1.1 Introduction
Distributed energy resources (DERs), such as, micro turbine, fuel cells, photovoltaic (PV) and wind energy systems play a crucial role in the alley towards future energy landscape [1–5]. These renewable energy based distributed generation (DG) units have gained popularity with the promotion of policies by government, including feed-in-tariffs, renewable portfolio standards, tradable green certificates, investment tax credits, and capital subsidies [1]. With the technological growth, there is a massive transformation of passive distribution networks into active distribution networks (ADN) accompanied by the role of more energy collection and storage [1,2,4].
Although the DG units play an important role in reducing pollution and enhance the economics by minimizing transmission loss, the intermittent nature of DG units brings several crucial challenges to the operation of power systems [6–8]. Some of the key challenges faced by the operator are in- verse/reverse power flow, temporary over/under voltage, line congestion, frequency stability issues, protection equipment design to meet the reliability standards, voltage fluctuations and its regulation, etc. Voltage regulation problem is identified as one of the main challenges in ADN that restrict the incorporation of DG units [6,8–10]. Basically, DG units due to their bidirectional energy flow uplift the voltage across feeders. Besides, DG units due to their intermittent power fluctuations and pen- etration to LV/MV network cause severe voltage excursions. Traditionally, DG units are asked by distribution network operator (DNO) to operate at zero reactive power, which limits the application of DG units as voltage control devices. However, with the advancements in technology, the level of complexity of ADN has increased manifold with a wide variety of DG units, energy storage system (ESS), controllable loads, and voltage regulators. Moreover, typical conventional techniques, such as, on-load tap changer (OLTC) and capacitors switching may not be sufficient enough to regulate voltages under inverter dominated grid. Consequently, the usage of DG units as voltage regulators requires urgent investigation.
The different control elements that are used as voltage regulators can be classified into (i) legacy voltage control devices and
(ii) power electronics interfaced devices.
The OLTC, set-voltage regulators, shunt capacitors fall under legacy voltage regulation devices.
1. Introduction
The manipulation of active and reactive power of power electronics interfaced devices, such as, PV, EV, ESS, distribution static synchronous compensator (DSTATCOM) and other custom power devices, controllable loads act as voltage violation mitigation techniques.
An illustration of the voltage control problem in a small distribution network with wide-variety of distributed energy resources is depicted in Fig. 1.1. Initially, the DNO defines a target voltage for each bus in the network. The security or economical purpose, e.g., network losses minimization might be the basis of choosing the target voltages. However, reaching the actual target values is impractical and likely infeasible. Thereby, the network voltages are kept within some limits around the target values. These limits are referred to as normal operation limits. Maintaining the voltages within the prescribed limits is one of the main objectives of the controller. While the voltages of some buses fall in the undesirable region, the controller uses the minimum control actions to bring these voltages within the acceptable limits. As the voltages cross the emergency limits, the controller uses all its efforts to maintain voltages in the specified band of operation.
Loads
Normal Operation
Limits Bus 1
Bus 2
Bus 3 Bus 4
Bus 5 Bus 6
HV/LV Transformer With OLTC
DSTATCOM
DG
EV DG
EV Aggregator Loads Loads
Voltages
Bus Number Undesirable Region
Undesirable Region Unacceptable
Region
Unacceptable Region
Targeted Voltages
}
Emergency Limit
Emergency Limit DG
Figure 1.1: An illustration of voltage control problem of active distribution network.
Over the past decades, focus has been made on the control schemes to maintain the voltages within an acceptable level rather than investing in reconfiguration of the whole network to accom- modate DG units. There has been a growing emphasis on centralized, decentralized, and distributed control schemes in the distribution network housed with large number of DER units [6–11]. As shown
1.1 Introduction
calculates the control actions for all the single units at a single point. This scheme requires an ex- tensive communication infrastructure. Decentralized and distributed control structures do not require a central controller. Fully decentralized control scheme relies on the local controller based on local information. While the decentralized controller is unaware of the information of the system and other units, the distributed controller performs control calculations by gathering information and measure- ments from the neighboring controllers. An illustrative example of different control schemes used in a small distribution network is shown in Fig. 1.2.
Bus 1
Bus 2
Bus 3 Bus 4
Bus 5 Bus 6
HV/LV Transformer With OLTC
DG
EV DG
EV Aggregator Loads Flexible
Loads
DSTATCOM
CENTRALIZED CONTROL
DE-CENTRALIZED CONTROL
DISTRIBUTED CONTROL
DIFFERENT CONTROL ELEMENTS
DIFFERENT CONTROL ELEMENTS:
OLTC DG DSTATCOM EV AGGREGATORS
FLEXIBLE LOADS
DG EV
DSTATCOM FLEXIBLE LOADS
Group-1:
DG-1 DSTATCOM
FLEXIBLE LOADS
Group-2:
EV DG-2 FLEXIBLE
LOADS
Communication OLTC
Group-3:
OLTC DG-3 FLEXIBLE
LOADS DG
DISTRIBUTED
CONTROL DISTRIBUTED CONTROL DE-CENTRALIZED
CONTROL DE-CENTRALIZED
CONTROL DE-CENTRALIZED
CONTROL DE-CENTRALIZED
CONTROL
Communication
Loads
Figure 1.2: Different control schemes used in active distribution networks: (i) centralized, (ii) decentralized (iii) distributed.
Although each control scheme has its own set of advantages and disadvantages, the usage of multi- layer/hierarchical [12–14] control structure is seen frequently in recent literature as the standardized solution to the efficient ADN energy management. The hierarchical control structure distributes the control functions into local controllers and upper level controllers, so that the complete system operates in a more efficient way [1–4]. Further, the multi-layer control structure is restructured to a multi-level hierarchy where decentralized and distributed control schemes occupy the local level control [7,13]
and centralized control scheme occupies the upper level control.
The primary, secondary and tertiary control levels constitute the multi-level control structure. The
1. Introduction
Time Frame: Seconds to minutes Tertiary Control Functionalities:
Overall network management policies, Neighbouring grid status,
Optimal Power Flow
Time Frame: 100s of milliseconds Secondary Control Functionalities:
Compensating voltage and frequency deviation caused by primary control
Time Frame: 10s of milliseconds Primary Control Functionalities Voltage and Frequency stability,
Plug and play capability, Avoiding circulating current
Distribution System Control
Element 1 Upper
Level controllers
Control Element
N
Figure 1.3: Multi-level control structure implementation in active distribution network.
primary control is basically a decentralized local control method which features the fastest response.
Master-slave and peer-to-peer control are the two main methods for primary control [1,2]. Secondary control is used to compensate the frequency and voltage deviations induced by primary control and to realize a prescribed power sharing scheme among DG units. Tertiary control enables an optimal operation of the system on a longer time scale. Both secondary and tertiary control structures work in coordination to form the energy management system (EMS) [1]. However, the definitions of these control schemes vary in different literature. For example, in some papers, tertiary control is established as an entity responsible for coordinating the operation of multiple microgrids interacting with one another in the system [1]. In that scenario, the secondary control structure is entrusted with added functionality apart from regulating voltage and frequency. The choice of the control structure varies according to the type of microgrids (residential, commercial, or military), and the legal and physical features (location, ownership, size, and topology) [2]. A schematic of hierarchical control structure with specific objectives and operation times is depicted in Fig. 1.3 [4].