Smart Grids: Embrace Time Sync, Rethink Today’s Standards
It seems that there is something wrong with current system’s interoperability. In the beginning, there were plenty of presentations about how smooth and reliable future operations would be because of interoperability. Today, the initial skepticism of the realists is being realized.
Savings in Interoperable Smart Grid Systems
Although there were plenty of installations and rollouts realized, no summary results have been announced about the amounts of data transferred (let’s assume more than one reading daily), communication range, real transmission rates, latency, etc.
Though no information about results have appeared it is possible to find information about savings within interoperable smart grid (SG) systems. The big surprise is that this information does not come from a single utility, but has been released by the European Commission.
If power distributors want to operate in a smart way, it is not enough to have customers with meters equipped with a relay that switches according to tariff. If customers do not connect this functionality with their home infrastructure and do not change consumption behavior, the paper results vary from real life in a dramatic way. The only ones who benefit are thus the meter manufacturers.
Rethinking Today’s Standards for Grid Stability
Paradoxically, recent rollouts have highlighted one fact: interoperable SG systems and the way they are designed today, are not able to ensure power network stability and safety in power supply. In order to change this situation, it is necessary to fulfill SG components with reasonable content. One of the most basic characteristics seems to be good synchronization of subordinate PLC networks for communication with meters.
This is therefore, not a discussion about the intelligence of the device. The important issue here is that devices behave in a coordinated way within the smart grid. They have to have synchronized time, in order to benefit from the predictable behavior of the smart grid, and ensure mutual cooperation of its components.
Synchronization will help in the following areas:
- Time control of the quality for data packet transmissions (TDM – time-division multiplex): The lower the time uncertainty is, the better because single devices on PLC network can transmit data with a lower time gap without mutual disturbance, which leads to increase in network throughput.
- Exact measurement of immediate voltages and currents: If we analyze these values, we are able to determine real distribution load, and transformers supply. Furthermore, if Kirchhoff’s circuit laws for node currents and circuit voltage are applied, we can precisely find out, where non-authorized consumption is taking place. Taking into account that non-technical losses reach 10-50% in Europe (increasing in the south-east), it is a really important issue.
- All meters are able to measure current or voltage: Values measured in a synchronous way can be stored in a FIFO register, and in case the operator needs to analyze problems in a given locality, data stored in the meter (including time stamps) will be transferred to him.
- Synchronization impulses can be used to indicate the single phases that supply a meter (See Picture 1): It doesn’t matter if the supply is one-phased or poly-phased. We can thus detect current asymmetry arising at customers with one-phased appliances.
Each phase has its own color. At the exact moment that the voltage goes through zero, the signal is injected into this phase (because of coupling, it is also transmitted into other phases, but only one of them has a signal in zero crossing the appropriate phase voltage).
Synchronization Course and Islandic Characteristics
The system of data reading and transmission to and from customers has an islandic characteristic with one center: distribution transformer supply. There is a device within this transformer (data concentrator) that communicates over low voltage power lines with customers’ smart meters. The whole power distribution low voltage network is set of these islands.
Synchronization happens on two levels. First of all, the data concentrator is synchronized and, subsequently, synchronization is transferred to individual meters.
This suggested solution has a couple of advantages:
- The synchronization impulse is precisely defined, and is transmitted in the zero-crossing moment of a particular phased voltage including time stamp.
- Standards for synchro phasor systems (applied for very high voltage system control) accept deviation lower than 1%. Thus for 50 Hz frequency, uncertainty of ± 30 µs is allowed, which is within accepted accuracy for low voltage systems (own synchronization for phasor measuring has significantly higher accuracy: ± 1 µs).
Calculating Synchronization Delays
In order to calculate the delay that is caused by characteristics of power lines, we will assume a constant k = 0.6 speed of light, and maximal length of low voltage lines that due to the guarantee of maximum losses does not exceed ca. 2 km. Time of network period reaches 0.02 s.
It means that it is possible to reliably realize synchronization of devices supplied from LV network with accuracy of ±50 µs for all devices. In comparison, for example PRIME standard synchronizes subordinates end devices with data packet transmitted over the power lines.
Limitations of PRIME Standard Accuracy
If the packet goes through without the signal repeating (usually the number of repeating reaches 4-8 times), the accuracy reaches ± 0.01 s under very favorable conditions. However, there is a big weakness of this ‘standard’: the number above represents basic accuracy and due to signal repeating, there is a significant time shift at single meters.
Usual accuracy within the PRIME meters thus reach ±1 sec. This system is suitable for tariff switching. It is, however, not applicable for network control (see above).
Methods for Synchronizing Data Concentrators
Let’s go back to data concentrator synchronization. There are a couple of ways to synchronize data concentrators. If the data concentrator is connected directly (via cable) to internet, it is possible to use the NTP or PTP protocols that are used for time synchronization of devices connected to internet. If connectivity is wireless (GPRS, or EDGE) through mobile operators, we cannot use this channel for synchronization because of undefined latency.
The other choice is synchronization via GPS (or similar) signal. The accuracy is similar, or even better than NTP, or PTP services. The only disadvantage is that this signal is not guaranteed and its owner can switch it off without any warning.
Defending Science Against Bureaucratic Control
Beware of a system that works on the basis of physical laws if it is controlled by bureaucrats. Often, they present decisions that are reviewed only by themselves without the necessary knowledge and they naively hope it will operate correctly over a long time period.
Luckily, the power industry generally defends itself against these pressures. The fact however remains, that this is only question of time until it will be defeated. If this happens, it is necessary to be ready with technical solutions based on the laws of science, not on pretty pictures and presentations that are not useable in practice.