Controlling the Production of Dispersed Energy Sources

In last two decades the number of dispersed renewable energy sources (like small photo-voltaic units or small wind generators) has grown rapidly, partially due to environmental awareness of the public and partially due to high subsidies which made the investment into such sources very profitable.

From the grid perspective these sources still represent a small portion of produced energy which is in normal circumstances consumed locally (local substation), but the short-term power they can deliver and the unpredictable nature of it is already causing problems to DSOs (Distribution System Operator) and in some cases even to TSOs (Transmission System Operator). Main problem of these sources is their unpredictability and intractability.

If man can’t do much about predictability, it can address the tractability – ability to control and observe (measure) the source state. The key is to get relevant data (e.g. current production power) as quickly as possible, as often as possible (e.g. every minute with few seconds latency).

Problem

TSOs are responsible for the control of frequency which is achieved by constantly measuring and calculating the grid balance (production – consumption ± cross-border exchange) and comparing it to the scheduled balance. The difference (error) is then sent to regulation units (dedicated generators, virtual power plans, etc.) trying to compensate the error. In the “conventional grid” achieving this is not a problem because the TSO has an on-line access to substations, power plants (the big ones) and transmission lines but in the “modern grid” this is not a case due to dispersed production sources which are spoiling the first part of equation by bringing certain unpredictability. Therefore, TSOs should also get on-line (near-on-line) data from small power plants which would help them to get more accurate production state.

Traditional solution

Getting data from substations and big power plants is solved by RTUs and smart meters connected to dedicated and highly reliable communication lines (multiplexed serial or IP over fibre optics) but getting data from dispersed production units is not so easy. Pulling a dedicated fibre optics line to a small photo-voltaic power plant in a rural area and equipping it with (expensive) RTU isn’t economically feasible. The only viable option left is by using smart meters capable of measuring in short intervals (1 minute) connected to some public IP infrastructure (e.g. GPRS).

Most small power plants are already equipped with smart meters and communication modules enabling DSOs to collect data required for billing, time synchronization and maintenance but there are some problems:

· Single communication line (GPRS with private APN for utilities only)
· Low frequency of reading (typically once per day)
· “PULL” nature of metering protocols (DLMS, M-BUS)

To allow TSOs to read meters while keeping basic functionality of daily reading by DSOs an additional communication line with dedicated GPRS APN should be added and meters should be equipped with dedicated “high-frequency” load profile (1 minute).

In theory this should work but in practice communication over wireless IP, especially over 2.5 G (like GPRS/EDGE) is not as smooth as over wired IP networks. This becomes evident during communication with a typical “PULL” based protocol (e.g. DLMS) due to a lot of handshaking causing a long round-trip time.

This configuration is also relatively expensive (two communication modules, two APNs, two Head-End Systems) and difficult to maintain.

Are there any good alternatives? Yes, IoT!

IoT solution

Fortunately, many meters on the market already support PUSH mode which allows meters to push data on special events (e.g. power-on, communication established, loss of phase) and periodically.

PUSH mode solves two problems:
· Meter initiated connection with single message sent (without handshaking) on wireless networks is faster and more reliable
· Data are sent when they are available (immediately after load profile period reset)

Typical round-trip between meter connected to GPRS network and server connected to internet is below 2 seconds. Despite PUSH mode these meters are still capable of communicating in PULL mode for time synchronization or maintenance.

What about multiple communication lines? Well, here comes the IoT part. DLMS/COSEM push mode is not an IoT protocol like MQTT, AMQP, CoAP. Messages are pushed directly to the TCP broker without quality of service, payload is COSEM APDU (binary data, very condensed), etc. But non-IoT devices can be transformed into IoT ones by using the IoT gateway.

The IoT gateway should be able to:

· catch pushed messages from meters
· transform them into readable form like JSON or XML and
· publish them to the MQTT broker

use-case

When messages arrive to MQTT broker they can be securely and reliably delivered to multiple clients, including TSO, DSO or any other interested party. 🙂

The whole process happens in few seconds! 🙂

The solution is price efficient! 🙂