grid connected generator, inverter, voltage stability

The type of generation technology adopted determines the behaviour of EG in a distribution system. The major differ-ence between the synchronous generator and the induction generator is that the induction generator can only operate on the circular locus and so there is always be a defined relation-ship between real power (P) and reactive power (Q). Hence, the independent control of Power Factor in an induction gen-erator is not possible [2]. This independent control of P and Q make synchronous generators attractive for embedded genera-tion schemes. Electronic Inverter Systems, however, introduce power quality problems into the system [4].

the system has become unstable. This point is called the Criti-cal point. Hence, the curve can be used to determine the sys-tem’s critical operating voltage and collapse margin. Gener-ally, operating points above the critical point signifies a stable system. If the operating points are below the critical point, the system is diagnosed to be in an unstable condition [5].

2.3.2 QV Curves

Voltage stability depends on how the variations in Q and P affect the voltages at the load buses. The influence of reactive power characteristics of devices at the receiving end (loads or compensating devices) is more apparent in a QV relationship. It shows the sensitivity and variation of bus voltages with respect to reactive power injections or absorptions [5]. Figure 2 shows a typical QV curve, which is usually generated by a series of load-flow solutions. Figure 2 shows a voltage stabil-ity limit at the point where the derivative dQ/dV is zero. This point also defines the minimum reactive power requirement for a stable operation [5].

An increase in Q will result an increase in voltage during normal operating conditions. Hence, if the operating point is on the right side of the curve, the system is said to be stable. Conversely, operating points in the left side of the graph are deemed to be unstable.

2.3 Voltage Stability

A system experiences a state of voltage instability when there is a progressive or uncontrollable drop in voltage magnitude after a disturbance, increase in load demand or change in op-erating condition [5]. The main factor, which causes these unacceptable voltage profiles, is the inability of the distribu-tion system to meet the demand for reactive power.

Under normal operating conditions, the bus voltage magnitude (V) increases as Q injected at the same bus is increased. How-ever, when V of any one of the system’s buses decreases with the increase in Q for that same bus, the system is said to be unstable [5].

Although the voltage instability is a localised problem, its impact on the system can be wide spread as it depends on the relationship between transmitted P, injected Q and receiving end V. These relationships play an important role in the stabil-ity analysis and can be displayed graphically.

2.3.1 PV Curves

When considering voltage stability, the relationship between transmitted P and receiving end V is of interest. The voltage stability analysis process involves the transfer of P from one region of a system to another, and monitoring the effects to the system voltages, V. This type of analysis is commonly referred to as a PV study [5].

Fig. 2. Typical Reactive Power-Voltage (QV) characteristic Curve

2.4 Impacts of EG

Connecting a generation scheme to a distribution network will affect the operation and performance of the network depend-ing on the scheme and rating of the generator itself [6]. The impacts are as follows:

2.4.1 Power Flows

The significant penetration of embedded generation reverses the power flow and the network is no longer a passive circuit supplying loads. It becomes an active system with power flows and voltages determined by the generation as well as the loads [7]. In these cases, the generator exports excessive power to all the loads on the system to which it is connected. The surplus power is transferred into a higher voltage system.

Fig. 1. Typical Power-Voltage (PV) characteristic curve

The Figure 1 shows a typical PV curve. It represents the varia-tion in voltage at a particular bus as a function of the total active power supplied to loads or sinking areas. It can be seen that at the “knee” of the PV curve, the voltage drops rapidly when there is an increase in the load demand. Load-flow solu-tions do not converge beyond this point, which indicates that

2.4.2 Network Losses

EG will have an impact on losses in a network. The strategic placement of EG on the network can reduce losses normally