November 2022 - SKYNEXT Tech.

# PMTR_001_SRV_001 V0.22

Disclaimer

PMTR_001_SRV IS NOT INTENDED FOR DISTRIBUTION / POWER UTILITY NETWORKS !

It should be installed on the customer side of the power utility meter. This project is open source and implementation should be done according to local electrical code.

It is designed to operate on high voltages up to 400V AC. These voltage levels may cause serious bodily harm or death. You should have the necessary qualifications to operate and design at these voltages.

There is no guarantee or responsibility whatsoever with this project from our part. If you wish to design and implement your own devices based on the information presented thereof, You do this at your own risk.

Description

PMTR (Power Meter, Telemetry & Relay) is an open source project that allows precise monitoring of mains parameters (voltage, current, active power, energy, power factor and frequency) and allows to act on external relays based on formula triggers (ex: overcurrent or overvoltage). It is a client/server project with a Web interface for data display, logging and device configuration.It uses power line communication and the industrial proved Modbus protocol to transfer data between the client and servers; no need to have ethernet or wifi at your main power panel, although LoRa telemetry is envisioned in future iterations.
It is modular and robust in approach, using off the shelf components available on local or Chinese or vendor sites to ease replacement of parts if need arise, and thus lower TCO for many years.
The server(s), (PMTR_001_SRV) is an Arduino based device that performs the power measurements and relay operations. Its configuration is pushed by a client.
The client(s) (PMTR_001_CLI) is a Raspberry Pi device that grabs power telemetry and pushes configuration and time synchronisation to the server(s) device. As stated above, it does not rely on an Ethernet connection, just a Power line communication based on the ES-1642 NC module from EastSoft.
Relay actions are used mainly to protect your equipment (for industrial settings) and control energy costs (ex: hotel industry, power charging for EV), It will also allow to cut/enable power at specific time of the day.
This project uses the Modbus RTU protocol, and allow several servers, as Modbus RTU is a shared bus protocol. Thus, you can monitor individual circuit legs or even devices on power subpanels.
Two flavour of servers are being developped : A single phase/device meter, and a 3 phase meter. They are rugged devices and implement ceramic fuses for ovecurrent protection and varistors for overvoltage protection. Metering is based on a modified PZEM-004t v3.0 modules that can measure line to line voltages up to 437V AC.

For more information on the client, check: https://www.github.com/rodv92/PMTR_001_CLI

For more code information on the modbus server, check: https://github.com/rodv92/PMTR_001_SRV

Current State of the project.

A single phase proof of concept with a Raspberry pi client and a single phase power meter was done and shows good results. The server code integrate formula based actions, and time synchronization. A basic web chart of power parameters and logging into a MariaDB database is part of this POC.

What has been done :

Prototype server for 3 phase line/line measurement and line/neutral measurement (with 400V support, still needs thorough testing)
ADG333 serial line switching support added to query sequentially all three PZEM-004t V3 modules
DS3231 RTC time-keeping support added.
USB Serial shell added to print current time, set time, get PZEM-004t metrics (instantaneous and moving average), dump circular buffer, dump EEPROM event log.
EEPROM logging of important events, for further inquiry by Modbus client or through serial shell
Modbus RTU device address DIP switch selector added plus code support.

Priority tasks as of August 2024:

Add supercapacitor bank to provide limited time power to the 12V bus and 3.3/5V buses in case of power outage. This will allow communication to the Modbus client of black-out or brown-out events, or AC/DC converter failure. Note that the PZEM-004t v3.0 metering unit is powered through L/N lines, not through the UART connector. The 5V/GND lines on the UART connector only provide power to the optocouplers. Low scale voltage before the library returns NaN is around 60V AC, While the AC/DC converters YHT4S operation low voltage threshold is 85V AC. This feature would thus improve metering range in case of brown-outs.
Test relay actions (using SSR / TRIACS) first, such as Fotek units)
Harden and test formula actions where conflicting pins are used with different actions.
Test communication over long runs of power line wire and over electrically noisy environments.
Make the raspberry pi (wifi, serial) configuration seamless.
Expand the raspberry pi client to query several servers with auto-detection of servers residing on the Power Line Communication bus.
Add LoRa support for failsafe communication in case of power line communication failure.
Add a MQTT layer on the raspberry pi for upstream telemetry reporting.
keep in touch with PeaceFair, the maker of the PZEM-004t V3.0 to make a hardened 400V module (there are still some issues regarding with high voltage tolerance).
Enclosure design for prototype, ideally DIN Rail compatible.
Add LCD Display and keys.

INSTALLATION and SETUP

typical use case for 3 phase delta-wired measurement:

This use case shows the installation of the PMTR_001_SRV for monitoring voltage and current drawn by a 3 phase induction motor, and all remaining power parameters.

Note that in that example, since the motor is a 3 phase delta wired device, and in the absence of a neutral return path, PMTR_001_SRV measurement mode rocker switches should be set for line-line measurement.

The setup contains a single digital pin operating a 5V relay (NO) that provides in turn power to a contactor (NO) that power ups (or down) the induction motor.
It is then trivial to use logic to specify a relay action that evaluates and power ups or down the motor upon specific power conditions. More on relay actions below.
Also note that the PMTR_001_SRV and PMTR_001_CLI are connected on the same line (L1) for PLC communication to occur.

Other installation best practices & Miscellanous

PMTR_001_SRV Contains a ES-1642 NC PLC modem (Power Line Communication). No Line filter should be present between the server and the client, the client would be unable to communicate with the server.
PMTR_001_SRV can operate over IT, TT, TN-C TN-CS earthing schemes.
As fort the prototype, it will be designed aroud a non metallic chassis, unless EMI hardening mandates to do so. so, PMTR_001_SRV does not require connection to the PEN.
PMTR_001_SRV implements ceramic fuse protection for overcurrent and varistors for overvoltage protection.
PMTR_001_SRV will be packaged inside a standard DIN Rail plastic enclosure for ease of installation inside a power distribution main panel.
Check the HW folder of this project for the terminal pin references.

Installation Steps

Disconnect power to the L1 L2 L3 lines powering the meter or to the VFD for safe installation of the current coils and metering unit
Make sure the low voltage board / Arduino is not connected to the 5V and GND lines (and JH 4pin cables linking the low voltage mezzanine board to the high voltage board are NOT installed)
Setup Modbus Address using the 8 position DIP switch to any value between 1 and 254. 0 is a broadcast address. 255 is a bridge address. Do not use these on the PMTR_001_SRV2) Set the 3 jumpers to LN metering – default – (line to neutral) or LL metering (line to line) Usually LN is preferred if the metering is on a standard european consumer network. LL metering is usually reserved for Variable Frequency Drive metering, or for star/delta starters, or on the secondary of a transformer with delta outputs. In that case, skip step 4.
After each phase is disconnected from the terminals, insert the current coil over the wire and reattach to the input and output terminals. repeat for each phase.
Connect CT- and CT+ wires to the appropriate PMTR-001-SRV terminals. Repeat for each phase
Connect neutral wire to the PMTR_001_SRV neutral terminal
Connect L1 L2 L3. It is advisable to install L1 L2 L3 while the upstream circuit breaker is off, screw terminals are high voltage, always use an electrician screwdriver or HV gloves. When connecting L1, the unit should power up, unless on a VFD where you need at least two phases. You can test that all DC power supplies are providing backup power by connecting them individually. PMTR_001_SRV should work with either L1, L2 or L3 connected in LN metering.
If a backup / battery supply is available you can connect it to AUX12VDC to the positive terminal and GND to the negative terminal
Set the DC step down module potentiometer to output 5V DC !
Screw the arduino mezzanine to the low voltage mezzanine, screw the serial lines, DIP switch lines, 5V and GND lines going from the Arduino to the low voltage mezzanine.
Screw the low voltage mezzanine board with the Arduino mezzanine already attached to it to the high voltage board and connect the 5V and GND lines to the arduino mezzanine board.
Tie the SEND terminal to any L1 L2 or L3 phase, now the Rasperry Pi (PMTR_001_CLI) can be connected to the same phase and neutral as the PMTR_001_SRV and it should set the time up.
Note: as of today, the PLC modem use line to neutral signalling. line to line signalling still needs testing.
Repeat the same actions above on any other PMTR_001_SRV to install on the network. use the same phase line for all the PMTR_001_SRV that you install
Relay connection : PMTR_001_SRV can supply up to 0.8A at 12V over the 12VDC_OUT terminal, Electromechanical relays can be powered by 12V IF the 5V relay action pin voltage is sufficient to be recognized as logical HIGH by the relay. Remember that The relay action pins on Arduino cannot supply more than 20mA continuous current. Always use relay modules with a separate power pin. Modules also take care of the inductive kickback. On the other hand, Digital pin action pin sourcing current is sufficient for SSR (TRIAC modules such as Fotek), and thus does not require external power.
Note : PMTR_001_SRV can supply up to 1.7A at 5V over the 5VDC_OUT terminal.
Proceed to PMTR_001_CLI Readme to finish setup.

Modified PZEM-004t v3.0 information and general PZEM-004t v3.0 limitations

–
The original PZEM-004t v3.0 can measure up to 267V RMS.
Here, The metering is done through a modified PZEM-004t v3 for up to European phase to phase voltage tolerance :up to 427V RMS with 0.2V resolution. Compared to the non-modified version, This ALSO allows phase to phase metering, and phase to neutral in case of neutral fault.Power, Energy, power factor resolution are doubled due to the fact that their computing involve instantaneous voltage measurement which is affected by the PZEM modification.
Otherwise, the floors, ceilings of the remaining parameters conforms to the PZEM-004T v3.0 datasheet of this day.It is expected that frequency measurement is not affected by the PZEM-004t v3.0 HV modification.

As for power factor, leading or lagging info is not provided, and neither power flow direction.

As of August 2024, no new version of PZEM-004t with 400V support seems to have hit the market.

PMTR_001_SRV does not measure the phase shift between phases.

PMTR_001_SRV performs instantaneous measurement as fast as the 9600 bps serial interface allows, which is a little less than 100 bytes per second (all parameters read). When performing 3 phase metering the PMTR_001_SRV performs serial port switching (using an ADG333A Quad SPDT switch) So it shares a single serial port on the Arduino for 3 PZEM-004T modules. This lowers the read rate to less than 33 bytes per second on average, which is still sufficient to get all line parameters with a polling of the 3 phases each second.
Sampling is thus done each second, and these samples are the basis for moving average computations.
The device has 5 memory slots for Moving Average information of all parameters (except energy)For instance, We could store the Moving Average over 5, 10, 20, 30, or 60 seconds.
The incentive behind moving average calculation is to apply low pass filtering and filter any spurious spike that would command an inappropriate relay action.

Relay Actions

A ‘relay action’ is a set of the following parameters :

An expression : using tokens representing PMTR_001_SRV parameters, and a set of inequalities and logic.- A moving average to use for expression evaluation (except energy which is never averaged)
A dry run bit (Yes/No), which tells whether to perform or not the relay action, while still logging all action behaviour
A recovery period during which the expression must keep evaluating to false before reverting relay action. This hysteresis is implemented for protecting devices which should not be turned on/off too frequently.
A HIGH or LOW state to command the relay when expression evaluates to true. (on false evaluation the state commanded will the be opposite ) nb : This evaluation from memory is the first action to be executed at boot up.
A list of pins on which DIGITALWRITE Will be performed.It is recommended that each action activates a separate set of pins, otherwise the last evaluated actions will have precedence and overwrite the status of pins.

There are 5 actions slots.

Examples of valid expressions :

(L1U > 253) || (L2U > 253)) || (L3U > 253)
LII > 16
LIp > 1200
L2e > 10
(L1f < 49.1) && (L2f < 49.1) && (L3f < 49.1)

Ex : L1U is phase to neutral L1 RMS voltage, L1I is phase to neutral RMS current, L2f is L2 frequency, L2e is energy counter for L2. Units use are SI, excepts for energy which is in kWh.

Let’s parse these formulas further.
First case formula would be the detection of a voltage swell over 253V over L1, L2 and L3 at the same time. The second formula equates to a current of more than 16A on L1. Next formula is an active power of more than 1200VA on L1. Next, A total energy consumed of more than 10kW over L2. Finally a frequency sag of 49.1 Hz over any of the phases.

Neutral fault detection

This feature is experimental and has not been yet implemented.

Broken neutral (floating) fault in three phase systems can have disastrous consequences by essentially rewiring the loads in a delta fashion.

Depending on the location of the fault and earthing arrangements, voltage imbalances arise, and the neutral point is shifted and may be subjected to voltages relative to earth that may be unsafe.

A sucessful neutral fault detection without false positives is not trivial based on a limited number of voltage readings (three power meters only), absence of phase angle information, and absence of a neutral to separate ground electrode voltage measurement

However, using magnitudes of voltages only and their variation in the time domain according to load variation, we could tentatively propose an lightweight algorithm suitable for MCUs to detect such occurrences.

Keeping in mind that if the load are reasonably balanced the fault will be harder to detect, until a disturbance in the balance of loads will unbalance voltages.

Generally speaking, a broken neutral condition will translate in an increase of one phase voltage, while the other two will decrease with reference to the nominal value, or one will decrease while the other two will increase.

A lightweight pre-check for discriminating between single phase power disturbances and neutral fault would be to check for any voltage exceeding a 10% nominal value deviation, and then proceed to check remaining voltages for disturbances.

A valuable tool for analysis of neutral fault conditions is the three phase voltage triangle. By using basic trigonometric identities of the equilateral triangle, we can characterize the least possible deviation in absolute values of the other two voltages assuming epsilon is the deviation factor from nominal value for the upmost voltage.

$$ \epsilon = 1.1 $$

$$ V_{p} = 230V $$ is the nominal phase RMS voltage

$${V_p}^{‘} = \frac{V_p \sqrt{4+12\epsilon^2}}{4\epsilon} $$ is the least possible deviation from nominal values for the two remaining voltages.

Thus we can write : $$ V_pdev >= |V_p – {V_p}^{‘}| $$

So, in terms of ratio we have $$ \frac{V_p^{‘}}{V_p} $$

The algorithm has to check that the remaining two voltages is higher or equal to that ratio for the first voltage and lower or equal to that ratio for the second voltage. Once this pre-characterization is done, a second part, more computationally intensive part of the algorithm can kick-in, to check that the sum angle derived from magnitude of the phase voltages is still close to 360° degrees, while analyzing voltages changes in the time domain.

Phase angles can be derived from magnitudes using sine and cosine trigonometric identities of the triangle. voltages fluctuations on the three phases with time that keep their sum angle close to 360° can be assigned a score for each angle sum characterization. With a value close to 360° giving a positive score, and a value diverging from 360° giving a negative score.

A sum of absolute values differences between discrete time samples of the three voltages can assign a weight to the characterization. This characterization puts into evidence the variation of load impedance in a broken neutral condition, and gives more credence to the computation if the angle sum keeps getting close to 360°

Once a positive score compounded by the previous weight reach a threshold, a broken neutral condition can be raised.

There are caveats however. a robust algorithm must assess the stability of the line voltages over time, preferably over a long term period. Using only three power line meters, a common occurence of unstable line voltages will degrade significantly the predictive abilities of the algorithm.

Hardware in-depth information

AC protection features

20mm x 5mm 0.1 A ceramic fuses, fast blow 250V, this for each phase L1 L2 L3 and neutral N.

14D481K varistors between each phase and neutral, and for protection of the PLC module.

AC/DC conversion

Power is provided by YHT4S12V/10W modules AC 400V tolerant.the three modules are connnected line to neutral and should be tolerant to neutral fault and up to 2 phase faults.

DC Bus

Matched Schottky diodes are used to improve power sharing betweeen the modules at 11.5V after the diode drop. a single LM2596 dc step down module provides 5V to the DC bus for the Arduino, PZEMs and ES-1642NC 5V line. ES-1642NC also needs 12V power.

PLC

ES-1642NC uses a separate hardware serial port for modbus communication over one phase and neutral, or a separate communication bus. line to line signalling is not yet tested. The serial line speed is 9600 bps. the PLC speed is between 2.5 to 4.5 kbps. Check Taidacent/Eastsoft product page for more information.

MCU

The main controller is an Arduino mega 2560 it is on a separate mezzanine board than the HV parts / PZEM Modules.
The ADG333 IC allows routing a single hardware serial port to the three PZEM modules. Querying is continuous and sequential, With a polling interval of 333ms between each module, giving a net polling interval of 1 second.
It could be technically feasible to have all PZEM modules on the same RX/TX serial TTL pair, but that would need to ensure that the optocouplers do not overload the RX/TX current sourcing capabilities, and also would need a factory preset of the PZEM-004t V3.0 addresses. Note that PZEM-004t V3.0 itself is a Modbus device.
The server itself encapsulate telemetry queries and present the 3 PZEM-004t V3.0 as a single Modbus server device to the Raspberry Pi client, although with Modbus register addresses remapped.
Thus, it seems more adequate to use the same Modbus address on each module, and switch them using an IC switch like the ADG333.

Time keeping & Synchronisation

A real time clock (RTC) 3231 mini module is used to keep track of time and the synchronization is done over PLC / Modbus.
Modbus address encoding of the server is done through a 8 bit DIP switch encoder.

Software (Github repositories)

Most of the single phase functionality is POC tested.three phase hardware testing and hardening is still in progress.Software is still in POC phase. Consider the code to be in alpha stage.

https://github.com/rodv92/PMTR_001_SRV

https://github.com/rodv92/PMTR_001_CLI