EMO-L DER Management

Genset and CHP Control

A Genset is a combination of an engine (prime mover) and an alternator. The engine converts the chemical energy of a fuel (diesel, gasoline, or gas) into mechanical energy. The mechanical energy produced by the engine is used to spin the alternator rotor through the magnetic field between the rotor and stator, creates a voltage on the alternator stator due to the electromagnetic induction phenomenon, converting mechanical energy to electrical energy. An alternator is made of two main parts: rotor and a stator. When connected to a load, which demands for current flow, then the generator produces power.

Cogeneration or Combined Heat and Power (CHP) is the use of a heat engine to generate electricity and useful heat at the same time at high efficiency. With on-site power production, losses are minimized and heat wasted is applied to facility loads in the form of process heating, steam, hot water, or even chilled water. CHP can be located at an individual facility or building, or it can be a district energy, Microgrid, and/or utility resource that provides power and thermal energy to multiple end users. CHP can be paired with other distributed energy technologies like solar photovoltaics and energy storage.

In this EMO-L guide, a CHP is seen as a Genset. Only the electrical behavior of the CHP is considered and controlled by the algorithm.

A Fuel System

B Control Panel

C Alternator

D Voltage Regulator

E Exhaust System

F Engine

G Lubrication System

H Radiator

I Base Frame

Generator Protection

The generators are protected by dedicated protections. For medium size generators, the following protections are used:

• Phase to phase and phase to earth over current.

• Percentage biased differential

• Negative sequence over current

• Overload

• Stator frame fault

• Rotor frame fault

• Reverse active power

• Reverse reactive power or loss of field

• Loss of synchronization

• Over and under voltage

• Over and under frequency

• Overheating of bearings

It is recommended to check with the manufacturer ability in providing a short circuit current ensuring the operation of the phase-to-phase short circuit protection when ordering a generator(s). In case of difficulties, the boosting of the generator’s excitation is required and shall be specified.

Generator Voltage and Frequency Control

The voltage and the frequency are controlled by the primary regulator(s) of the generator(s) provided by the genset manufacturer. The frequency is controlled by the speed regulator(s), while the voltage is controlled by the excitation regulator(s). These two regulations are considered as primary regulations while different operating modes are provided:

• Frequency by the speed regulator or governor, is to control the rotational speed of the engine linked to the alternator rotor. The frequency of the AC voltage is directly linked to the engine speed. Among the external control mode provided, the main modes used are:

  • Isochronous or constant frequency mode

  • Droop frequency / active power mode (F/P)

  • Constant active power set point

• Voltage by the AVR or AVC regulator, is to control the excitation voltage of the genset rotor. Several external control modes can be used:

  • Constant voltage mode

  • Droop voltage or reactive power (V/Q mode)

  • Constant reactive power setpoint

  • Power factor mode (PF mode)

Droop Curve

Droop control is a technique to control synchronous generators in electric grids. The droop curve represents how much the frequency and voltage are allowed to deviate from their nominal values to account for changes in power demands. In the most common type of droop control, frequency and voltage vary linearly with respect to active and reactive power.

The droop function is the control compensation applied by the genset controller when a power disturbance appears within the electrical network. It can be applied either on Frequency or active power compensation or on Voltage or reactive power compensation.

All active (reactive) power increase induces a frequency (voltage) decrease. The droop curve is characterized by its slope. For example, with a nominal frequency of 50Hz, a slope of 4% ensures a frequency between 49 Hz and 51 Hz along the full power range of the generator.

Inverter Control

An inverter is characterized by its capability to be a voltage source and a frequency reference.

The following two types of DER inverters are available:

• Grid-tied or Grid Following

• Grid Forming

Grid-tied or Grid Following

The inverter should be connected to a voltage source to be able to inject power and it is made to disconnect from the grid whenever voltage or frequency is more than acceptable boundaries. In case of PV inverters, the inverter is seen as a current source. The two types of inverters are:

• Grid-feeding - imposes its output active and/or reactive power to the connected grid at any voltage frequency and amplitude.

• Grid-supporting - adjusts its output active and/or reactive power to support grid voltage amplitude and/or frequency.

Grid-Forming

The inverter does not need the initial presence of a voltage source and can operate alone during Microgrid islanded mode or in parallel with other grid forming DER, contributing to the regulation of the frequency and voltage. In that case, the inverter is seen as a voltage source and becomes a voltage and frequency reference for the other DER and loads connected to the same Microgrid.

When connected to the Grid, the voltage and frequency reference is imposed by the grid itself. Inverter with grid-forming capability only (without any other feature) has no capacity to impose its own voltage and frequency reference against the grid. In Grid-Connected mode, when transition to the grid connected mode on a close transition, EMO switches the grid - forming inverter in VSG or droop - mode. Grid-forming inverter must change its operating mode, from voltage source operating mode to current source operating mode. The inverter in grid following mode allows the power control of the inverter according to customer use cases.

Control Mode

From a Microgrid operation perspective, the control of operating modes are focused on the inverter.

The following three modes are commonly identified:

• Active – reactive power (P/Q) control mode

• Voltage – frequency (V/f) control mode

• Droop/VSG control mode

Active – Reactive Power (P/Q) Control Mode

The inverter is Grid-Following or grid-tied. The inverter matches the busbar voltage and frequency, and generates power based on a P and Q set points provided by the control. Before the inverter can start, a grid-forming energy source, like a genset or a grid mains connection, must be connected to the busbar.

If the grid-forming genset or mains trips, the inverter cannot keep the voltage and frequency stable. The inverter therefore also stops power injection within the busbar it is connected to, which causes a blackout.

Voltage – Frequency (V/F) Control Mode

The inverter is Grid-Forming, meaning a voltage source. The inverter tries to keep the output voltage and frequency constant (usually at the nominal voltage and frequency defined through the settings of the inverter). This allows the inverter to generate power and frequency without any gensets or mains connected to the busbar.

However, since the inverter keeps the voltage and frequency constant, the inverter generally cannot be connected in parallel with a genset or grid mains connection. The constant voltage and frequency would conflict with other voltage and frequency sources. Therefore, if a genset or mains is connected to busbar, the inverter must generally switch back to P/Q mode. Depending on the inverter, a blackout might be required. Some inverters support running in V/f mode while parallel with other identical inverters (which are also in V/F mode).

Droop/VSG Control Mode

In inverters, droop control mode is also called VSG mode (Virtual Synchronous Generator mode). The inverter acts like a generator, which allows the inverter to always be grid-forming. In addition, in droop mode, the output voltage and frequency are controlled using predefined droop slopes. This allows the inverter to be connected in parallel with gensets and sometimes in mains connection.

EMO-L DER Regulations

When several DER are interconnected, being or not connected to the utility grid, they cannot work independently, otherwise they would oppose their actions. In this case, a secondary level control loop is necessary to coordinate them. This is one purpose of EMO-L.

The secondary regulation achieved by EMO-L, introducing a split with the primary regulations achieved by the DER controllers themselves, allows the synchronization of all the DER interconnected in parallel to achieve the common target of the Microgrid installation. If all primary regulations can be directly and locally set by an operator through the DER HMI, the secondary regulation replaces the human interaction with these primary regulations. This automation layer controls the primary regulations.

The secondary regulation is not as fast as the primary one, however it allows to improve the control of the operating points of the DER and to improve the global network’s electrical stability and the efficiency of the DER units.

One prior function of EMO-L is to regulate the frequency and the voltage on each island (electrical sub-network) to maintain them within their respective predefined acceptable ranges. This regulation is essential to keep the electrical and electromechanical machines, as well as protection IED, in their nominal electrical conditions of running.

When a voltage source DER at a consumer’s Microgrid installation operates in island mode (utility power supply disconnected), the voltage and the frequency at the main busbar the DER is connected to are both fixed by the DER and its control system. The DER operates in Voltage or Frequency (V/F) mode.

When main island busbar is connected to the utility power supply, the voltage and the frequency are both fixed by the utility. The control system of the DER with grid forming capabilities (generator or BESS) must be switched from Voltage or Frequency mode (V/F control mode) to Active power or Reactive power mode (P/Q control mode). When considering an inverter base DER, this control mode modification may be accompanied with a change of source mode.

EMO-L F/V Control Mode

When there is no active grid connection in the Microgrid electrical network, at least one DER acts as the voltage and frequency reference to ensure service continuity while the Microgrid operates in island mode. This responsibility is fulfilled by either a generator or a battery energy storage system (BESS) with grid-forming capability. In Microgrid operations, this reference voltage and frequency source is referred to as the anchor source. Any load power fluctuations are managed by this single anchor source. In the case of using one DER as the anchor source in an island, the operating mode for a generator is F/V isochronous mode, while for a BESS, it is grid-forming mode.

Some Microgrid installations require multiple DER to serve as the voltage and frequency reference, mainly because load power fluctuations can exceed the capacity of a single DER. In this case, EMO-L ensures the synchronization of voltage and frequency across all these DER. To achieve this synchronization, all DER contributing to voltage and frequency regulation should be set to (F/P) and (V/Q) droop modes for effective management. When referring to battery energy storage systems (BESS), the droop mode is known as VSG.

In island mode, for the power sources that regulate the voltage and frequency of the island busbar, EMO-L applies regulations to compensate for the operating points sliding along the droop curve of each DER. This compensation is applied either to frequency and active power or to voltage and reactive power.

The above figure illustrate behavior of the (F/P) and (V/Q) working points of threeDER connected in parallel, when a power demand, active or reactive, appears within the electrical network. (Fnom(t0)/Pi(t0)) and (Vnom(t0)/Qi(t0)) are the initial (F/P) and (V/Q) operating points before the power demand appears.

Assumption is made that at each steady state operating point reached, the power SoC of each DER controlled in that (F/V) control mode is the same. In other words, the rate of the current power delivered by the maximal power that can be delivered, has the same value for each DER. We will see how EMO-L is ensuring this condition by applying its regulation.

In the present example, three DER are connected to the same busbar and are sharing the same voltage and frequency. Then an active or reactive power demand increase appears, inducing a decrease of the frequency or voltage of the main busbar the DER in (F/V) control mode are connected to. Thanks to the primary regulation of each DER, the (F/P) and (V/Q) operating points slide along their relative droop curve, reaching the next operating points (F(t1)/P1,2,3(t1)) and (V(t1)/Q1,2,3(t1)). The droop curve characteristic being different for each DER, the power SoC reached by each primary regulation is different. Lower is the rate of the droop curve, higher is at t1 the power contribution of the DER after the power demand change.

EMO-L then applies a secondary regulation on each DER to come back to the nominal frequency and voltage for the island. This regulation applies in the two axes of the (F/P) and (V/Q) droop curves, reaching for each DER involved the same final operating points at t2 (Fnom(t2)/Pf(t2)) and (Vnom(t2)/Qf(t2)):

• Vertical axis: the compensation helps to get back the operating points to the nominal frequency and voltage.

• Horizontal axis: a (P/Q) load sharing compensation is applied so that the power SoC will be the same for each DER contributing to the (V/F) regulation of the busbar.

This regulation can be interpreted as a vertical sliding of each DER’s droop curve, within the range as shown on the below figure:

The below figure illustrates the equilibrium strategy applied by EMO-L on the power SoC of all the DER working in parallel. The width of the rectangle represents the DER power capacity. From the departure condition on the left hand, in which the power share is uneven, the load sharing function lowers the production of the first two DER and raises the production of the third one to reach a global production distribution proportional to the DER respective power capacities (both active and reactive powers).

EMO-L P/Q Control Mode or Load Sharing

The function of the P/Q control mode or load sharing function, is to control the exchange of active and reactive power within the installation and with the utility. This function is required to maintain continuity and stability of the power supply without overloading problems. EMO-L sharing function monitors the electrical network mainly through the information provided by the topological engine. The load sharing function considers the DER’s capability, the generated power, the DER’s operating modes, the imported/exported power of the active grid connection.

The following characteristics are taken into consideration when controlling a DER:

• Minimum and maximum power. If the limit is reached, the load sharing function stops sending control that decreases or increases the production and generates an alarm in the HMI.

• Maximum acceptable raising and lowering of power production (also called step up or down or incremental or decremental reserve margin). The load sharing is performed in several steps, compatible with the acceptable limits, to reach the final target.

• Minimum production level at which the load sharing function can operate.

• Minimum production level at which the generator can be disconnected.

The EMO-L controller takes into consideration the overall electrical network and accordingly adapts the DER’s targets in active power and reactive power while respecting the DER’s capacities and limits. The regulation is performed independently on each island (electrical sub-network) of the Microgrid installation.

The following principle of operations are used in most of the applications:

• The amount of the active and reactive power exchanged with the utility are set by the operator. The settings may be specified by the utility according to the grid code.

• The control system maintains the values of the exchange at the required values by acting on the controls of the DER.

• The control system computes and shares of the required active and reactive powers between the DER remained in operation.

Through this P/Q control mode, EMO-L:

• Prioritizes the usage of renewable energies, minimizing the consumption of the generators.

• Curtails the PV to protect either a BESS which is already full or to avoid injecting reverse power to the generator.

• Smooth the PV fluctuation by driving accordingly the BESS to avoid huge power fluctuation on the generator.

When the Microgrid is connected to the grid, the P/Q control mode allows the following:

• Strictly limit the value of the active power imported from the utility at the amount which cannot be provided by the DER when the demand of the installation exceeds their capability.

• Maintain at zero the imported active power, when the demand of the installation remains below the capability of the DER.

• Maintain the power factor of the installation at the contractual value specified by the utility.

• When the capability of the DER in providing reactive power is exceeded, the additional reactive power required to comply with the contractual power factor shall be supplied by a dedicated capacitor bank.

In grid connected mode, the grid power import export regulation is intrinsic of P/Q regulation of EMO-L. In the same way, this P/Q regulation applied at the point of common coupling with the grid maintains the power factor of the site within the limit defined by the utility, according to the grid code applied.

When the Microgrid or the island within the Microgrid is in the islanded mode, the P/Q control function:

• Shares/balances active and reactive power generation.

• Contributes to the voltage and frequency regulation.

EMO-L can prioritize reactive power as per type of the DER. Add all the control done to target power management according to DER present.

Spinning Reserve Management

EMO-L is designed to manage several generators in parallel, with or without the grid. EMO-L can also operate BESS.

The EMO-L automation analyzes the spinning reserve power of rotating machines to ensure power stability and meet operator criteria. Based on the computation of active and reactive power needs, EMO-L calculates the available spinning reserve on the Microgrid. If the spinning reserve is over a threshold defined by operator, then EMO-L will ask for a new energy source to start to increase the energy source capacity. The decision process takes account the power generation capacity of the BESS being connected to the Microgrid.

The start and stop command of the genset takes account the starting phase delay to get the new generator elected fully operational. EMO-L automation automatically elects rotating machine to start when production becomes insufficient. At the same time, it elects rotating machine to stop when there is an overproduction situation.

Battery Energy Storage System Management

The BESS must not go below a minimum and must not exceed a maximum State of Charge (SoC). Minimum and maximum SoC settings are customizable and are to be defined based on project requirements.

• The Microgrid controller provides fast load shedding (if there is a battery system and if we need more power than it can deliver) of a prioritized list of electrical loads and the energy storage system with grid-forming inverter switches from CSI to VSI mode, for the remaining loads to stay on-line connected.

• State of Charge (SoC) Management. EMO-L engine embeds algorithms to analyze the SoC of the energy storage system and optimize power delivery by considering the state of the charge values for the load sharing strategy to be deployed by EMO-L engine.

  NOTE: Operator can set minimum or maximum energy storage percentage and limit energy storage per DER.

EMO-L monitors the SoC of the BESS. It manages and control the determined minimum and maximum SoC in order to increase or discrease PV production to help avoid damage due to BESS overcharging. The Microgrid controller manages and controls the energy import or export capacities of the BESS.

PV Inverter Curtailment

Photo Voltaic (PV) power production depends on the real-time sun irradiance high, which on a time scale of a few minutes is deemed as unpredictable. Curtailing feature applies to projects integrating PV systems and need to curtail PV power production (or limit energy exported from PV system to the Microgrid electrical network) due to system stability conditions or grid codes constraints.

The Microgrid controller manages PV production and can shed PV energy production in specific cases (for example instability, energy production excess). The EMO-L engine manages PV sources curtailing based on topological, power balance and forecasting constraints.