review and testing of steffes electric thermal storage...

Review and Testing of Steffes Electric Thermal Storage Unit with Grid‐Interactive Frequency Regulation

Authors(s):

Richard Wies, Associate Professor, PI, University of Alaska Fairbanks

Nicholas Janssen, Research Assistant,

University of Alaska Fairbanks

Submitting Organization(s): University of Alaska Fairbanks

Institute of Northern Engineering PO Box 755910

Fairbanks, AK 99775‐5910

Prepared for: Intelligent Energy Systems, LLC

Dennis Meiners, Principal Under

EETF Grant #7310049: “Small Community Self‐Regulating Grid”

Subrecipient/Funding Organization:

Alaska Energy Authority 813 West Northern Lights Blvd.

Anchorage, AK 99503

Last Revised: Wednesday, January 22, 2014

ii

Cover Photo Description and Credits

Cover: Steffes ETS Grid Frequency Regulation Test Setup at UAF Electric Power Lab Photo Credit: Nicholas Janssen, UAF INE Graduate Research Assistant

iii

Table of Contents

Cover Photo Description and Credits ............................................................................................. ii

Table of Contents ........................................................................................................................... iii

List of Figures ................................................................................................................................ iv

List of Tables .................................................................................................................................. v

Acknowledgements ........................................................................................................................ vi

Summary of Findings ................................................................................................................... viii

CHAPTER 1 – INTRODUCTION AND BACKGROUND .......................................................... 9

Problem Statement and Objective ............................................................................................... 9

Scope of Work ............................................................................................................................ 9

Grid Frequency Regulation ....................................................................................................... 10

Electric Thermal Storage (ETS) ............................................................................................... 12

Control Methodologies ......................................................................................................... 13 Thermostatic Control ........................................................................................................ 13 Charge Level Control ........................................................................................................ 13 Electric Load Control ........................................................................................................ 14

Grid-Interactive ETS (GETS) ............................................................................................... 14

CHAPTER 2 – TESTING AND MEASUREMENT METHODS ............................................... 16

Laboratory Test Setup ............................................................................................................... 16

Testing Procedure ..................................................................................................................... 18

Measurement Methods .............................................................................................................. 19

CHAPTER 3 – OBSERVATIONS AND RESULTS ................................................................... 20

CHAPTER 4 – CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ....... 27

Conclusions ............................................................................................................................... 27

Recommendations for Future Work ......................................................................................... 28

REFERENCES ............................................................................................................................. 30

APPENDIX A: Steffes 2102 Specification and Data Sheets ........................................................ 31

APPENDIX B: Measurement Instrumentation Specification and Data Sheets ............................ 35

Fluke i410 Current Clamp ........................................................................................................ 36

Tektronix 5200A High Voltage Differential Probe .................................................................. 37

APPENDIX C: Additional Scenario Results for ETS GETS Controller ...................................... 38

iv

List of Figures

Figure 1: Example energy balance of a high-penetration wind-diesel power system. ................. 11 Figure 2: Typical ETS room unit [2]. ........................................................................................... 12 Figure 3: Grid-Interactive ETS (GETS) controller proposed response. ....................................... 14 Figure 4: Laboratory setup circuit diagram. ................................................................................. 16 Figure 5: Steffes ETS laboratory test setup showing the major components. .............................. 17 Figure 6: Steady state GETS system response. (No manual adjustments). .................................. 21 Figure 7: System response to a frequency swell (field perturbed). ............................................... 21 Figure 8: System response to dips in frequency (field perturbed). ............................................... 22 Figure 9: Typical observation with real power peaks shown (no field adjustments). .................. 23 Figure 10: Switching delays between zero and full load (no field adjustments). ......................... 24 Figure 11: Switching delays between 3/4 load and full load (no field adjustments). ................... 24 Figure 12: Visible time delays in the fast switching of small load. .............................................. 25 Figure 13: Visible time delays in the fast switching of medium load. ......................................... 26 Figure 14: Data group “Nov01”. No field adjustments, 23 Ohm load. ........................................ 39 Figure 15: Data group “Nov02”. No field adjustments, 23 Ohm load. ........................................ 40 Figure 16: Data group “Nov03”. Field adjustments made, 23 Ohm load. .................................... 41 Figure 17: Data group “Nov04”. Field adjustments made, 23 Ohm load. .................................... 42 Figure 18: Data group “Nov05”. Field adjustments made, 23 Ohm load. .................................... 43 Figure 19: Data group “Nov06”. No field adjustments, 30 Ohm load. ........................................ 44 Figure 20: Data group “Nov07”. Field adjustments made, 60 Ohm load. .................................... 45 Figure 21: Data group “Nov08”. Field adjustments made, 60 Ohm load. .................................... 46 Figure 22: Data group “Nov09”. No field adjustments, 60 Ohm load. ........................................ 47 Figure 23: Data group “Nov10”. No field adjustments, 60 Ohm load. ........................................ 48 Figure 24: Data group “Nov11”. Field adjustments made, phases ‘b’ and ‘c’ open. ................... 49 Figure 25: Data group “Dec01”. No field adjustments, phase ‘b’ unloaded. ............................... 50 Figure 26: Data group “Dec02”. Field adjustments made, phase ‘b’ unloaded. ........................... 51 Figure 27: Data group “Dec03”. Field adjustments made, phase ‘b’ unloaded. ........................... 52

v

List of Tables

Table 1 - GETS Controller Response Times ................................................................................ 22

vi

Acknowledgements

The testing, measurements, and observations of the Steffes Electric-Thermal Storage unit

with grid-interactive frequency regulation reported in this document were performed by the

Institute of Northern Engineering, University of Alaska Fairbanks under subcontract from

Intelligent Energy Systems, LLC with an Emerging Energy Technology Fund (EETF) Grant

#7310049: “Small Community Self‐Regulating Grid” from the Alaska Energy Authority.

Richard W. Wies, Associate Professor of Electrical and Computer Engineering, University of

Alaska Fairbanks, was the principal investigator. The second contributing author of this report is

Nicholas Janssen, Graduate Research Assistant.

The testing and measurements were performed in the Electric Power Systems Laboratory at

the University of Alaska Fairbanks under the direct supervision of Associate Professor Richard

Wies using available university equipment and supplies.

vii

Abstract

This report documents the findings of a laboratory analysis of a novel grid-interactive

electric-thermal storage (GETS) controller installed in a Steffes 2102 electric-thermal storage

(ETS) unit. The objective of this analysis was to document and describe the frequency response

of an isolated grid driven by a rotating prime mover with an ETS unit connected as a self-

regulating load. The controller is said to respond to changes in grid frequency by activating and

deactivating individual resistive heating elements in order to maintain a balance of real power in

the system. Measurements taken on a laboratory setup confirm that the unit is responding to

changes in grid frequency. The time series data are presented and discussed, though conclusions

about the performance of a network of ETS units in a full-scale grid application are difficult to

make without the development of a simulation model and testing in an Alaska village with a

hybrid wind-diesel system.

viii

Summary of Findings

The ultimate question this project sought to answer was, “Does the grid-interactive electric-

thermal storage (GETS) controller respond to changes in grid frequency in an isolated grid?”

The response of a single ETS unit with the GETS controller to manual frequency changes

was demonstrated in the laboratory using a standalone AC synchronous generator driven by a

DC motor. The results of this testing showed that the controller does respond to deviations from

60 Hz. The collected data provided some insight into the rate and nature of the controller

response.

The GETS controller responded to changes in system frequency f as described below:

1) f < 56.0 Hz: unit shut off with Error 20 due to under frequency condition,

2) 56.0 Hz f 60 Hz: unit stayed active with all resistive elements off,

3) 60 Hz < f < 60.5 Hz: unit cycled on and off resistive elements trying to overcome

generator inertia to correct system frequency to 60 Hz, and

4) f 60.5 Hz: unit remained on with all for resistive elements active.

Analysis of the laboratory results showed that rises in system frequency above 60 Hz were

met by additional load applied by the GETS controller. Similarly, when the unit was loaded and

the frequency dropped below 60 Hz, the controller responded by switching the load back off.

The GETS controller acting on its own was never able to adjust the output to a steady 60 Hz

waveform when the system frequency was between 60 Hz and 60.5 Hz. It was also not clear in

the data that there were four discrete resistive elements turning on and off at a rate proportional

to the frequency shift. The unit tended to alternate between two discrete and disparate loading

levels when attempting to make a frequency correction. Ramp rates and other patterns, such as

obvious time delays, were evident in the response of the ETS heater and are analyzed and

discussed in more detail within the report.

The authors recommended further study into the effects of a large network of self-regulating

ETS units on isolated medium and high-penetration wind-diesel grid stability. Both computer

modelling and in-situ testing would reveal some of the inherent challenges of regulating grid

frequency with multiple self-sensing ETS units with GETS controllers in an isolated grid.

9

CHAPTER 1 – INTRODUCTION AND BACKGROUND

Problem Statement and Objective

Integration of wind turbine generators into electrical grids in Alaska villages could alleviate

the high cost of imported diesel fuel and the associated electric energy prices. However, stability

problems are highly prevalent in systems that approach a medium to high level of wind energy

penetration. Secondary loads are necessary in order to absorb swells in wind energy that result in

grid frequencies above 60 Hz. Ideally, the central power plant, comprised of Diesel Electric

Generation (DEG), would be responsible for grid frequency regulation so long as the total wind

generation does not exceed demand. One proposed method for utilizing excess wind energy for

heating and grid frequency regulation is to power a network of Electric-Thermal Storage (ETS)

devices in village residences with thermostatic heating, charge, and electric load controllers.

ETS masonry heaters use resistive heating elements to charge a “core” of ceramic storage

bricks when the energy is either cheaper or more available. When the home’s thermostat calls for

heat, a blower is activated and air is forced through the ceramic core, and then into the room.

This provides space heat to village residences using excess wind energy that would otherwise

have no application. A grid-interactive ETS (GETS) controller has been added to the unit tested

in this study which was designed to sense grid frequency, and then add resistive electric load to

aid in stabilizing the grid frequency at 60 Hz when increases in wind generation or decreases in

load occur.

The objective of this study was to review, test, and evaluate a Steffes 2102 ETS with a grid-

interactive controller as a secondary load for frequency regulation in medium to high-penetration

wind-diesel grids in Alaska villages. A scope of work for this study is provided in the next

section.

Scope of Work

The scope of work for University of Alaska Fairbanks (UAF) Institute of Northern (INE)

under subcontract with Intelligent Energy Systems (IES) under an Emerging Energy Technology

Fund (EETF) Grant #7310049 “Small Community Self-Regulating Grid,” from the Alaska

Energy Authority (AEA) includes the review and testing of a single Steffes 2102 ETS unit with a

grid-interactive controller. The original grant included a provision to test and verify the operation

10

of the unit with the proposed self-regulating grid controller at the Alaska Center for Energy and

Power (ACEP) Power Systems Integration Laboratory (PSIL)) located on the UAF campus. An

amendment to that provision was proposed and adopted to conduct the testing in the UAF

Electric Power Systems Research laboratory given the small capacity of the single unit and the

need for more than a single day lease for testing.

The steps used to test and verify the operation of the Steffes 2102 ETS unit with the GETS

controller include the following.

1) Procure a Steffes ETS from Steffes through IES with the proposed GETS controller.

2) Power the unit with an isolated variable frequency three-phase AC generator.

3) Take the unit with the GETS controller through a series of positive and negative load

ramps in steps to observe the frequency response to load changes.

4) Record all pertinent data including voltage, current, power, and frequency.

5) Analyze the data to determine if the single Steffes ETS unit with the GETS controller

responds sufficiently to changes in load to regulate the system grid frequency.

Before describing the laboratory test setup, testing procedure, and measurement methods,

some background on grid frequency regulation and ETS control methodologies will be

presented.

Grid Frequency Regulation

Maintaining a constant grid frequency, particularly in an isolated high-penetration wind-

diesel grid, is the challenge of establishing a real time balance between real power generation

and demand. The three main variables are the wind power, the consumer electrical demand, and

the consumer heat demand.

If the amount of real power generation exceeds the demand at any given instant, the resulting

power imbalance will tend to accelerate any rotating masses in the system. This will result in

frequency swings above the nominal 60 Hz. Pseudo-instantaneous increases in wind power

generation and decreases in load are the primary causes.

In order to prevent these frequency swings, a high-penetration wind-diesel hybrid power

system must make use of a secondary (or dump) load (see Figure 1) in order to absorb any excess

11

wind power. A resistive load bank or electric boiler may be used for this purpose. However, if

there is no demand for this heat, the energy is wasted. Some systems may use an electrical

energy storage system, such as a lead-acid battery, but these add complexity and cost. There is

also no way for a fully-charged battery to absorb any additional energy.

Figure 1: Example energy balance of a high-penetration wind-diesel power system.

Some cold-climate communities, such as those located in rural Alaska and Canada, may

benefit from the use of medium to high-penetration wind-diesel systems and the dumped waste

heat. It is common for some remote diesel plants in these regions to operate a waste heat

recovery loop and provide space heating utility to a nearby community building [1]. However,

this solution hinges upon the geographic proximity of the diesel plant to a sizeable heat demand.

A wind-diesel power grid that could simultaneously maximize the use of available wind power

while also satisfying the heating demand of the rest of the community would be highly valuable

to the residents.

12

Electric Thermal Storage (ETS)

Masonry ETS units (see Figure 2) are space heaters consisting of a mass of ceramic bricks

(12) heated by resistive heating elements (14) [2]. When the space demands additional heat, a fan

(16) turns on and circulates room air (10) through the heated bricks. These units are designed to

operate in regions with “off-peak” electricity rates in order to help level load demand during

heating seasons (demand-side management). Since they are capable of delivering heat on

demand, storing heat energy, and absorbing electrical energy, they may be a well-suited

secondary load for isolated high-penetration wind-diesel systems.

Figure 2: Typical ETS room unit [2].

The ETS unit provided for review and testing for this application was the Steffes 2102. The

unit is powered by common 120 VAC with 4 sets of resistive elements for a total absorptive

capacity of 1.3 kW with manufacturer’s data and specifications as shown in Appendix A [2]. In

order to test the ETS unit, the control methodologies for delivering heat, storing heat, and

controlling electric load were reviewed and understood as described in the following sections.

13

Control Methodologies

A central challenge of implementing a network of ETS units in a high-penetration wind-

diesel grid is unit control. Much of the focus with this application of ETS units is on

determination of the optimal number of storage units to satisfy the heat demand while ensuring

enough capacity to consistently store excess wind power [3]. However, for delivering heat on

demand, storing heat energy, and absorbing electrical energy, the ETS units should have

controllers to optimize wind-diesel system performance and use of excess wind energy. The

three types of control related to delivering heat on demand, storing heat energy, and absorbing

electrical energy, respectively, are: 1) thermostatic, 2) charge level, and 3) electric load.

Thermostatic Control

All ETS units are controlled by a thermostat which ensures room temperature is maintained

by turning on a fan to pull air through the heated bricks and force heated air into the interior of

the residence. Since indoor temperatures are directly proportional to outdoor temperatures,

thermostatic control is dependent on the availability of stored heat (charge level control).

Charge Level Control

Charge level control of ETS units involves adjusting the target charge level and charging

rate. Two common methods of charge control used on the ETS units are: 1) charge level

adjustment with outdoor temperature, and 2) full charge level set point with off-peak charging.

In the first method the charge level is adjusted based on the outdoor temperature [4]. Lower

outdoor temperatures indicate a higher demand for space heat. This ensures the unit is adequately

charged in the case of a “cold snap”, when the heat is needed most.

In the second method a full charge level set point is maintained, but charging is limited to

off-peak hours [4]. This ensures there is always enough heating potential to satisfy the next day’s

demand. However, maintaining a high charge level is not necessarily beneficial for the utility. It

is in their best interest that a certain portion of the ETS units remain empty. The challenge is to

determine how many ETS units of a certain power level and storage capacity are needed in order

to provide benefit to both the consumer and the grid.

From the standpoint of ETS charging, the utility needs sufficient storage capacity in order to

absorb anticipated wind events. Maintaining an optimum charge level is imperative to both the

customer and the utility, since the aggregate system storage capacity must be sufficient in times

14

of high wind or low demand [3]. This is where electric load control can play a key role in

absorbing excess electric energy from wind such that grid frequency is maintained.

Electric Load Control

Electric load control of ETS units involves adjusting the number of active resistive heating

elements in response to a system frequency increase above 60 Hz. The additional system load

assists generation controls in maintaining the grid frequency. The increase in system frequency is

normally due to an increase in wind generation or a decrease in system load. Maintaining grid

frequency stability is critical to the operation of a high penetration wind-diesel system as

discussed in Chapter 1. Pursuant to the application of the ETS units in high-penetration wind-

diesel systems for maintaining grid frequency, a grid-interactive controller was developed by the

Steffes Corporation for the purpose of adding resistive elements (electric load) in response to

increases in frequency. This grid-interactive controller is the subject of this review and testing.

Grid-Interactive ETS (GETS)

A grid-interactive ETS (GETS) controller was developed which is said to sense the

frequency of the current powering the ETS unit, and activate resistive elements within the bricks

using controlled relays as the frequency increases above 60 Hz. The controller should

incrementally load or fully unload the unit in response to frequencies above and below 60 Hz,

respectively. The GETS controller was programmed to incrementally activate up to four resistive

elements to increase the electric load on the system in proportion to the frequency increase at a

rate of 25 % power capacity per 0.125 Hz between 60 and 60.5 Hz as shown in Figure 3.

Figure 3: Grid-Interactive ETS (GETS) controller proposed response.

15

Many of these ETS units with a grid-interactive control would be distributed throughout a

village power grid. In this study only a single ETS unit was tested to determine if the GETS

controller responded and at what rate to changes in frequencies above and below 60 Hz. The

question remains as to how a network of the ETS units would affect the dynamics of an isolated

high-penetration wind-diesel grid when individually responding to frequency increases above 60

Hz at the same time. Answering this question would be best suited for a dynamic simulation

model and an actual installation of these units in an isolated medium to high-penetration wind-

diesel system in an Alaska village.

Chapter 2 provides a detailed description of the laboratory test setup, testing procedure, and

measurement methods used to evaluate the performance of the single ETS unit with the grid-

interactive controller as supplied by the manufacturer. The results of the lab testing will then be

discussed in Chapter 3. Finally, in Chapter 4, recommendations will be provided and conclusions

drawn from our review and testing of the ETS unit with the grid-interactive controller.

16

CHAPTER 2 – TESTING AND MEASUREMENT METHODS

In order to evaluate the response of the GETS controller, a lab-scale three-phase 208-V mini

grid was established. Measurement devices for current and voltage also had to be installed along

with a way of sampling the waveforms at a moderately high frequency. Perturbations were

introduced to the system in order to simulate changes in wind power or electrical load. The

response of the GETS controller was subsequently measured throughout the duration of the

resulting system response. Of primary interest were frequency, voltage, and current.

Laboratory Test Setup

The laboratory setup used for testing the ETS unit is shown in Figures 4 and 5. The

synchronous machine (AC generator) is driven by a 5-kW, 120-V shunt-connected DC motor.

The 5 kW DC motor and synchronous machine are mechanically coupled by a common drive

shaft. Collectively they represent the DEG that would supply power to the village grid.

Figure 4: Steffes ETS laboratory test setup circuit diagram.

17

Figure 5: Steffes ETS laboratory test setup showing the major components.

18

The field currents of the DC motor and AC synchronous generator are independently

controlled by variable resistors. Adjustments in current to the DC motor field winding allow for

manual frequency control through shaft speed. Adjustments in current to the AC synchronous

machine field winding allow for manual voltage control. The loads connected to phases ‘b’ and

‘c’ of the AC synchronous generator are switchable resistive load banks. The Steffes 2102 ETS

unit equipped with the GETS controller is connected between phase ‘a’ and the neutral. Current

and voltage measurement instruments were placed on all three phases of the generator bus.

It should be noted that there are no automatic controls on either the voltage or frequency of

the system. Any changes to the system voltage and frequency had to be made manually by

adjustment of the AC synchronous machine field resistance and the shunt DC motor field

resistance, respectively.

Testing Procedure

The procedure used to test the GETS controller is outlined in the following steps.

1) Motor Startup – First, the shunt field and armature windings of the DC motor were energized.

As current was diverted from the field to the armature by increasing the field resistance, the shaft

speed approached 1200 RPM (synchronous speed for the AC generator). A handheld light-pulse

tachometer was used to confirm the shaft speed.

2) Generator Startup – Once the motor reached 1200 RPM, the synchronous generator field

current was gradually increased by decreasing the generator field resistance to bring the line-line

voltage up to 208 VAC, as measured by fixed instrumentation. Some back and forth adjustments

to both AC synchronous machine field and the shunt DC motor field resistors was needed before

a steady three-phase, 60 Hz, 208 VAC waveform was obtained.

3) Load Activation – The load was applied by closing the breaker and placing 23 ohms on phases

‘b’ and ‘c’ of the generator, with the Steffes 2102 GETS on phase ‘a’ (see Figure 5).

4) Field Adjustment – Having no automatic voltage regulator (AVR) or speed control, manual

adjustments had to be made with every change in load. Application of the initial load would result

in an immediate drop in frequency and voltage.

5) System Perturbation – Three cases were tested:

i) steady state response of the GETS controller (no field adjustments made),

ii) deliberate system perturbations by manual adjustment of the DC motor field, and

iii) deliberate system perturbations by manual removal of the initial load on phases ‘b’ and ‘c’.

19

Measurement of the voltage and current waveforms was performed to evaluate the subsequent

system dynamics throughout the duration of the resulting response.

Measurement Methods

Current and voltage measurements were taken on all three phases of the generator bus (see

Figures 4 and 5). Fluke i410 AC/DC Hall Effect current clamps (see Appendix B) with a 1 mV/A

output were used to measure current in all three phases. Tektronix P5200A high voltage

differential probes with 500X attenuation (see Appendix B) were placed between each phase and

the neutral point for a nominal L-N voltage of 120 Vrms.

Automatic measurements of all six channels were performed by using a National Instruments

PCI-6221 Data Acquisition System (DAQ) and breakout box with a LabView interface. The

outputs of the current and voltage probes were 256-bit 0-5 V signals, which were routed directly

to the breakout box and sampled at 6000 Hz.

Data post-processing was performed on the six channels of data (three phases of voltage and

three phases of current) using MATLAB in order to generate the following time series signals.

RMS Voltages [V] - Calculated in a sliding window of 600 samples (6 cycles).

RMS Currents [A] - Calculated in a sliding window of 600 samples (6 cycles).

Frequency [Hz] - The zero-crossings of the voltage waveforms in each phase were

interpolated in order to find the “instantaneous” value of the frequency.

Real Power [W] – The product of RMS voltage and RMS current was calculated

assuming unity power factor load.

The resulting time series signals for the three cases listed under Testing Procedure: 5) System

Perturbations were plotted as shown in the following chapter and used to make observations

about the response of the GETS controller to system frequency changes.

20

CHAPTER 3 – OBSERVATIONS AND RESULTS

Several observations about the response of the GETS controller were formulated upon

inspection of the results.

1) The GETS controller responded to adjustments in system loading based on measured

frequency with discrete levels of applied or removed load.

2) The controller responded to swells in frequency by adding load until fully loaded.

3) The controller responded to dips in frequency by removing load until fully unloaded.

4) Time delays were present in the controller response.

a) Delays of 1.00 s are observed when the controller switches from no load to full load.

b) Delays of 2.00 s are observed when the controller switches from ¾ load to full load.

5) Fast switching (0.10 s) of small to medium loads was observed.

a) Small (< 1 A) loads switch off for 0.10 s.

b) Medium (≈ 3 A) loads switch on for 0.10 s.

6) The controller cycled the ETS loads on and off at two discrete levels when the system

frequency was between 60 and 60.5 Hz.

Figure 6 shows a portion of the system response with no manual adjustments or interaction of

any kind. The changes in frequency and current were solely a result of the GETS controller

switching load on and off. Phase ‘a’ shows the current drawn by the Steffes heater, which

appears to rise and fall between about 2 A and 10 A. The range of frequency swings was from

about 57 Hz to just over 60.5 Hz.

In order to test system response time, deliberate perturbations in the DC motor field were

performed to adjust shaft speed and thus the output frequency of the AC generator. Initially, the

system frequency in Figure 7 was at 59.95 Hz with no ETS load. The DC motor field current

adjustment to increase shaft speed caused a rise in frequency to 60.25 Hz at t = 9.98 s. No load

was applied until t = 10.53 s: a delay of 0.55 s. By this time, the system frequency had risen to

well above 63 Hz. Also note that the initially applied load was only 5 amps (50% loading). By

the time all elements were activated (t = 11.54 s), a delay of 1.51 s had passed since the increase

in frequency was initiated.

21

Figure 6: Steady state GETS system response. (No manual adjustments).

Figure 7: System response to a frequency swell (field perturbed).

4 6 8 10 12 14

57

58

59

60

61

Time (s)

Fre

quen

cy (

Hz)

Frequency of Voltages

Fan

FbnFcn

4 6 8 10 12 14

2

4

6

8

10

Time (s)

RM

S C

urre

nt (

A)

RMS Line Currents

IaRMS

IbRMSIcRMS

10 10.5 11 11.5 12 12.5

58

60

62

64

66

68

X: 10.03Y: 60.5

Time (s)

Fre

quen

cy (

Hz)


X: 9.984Y: 60.25

X: 9.809Y: 59.95

Fan

FbnFcn

10 10.5 11 11.5 12 12.5

2

4

6

8

10

12

14

X: 11.54Y: 6.018

Time (s)

RM

S C

urre

nt

RMS Line Currents

X: 10.53Y: 1.745

IaRMS

IbRMSIcRMS

22

Next the controller response to under-frequency was observed. Figure 8 shows the system at

a stable 60.5 Hz with the ETS unit fully loaded. As the field on the DC motor was adjusted to

reduce the shaft speed, the AC generator frequency began to dip. At t = 2.16 s, the frequency had

already dipped to 60.0 Hz. At this point, the ETS unit should have been fully unloaded, but did

not do so until 1.346 s later at t = 3.51 s.

Figure 8: System response to a frequency dip (field perturbed).

In summary, Table 1 shows the controller’s frequency response times and ramp rates for

frequency swells and dips as observed in Figures 7 and 8, respectively. “Time to first response”

was the difference in time between the first point at which the controller could take any action

and the point at which any action was observed. “Time to appropriate response” was the time it

takes the controller to respond with an appropriate amount of loading/unloading according to the

specifications in Figure 3. The implications of these delays are discussed later in this section.

Table 1 - GETS Controller Response Times

Frequency swell Frequency dip Time to first response (s) 0.58 0.49

Time to appropriate response (s) 1.51 1.35 Ramp rate (Hz/s) 8.00 2.67

1 1.5 2 2.5 3 3.5 4 4.5

56

58

60

62

X: 2.16Y: 60

Time (s)

Fre

quen

cy (

Hz)


Fan

FbnFcn

1 1.5 2 2.5 3 3.5 4 4.50

5

10

15

X: 3.506Y: 8.832

Time (s)

RM

S C

urre

nt

RMS Line Currents

IaRMS

IbRMSIcRMS

23

As seen in Figure 6, the system cycled between at least two discrete levels of loading (2 A

and 10 A). Since the system has four elements, more than just these two discrete levels could be

seen in other scenarios as would be expected. However, it was not obvious in the data that four

discrete, equally-sized resistive elements were switching. In the absence of an AVR, it was better

to view these loading levels in terms of real power (product of current and voltage) rather than as

currents (A).

The following (Figure 9) shows a typical example of one of the eleven recorded scenarios.

Peaks in real power could only be seen at 200 W and 1100 W corresponding to no load (f 60

Hz) and full load (f 60.5 Hz), respectively. Power peaks between 200 W and 1100 W were not

visible here, and were scarcely present in some of the other recorded scenarios (see Appendix C).

Figure 9: Typical observation with real power peaks shown (no field adjustments).

Also present in the frequency response data were what appear to be fixed time delays in the

switching of the loads. When the controller switched from zero to full load, the load was

maintained for a minimum of 1.00 s (see Figure 10). There also appeared to be a minimum

amount of time of about 2.00 s maintained at no load. In addition, when the controller switched

between ¾ and full load (Figure 11), the load was maintained for 2.00 s.

-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy

Real Power Histogram

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Pow

er (

W)

Real Power (W)

Pa

Pb

Pc

24

Figure 10: Switching delays between zero and full load (no field adjustments).

Figure 11: Switching delays between 3/4 load and full load (no field adjustments).

4 6 8 10 12 14 16

56

58

60

62

Time (s)

Fre

quen

cy (

Hz)


Fan

FbnFcn

4 6 8 10 12 14 160

5

10

15

X: 4.338Y: 9.53

Time (s)

RM

S C

urre

nt

RMS Line Currents

X: 3.338Y: 2.044

X: 13.34Y: 9.707

X: 12.34Y: 1.869

X: 7.332Y: 10.29

X: 6.332Y: 1.501

X: 10.34Y: 9.214

X: 9.34Y: 1.642

IaRMS

IbRMSIcRMS

2 4 6 8 10 1255

60

65

70

Time (s)

Fre

quen

cy (

Hz)


Fan

FbnFcn

2 4 6 8 10 12

2

4

6

8

10

12

X: 10.51Y: 8.538

Time (s)

RM

S C

urre

nt

RMS Line Currents

X: 9.483Y: 9.314

X: 7.483Y: 8.062

X: 6.505Y: 10.02

X: 4.505Y: 8.349

X: 3.483Y: 10.37

X: 1.483Y: 8.393

IaRMS

IbRMSIcRMS

25

Also seen in the data were relatively fast switching transients of small load currents (< 1 A)

that appeared to be the control unit’s load demand (see Figure 12). These switching transients

were likely a characteristic of the control units design with the small load currents OFF for a

predetermined time of 0.10 s. This cycling on and off represented the control unit switching to

attempt to engage load in response to an over-frequency condition between 60.0 Hz and 60.5 Hz.

The control unit was never able to correct the AC synchronous generator frequency to 60 Hz

which was manually set to 60.3 Hz through the adjustment of the shunt field of the DC drive

motor.

Figure 12: Visible time delays in the fast switching of small load.

In addition, there was a slightly larger (≈ 3 A) load that was turned ON at the same rate (see

Figure 13). However, this load was kept ON for exactly 0.10 s. This cycling on and off

represented the control unit switching to attempt to disengage load in response to an under-

frequency condition (f < 60.0 Hz).

2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.459

60

61

62

Time (s)

Fre

quen

cy (

Hz)


Fan

FbnFcn

2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4

8

9

10

11

X: 2.69Y: 9.76

Time (s)

RM

S C

urre

nt

RMS Line Currents

X: 2.79Y: 9.163

X: 2.97Y: 10.08

X: 3.07Y: 9.496

X: 3.251Y: 10.54

X: 3.351Y: 9.883

IaRMS

IbRMSIcRMS

26

Figure 13: Visible time delays in the fast switching of medium load.

A comprehensive set of plots at full test time length for all scenarios tested were produced as

shown in Figures 14 through 27 of Appendix C.

Conclusions and the authors’ recommendations for future work based on results and

observations presented above are discussed in the following chapter.

3.2 3.4 3.6 3.8 4 4.258.5

59

59.5

60

60.5

61

Time (s)

Fre

quen

cy (

Hz)


Fan

FbnFcn

3.2 3.4 3.6 3.8 4 4.2

1

2

3

4

5

Time (s)

RM

S C

urre

nt

X: 3.926Y: 4.373

X: 3.608Y: 1.574

X: 3.507Y: 4.389

X: 3.407Y: 1.491

X: 3.29Y: 4.421

X: 3.19Y: 1.368

X: 3.708Y: 4.421

X: 3.826Y: 1.698

RMS Line Currents

IaRMS

IbRMSIcRMS

27

CHAPTER 4 – CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

Conclusions and recommendations for future work based on our review and testing of the

Steffes 2102 ETS with the GETS controller are provided in the following sections.

Conclusions

In this study an individual Steffes 2102 ETS with a GETS controller was reviewed and tested

for response to changes in frequency above and below 60 Hz. The results of the laboratory test

revealed that the GETS controller responded to changes in frequency. In general our findings

suggest that although the Steffes 2102 ETS with the GETS controller responds to frequencies

above 60 Hz by switching on resistive load, we cannot confirm that it responds as indicated with

incremental load based on the magnitude of the grid frequency shift with our test setup.

Limitations in our test setup prevented the assertion of any conclusions about how a network of

these units would respond on a larger grid.

There were three major differences between the test setup and a realistic isolated medium to

high-penetration wind-diesel grid. These differences should be considered when drawing any

conclusions about the data presented in this study.

First, there was no automatic speed controller or voltage regulator on the motor/generator

system. These devices are present on all DEGs and are responsible for controlling grid frequency

and voltage. Without these devices in the laboratory test setup, we could only see how the GETS

controller takes action as a result of certain frequency excursions. We could not make any

conclusions about its ability to assist with grid frequency regulation.

Second, the capacity of motor/generator in proportion to the load capacity of the Steffes 2102

unit was much larger than a village power system with DEGs. By the time a load correction was

made by the controller, the system had already slowed down or sped up well beyond the

allowable frequency limits of a typical grid. This gives the appearance of poor controller

performance (see Figure 6), however, the controller’s response in a situation with more base load

and generation present could improve.

Finally, we were only able to test a single unit on a single phase of the three-phase AC

synchronous generator. This architecture was very different from an actual village electrical

power system with a network of ETS units—each attempting to regulate grid frequency.

28

Understanding these differences was important for interpreting the results of this study.

Visible time delays were present (see Figures 10 and 11) which appeared to have negative

consequences on the laboratory test grid’s system frequency. These time delays could contribute

to grid instability in a larger medium to high-penetration wind-diesel system where it is

important that not all ETS units respond at the same time to pseudo-instantaneous changes in

generation or load. However, the ETS units should respond faster than the DEG. Since the DEG

cannot tell the difference between primary consumer load and secondary ETS load, the ETS

units must unload before the DEG “picks them up” and makes them a part of the primary load.

In conclusion, based on the authors’ review and testing of a single Steffes 2102 ETS unit

with GETS control, while the unit response to frequency shifts was evident, we could not

confirm the incremental load versus frequency response with our test setup. However, based on

our results, recommendations for future testing of ETS units with grid-interactive control were

proposed as discussed in the next section.

Recommendations for Future Work

The concept of the grid-interactive control with multiple ETS units in an isolated wind-diesel

grid should be further investigated through in-situ testing and modelling to determine their effect

on system stability. Two courses of action are recommended in order to further validate the

concept of the ETS with a self-regulating GETS controller.

1) A network of these units needs to be installed on an actual village wind-diesel grid to

collect data and observe the system frequency response. It was not clear from our testing

how the timing of the units in response to frequency changes above and below 60 Hz

would affect system transient stability. System stability could be an issue with the GETS

control competing against the response of the DEG(s) speed control and other ETS units

with GETS control in the network.

2) Secondly, a modelling study should be conducted to improve our understanding of the

effects of time delays, loading levels, and ETS unit cooperation. Such a study would

require an accurate wind-diesel system model and additional knowledge of how the

GETS controller would respond to changes in wind generation from induction machines,

diesel generation from three-phase AC synchronous machines, and system load.

29

In closing, further study of these ETS units with GETS controllers for isolated medium to

high-penetration wind-diesel grids is required to determine their effectiveness in stabilizing grid

frequency.

30

REFERENCES

[1] Isherwood, W., Smith, J. R., Aceves, S. M., Berry, G., Clark, W., Johnson, R., Das, D., Goering, D., & Seifert, R. (2000). Remote power systems with advanced storage technologies for Alaskan villages. Energy: The International Journal, 25(10), 1005–1020.

[2] Steffes 2102 Specification and Data Sheets. Retrieved December 18th, 2013, from the

Steffes Corporation website: http://www.steffes.com/off-peak-heating/residential-systems.html

[3] Hughes, L. (2010). Meeting residential space heating demand with wind-generated

electricity. Renewable Energy: An International Journal, 35(8), 1765–1772. [4] Bedouani, B. Y., Moreau, A., Parent, M. & Labrecque, B. (2001). Central electric

thermal storage (ETS) feasibility for residential applications: Part 1. Numerical and experimental study. International Journal of Energy Research, 25(1), 53–72. doi: 10.1002/1099-114X(200101)25:1<53::AID-ER610>3.0.CO;2-T

[5] Fluke i410 Current Clamp Specification and Data Sheet. Retrieved December 22nd, 2013,

Fluke Company website: http://www.fluke.com/fluke/inen/accessories/Current-clamps/i410.htm?PID=56301

[6] Tektronix 5200A High Voltage Differential Probe Specification and Data Sheet. Retrieved December 22nd, 2013, from the Tektronix Company website: http://www.tek.com/differential-probe-high-voltage

31

APPENDIX A: Steffes 2102 Specification and Data Sheets

Steffes 2102

35

APPENDIX B: Measurement Instrumentation Specification and Data Sheets

Fluke i410 AC/DC Current Clamp

Tektronix P5200A High Voltage Differential Probe

36

Fluke i410 Current Clamp

Specifications

Measurement type Hall sensor

Nominal current range 400 A, AC/DC

Continuous current range 1 A - 400 A AC/DC

Maximum Non-Destructive Current

400 A

Lowest measurable current

0.5 A

Basic Accuracy 3.5% + 0.5 A (% reading + floorspec)

Useable frequency DC - 3 kHz

Output level(s) 1 mV/A

Zero error adjustment Yes

Safety Specifications

Safety CAT III, 600 V

Maximum voltage 600 V

Mechanical & General Specifications

Warranty 1 year

Battery Life 9 V, 60 h

Maximum conductor diameter

30 mm 2 x 25 mm

Output cable length 1.6 m

Shrouded banana plugs Yes

37

Tektronix 5200A High Voltage Differential Probe

P5200A

Attenuation 50X / 500X

Differential Voltage 500X: ±1300 V 50X: ±130 V

Common Mode Voltage ±1300 V

Maximum Input Voltage-to-Earth 1000 V CAT II

Bandwidth 50 MHz

Differential Input Impedance 10 MΩ || 2 pF

Input Impedance between each Input and Ground 5 MΩ || 4 pF

Typical CMRR DC: >80 dB 100 kHz: >60 dB 3.2 MHz: >30 dB 50 MHz: >26 dB

Cable length 1.8 m

1. The differential voltage is the maximum measurable range between the (+) and (-) input leads of the probe. Beyond these limits, the output could be clipped.

2. The maximum common mode voltage and maximum input voltage-to-earth (RMS) are the maximum voltages that each input lead (+/-) can be from ground.

38

APPENDIX C: Additional Scenario Results for ETS GETS Controller Figure 14: Data group “Nov01”. No field adjustments, 23 Ohm load. ...................................39

Figure 15: Data group “Nov02”. No field adjustments, 23 Ohm load. ...................................40

Figure 16: Data group “Nov03”. Field adjustments made, 23 Ohm load. ...............................41








Figure 24: Data group “Nov11”. Field adjustments made, phases ‘b’ and ‘c’ open. ..............49

Figure 25: Data group “Dec01”. No field adjustments, phase ‘b’ unloaded. ..........................50

Figure 26: Data group “Dec02”. Field adjustments made, phase ‘b’ unloaded. ......................51

Figure 27: Data group “Dec03”. Field adjustments made, phase ‘b’ unloaded. ......................52

39

Figure 14: Data group “Nov01”. No field adjustments, 23 Ohm load.

-200 0 200 400 600 800 1000 1200 1400 16000

1

2

3

4

5

6x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2059.5

60

60.5

61

61.5

62

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

40


-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2059

60

61

62

63

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

41

Figure 16: Data group “Nov03”. Field adjustments made, 23 Ohm load.

-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2059.5

60

60.5

61

61.5

62

62.5

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

42


-200 0 200 400 600 800 1000 1200 1400 16000

0.5

1

1.5

2

2.5

3x 10

4

Real Power (W)

Fre

quen

cy


0 1 2 3 4 5 6 7 8 9 1057

58

59

60

61

62

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 1 2 3 4 5 6 7 8 9 100

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

43


-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2056

58

60

62

64

66

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

44


-200 0 200 400 600 800 1000 1200 1400 16000

5

10

15x 10

4

Real Power (W)

Fre

quen

cy


0 5 10 15 20 25 30 35 4057

58

59

60

61

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fbfc

0 5 10 15 20 25 30 35 400

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

45


-200 0 200 400 600 800 1000 1200 1400 16000

1

2

3

4

5

6x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2056

58

60

62

64

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

46


-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2058

58.5

59

59.5

60

60.5

61

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

47


-200 0 200 400 600 800 1000 1200 1400 16000

1

2

3

4

5x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2059

60

61

62

63

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

48


-200 0 200 400 600 800 1000 1200 1400 16000

1

2

3

4

5

6x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2058

59

60

61

62

63

64

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

49

Figure 24: Data group “Nov11”. Field adjustments made, phases ‘b’ and ‘c’ open.

-200 0 200 400 600 800 1000 1200 1400 16000

1

2

3

4

5x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2056

58

60

62

64

66

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

50

Figure 25: Data group “Dec01”. No field adjustments, phase ‘b’ unloaded.

-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy


0 2 4 6 8 10 12 14 16 18 2056

58

60

62

64

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 2 4 6 8 10 12 14 16 18 200

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

51

Figure 26: Data group “Dec02”. Field adjustments made, phase ‘b’ unloaded.

-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8x 10

4

Real Power (W)

Fre

quen

cy


0 5 10 15 20 25 3056

58

60

62

64

66

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 5 10 15 20 25 300

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

52

Figure 27: Data group “Dec03”. Field adjustments made, phase ‘b’ unloaded.

-200 0 200 400 600 800 1000 1200 1400 16000

2

4

6

8

10

12x 10

4

Real Power (W)

Fre

quen

cy


0 5 10 15 20 25 3054

56

58

60

62

64

Time (s)

Fre

quen

cy (

Hz)

Frequency

fa

fb

fc

0 5 10 15 20 25 300

500

1000

1500

Time (s)

Rea

l Pow

er (

W)

Real Power

Pa

Pb

Pc

review and testing of steffes electric thermal storage...

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