DPL: (re-)send power limits periodically

avoid staleness in case the same power limit is calculated over and over
again, hence no new power limit value is calculated and hence no power
limit command is sent to the inverter. staleness occurs in this case if
the first power limit command to establish the respective limit was not
received by the inverter. one can easily simulate a situation where the
same power limit is caluclated over and over again: with a battery above
the start threshold, set a very low upper power limit for the inverter
(DPL setting). that value will be used as the limit as long as the power
meter reading is larger than that.

we could also check the limit reported by the inverter. however, that
value is in percent of the inverter's max AC output, and is often not
the same value as we requested as the limit, but slightly off. we then
would have to decide how much deviation is okay, which is unreasonably
complicated.

closes #478.

include/PowerLimiter.h

@@ -64,6 +64,7 @@ class PowerLimiterClass {
 
 private:
     int32_t _lastRequestedPowerLimit = 0;
+    uint32_t _lastPowerLimitMillis = 0;
     uint32_t _shutdownTimeout = 0;
     Status _lastStatus = Status::Initializing;
     uint32_t _lastStatusPrinted = 0;

src/PowerLimiter.cpp

@@ -496,6 +496,7 @@ void PowerLimiterClass::commitPowerLimit(std::shared_ptr<InverterAbstract> inver
             PowerLimitControlType::AbsolutNonPersistent);
 
     _lastRequestedPowerLimit = limit;
+    _lastPowerLimitMillis = millis();
 
     // enable power production only after setting the desired limit,
     // such that an older, greater limit will not cause power spikes.
@@ -546,17 +547,23 @@ bool PowerLimiterClass::setNewPowerLimit(std::shared_ptr<InverterAbstract> inver
 
     // Check if the new value is within the limits of the hysteresis
     auto diff = std::abs(effPowerLimit - _lastRequestedPowerLimit);
-    if ( diff < config.PowerLimiter_TargetPowerConsumptionHysteresis) {
+    auto hysteresis = config.PowerLimiter_TargetPowerConsumptionHysteresis;
+
+    // (re-)send power limit in case the last was sent a long time ago. avoids
+    // staleness in case a power limit update was not received by the inverter.
+    auto ageMillis = millis() - _lastPowerLimitMillis;
+
+    if (diff < hysteresis && ageMillis < 60 * 1000) {
         if (_verboseLogging) {
-            MessageOutput.printf("[DPL::setNewPowerLimit] reusing old limit: %d W, diff: %d W, hysteresis: %d W\r\n",
-                    _lastRequestedPowerLimit, diff, config.PowerLimiter_TargetPowerConsumptionHysteresis);
+            MessageOutput.printf("[DPL::setNewPowerLimit] requested: %d W, last limit: %d W, diff: %d W, hysteresis: %d W, age: %ld ms\r\n",
+                    newPowerLimit, _lastRequestedPowerLimit, diff, hysteresis, ageMillis);
         }
         return false;
     }
 
     if (_verboseLogging) {
-        MessageOutput.printf("[DPL::setNewPowerLimit] using new limit: %d W, requested power limit: %d W\r\n",
-                effPowerLimit, newPowerLimit);
+        MessageOutput.printf("[DPL::setNewPowerLimit] requested: %d W, (re-)sending limit: %d W\r\n",
+                newPowerLimit, effPowerLimit);
     }
 
     commitPowerLimit(inverter, effPowerLimit, true);