# Advanced Control and Fault Detection Strategies for District Heating and Cooling Systems--A Review
## 2 State of the Art Control Strategies for District Heating (DH) and District Cooling (DC) Systems
### Basic Control Strategies in Traditional DH/DC Systems  
Six independent basic control strategies operate simultaneously in a traditional DH network, as depicted in Figure 1 and listed in the following. These are usually implemented in the form of classical control such as rule-based or PID. According to [PERSON] and [PERSON] (2014) [25], the ones from A to E are implemented at the production and pumping station level, whereas the control strategy F is implemented at the user substation level. A. Supply temperature control: The goal of this control is to module the thermal power at the heat generation plant to reach a fixed supply temperature set-point. The latter is generally determined by means of a heating curve as a function of the external ambient temperature, and it is high enough to satisfy the temperature level required for all the customer-sited heat emission systems. B. Minimum supply temperature control: This control is important, especially in summer, when the DH system operates to supply only the DHW load. During the night,for instance, a by-pass between the supply and return pipelines at the end of the network allows maintaining a small recirculation in order that the supply temperature always satisfies the comfort level for the users. C. Minimum differential pressure control: It is essential to maintain a minimum differential pressure between the supply and the return pipelines at the farthest substation from the network pumping station. In this way it is assured that the pressure head is enough to supply all the substations in the between. D. Maximum pressure control: The pumps are controlled to avoid over-pressure at the supply line that could damage the network. E. Minimum pressure control: The minimum pressure of the network that occurs at the suction of the pumps is controlled to avoid cavitation and damage of components. F. Heat demand and flow control: These control strategies are implemented at the user substation level. For instance, a climatic control can be implemented to set the supply temperature on the substation's secondary side. A modulating valve is then used to achieve this set-point by controlling the flow rate on the substation's primary side. Thermal energy storage (TES) systems based on short-term tanks provide several advantages in traditional DH/DC networks. They allow equalizing the load of the heat production plants, while heat demand by consumers is fluctuating, and covering the peak demand by avoiding that peak generation units switch on with high operational costs. Moreover, TES systems allow operating the system when the heat demand is below the technical minimum of the heat production plant and increasing the flexibility of combined heat and power (CHP) systems by permitting electricity generation during off-peak hours of the heat load without wasting the heat produced. Several temperature meters are usually installed along the height of a tank TES to monitor the temperature stratification and state of charge. To exploit the capacity of TES directly connected to the DH network such as in Figure 1, few further control measures are needed: the charging and discharging phases are activated automatically by generating the right amount of positive/negative differential pressure at the connections of the TES at the supply and return DH pipelines by controlling properly the network and production pumps. Moreover, usually, a control to limit the minimum temperature at the top during discharging and the maximum temperature at the bottom during charging can be beneficial to avoid thermocline destruction. The same basic control strategies shown in this section for traditional DH applications are also valid for DC systems. The latter usually operate with a smaller temperature difference between supply and return concerning DH, with consequent larger flow rates and pumping  
Figure 1: Basic control strategies in traditional district heating (DH) systems. electricity consumption to supply the same thermal power.