WAMSTER Architecture Details

Introduction

What is Wamster?

Wamster is a synchrophasor measurement and storage system which includes all equipment necessary to create an ad-hoc PMU wide-area network. PMU devices are designed with portability and quick deployment in mind, while the concentrator server provides easy web access, event triggering and data export.

Figure 1. WAMSTER system overview. Handheld STER PMU devices (shown on the left) communicate with the server over the mobile (GPRS) network. WAMSTER server processes received synchrophasors, drives them through the event triggering system, stores them into the database (shown at the bottom), sends notification messages to clients, and renders an online interface for web clients (right).

What are the benefits?

Compared to a classic PMU/PDC system, Wamster allows less networking infrastructure, reduces management costs and speeds up deployment. STER PMU devices are typically deployed within 15 minutes of arrival on site. Measurements are immediately available to all team members from any web- enabled device. Customer does not need a dedicated PDC device, which results in less administration.

Custom communication protocol adapts the reporting speed to network conditions. The protocol allows retrieving historical data from PMU’s local memory in case of network outage. Local SD flash memory is enough for cyclically storing synchrophasor data for the last 4 months.

Wamster components are also compatible and interoperable with other equipment: STER PMU devices are IEEE C37.118 compliant and can work in Ethernet or serial mode like a standard PMU device. Also, Wamster server can be configured to exchange data with other PDCs and customer devices.

With Wamster, there are also no hidden costs: all the equipment necessary to start measuring and transmitting data (apart from a SIM card) is included with each device: cables, clamps and measurement transformers, GPS clock for time synchronization, GPRS/UMTS or Ethernet modem, and other auxiliary equipment.

Who should use Wamster?

Wamster is ideal for projects involving synchrophasor baselining and model research and estimation at geographically distributed locations, where hard-wired networking is not feasible. Wired networking requires substantial planning effort, as well as security considerations, especially when reusing existing networking infrastructure.

Projects with quickly changing requirements and locations will benefit from quick deployment, while small and midsize R&D teams can utilize various online analysis capabilities. Online web access to the data and e-mail event reporting is especially useful when working with larger and distributed teams.

Wamster is a cost effective solution for academic research and education, where removing the need for networking infrastructure and PDC equipment is of great benefit as well.

Communication Layer

Introduction

One of the main benefits of Wamster is the possibility of using unreliable GPRS/UMTS networks as the communication channel. There are several system features which work cooperatively to make this feasible, some of them already mentioned in the introduction:

• PMU devices are equipped with non-volatile (flash) memory capable of storing 4 months of data locally while communication is offline.
• Device is powered with rechargeable batteries, allowing up to 4 hours of autonomy during power outages. Modem is also supplied through the device, allowing the communication to work during blackouts.
• Each PMU device always stores data locally at full resolution for the specified grid frequency (50 or 60 fps). However, transmitted frame resolution can be adjusted to meet user needs and current communication conditions, as requested by the server or a user.
• If one of more frames is dropped during communication, Wamster communication protocol allows the server to negotiate resolution and request those frames from the device’s local memory. The same principle is used to fill missing data if the default reporting speed is lower.
• Full resolution frames are also automatically requested whenever an event is detected, in order to speed up the analysis process.

The following chapter describes communication layer internals.

Communication Cycle

Communication cycle starts with the device connecting to the configured TCP socket on the Wamster server.

On the top of the Wamster communication module sits the TCP/IP layer, which handles incoming connections from all PMU devices. Whenever a device is connected, it immediately transmits its settings and handshake information, including its ID, firmware version, selected grid frequency, current GPS location and various internal log messages. This data allows server to determine device status, prepare the processing pipeline, or discard the connection if needed.

During this connection phase, Wamster server queries the database about any missing (not received) synchrophasor frames for the last 24 hours (by default), and prepares data parsers for the specified protocol type and version. After several received frames (in order to determine actual network throughput), Wamster will request missing frames from the device at the resolution configured by the user for the specified device.

Whenever a connection is dropped, device will continue measuring and storing data locally. As soon as the connection is reestablished, server will request all frames that weren’t received during the network outage.

Adapting to Network Conditions

Wamster automatically adapts the default reporting speed for historical or real-time frames in three cases:

• Whenever an event is detected, Wamster requests the device to resend frames for the pre- and post-trigger time range at full resolution, and notifies the user about the event.
• User can compare full resolution measurements for two PMU devices through the web interface. Wamster will send a request to both devices to increase their reporting speed to 50/60 fps while the analysis is active.
• If network conditions don’t permit the device to transmit synchrophasors at the user configured transmit resolution for a longer period of time, Wamster will automatically gradually decrease real-time reporting speed until conditions improve or it reaches the minimum speed (1 fps). When network conditions improve, Wamster will again recollect all frames stored with lower resolution to meet the user specified speed.

For high-speed Ethernet connections, default reporting resolution can be set to nominal grid frequency (50 or 60 fps), in which case detailed synchrophasor data will be available immediately.

Remote Firmware Upgrade

Wamster communication protocol also allows device firmware to be upgraded remotely, over the GPRS network. This is especially important for customers who want custom protocols and functionalities added to their devices, as it allows central upgrade of all devices in the field, without the need for physical access to devices.

It also simplifies scenarios where devices are relocated frequently and

It typically takes less than a minute to transmit the firmware over a GPRS connection and reinitialize the device, during which time the device temporarily suspends measurements.

Concentrator Components

Configurable Wamster architecture allows various processing scenarios for the incoming synchrophasor data, versatile extension possibilities and custom data adapters.

A processing pipeline is assigned to each device when it connects to the server, which includes following actions:

• communication monitor monitors communication status, adapts the communication system to network conditions and tracks all pending requests;
• event detector processes synchrophasor frames as they are received and detects events according to user defined rules;
• status monitor monitors PMU status, reports system events and creates statistics;
• export manager coordinates detailed export merged from multiple PMU devices;
• firmware updater manages remote updates for all devices
• remote device manager allows remote user interaction with the device.

All components can send notifications to the e-mail/SMS reporting system.

Figure 2. Wamster server architecture

Communication Monitor

This module contains components responsible for most of the functionality described in the previous chapter. At its core is the highly configurable state machine which sends and keeps track of pending requests, using information supplied from various pipeline components.

Each component initiates requests for a range of frames at a desired resolution. Also, the component defines either strict or relaxed download policy. If the policy is relaxed, Wamster can decrease frame resolution during requests if network conditions demand it. Some components (like export manager, responsible for user requested exports) will always create strict policy requests to ensure that their resolution meets user requirements.

Communication monitor also adjusts current (real-time) resolution according to various configurable rules: time between two frames, detected increase in dropped frames, rate at which the frames are being received, etc.

Event Detector

This module uses defined processing rules and thresholds to generate synchrophasor events. Data preprocessing is first applied to incoming frames to provide quantities for the analyzer, which are then checked against defined thresholds or compared between multiple devices.

When a threshold is exceeded, an event is generated and stored in the database. If current reporting resolution is lower than the grid frequency, a request is created and sent to the device by the communication monitor. Notification e-mail is also sent to users configured for this device.

PMU Status Monitor

This component monitors internal status of PMU devices: GPS clock status, SD flash card capacity, battery level and charger status, and internal device events. System events are also generated, stored in the database, and reported to administrator e-mail addresses.

Export Manager

Using the web interface, user can select a time range for export from one or more PMU devices. When a data export job is requested, export manager will scan the database for all devices involved in the export, and define a list of frames to be requested by communication monitor. It will also track and report individual download progress, and finally create the output stream when all data is collected.

When defining the export job, user can decide if data should be retrieved from the server database only, without sending requests to the devices. By default, data exporting includes collecting all missing data from devices' local memory.

Remote Device Manager

Remote device manager allows remote interaction with the device, by sending keystrokes and receiving device screen contents. Typical usage includes verification and change of measurement settings from a remote (web) location.

Database and Storage

By implementing database as a service over the online web interface, Wamster eliminates administrative IT costs, and allows future scalability to be completely transparent to the end user. Data is securely stored and backed up at a remote location, accessible at any time from any location.

Underlying Technology

WAMSTER supports several types of database systems. By default, it uses instances of Microsoft SQL Server 2008 R2 Enterprise Edition as the underlying storage technology, with separate monthly databases for each PMU device. This allows scalability, availability, flexible migration and data backups, as well as full separation between clients (multi-tenancy). This provides full separation at the file system level, ensuring that each client can have their own data files ready within minutes, should they desire this data.

The data layer is organized to allow per-client, or even per-device storage differentiation, which means that it's possible to use multiple different database systems or architectures, depending on client needs and preferences. This includes using different relational DBMSs like Oracle, NoSQL databases or fully distributed DBMSs like Apache Cassandra.

Synchrophasor measurements are stored with an IEEE C37.118 compliant UTC timestamp (time quality info, leap second support, etc.), which allows system integration with other PDC systems. Measurements can be grouped by configurable measurement points, which are in turn related to PMU device entries in the database. All data processing rules, analysis rules and thresholds, as well as all events detected during online operation are also stored inside the same database and hold reference to related measurement entries, along with PMU device data and their GPS locations, which ensures consistency when data is archived or migrated.

Partitioning

To provide easier management and maintenance of vast amounts of stored data, time-based partitioning strategies are employed for time-value series, based on monthly intervals and individual PMU devices. Partitioned views allow higher insert throughput for real-time data during historical data queries, allowing online index maintenance jobs to perform more quickly and simplifying archiving.

Scalability

Storage system has been designed and implemented with scalability in mind. As the network grows, database system can easily accommodate arbitrarily large numbers of devices, by scaling out using distributed partitioned views and custom data-dependent routing strategies.

Backups

WAMSTER databases are regularly backed up to prevent data loss in case of failures. Additionally, given the fact that WAMSTER’s online database doesn’t usually contain full resolution data (depending on the configured reporting resolution), it is important to note that all PMU devices store full-resolution measurements on their SD cards, creating an additional remote copy of all measurements.

Clients always have physical access to these full-resolution measurements, can replace these SD cards in regular intervals (5 months for 32GB cards) and archive them at their own locations.

Additionally, full resolution SD cards can be uploaded to our servers to be used as virtual devices by WAMSTER, allowing online access to all full-resolution measurements.