Avoid These Pitfalls When Implementing Power Distribution Automation with Siemens PLCs!

Power distribution automation enhances power supply reliability and operational efficiency. This article introduces an implementation plan based on Siemens PLCs, focusing on hardware selection, program design, and practical applications.

1. Hardware Configuration

PLC and Expansion Module Selection

The power distribution automation system has high requirements for real-time performance and reliability. It is recommended to use the Siemens S7-1500 series PLC, with the following specific configuration:

• CPU: 1515-2 PN (with dual Ethernet ports, supports redundant communication)

• DI Module: SM521 DI32×DC24V (2 units)

• DO Module: SM522 DO32×DC24V/0.5A (2 units)

• AI Module: SM531 AI8×13Bit (2 units)

• AO Module: SM532 AO4×16Bit (1 unit)

I/O Point Allocation Table

| Device Name | Signal Type | PLC Address | Function Description |

|———|———|———|———-|

| Circuit Breaker Status | DI | I0.0-I0.7 | 8-channel circuit breaker open/close status |

| Overcurrent Alarm | DI | I1.0-I1.7 | 8-channel overcurrent protection signal |

| Circuit Breaker Control | DO | Q0.0-Q0.7 | 8-channel open/close control |

| Alarm Indication | DO | Q1.0-Q1.7 | Fault alarm output |

| Current Signal | AI | IW64-IW78 | 8-channel current acquisition |

| Voltage Signal | AI | IW80-IW86 | 3-phase voltage + zero-sequence voltage |

| Remote Control Signal | AO | QW96-QW98 | Output for speed controllers, etc. |

Peripheral Device Selection Criteria

1. Current Transformer: Use high-precision transformers (0.2S grade) to ensure accurate current acquisition

2. Voltage Transmitter: Use isolated transmitters that output 4-20mA standard signals

3. Touch Screen: TP1200 Comfort, 12-inch human-machine interface, supports multi-touch

4. Communication Module: CP1543-1 industrial Ethernet module, supports PROFINET and Modbus TCP

Key Points for System Wiring

• Separate strong and weak currents, with a distance greater than 30cm

• Use shielded twisted pairs for analog signals, grounded at one end

• Use independent relay isolation for digital signals

• Use UPS for PLC power supply to ensure 30 minutes of operation after power failure

2. Control Program Design

Variable Definition Specifications

// Global Variable DB

DATA_BLOCK "dbGlobal"

STRUCT

  // System Status

  sysStarted : BOOL;        // System startup flag

  sysRunning : BOOL;        // System running status

  sysFault : BOOL;          // System fault flag


  // Circuit Breaker Status

  CB_Status : ARRAY[1..8] OF STRUCT
    position : BOOL;        // Position status (0-open, 1-closed)
    ready : BOOL;           // Ready status
    fault : BOOL;           // Fault status
    current : REAL;         // Real-time current value
    overload : BOOL;        // Overload flag
  END_STRUCT;


  // System Parameters

  param : STRUCT
    currentLimit : REAL;    // Current limit
    voltageLimit : REAL;    // Voltage limit
    delayTime : TIME;       // Action delay
  END_STRUCT;

END_STRUCT;

Program Architecture Design

The layered structure includes:

1. OB1 Main Loop: Calls various function blocks and manages system status

2. FB1_DataAcq: Data acquisition function block

3. FB2_LogicCtrl: Control logic function block

4. FB3_FaultDetect: Fault detection function block

5. FB4_Protection: Protection function block

6. FB5_Communication: Communication management function block

Function Block Design

FUNCTION_BLOCK "FB2_LogicCtrl"

VAR_INPUT

  enable : BOOL;            // Enable signal

  cbCmd : ARRAY[1..8] OF BOOL;  // Circuit breaker control commands

END_VAR

VAR_OUTPUT

  cbCtrl : ARRAY[1..8] OF BOOL; // Circuit breaker control outputs

  ctrlStatus : WORD;        // Control status word

END_VAR

VAR

  cmdTimer : TON;           // Command timer

  prevCmd : ARRAY[1..8] OF BOOL;  // Previous command

END_VAR


// Control logic processing

IF enable THEN

  FOR i := 1 TO 8 DO

    // Detect changes in control commands

    IF cbCmd[i] <> prevCmd[i] THEN

      // Start timer to avoid misoperation

      cmdTimer(IN := TRUE, PT := T#100ms);


      IF cmdTimer.Q THEN

        // Output control command

        cbCtrl[i] := cbCmd[i];
        // Record command status

        prevCmd[i] := cbCmd[i];
        // Reset timer

        cmdTimer(IN := FALSE);

      END_IF;

    END_IF;

  END_FOR;

ELSE

  // Clear all outputs when the system is disabled

  FOR i := 1 TO 8 DO

    cbCtrl[i] := FALSE;

  END_FOR;

END_IF;

Status Control Design

// System State Machine

CASE systemState OF

  0: // Initialization State

    IF initComplete THEN

      systemState := 1;

    END_IF;


  1: // Standby State

    IF startButton THEN

      systemState := 2;

    END_IF;


  2: // Running State

    IF stopButton THEN

      systemState := 1;

    ELSIF faultDetected THEN

      systemState := 3;

    END_IF;


  3: // Fault State

    IF faultReset AND NOT faultDetected THEN

      systemState := 1;

    END_IF;

END_CASE;

3. Fault Diagnosis and Troubleshooting

Common Fault Analysis

1. Communication Fault

• Symptoms: HMI cannot display data, communication indicator flashes

• Causes: Loose network cable, IP address conflict, switch failure

• Solution: Check network connection, perform ping test, check PLC diagnostic buffer

• Symptoms: No action from circuit breaker after issuing control command

• Causes: Broken control circuit, coil failure, auxiliary contact issues

• Solution: Check control circuit voltage, measure coil resistance, check feedback signal

• Symptoms: Abnormal current/voltage display values

• Causes: Sensor failure, wiring errors, parameter setting errors

• Solution: Verify sensors, check wiring, correct range parameters

Using Diagnostic Tools

// Fault Diagnosis Function Block

FUNCTION_BLOCK "FB3_FaultDetect"

VAR_INPUT

  cbStatus : ARRAY[1..8] OF BOOL;

  cbFeedback : ARRAY[1..8] OF BOOL;

  currentValue : ARRAY[1..8] OF REAL;

END_VAR

VAR_OUTPUT

  faultCode : ARRAY[1..8] OF WORD;

  alarmActive : BOOL;

END_VAR


// Diagnosis Logic

FOR i := 1 TO 8 DO

  // Check for inconsistencies between control and feedback

  IF cbStatus[i] <> cbFeedback[i] THEN

    faultCode[i].%X0 := TRUE;  // Position inconsistency fault

  END_IF;


  // Check for overcurrent

  IF currentValue[i] > 1000.0 THEN

    faultCode[i].%X1 := TRUE;  // Overcurrent fault

  END_IF;


  // Update overall alarm status

  IF faultCode[i] <> 0 THEN

    alarmActive := TRUE;

  END_IF;

END_FOR;

4. User Interface Design

Interface Layout Description

1. Main Interface: Displays system overview, including single-line diagrams, key parameters, and alarm status

2. Control Interface: Circuit breaker control, parameter settings, manual/automatic switching

3. Trend Interface: Current/voltage curves, historical data queries

4. Alarm Interface: Current alarms, historical alarms, alarm statistics

Parameter Setting Description

The parameter setting interface includes:

- Current limit setting: 0-2000A, step 0.1A

- Voltage limit setting: 0-1000V, step 0.1V

- Action delay setting: 0-10s, step 0.1s

- Communication parameters: IP address, port number, timeout

Operation Monitoring Description

The real-time monitoring system includes:

• Circuit breaker status indication (open/close, fault)

• Real-time display of electrical parameters (current, voltage, power)

• System status indication (running, stopped, fault)

• Communication status display

Alarm Handling Description

Alarm handling process:

1. After an alarm is triggered, the HMI pops up an alarm window

2. Audible and visual alarm is activated

3. Record alarm information in the historical database

4. The operator confirms the alarm and takes action

5. After troubleshooting, manually reset the alarm

5. System Debugging Methods

Step-by-Step Debugging Method

1. Hardware Testing

• Check power supply voltage

• Test I/O channels

• Verify communication connections

// Debugging Function Block

FUNCTION_BLOCK "FB_Debug"

VAR_INPUT

  debugEnable : BOOL;

  testChannel : INT;

END_VAR

VAR_OUTPUT

  testResult : BOOL;

END_VAR


IF debugEnable THEN

  CASE testChannel OF

    1: // Test digital input

      testResult := TestDI();

    2: // Test digital output

      testResult := TestDO();

    3: // Test analog input

      testResult := TestAI();

  END_CASE;

END_IF;

• Individual device testing

• Interlocking function testing

• Protection function testing

Parameter Tuning Steps

1. Set basic parameters (current limit, voltage limit)

2. Calibrate analog inputs (zero point, full scale)

3. Adjust control parameters (PID parameters, delay time)

4. Optimize communication parameters (polling cycle, timeout)

Abnormal Simulation Testing

• Simulate circuit breaker failure

• Simulate overcurrent conditions

• Simulate communication interruption

• Simulate power supply failure

Performance Verification Key Points

• Response time testing (<100ms)

• Communication stability testing (packet loss rate <0.1%)

• System load testing (CPU usage <60%)

• Long-term operational stability

This article introduces the key points of designing a power distribution automation system based on Siemens PLCs, covering hardware selection, program design, interface development, and debugging methods. Colleagues are welcome to exchange ideas and discuss to jointly improve the level of power distribution automation.