DISTRIBUTED GENERATION IN SYNERGEE VERSION 4.0

DISTRIBUTED GENERATION IN SYNERGEE VERSION 4.0
Different types of generators provided by synergee for simulation and analysis.

Distributed Generation is defined as the generation of energy close to the point of use. Distributed generation has many advantages:

• Reduction in building new transmission/distribution lines or upgrading existing lines.
• Configured to meet peak power needs.
• Configured to provide premium power (when coupled with uninterruptable power supply).
• Well-suited for renewable energy technologies (located close to the user and can be installed in small increments to match the load requirement of the customer)

SynerGEE supports detailed by-phase models of generators. There are four types of models.

• Induction – The machine is modelled with passive components. A variable resistance represents the electro-mechanical coupling through the notion of slip. The output power in kW can be specified.
• Synchronous – Series winding impedances and a back EMF are used to represent the machine’s behaviour. The machine responds to the voltage and output power settings.
• Inverter PQ – The PQ model behaves just like a negative constant power load. Future development on fault contribution.
• Constant PQ – The PQ model behaves just like a negative constant power load.

Equipment Database (Generator Instance)
The rated kW, kV, and pf are nameplate values for the generator. The rated pf is the power factor output of the machine at rated kW and kV.

Xd” is the subtransient reactance of synchronous and induction machines respectively. These are the impedances seen looking into the machine at the instance of a fault. The winding impedances are used in unbalanced load-flow calculations. They are specified in percent, based on the machine’s rating.

The PT ratio is required for synchronous machines. The PT ratio is used to tie generator’s voltage settings to the actual terminal voltage seen during an analysis.

Synchronous Generators
Synchronous generators provide their own excitation and can be operated in isolation from the utility grid or be used in emergency power applications. If intertied to the utility grid, then the cogeneration system control must provide a means to synchronize the generator’s voltage, frequency and phase angle with those of the utility grid prior to interconnection. Once the cogeneration system and the grid are intertied, the generator frequency will be controlled by the grid.

Under steady-state, the speed of a synchronous machine is proportional to the frequency of the armature current. As DC excitation is applied to the field winding. AC current flows through the armature winding. The armature is typically wound on the stator and is usually three-phase.

The exciter is a DC generator typically on the same shaft as the motor. Usually, the voltage and frequency at the armature terminals are fixed by the connected system. Typically, the generator cannot affect terminal voltage or frequency. The rotor must turn at a precise synchronous speed.

Induction Generator
The induction generator is nothing more than an induction motor driven above its synchronous speed by an amount not exceeding the full load slip the unit would have as a motor. The induction generator requires one additional item before it can produce power – it requires a source of leading VAR’s for excitation. The VAR’s may be supplied by capacitors (this requires complex control) or from the utility grid. Induction generators are inexpensive and simple machines, however, they offer little control over their output. The induction generator requires no separate DC excitation, regulator controls, frequency control or governor.

They are popular in smaller cogeneration systems. Their major disadvantage is that they cannot operate in isolation from an external reactive power source.

Wind
These are active models that respond to distribution system conditions and contribute to fault current levels.

• Cut-in Speed – The minimum wind speed at which the wind turbine will generate usable power.
• Rated Speed – the minimum wind speed at which the wind turbine will generate its designated rated power.
• Furling Speed – The speed to turn the rotor out of the wind (Cut-out Speed)

Inverter PQ
The solar generator model can be modelled as Inverter PQ to simulate the effects of a PV installation. They react as PQ loads and do not actively respond to distribution system conditions. Their output power value will be the user-specified value. (Future Fault Contribution = Zero sequence impedance contribution).

Constant PQ
The solar generator model can be modelled as Constant PQ to simulate the effects of a PV installation. They react as PQ loads and do not actively respond to distribution system conditions. Their output power value will be the user-specified value (Fault Contribution = Load Current).

Model Database (Generator Instance)
The output power is entered as a percentage of the rated power of the generator. The slip or rotor angle of the machine is adjusted to achieve a total output power that matches this value.

The output power may be unbalanced during a by-phase analysis. The voltage setting is the desired terminal voltage for a synchronous machine. During analysis, the field current is adjusted within the field limits to attempt to reach this value. The metering phase specifies the phase to which the PT is connected for voltage metering.

The grounding impedance is used for active generator models in both load-flow and fault studies. Residue from unbalanced load current and/or fault current has a direct effect on generator terminal voltage. The grounding impedance also has the effect of reducing this current.

A synchronous or induction generator can be treated as a PQ load.