher power transfer, which is particularly important for interconnections. Early detection of insecure or unstable conditions by the WAM system can provide early detection of the system trending towards an insecure state (that is, not meeting contingency requirements), or unstable state, and will assist operators in making informed remedial decisions.

Post Event Analysis: Post event analysis after a disturbance on the network can make use of the WAM measurements to determine the sequence of events that led to the cause of the disturbance. If PMUs retain power during a black start (where all generators have had to shut down due to loss of grid stability) the frequency and phasor angle information can assist system operators to conduct the restoration process.

WAM data access and presentation: Besides engineers accessing the data, the requirements of system planners, system operators, and protection engineers are being gathered in order to develop an appropriate visualisation front-end for their use of phasor measurement data. System planners are interested in historical data to verify planning models. System operators are interested in recent history and near real-time data to complement their existing understanding of the operation of the network that is gained through SCADA measurements, and some post-fault view of events. Protection engineers are interested in post event (fault) waveform information.